Acoustic Siren for Generating Wide-Band Noise

THE JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA VOLUME 35, NUMBER 2 FEBRUARY 1963

Acoustic Siren for Generating Wide-Band Noise*

J. N. COLE, R. G. POWELL, H. L. OESTREICHER, AND H. E. VON GIERKE

Bio-Acoustics Branch, Aerospace Medical Research Laboratories, Wright-Patterson Air Force Base, Ohio (Received 25 April 1962)

Principle, theory, and an experimental development program for a new type of siren, capable of generating wide-band noise, are discussed with special emphasis on its application to the economic and realistic simulation of high-intensity jet and rocket-noise environments. In contrast to conventional sirens with a single rotor, this wide-band noise siren uses a series of overlapping, slotted rotors rotating at different speeds to produce the modulation of an air flow through a nozzle. The sound field generated by this siren can, mathe- matically, best be approximated by an "almost periodic" function; that is, a function whose spectrum is a line spectrum but with infinitely many lines in each interval of frequency. A series for the power spectrum can be derived and used to guide the design. For most practical purposes, the resulting acoustic field radiated by the siren represents random noise. Data on acoustic power, spectrum, efficiency of noise generation, and fine structure of the noise for various experimental siren types are presented. The potential value of this principle for large-scale installations (sonic fatigue and missile-component testing and bio-acoustic applications) is evaluated.

INTRODUCTION

HE need for testing man, materials, electronic and mechanical components, and even full-scale vehi-

cle structures under the adverse acoustical environments

of aerospace operations increases continually, and the noise levels to which missiles, launch-site equipment, and aircraft are exposed become more intense from year to year with the increase in power of the propulsion systems. 1,•' In order to study the causes of failure, improve the design of components, and assure over-all system reliability, the capability to simulate acoustical environments in the laboratory realistically, economi- cally, and under well-controlled conditions is a necessity. a The high-intensity sound fields to be simulated over large test areas or volumes range in rms sound-pressure

* Some of the results reported in this paper were presented at the 56th meeting, 1958, and the 62nd meeting, 1961, of the Acoustical Society of America and at the 8th Annual Air Force Science and Engineering Symposium, 1961.

• Proceedings of the Symposium on Fatigue of Aircraft Struc- tures, WADC TR 59-507 (August 1959).

• H. E. yon Gierke, "Recent Advances and Problems in Aviation Acoustics," in Proceedings of the Third International Congress on Acoustics, Stuttgart, Germany, 1959 (The Elsevier Publishing Company, Holland, 1960).

a D. M. Forney, Jr., "Acoustical Fatigue Test Procedures Used in Aircraft Industry and Their Limitations," in WADC TR 59-676, WADC-University of Minnesota Conference on Acoustical Fatigue, edited by W. J. Trapp and D. M. Forney, Jr. (March 1961), pp. 339-378.

levels from 150 to 175 dB re 0.0002 microbar and are

wide-band, random-type noises with a high peak factor. 4 Attempts to simulate such broad-band noise at very

high intensities with electrodynamic transducers, air jets, small pulse jets, and air-modulated loudspeakers have not been completely successful. Either the sources did not fulfill the very high sound-pressure or sound- power requirements, or they promised to do it only with such low efficiency that economical or other practical considerations limited their usefulness. •,5 The low-

frequency part of the desired noise spectrum was especially difficult and uneconomical to produce. The only effective and economical sound generator available was the siren producing strictly periodic sounds. Its principle, the periodic modulation of an airstream by relatively inexpensive, mechanically rotating machinery, has been studied and applied in acoustics extensively, whenever sounds of high intensity were required. ø A mechanical-acoustical efficiency for sound generation of almost 100% can be achieved with such sirens producing square-wave waveforms. If a sinusoidal sound generator

4 t½ H. E. von Gierke, Types of Pressure Fields of Interest in Acoustical Fatigue ProBlems," in WADC TR 59-676, WADC- University of Minnesota Conference on Acoustical Fatigue, edited by W. J. Trapp and D. M. Forney, Jr. (March 1961), pp. 57-84.

5 W. H. Mayes and P.M. Edge, Jr., J. Acoust. Soc. Am. (A) 33, No. 6, 846 (1961).

6 T. F. Hueter and R. H. Bolt, Sorties (John Wiley & Sons, Inc., New York, 1955).

173

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21:28:14

174 COLE, POWELL, OESTREICHER, AND YON GIERKE

without distortion is desired, the theoretical maximum efficiency is 50%. As a consequence, several high- intensity siren facilities have been built and are in use for acoustical fatigue testing and the environmental testing of equipment and components. a

Although the value of discrete frequency testing for all these applications cannot be denied, the results obtained require mathematical interpretation or the assumption of specific conditions before they can be applied to predict the behavior of components in a random-type noise field. Nonlinear responses of structures make the interpretation of discrete frequency tests for random-noise conditions extremely difficult. The quantitative guidance obtained from pure-tone siren tests to predict the response to random acoustic loading is marginal with the present state of the art. Such a statement does not deny the usefulness of such tests for more basic research studies and for rough engineering comparisons of similar structures. There is no question that basic and applied research in this area needs an economical laboratory tool for producing random-type, high-intensity noise. Similarly, such a tool is desirable for over-all tests on whole missiles or aircraft structures, and for checking over-all performance of mechanical or electrical systems, and of whole space capsules, with or without man, under the simulated acoustical stresses of flight operation.

Recent progress in the development of electro- pneumatic transducers has resulted in the availability of generators that can produce moderatdy high-intensity, broad-band random noise. 7.8 These devices offer a means of simulating random acoustic environments within certain limitations of power and frequency range. Their application, however, for simulation of the very high-intensity levels sometimes required over large areas or in large volumes becomes question- able in terms of service life, maintenance, economy, and space and volume requirements.

The use of amplitude-modulated, discrete frequency sirens to produce side bands of noise for simulation of random acoustic environments is being considered. 9 The large number of such sirens necessary to cover an ade- quate frequency range, plus the requirements for the necessary control systems, poses practical problems when considering this approach toward broad-band noise simulation.

The wide-band siren to be discussed here constitutes

another new approach toward the development of a noise source to solve the problems outlined above. It employs the advantages of the conventional siren to produce broad-band noise with characteristics similar

? J. K. Hilllard and W. T. Fiala, "Methods of Generating In- creased Sonic Power for Environmental Testing," Ling-Altec Re- search, Division of Ling-Altec Electronics, Inc. (unnumbered report).

8 D.C. Skilling, "Norair Air Modulator Acoustic Generator," Norair Report NB 60-234 (August 1960).

9 C. H. Allen and B. G. Watters, J. Acoust. Soc. Am. 31, No. 4, 463-469 (1959).

I- ROTO RS (4) 2-S LOTS

3-NOZZLE

4-SHAFTS (4) 5-DRIVEN PULLEYS

PORT DIAMETER -0.28 INCH

ROTOR DIAMETER- 2 INCHF_•

ß ,e---. 5

Fro. 1. First experimental model of wide-band noise siren. The mass flow of air issuing from the nozzle (3) is modulated by the four rotors (1). The rotors are rotated at different speeds.

to jet and rocket noise. •ø.u This new type of siren employs rotating machinery to accomplish efficient and irregular modulation of an airstream. A new class of sounds can so be produced by sirens, and the noises from the models of the wide-band noise siren investi-

gated to date are only specific examples of such sounds. The basic principle of the wide-band noise siren can

best be seen from the first small-scale model of the siren

shown in Fig. 1. Complex modulation of an airstream issuing from a nozzle is achieved mechanically by a series of overlapping rotors irregularly slotted and rotated at speed ratios such that any instantaneous combination of rotor positions will not repeat except over long time intervals. Preliminary calculations and experiments with such a siren indicated that wide-band power spectra similar to jet and rocket-noise spectra can be obtained, and that the principle showed enough promise to warrant a more detailed program to study the practical potential of such a siren. •2.• The purposes of this program were: (a) to develop a theoretical under- standing of this noise-siren principle; (b) to study the merits of various design configurations proposed; (c) to investigate the dependence of the wide-band noise produced on the basic mechanical and airflow characteristics of the siren; and (d) to assess the potential of the siren for large-scale environmental testing, with special emphasis on studying the efficiency of noise generation.

14 15 The material to be presented . discusses briefly the •0 H. E. yon Gierke and J. N. Cole, Acoustic Siren for Generating

Wide Band Noise, U.S. Patent 2,912,958 (17 November 1959). • J. N. Cole and H. E. yon Gierke, Acoustic Siren for Generating

Wide Band Noise with Improved Efficiency, U.S. Patent Applica- tion Ser. No. 8466 (12 February 1960).

•2 j. N. Cole, H. E. yon Gierke, H. L. Oestreicher, and R. G. Powell, "Simulation of Random Acoustic Environments by a Wide Band Noise Siren," Bulletin No. 27, Part II. Shock, Vibra- tion, and Associated Environments, Office of the Secretary of Defense (1959).

• H. E. yon Gierke, J. N. Cole, and H. L. Oestreicher, J. Acoust. Soc. Am. (A) 31, No. 1, 116-117 (1959).

•4 j. N. Cole, H. L. Oestreicher, R. G. Powell, and H. E. yon Gierke, J. Acoust. Soc. Am. (A) 33, No. 11, 1675 (1961).

•* J. N. Cole, R. G. Powell, H. L. Oestreicher, and H. E. yon


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WIDE-BAND ACOUSTIC SIREN 175

main results of these studies by using representative data and answers the questions raised insofar as it is possible today.

I. THEORY

The generation of a large variety of sounds by means of the highly effective principle of airstream modulation can be achieved in various ways. The siren principle described in this report uses relatively inexpensive rotating machinery to generate a class of irregular sounds which have a power spectrum similar to the wide-band noise that is frequently encountered in practical environments.

A conventional siren with one or more openings ø achieves the modulation of an airstream simply by periodically interrupting it. If A•(t) is the modulated cross section of the airstream at time t, then, for a conventional siren, A•(t) is a periodic quasitrapezoidal function. If the times during which the rotor leaves the port either completely open or completely closed are long compared to the times during which the port is partially covered, then A•(t) can be considered a rectangular pulse (upper part of Fig. 2). Let T• be the period and q• the time during which the nozzle is open. Both quantities are determined by rotor speed and the dimen- sions of the openings in the rotor. If we now consider two overlapping rotors with different openings A•(t) and A,.(t), turning with different speeds, then the open- nozzle area A, i.e., the cross section of the airstream, is simply given by

A(t)=A•(t).A,.(t) (1)

because A (t) equals the full-nozzle area when both rotors are open, and A (t)-0 when at least one rotor is closed. A (t) is, therefore, also a sequence of rectangular pulses, but with pulses differently shaped and unequally spaced. Figure 2 shows the function A (t) for a two-wheel siren. The extension to a siren which has 3, 4... j-.. wheels is obvious: A (t) is the product of all the As(t).

From the function describing the modulation of the nozzle area, we calculate the corresponding sound pressure under somewhat idealized conditions. Assuming that the chamber pressure is sufficiently large so that the reaction of the sound field is negligible and that the siren radiates like a piston, then the exit velocity is independent of frequency, and the sound pressure in the farfield of a siren without horn is given by

p(t)=C.dA(t)/dt, (2)

for wavelengths for which the siren can be considered a point source.

If the siren is fitted with an ideally matched horn, the sound pressure in the farfield is given by

ph(t)=Ch'A(t), (3)

Gierke, "Rocket and Jet Noise Simulation by the New Wide Band Noise Siren," ASD TR 61-394, Vol. 1 (September 1961).

i

i

FIO. 2. Jet-exit area as function of time for two rotors modulating individually (upper two functions A • and A2) and together (lower function A -- A •. A 0.

where C and Ch are constants, depending on the operating conditions.

By means of Eqs. (1) and (2), it is now easy to calculate the power spectrum of the idealized two-rotor siren. If T• and T,. are the periods of the two rotors, then the Fourier series of the two rectangular pulse functions of Fig. 2 are'

+• 2•r A•(t)= • a•,, exp(ico•nt), co•=2•rf•=--;

n•-• T1

+• 2•- A•.(/)= • a•.• exp(i•nt), •=2•f•=--; (4)

aj.•= (1/mr) sin (ncosqj/2) exp(--incoyqy/2)

(/=1, 2).

The periodic functions A•(t) and A,.(t) have discrete spectra, which are plotted in the upper part of Fig. 3. The Fourier series of A (t) follows easily from Eq. (4).

A (t) =A•(t).A2(t) = • a•,,•a,., •,1 • • 2-•--•

X expi(n•co•+n2w2)t. (5)

The function represented by Eq. (5) has, as is seen from the formula, a discrete spectrum for any values of •, •=, but is periodic only if the ratio •/•= is rational. If this ratio is irrational, however, then the function is not periodic, but still has a discrete spectrum. Functions of this t•e are called "almost periodic functions" and occupy a position between the perodic functions and the nonperiodic functions with continuous spectrum. In


21:28:14

176 COLE, POWELL, OESTREICHER, AND vos GIERKE

0.5

I

a) e in •, { Ai ('J') = Z ,Oi,n Ap_(f):-• ae,n ein '":' f _

' 1000 2000 3000

FREQUENCY IN C P$

IbmJ

,•(t) = ,•,(t) ß % (t)

= n • bn n2ei(nl •, +n z • )f i,n• I'

1000 2000 FREQUENCY IN C P$

3000

Fro. 3. Spectra of functions A•, A2, and A =A•.A2 describing the modulation of the nozzle-exit area (f• =330 cps, f•= 245 cps).

the case of a periodic function, Eq. (5) is an ordinary Fourier series, and the spectral lines are separated by 1IT if T is the period. If w•/w.o is irrational, the spectral lines are everywhere dense, i.e., in every interval are infinitely many lines, but the total energy is, of course, still finite. We assume for the theoretical treatment of

the wide-band siren an irrational ratio of w•/w2, but, for practical purposes, where limited accuracy makes a distinction between rational and irrational numbers

impossible, any rational ratio which gives a sufficiently large period of A (t) will be just as good.

Equation (5) shows the basic principle of the siren: A(t) is obtained by modulating every harmonic component of A •(t) with every component of A2(t). Each

> • 14

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6 o 4

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..i t3 i I • i i ,I i.d 75 150 300 6 0 1200 2400 4800 n- 150 300 600 1200 2400 4800 9600

OCTAVE BAND FREQUENCY IN CP5

Fro. 4. Calculated, relative acoustic power spectrum for a two-rotor siren with the nozzle-exit modulation A described in Fig. 2.

line in the spectrum of A x(t) or A•t) is therefore split into infinitely many lines, and the energy which was originally contained in the fundamentals and the higher harmonics of A x(t) and A•t) is spread over larger frequency intervals. This modulation and spreading effect is illustrated by the lower part of Fig. 3, where the spectrum of the function A(t) is plotted. If w•/w•is irrational, an infinite number of lines is in every interval, but, of course, only a finite number of them contributes significantly to the energy.

Figure 4 represents the relative acoustic power-level spectrum which is obtained from Fig. 3 corresponding to Eq. (2) and shows the relative distribution of energy in octave bands for the particular numerical example used. For this calculation, the siren was assumed to be a simple source radiator.

This example of the two-rotor siren shows the basic mechanism which shifts energy by modulation from lower to higher frequency bands. If more than two rotors are used on the siren, each additional rotor again modulates the spectrum with all harmonics and widens the power spectrum still further.

II. DESCRIPTION OF DEVELOPMENTAL MODELS

Many mechanical arrangements can be used to achieve the "almost periodic" mechanical modulation of air flow described in the preceding section. Two basic

5 5

Fro. 5. Four-rotor wide-band noise siren with one nozzle (1). The rotors (2) with the irregular slots (3) are mounted on two sets of coaxial shafts (4) driven at various speeds by pulleys](5).


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(a)

Fro. 6. (a) Wide-band noise siren with coaxially mounted rotors (sche- matic). The rotors (2) have approximately the same diameter as the air stream (1). The modulated air stream can easily be coupled to an acoustic horn (8). The rotors are mounted free wheeling on one fixed shaft (4) and are driven by belts (6) in a groove on their circumference [support for shaft (5), drive pulley (7)']. (b) Coaxial design of model siren generating wide-band noise. Over-all view of "basic siren

model" with only one port loaded with horn type (b) in Fig. 7. (c) Coaxial model of wide-band noise siren. Cross- sectional side view.

TOP PLATE SPACER,---• REAR -TOP JOINER PLATE

DIRECTION OF AIR

t-- HOSE ADAPTER

(b)

•-- TOP PLATE SlaACER, FRONT

ß

•HORN ADAPTER RING

•FRONT HOUSING

BEARING RETAINER RINGS

FRONT SHAFT SUPPORT

FRONT HOUSING

ZB ..... BASE ..... ZBASE .... E ZBOTTOM .BASE,, FRONT (½)

variations are shown in simplified form in Figs. 5 and 6. In each case, air is ejected through one or more ports which are opened and closed, i.e., modulated by a series of rotors. Although a siren with only two rotors can produce broad-band type spectra as discussed in the theoretical example, a larger number of rotors will further modulate with all harmonics and correspond- ingly further widen the power spectrum. The four-rotor siren proved to be an engineering compromise between mechanical complexity and the desired fine-structure characteristics of the power spectrum.

Several prototype models of the wide-band siren have been constructed and evaluated at the Aerospace Medical Research Laboratories, dating from 1956. The first basic models were mechanically similar to the one model shown in Fig. 1. Note that individual drive shafts were used (motor not shown) and that the rotors were constructed of plastic. A one-half horsepower motor drove all four rotors through a series of different diameter drive pulleys. Air requirements were easily satisfied by a standard laboratory bench supply (75 psi, 7 hp) and resulted in a maximum over-all sound-pressure levels of 115 dB (re 0.0002 dyn/cm 2) within four feet of the unit. This model, although of small power output, verified that a broad-band type spectrum could be produced and shaped within limits by rotor-speed control.

A later model, Fig. 5, was designed not only to give

the flexibility desired in an experimental model, but also to yield acoustic power outputs for actual use in biological and equipment environmental testing. This model had two sets of coaxial shafts with one motor

drive per set (approximately 3/4 hp each), giving a total of four rotors with individual speed control achievable through motor speed and/or drive-pulley diameter variations. This model was used for several years in a variety of configurations employing rotors with different modulation functions and horn-loading arrangements. The preliminary report on the siren made in 1958 gave some of the experimental results obtained with this particular model along with a presentation of its noise- generating mechanism. 12

After gaining some experience with the earlier models, a more efficient and better mechanically engineered siren of larger acoustic power output was constructed during January-April 1959. Reference to Fig. 6 shows the design to be a coaxial type with four rotors, each rim driven on a single fixed shaft. Figure 6(c) is a detailed cross-sectional view of this model. Two 1/2-hp varidrive motor units each power two rotors through belt drives. Rotor speeds can be individually regulated by motor speed and pulley-size changes.

As is the case with all of the models, the acoustic output of the siren relates in part to the load impedance seen by the ports. This load is represented primarily by the horn configuration and the characteristics of the


21:28:14

178 COLE, POWELL, OESTREICHER, AND yon GIERKE

5.5-INCH THROAT DIAMETER 30-INCH MOUTH DIAMETER 34-INCH LENGTH

TYPE (b)

;•.5-1NCH THROAT DIAMETER 30-INCH MOUTH DIAMETER 4B-INCH LENGTH

Fro. 7. Typical horns used with siren models. To avoid sonic fatigue from the high sound-pressure levels, the spun aluminum horns are covered with damping material.

space into which the horn is operating. Several types of horn-loading arrangements of this coaxial model were evaluated (Fig. 7).

The coaxial model just described represents the final basic design, or model, which this laboratory has constructed. Larger models of the wide-band siren are produced commercially and have also been built by other government organizations. One model under current development for the Air Force High Intensity Sonic Test Facility at Wright-Patterson Air Force Base will have an estimated maximum acoustic output of 50 000 W. A smaller model is in current use by this same organization. •6

III. EXPERIMENTAL METHODS

For purposes of simplification, the experimental results presented in this report will be primarily those obtained in evaluating one particular model configuration consisting of the coaxial model as shown in Fig. 6(b) with only one of its eight ports open and loaded by horn (b) in Fig. 7. This model configuration is hereby defined as the "basic siren model" and will be referred

to as such in the following discussions. The performance data on this basic siren, although only a small percent- age of the total data taken on the different models during the course of this development program, fairly well represent the type of noise generation achievable by the principle of cross modulation with a multirotor siren. The efficiencies of these prototype sirens were minor considerations in their design, since the aim o[ the development study was not to achieve a maximum sound-power output for a fixed air-supply capability,

•6 j.p. Henderson and K. M. Hankel, "Design and Performance Data of a Unique Broad Band Acoustic Test Facility," unofficial report, Aeronautical Systems Division (ASTEV), Wright- Patterson Air Force Base, Ohio.

but was to investigate the spectral character of the noise produced to substantiate, if possible, the theoretical concept of noise generation by this principle.

A. Measurement and Evaluation Methods

During the development of the siren, various methods evolved for experimentally measuring and evaluating its performance. Acoustic power output, spectral distribution, directivity, efficiency, and other measures of acoustic characteristics have been related to measured, mechanical, operating conditions of the siren such as mechanical power, rotor speeds, and horn configurations. Because of the many variables, a prohibitively large amount of data would be required to document fully the performance of any particular siren design.

Rather than attempt to evaluate the effects of all possible parameters, only the more significant variables were extensively investigated. Other variables were evaluated on a limited basis. The instrumentation and methods used to determine the mechanical-acoustical

performance of the siren will now be discussed briefly.

B. Rotor Speeds

Since one factor determining the siren's output characteristics is rotor speed, a convenient and reliable method of monitoring these speeds was essential. This function was accomplished for the siren shown in Fig. 5 by using commutator-type switches with frequency outputs proportional to the various rotor speeds. Elec- tronic frequency-meter readout was employed. Me- chanical tachometers were utilized on the drive-motor

shafts of the coaxial model, Fig. 6. Individual rotor- speed control was achieved on all models with both motor-speed control and the use of different drive-pulley arrangements. True rotor speeds were determined through commutator and/or tachometer information plus knowledge of drive-to-driven pulley ratios. Actual rotor speeds were periodically checked, while under load by electronic stroboscope to verify the other methods.

In the presentation of data in this report, the rotational speeds of the four rotors of the coaxial model will be given in rpm and identified as rotors 1, 2, 3, and 4, progressing sequentially downstream with the airflow. In all cases adjacent rotors were driven in opposite directions.

C. Air-Flow Measurements

Air-flow measurements were made for various siren-

chamber pressures to obtain the weight flow and work input of the air stream, thereby enabling the calculation of mechanical power in the air stream. Two techniques were employed for determination of weight flow' (1) an orifice plate capable of measuring weight flows up to one pound per second, and (2) a blow-down procedure for weight flows greater than one pound per second. Also employed was a technique for determining air losses


21:28:14


due to air which does not come through the port, but escapes between and around the siren rotors.

The measurements taken for calculations of weight flow when the calibrated orifice plate was employed were the upstream temperature and pressure, differ- ential pressure across the orifice plate, and barometric pressure. Instrumentation employed was standard laboratory-type manometers, pressure gauges, thermo- couples, and a barometer. The same measuring instru- ments, plus a timer, were used when the blow-down procedure was employed. In this case, the necessary measurements were' the volume of the air-storage tanks, pressure and temperature of air in the tanks at the start and finish of the testing period, and the running time.

The siren models studied in this program were not designed for maximum efficiency. During siren operation, relatively large quantities of air were lost through leakage paths around and between adjacent rotors. The separation gap between adjacent rotors of the coaxial model was approximately 1/16 inch. These losses can be minimized in future models by proper design. There- fore, it was necessary to determine the magnitude of such losses with these present models so that a realistic value of siren efficiency could be determined for design purposes. The technique, in brief, was to measure weight-flow requirements of the coaxial model (Fig. 6) with no ports, one port, and eight ports open, main- taining the same chamber pressure, rotor speeds, etc. Differences in these weight-flow requirements for these different operating conditions gave an approximate measure of the relative amounts of air lost through the leakage paths compared to the air going through the rotor-modulated ports. Through careful calibration by this technique, the air leaking radially through the rotors was subtracted from the total air-flow requirements of the model.

Additional measurements of chamber temperature and pressure were taken for the calculation of the work input of the air stream for an assumed isentropic flow in the siren chamber.

D. Mechanical Power

To determine the efficiency of operation of the wide- band noise siren, the mechanical power Wm of the air stream must be calculated. This quantity was obtained by the product of the weight flow w and work input.

The weight-flow calculations for the orifice plate were from standard equations incorporating the characteristics of the calibrated plate. In the blow-down procedure, the weight flow was obtained through the use of the general gas laws and knowledge of the test- operation time. Having the storage-tank pressure and temperature at the beginning and end of each test, along with the tank volume, the weight of the expanded gas was calculated. The weight flow in pounds per second was then taken as the ratio of the total expanded gas

weight to the testing time for each particular test operation.

The equation used to obtain the work input for an isentropic flow through a nozzle was

Workinput: c•,rc[(Po/Pc) (K--I[K)_ 1'],

where Cp is the specific heat with constant pressure, Tc is the temperature in the siren chamber, Po is atmos- pheric pressure, Pc is siren chamber pressure, and K is the ratio of the specific heat of constant pressure to that of constant volume. •7 The use of absolute temperatures and pressures gave the work input in BTU's per pound.

To obtain the air-stream mechanical power in watts for comparison to the acoustic power in watts produced by the siren, it was necessary to calculate the product of the weight flow, the work input, and a conversion factor.

E. Sound-Pressure Level

The basic acoustic measurement made was that of

the sound-pressure level (SPL, in dB re 0.0002 dyn/cm •) produced at particular locations around the siren source. Standard, commercial, laboratory-quality equipment shown schematically in Fig. 8 was utilized to take these measurements. The program control and transfer and analysis control functions illustrated are relay-logic circuits in the semi-automatic data-processing system used at the Aerospace Medical Research Laboratories. These logic circuits provide interlocking operational control between the several components of the system for fast and reliable data reduction. The compensating circuit automatically provides the necessary correction for minor variations in the frequency response of the equipment used, including the microphone transducer. In cases where measurements were made under condi*

tions of dynamic air flow, calibrated windscreens were employed.

F. Acoustic Power Level

One fundamental measure of the siren as a noise

source is its acoustic power-level spectrum as related to its mechanical operation. Acoustic power can be measured in several ways, each having certain advantages with respect to accuracy and case of measurement. Since the output power of the siren was important in establishing an efficiency figure, considerable effort was spent in evaluating several techniques for this measurement. To compare the different methods, one particular model configuration was evaluated by all of the various methods under the same operating conditions.

With the siren located in the free field of a 20X20

X20-ft anechoic chamber, the power was ascertained by two methods. First, it was assumed that all of the energy produced by the siren passed down the horn and that plane-wave conditions existed at the mouth. In that

17 H. J. Stoever, Essentials of Engineering Thermodynamics (John Wiley & Sons, Inc., New York, 1953), pp. 219-224.


21:28:14

180 COLE, POWELL, OESTREICHER, AND vo• GIERKE

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PEAK/PEAK VTVM TEKTRONIX SCOPE PANORAMIC ANALYZER

Fro. 8. Recording and analysis system.

case, the acoustic power level (PWL in dBp re 10-•'W) can be related to the SPL at the mouth by a factor proportional to the area of the mouth. Curve A in Fig. 9 is the power spectrum of the siren as determined by the "horn-mouth integration" method just described. A second method to obtain the power output under free- field conditions was by integrating the intensity of the sound over a closed surface in the far field around the

source, a very common procedure in acoustical investigations. •8 For convenience, a spherical surface was chosen, and the SPL's were determined at various positions in the far field of the siren on a 14-ft radius

sphere. The siren-sound field was assumed to be rota- tionally symmetrical about the siren centerline. Curve B of Fig. 9 is the PWL spectrum of the source as determined by this "far-field integration" method. Before discussing the differences between spectra A and B, two other methods of power measurements will be presented first.

Instead of being operated in a free-field environment, the siren was placed in a reverberation chamber which may partially be seen in Fig. 6(b). The reverberation time Tr, total absorption at, and room constant R of this chamber are given in Fig. 10 as a function of

160

140

130

120

II0

i I

+o

_

X-

, I I I I I I I • I • I I I I I • I • I • I I I I OVERALL I00 160 250 400 640 I000 1600 2500 4000 6400 I0,000

I/3 0 B CENTER FREQUENCY IN CPS

Fro. 9. Comparison of four methods used to obtain power-spectral data of the siren. Power level is plotted in « octave bands (OB).

• I • I • I f I I I • I I I • COAXIAL MODEL SIREN

EIGHT PORTS WITH ONE COMMON HORN CSEE FI CURE 4 Pc = Io psi 9

ROTOR RPM

I 1250 2 2480 3 770

4 4010

0 CURVE A "HORN MOUTH INTEGRATION" FREE FIELD

ß CURVE B "FAR FIELD INTEGRATION" FREE FIELD

+ CURVE C "HORN MOUTH INTEGRATION" REVERBERANT

X CURVE D "DIFFUSE FIELD INTEGRATION"REVERBERANT

•8 H. E. yon Gierke, in Handbook of Noise Control, edited by C. M. Harris (McGraw-Hill Book Company, Inc., New York, 1957).


21:28:14


frequency, along with some dimensional information. With the siren source operating into this reverberant- type load, there was some question as to whether or not the output would be different from that under free-field conditions because of differences in load impedance. However, measurement of the SPL distribution over the horn mouth and determination of PWL on an area- conversion basis as described earlier for the free-field

case yielded curve C in Fig. 9, which differs rather negligibly from spectrum A as determined under the free-field conditions. One would conclude, on the basis of this evaluation, that the power output of the siren was essentially the same under both free-field and reverberant-environment conditions.

A fourth and final method of power calculation was based on the fact that a sound source of given power, when operated in a reverberation chamber, establishes a diffuse sound field therein with a magnitude that can be related fairly simply to the ?WL spectrum of the source, knowing certain acoustical properties of the chamber. Utilizing this method, the siren was placed in the reverberation chamber that has the acoustic charac-

teristics shown in Fig. 10. The SPL in the diffuse region of the chamber was measured using a swinging pendu- lum boom with a circular-path diameter of approximately 8 ft and a rotational frequency of approximately 1/2 cps. Typical variations of levels in this diffuse region were 4-1.5 dB from 200-10 000 cps, and 4- 3 dB from 40-200 cps. The time samples of data analyzed were sufficiently long compared to the boom period to allow the integration of a significant rms value. The SPL spectrum so measured in the diffuse field was then converted to the PWL spectrum of the source by the factors given on the lower curve of Fig. 10. (These difference figures were obtained directly from the other data on the same figure applying standard room- acoustics theory. •ø) The PWL spectrum of the siren obtained by this "diffuse field-integration" technique is shown as curve D of Fig. 9. It should be remembered that the four spectra A, B, C, and D of this figure were all determined for one mechanical and flow operating condition of one particular siren configuration. Yet, two distinctly different spectra were obtained by the different methods of measurement. The "horn-mouth integration" method for both anechoic and reverberant con-

ditions, curves A and C, respectively, yielded very nearly the same spectrum. However, the "far-field integration" and "diffuse-field integration" techniques (curves B and D, respectively) gave another spectrum that was 5 dB lower in over-all level than the spectrum obtained by horn-mouth integration.

Since the far-field integration and the diffuse-field integration methods yielded essentially the same result, we considered that the assumptions made of pressure and velocity'•phase relationships at the mouth (i.e., assumption of plane-wave propagation) were invalid.

•0 L. L. Beranek, Acoustics (McGraw-Hill Book Company, Inc., New York, 1954), Chap. 10.

I I I 1 I I I I I I I I I 5 • VOLUME v--3000 FT3 BOUNDARY SURFACE AREA S ----1550 FT z 250

• • RELAT'IVE HUMIDITY ---- 47 • \ IT, AVERAGE BAS-•--•-••D /I

/ I MICROPHONE BOOM-,/3OB

d 3 •o

z z

o o

,o PWL -- SPL -- 10 LOGio •2+ OR IN THE DIFFUSE FIELD

[e] WHERE R- S• I-•

•_ O•

-5

40 64 I00 160 250 400 •0 I000 1600 2500 4000 6400 I0000

FREQUENCY IN CPS

BiD. 10. •coustic characteristics of reverberation chamber 1-23, •uiId•g •1, W-•F•, Ohio.

Furthermore, the efficiency data using acoustic power information taken by the horn-mouth integration method proved to be unrealistically high and would not correlate, as theoretically expected, with the pressure- ratio operating conditions. Therefore, all of the ?WL data presented in this report have been based on either the far-field integration or diffuse-field integration measurement techniques, or both, unless otherwise stated.

G. Mechanical-Acoustical Efficiency

The efficiency of the siren is the measure of its ability to convert mechanical power Wu to acoustical power W A. The methods used for determining both of these quantities have been discussed previously. Electrical power requirements to drive the rotors were measured and proved to be insignificant with respect to the power in the air flow for these particular models. Therefore, the efficiency in percent was assumed to be given by the ratio (WA/W•) X 100.

IV. EXPERIMENTAL DATA

A. Acoustic Power, Air Flow, and Efficiency

The total acoustic power output WA and efficiency r/ of the basic model siren are given in Fig. 11. Operating conditions of the model are given in the same figure. Total W.•, PWL, and r/are shown as functions of


21:28:14

182 COLE, POWELL, OESTREICHER, AND VON GIERKE

•ooo f • ß

320 145

•oo !- •_

3• I- •

'" o

3.2 [ 12.5 I I

0 2 4.

,020 ,023

513 1 4 0

. OUTPUT

6 8 I0 II; > ;>0 212 30 CHAMBER PRESSURE, Pc, IN psicj

I I I I I

.036 .050 .066 .089 0.1:>9

CORRECTED WEIGHT FLOW, W, IN LBS/SEC

I

380 10150 191;::>0 31130 6 4 O0 MECHANICAL POWER, WM, IN WATTS

Fro. 11. Total acoustic power output and efficiency of the basic siren model as a function of chamber pressure.

- 3O

-

-20•

-

- :•o•

-18 13

-16 • z

-I,4. -I

-

chamber pressure Pc. Additional data are also shown on air-weight flows w and mechanical power W•, corresponding to the output for this particular model. It should be noted that the efficiency peaks at about 30% for a chamber pressure of approximately 10 psig. Above this pressure the acoustic output continues to increase to a maximum of 150 dBp OAPWL at 30 psig. But, the efficiency of generation at these higher pressures is less, being approximately 15% at 30 psig. This means that larger power levels can be generated with some siren configurations by running at pressures greater than 10 psig if the air supply is no limitation. Where maximum levels are desired for a fixed-air capability, a larger number or physically larger sirens should be

employed, each to operate at about 10-psig chamber pressure for most efficient operation.

These efficiency values are based upon measured data from the basic model corrected for air-leakage losses in accord with the techniques described earlier under the evaluation methods. To be conservative in estimating air requirements, good engineering practice might dic- tate that the expected probable efficiencies would be one- half those indicated by Fig. 11 (i.e., 15% at 10 psig, Pc).

The spectral distribution of this power for the described operating conditions is essentially independent of Pc (Fig. 12). In the high end of the frequency spectrum, however, an increase in level proportional to Pc is caused by aerodynamic noise generation produced

[ I ' I ' I I I i I ; I • I • I I I i I I I •

O-

ROTOR RPM

• I 125o n 2 2480 • 3 770 0 -I0 - 4 401 0

z-2ø I n .

o• PS5, 9 o -3o

• •o

io

-40 --

-50 • I j I_ • I • I J I I I • , I • I I I • I I I , I00 180 aS0 400 840 •000 •600 2500 4000 6400 •0000

I/30B CENTER FREQUENCY IN CPS

Fro. 12. Normalized acoustic power spectrum of the basic siren model.


21:28:14


170

165

•160

155

150

Z

_1145

._1

._1

135

130

I I I I I I I [ I I I

ROTOR RPM

I 1250 _

2 2480 3 770

4 4010

_

_

_

_

_

17 INCHES UP HORN

PLANE OF HORN MOUTH

DIFFUSE FIELD _

REVERBERATION CHAMBER 1-23

125 I I I • I • • I I I I I I I I 0 2 4 6 I0 I I 16 18 20 22 24 ' 26 28 30

CHAMBER PRESSURE, Pc) IN psi 9

Fz(3. 13. Over-all sound-pressure levels produced by the basic siren model at several locations as a function of chamber pressure.

by increased flow conditions. This high-frequency noise was established as being generated by flow in the horn and not by air flows leaking radially between the rotors.

An analysis of the fine structure of the noise will be discussed later, including a higher resolution frequency analysis.

B. Sound-Pressure Levels

The sound-pressure levels (SPL) that the basic model can produce are related not only to the acoustic power output of the siren but also to the geometrical orienta- tion of the point in question to the source and to the type of acoustic environment in which the source is located. Over-all SPL's at the horn mouth, 17 in. up the horn, and in the diffuse field of the reverberation chamber (described by Fig. 10) are shown in Fig. 13 as a function of/'c.

The SPL spectrum at the horn mouth and the spectrum in the diffuse field of the reverberation chamber

are similar (Fig. 14), although the diffuse field spectrum has relatively, but only slightly, less high-frequency content because of absorption characteristics of the chamber. Increases in high-frequency aerodynamic noise generation with increasing chamber pressure also show in both spectra.

C. Influence of Rotor-Modulation Functions

The spectral distribution of energy in the output of the wide-band siren is directly related to the port-area modulation functions produced by the rotors. Therefore, rotor speeds and port arrangements on individual rotors will influence the power spectrum as discussed in the theoretical evaluation of the siren. Experimental results presented so far have been for one set of four rotors with particular rotational frequencies. Typical spectrum shaping by rotor-speed control using the basic model is illustrated by Fig. 15. Such shaping is significant but has definite limitations. Air-flow parameters and all other variables were not changed for this evaluation.

Further examples of the spectrum shaping that are possible by rotor-speed control are given in Fig. 16 for several earlier siren models. The effect of stopping one rotor, Fig. 16(a), was rather negligible, but, in this particular case, all eight ports of the coaxial model were operating into a common horn throat [horn (a) of Fig. 7-]. The same effect was observable on the basic siren by stopping one rotor in such a position that the port was not blocked, resulting in three-rotor modulation of the flow. Although only small differences were observable in a one-third octave-band resolution analysis


21:28:14

184 COLE, POWELL, OESTREICHER, AND voN GIERKE

- io

"' -20

1 m

• -3o

o

_•-4o

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ROTOR RPM

•/ • 2 24e0 /// • 3 770

4010

/ • P;:3o psi 9 IO

.

5

1-25

I , I • I • I , I , I , I i I , I , I , I

I00 160 250 400 640 I000 1600 2500 4000 6400 I0,000

Y3 OB CENTER FREQUENCY IN CPS

• (a)

-I0 -

-J -30

0

-40

-5O

I i I • I i I I I I i i I ' I ' I ' I '

• ROTOR RPM •////•$-' • 2 2480

• '</-4/,/22• 3 •o

(b)

3o psig zo

IO

I-5

I00 160 250 400 640 I000 1600 2500 4 000 6400 I0,000

• OB CENTER FREQUENCY IN CPS

Fro. 14. (a) Normalized SPL spectra of the basic siren model at the mouth of the horn. (b) Normalized SPL spectra of the basic siren model in the diffuse field of reverberation chamber 1-23.

150

140

•" 130

Z

-• 120

0

_(• •o

on CONSTANT Pc: Io psig R PM

ROTOR ß X 0 rl I 740 1250 740 1250

2 650 1550 3500 2480 3 460 770 460 770

4 1050 2510 5640 4010

IOO I I I • I I I i I i I i I I I I , I i I i I • I OVERALL I00 160 250 400 640 I000 1600 25 O0 4000 6 400 I0 000

I/3 OB CENTER FREQUENCY IN CPS

Fro. 15. Effect of rotor-speed variation on the acoustic power spectrum of the basic siren model.


21:28:14


, $j;•j• i I ' I ] J i I ' J i j ß I ! i I (•ON•T•NT Pc '" ICCp ROTOR RPM (___). • I 740

,'-- •_-- -• 2 3500 ,• • OA SPL= 155d B 3 460

4 1050

I , I , I , I , I , I , I , I , I , I • I I I •

50 80 125 200 320 i 500 1800 11250 12000 13200•500018000 •% •, Wo ,• •o •oo •o ,o• ,• •oo •ooo •oo ,oooo

I/3 OB CENTER FREQUENCY IN CP•

• 155 o 150

• 145

•,4o a3o 135 oo ø •n o 130

z_ IiO

.

i , i , i , i , i , i , i , i , i , i , i ß i ,

- CONSTANT • 10•DSlg RPM(.-.•_ -

[ , i , i , i , 1 , i , i , i , i , i , i , i ,

OA SPL = 154d

ROTOR • % ROTOR RPM ROTOR RPM I - - I 1250 CONSTANT I• = IOps;a I 2510 I 2510

140

RO•OR 135 • • ,•o . ' 2 '550 OA SPL = '5'

.-/ 3 770 O • 120 4 2105 • Z I , I , I , I . I , { , I , , , I I I • I , I , - 115

50 80 125 , •00 , 320 , 500 , 800 IR50 2•0 3200 5000 8000 •v '" 3 1550 •o d• ,6o ,•o •o •oo •o ,o%o Woo z•oo •oo , m , I m I m I & I m I m l, m I , I m I , I , I/3 OB CENTER FREQUENCY IN CPS I 50 80 125 200 320 500 800 I• { •000 3•00 5000 800• •g •2 ,& ,4o •o •o •o ,o• ,•oo •oo ,o•o •3o ,oooo, i , i , I , i , i , i , i , i , i , ! , i , i ,

CONSTANT P½:10 psl D

/__J 2 3500 3 3500

4 564o

I I 14 i I , I J I i I i I, I i •J i•10 i i iJO i 40 100 100 250 400 •0 1000 I 00 0 4 O0 0 10000 I/3 OB CENTER FREQUENCY IN CPS

155

150 X 145

•140

O • 135 •0 125

• •2o • 115 _z •o

(a)


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4 I00 160 250 400 640 I000 1600 ?-500 4 0 6400 I0000 I/3 08 CENTER FREQUENCY IN CPS

CONSTANT Pc ' 10 ps,q OA SPt_- 125dJ• RO?R ..... ' , , , , i , I I I ! i i I ' ' i RPM I ._J 120

?•o /

460 l

•o5 •oo

• ..• g5 i i , , , , I I I I I , I s I , I i I , I , I , I 40 64 I00 160 250 400 640 I000 1600 2500 4000 6400 I0000

I/30B CENTER FREQUENCY IN CPS

• l i , i , i , i , i , i I • , i , i , i , i , i , i CONSTANT Pc ' IO ps,q oa SPt_ • 136 dB ROTOR RPM

13 O r 3 2480 125 r

115 r II0J-

:Z 4 40l 0 4010

' ' I i I i I i I I I I I i I i I I I i I I I I _•3 40 64 I00 160 250 400 040 1000 1600 2500 4000 6400 I0000

I/3 013 CENTER FREQUENCY IN CPS

13C •125

oJ 120

115 OII0

105 •,oo

_

135

130 I;'5

õ,•o 115

I10

,o5

(b)

i , i , i , i , i , i i r , i , i , i--, i , i ,

CONSTANT Pc =10pslq OASPL = 132dJ• I:K)TOR RPM

I 2510

1550 1550 4 2510

, , ; ..... , ..... 0 I '2 ' 25100 ' 40100 ' •4'00 ' 40 4 100 I•)0 250 400 {•$0 I' 00 I 00 10000 •3 OB CEN'TER FREQUENCY IN CPS

I , I i I , I , I i I , I , • , •--, I , ! , • ,

CONSTANT Pc = 10 p•q oa SPL •- 137 dB ROTC•R RP•

I 5640

/• 2 3500 / \ • a asoo

20 I 614 I i0J0 I i J60 I [ I 4;0 6•J0 J I li00 25[00 I I 250 I I I I 40100 I I0000 1000 6400

I/30B CENTER FREQUENCY IN C1:)5

Fro. 16. (a) Further examples of spectrum shaping by rotor-speed control. Coaxial model with eight ports open, with common horn. (b) Further examples of spectrum shaping by rotor-speed control. Coaxial model with eight ports open, without horn.


21:28:14

186 COLE, POWELL, OESTREICHER, AND vo• GIERKE

z 150 o

• x o

• 140 .

a3 13o

z

i-- ipo

IiO

0 , I t4o

OVERALL

I ' 1 ' 1 ' I ' I ' I ' ' I • I ' I t I ' I I I I I

CONSTANT Pc = IOpag ANECHOIC CHAMBER

/• ' '"'0.,,,.•, •,.... ROTOR R PM ..ONE PORT - BASIC SIREN DESIGN I ß

'•.,,•-• 0'ø-'ø", 3 770 .

/.' • '•' • '• 4 40,0 -

./ ,' ,/;• ONE PORT •.X•. •'-•O•.• /Z'/ -- • • '-. •.__•. _

••. • / • ', .... '-.-•-" -, •-•' .' / --,x •TWO PORTS

•,,'•• '•• ,

, I , I , I , I , I , I , I , I ,, I , I , I , I I 64 I00 160 250 400 640 I000 1600 :::)500 4000 6400 I0000


17. Coaxial model. Effects of operating multiple ports into a common horn compared to individual port loading. SPL spectra at horn mouth.

of the spectrum, the four-rotor design was a compromise between mechanical complexity and smoothness of spectrum when examined with a 2% bandwidth resolution (sample given later).

The modulation function of the individual rotors will

influence siren output. During the study program, several different functions were evaluated affecting spectral content and efficiency. Although simple periodic functions on each rotor will theoretically produce wide-band type noise by virtue of the cross- modulation principle, irregular modulation functions, i.e., rotor functions with high harmonic content, result in cross-modulated signals with a wider and more uniform distribution of energy over the frequency spectrum.

Both theory and experience have indicated that a

total average, effective flow area of approximately 50% (product of the open area in percent of each rotor) will yield the maximum acoustic output. To achieve this modulation for a 4-rotor siren, each rotor must have approximately 80 to 90% open area. The closed portion of each rotor should be contained in one or two, or at most three, major "blades." Narrow-type blades that do not effectively close the ports contribute very little to the frequency content of the signal and, if anything, are a source of power loss because of additional flow turbulence.

Optimal rotor-design parameters to achieve specific spectra shapes have not been completely documented either by theory or by experiment. The problem is amenable to computer techniques. One experimental approach considered to be practical is the study of the

140

130

LiJ

Z 120

nn o IlO

I00

CONSTANT Pc = I0 psIg ROTOR R P M

I 1250

2 2480

3 770

4 4010

• , I i I I I I I I I I I I I - I I i I I I • I OVERALL ,00 160 150 400 640 I000 1600 2500 4000 6400 I0000

I/:30B CENTER FREQUENCY IN CPS

Fro. 18. Influence of different types of horn loading on one port of the coaxial model. Acoustic power spectra. [Horns (a) and (b) used are described in Fig. 7.-]


21:28:14


area-modulation function caused by different rotor designs using a light beam source rather than an air flow. The light passing through the series of rotors would be proportional to the area-modulation function of the port and could be measured by suitable photo- multiplier equipment.

D. Multiple Port and Horn Designs

Experimental data have been presented on the basic siren model, which by previous definition consists of one port of the coaxial siren operating into horn b, Fig. 7. To utilize fully the eight-port capability of the coaxial model, each individual port would have to be appropri- ately loaded. If each were loaded with a horn similar to that used on the basic model, the unit would then actually consist of eight of the basic one-port siren models. Acoustic power levels for this unit would be approximately 9 dB greater than those of the basic siren, and the SPL's in the reverberant chamber would also be similarly increased, assuming that volume and absorption changes in the chamber caused by the added horn volume were sufficiently small. Coupling these horns together to give SPL's much greater than those produced at the mouth or up the horn of the basic single-port siren would be difficult. Phase relationships between the different sources and reactive mutual

loading of sources would be some of the problems. As a simple approach to the utilization of all eight

ports of the coaxial model, all ports were loaded with one common horn, type (a) of Fig. 7. Results of this approach, as shown in Fig. 17, indicate that the increase in output is reasonably consistent with the number of ports feeding the horn (i.e., 3-dB increase per doubling of ports). However, the load impedance as seen from the ports is improper because of the throat mismatch resulting in lower output levels per port as compared to the one-port basic model which has a better horn-load characteristic. The effect of horn loading on the one-port siren is shown in Fig. 18 with the 5.5-in.-diam throat horn and the 2.5-in.-diam throat horn giving increases in over-all PWL output of 10 and 16 dB, respectively, compared to the no-horn case. The mismatch of the larger horn results in a 6-dB relative loss compared to the basic model design.

Since this relative loss is less than the increase caused

by additional ports operating into the larger horn, the coaxial unit with eight ports and one large horn can produce higher levels than the basic model with one port and the small throat horn. But, the efficiency of operation is much less than is obtainable by individual port loading. The use of horns with different impedance characteristics to also achieve spectrum control is quite feasible. The results presented herein are by no means definitive since horns with relatively high, low-frequency cutoffs were used, and only a limited number of commercially available horns was employed,

I I , I , I , I t I • I , I , f , t , t , t , I , I

OVERALL 40 I00 250 640 1600 4000 I0,000 64 160 400 I000 2500 6400


FIo. 19. Effect of changing the plenum chamber of the basic siren model.

Although not specifically a horn or port effect, the influence of the plenum chamber behind the rotors will be briefly mentioned here, since it also relates to the total impedance seen by the ports, but in the opposite or upstream direction of air flow. The load impedance must be high compared to the backward impedance to minimize any dissipation of acoustic power backwards into the chamber. Also, no major cavity resonances should be excited in the chamber. Although no special inlet muffler was designed for the siren, the effect on acoustic output of increasing the size by one order of magnitude and also changing the shape of the inlet chamber was evaluated. No significant change was measured, as indicated in Fig. 19.

One other experimental result concerning the horn was the determination of SPL gradient over the surface of the horn-mouth plane, Fig. 20. This variation in SPL was taken into account in the evaluation of PWL using the horn-mouth integration techniques. The large deviations from the levels at the horn center occurred at

positions a few inches from the rim of the horn mouth. Comments in this section on rotor shaping and modu-

lation functions should be considered together with port configurations, since the shape of the stator or port is intimately involved in determining the total modulation characteristic.

E. Directivity

The siren without horn loading is almost nondirec- tional in radiation for the low- and middle-frequency regions and exhibits only moderate directionality in the upper-frequency bands (Fig. 21, left side).

• 4 z

•Bo

i I'111'111'l'1111l'l'l'111 , _

' •/•//////////•

I lllllll, tlll'l'ltl'l'l'l•l OVERALL 40 I00 250 640 1600 4000 I0,000

64. 160 400 I000 2500 6400

I/3OB OF FREQUENCY IN CP$

Fzo. 20. Variation of sound-pressure level over surface of horn mouth of coaxial models.


21:28:14


+5

-5-

:FI0 -

•3 +5 -

_z o x

•:I0 -

m g +5 -

• o

-5 -

-10 -

_

i i i i i 1

100 CPS

500 CPS

5OOO CPS

ALL DIRECTIVlTY INDICES ARE BASED ON STANDARD

¾3 O B'S WITH CENTER AS SPECI FI ED.

I I I I I _ I

•00-400 Cy - CP 5OO 10,000

ARE BASED ON STANDARD I/3 OB'S WITH THE RANGE OF

CENTER FREQUENCY AS SPECIFIED

I I I I I I I I I ; I I 0 30 60 90 120 150 1150 0 30 60 0 120 150 180

ANGLE g IN DEGREES ANGLE O IN DEGREES

TOTAL OF 27 DATA POINTS TOTAL OF 3?_5 DATA POINTS

FIG. 21. Directivity indices of wide-band noise siren. Left side' model without horn. Right side' model with horn.

Addition of the horn to the siren not only increases the power output but also gives more pronounced directional radiation of the energy. The right side of Fig. 21 shows that the siren radiation with horn (either horn, Fig. 7) is increasingly directional with increasing frequency, a typical characteristic of horn-radiation patterns.

F. Fine Structure Analyses

In the analyses of complex signals, such as those produced by the wide-band noise siren, several means exist to quantitate the information contained therein. PWL and SPL spectrum analyses with a 1/3 octave- band resolution cannot by themselves describe completely the nature of the sound generated by the siren. For this reason, some fine structure anslyses of the

noise were made' spectral analyses with 2% bandwidth resolution and amplitude-density distributions.

A representative comparison between 1/3 octave band and 2% bandwidth (approximately 1/35 octave) analyses of the siren spectrum is shown in Fig. 22. Deviations of the 2% bandwidth levels from the 1/3 octave-band levels do not exceed +5 dB below 600 cps, nor +2 dB above 600 cps. The frequency scan rate of the 2% analyzer was very slow compared to the data sample time (4 sec) to allow a meaningful rms average to be obtained.

Finally to determine the statistical amplitude distribution of the siren noise, the amount of time the signal was within a given amplitude range (a-kAa), compared to the total time sample, was determined. Figures 23(a) and 23(b) show the relative statistical

-.

-- CONSTANT •Y• BANDWIDTH I/3 OCTAVE BANDWIDTH

CONSTANT PC = ,o ps•q ROTOR RPM i

3 770 4 4010

.... J , • ' • .... 400 600 0oo iooo 2OOO 4O0O eooo FRI-'-QgENC¾ IN CP5

22. Narrow-band analysis of noise at horn mouth of basic siren model compared to « octave-band analysis; spectrum levels.


21:28:14


Fro. 23. (a) Amplitude-density functions of wide-band noise siren. Coaxial model. 40-125 cps and 125-650 cps bands. (b) Ampli- tude-density functions produced by wide-band noise siren. Coaxial model. 650-2500 cps and 2500- 10 000 cps bands.

0.4

O. 3

no2 D ß

<• 0.1

AMPLITUDE/ RMS AMPLITUDE

0.5

I- 0.4

z

0.3

n 0.2.

I I I

2500-I0000 CPS _•A.. I I -7 -6 -5

I I { • I I 1

650-2500 CPS • -

e5OO-lOOOO cPs ---'x"•'"' • _

GAU

' I i .•- _ I _•,.-,• A --4. -3 --2 --I 0 I 2 3

AMPL{TUDE/? RMS AMPLITUDE

(a)

(b)

frequencies of filtered noise samples and in each figure the Gaussian distribution for comparison. It is seen that the higher frequency bands follow a distribution similar to a Gaussian distribution. The lower bands, particu- larly the lowest band, do not show a Gaussian distribution, but resemble the amplitude density of a sine wave in noise.

The over-all peak-pressure distribution of wide-band siren noise has been shown before to be comparable with the peak-pressure distribution of rocket noise. •' Occasional peaks exceeding the rms sound-pressure level by as much as 20 dB have been observed in the wide- band siren noise field.

V, POTENTIAL OF THE WIDE-BAND NOISE SIREN, EXISTING LARGE-SCALE MODELS, AND FUTURE DEVELOPMENTS

The experimental data on the wide-band noise siren confirmed the potential usefulness of this siren principle for large-scale, high-intensity noise-test facilities. Sound fields of high enough level with a spectral distribution and statistical character similar to jet or rocket noise can be produced. Although the fine structure of the noise reveals a few discrete frequency components in the low frequencies, these do not appear strong enough to be disturbing for many applications. In addition, it is

likely that these components could be avoided by changes in the siren design; if necessary, five-rotor sirens or a slight modulation of the rotor speeds with time, or shifting the range of the discrete frequency components to still lower frequencies could be considered. The siren- noise spectrum has appreciable energy in the frequencies below 100 cps. The rapid drop-off in this range of the power spectra presented is probably caused by the poor acoustic radiation conditions of the nozzle or horn

rather than by the principle of the siren. If necessary, it should be possible to design a siren with a horn large enough to have the maximum of the spectrum at much lower frequencies. This fact is important since rockets of the Saturn and Nova class will exhibit such spectra during the next decade.

The experiments reported indicate that it should be possible to build wide-band noise sirens with mechanical-acoustical efficiencies between 15 and 30%. The latter figure applies to a siren without air leakage. The efficiency of the small model sirens built to date is not yet so high. But since these models were designed primarily to study the principle of this new type of sound source and not to obtain maximum efficiency, this fact is not disturbing. The theoretical maximum efficiency of a wide-band noise siren has not yet been calculated. For a conventional siren generating sinu-


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165

160

155

150

145

140=

135

I I I [ [ I I I I I I [ I I , I , I [ I I 163

ACOUSTIC I POWER •<-.....••,,• SOUND-PRESS REU LEVEL IN m• I LEVEL ..<<..•• I •000,• •EVE,eE•*•,ON. ,• •

14• •

138 •

I .I I I I I I I I I I 1•3 0.1 I 10

TOTAL AIR WEIGHT FLOW, •, IN LBS/SEC

I I •

I I0 I00

NUMBER OF ONE-PORT TYPE ,SIRENS REQUIRED

I

I 2 4

NUMBER OF EIGHT-PORT TYPE SIRENS REQUIRED 8 12

Fro. 24. Generalized correlation between acoustic power output, average sound-pressure level in reverberation chamber (3000 fts), and air-weight flow. The curves are drawn for an acoustical-mechanical efficiency between 15 and 30%, a range which according to the present investigation appears feasible for future designs.

soidal pressure functions using a resistive control for the air flow, the calculated maximum efficiency is 50%; an efficiency of 40% has been measured on actual models. 2ø For a siren producing sinusoidal waveform with amplitude modulation, the efficiency dropped to 12.5%. 9 It appears as if the efficiency of the wide-band noise siren will be not too far from the efficiency of a pure-tone siren, a very encouraging and interesting result.

Figure 24 has been compiled to extrapolate from the experience obtained from the various small-scale model sirens to the sound output and air-flow requirements of larger models. It represents an estimate of the relation between the acoustic power level and the total air- weight flow for an assumed efficiency of 15 and 30%. The two other scales on the abscissa give the number of basic single-port siren models and the number of coaxial 8-port siren models (as described in this report) required to obtain a certain acoustic power level. The lower curve on the graph indicates the average rms sound-pressure level produced by these sirens in a 3000-ft a reverberation room. It must be kept in mind that these rms sound-pressure levels will be higher in a smaller room, and also that the instantaneous peak sound-pressure levels of the random-type noise will exceed the rms pressure level by as much as 20 dB. The highest sound-power level obtainable while operating the 8-port siren, described herein, at the most efficient

20 C. H. Allen and B. G. Watters, J. Acoust. Soc. Am. 31, No. 2, 177-185 (1959).

chamber pressure (i.e., •'•10 psig) is approximately 154 dBp (2.5 kW). At higher, but less efficient operating pressures around 30 psig, the maximum power output can reach 159 dBp (8 kW). The actual efficiency was less than that indicated in Fig. 24 due to air leakage, as discussed earlier. Nevertheless, the efficiency/weight flow values indicated in Fig. 24 do not appear unrealistic for future designs in view of the data presented. The final efficiency achievable will probably depend critically on the matching of horn sections to the individual ports and in the way these individual horn sections are con- ducted to the test room or test specimen. It is inherent in the principle of a multi-port wide-band noise siren that the sound pressures in the individual ports are not in phase with each other and that the matching of the ports to a common horn or, if the horn is section- alized all the way, to a common room might result in some losses.

During the development program of the wide-band noise siren described, the urgent need for a wide-band noise source led to the building of several medium-scale wide-band sirens in industrial test facilities. •ø.2• All of

these designs are scaled-up versions of the models shown in Figs. 5 and 6. The port diameters of these sirens were increased by a factor of approximately 3; the consequent increase of the nozzle area and the weight flow by approximately 10 led to siren models with acoustic power outputs of 10-20 kW. It might not be impossible to increase the efficiency of these models further to

• J. M. Otera and H. McGregor, Missile Component Acoustic Testing, Martin-Denver Report CR-60-22, Vol. 1 (August 1960).


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achieve output powers of close to 50 kW. These siren units are in use at several places in industry for testing structures and components of ballistic missiles and for evaluating electronic components. In general, it must be deplored that it was necessary for all of these larger siren units to be built quickly without much further development or study of the many still unknown problems and parameters. It is known that several problems relating to both efficiency and smoothness of spectra have developed as a result of scaling up these prototype models with little or no additional acoustical studies. In one case

the use of multiple-inlet ports effectively bypassed the principle of series modulation resulting in line-spectra- type outputs. These line spectra could be related directly to the modulation of the exit ports by the last downstream rotor. In effect, the last rotor was controlling the output because the lack of proper flow control by means of stator and/or rotor design was preventing true, effective cross modulation.

A 50 000-W unit using basically the coaxial design (Fig. 6) with considerably reduced air leakage losses is presently under development for the new Sonic Fatigue Facility at the Aeronautical Systems Division, Wright- Patterson Air Force Base, Ohio. It is believed that sirens of this size (20-50 kW) might constitute a convenient unit to use as building blocks for still higher sound- power installations. The relative merits of multinozzle and multirotor arrays compared to a simple combination of independent sirens must be evaluated. It is obvious that the first type of siren array promises additional advantages with respect to the fine structure of the over-all spectrum. But a large installation using many independent sirens producing spectra of slightly different composition would probably satisfy most practical requirements.

It is conceivable that wide-band noise sirens could be

designed in such a way that they could be converted by relatively small changes into discrete frequency sirens.

Logically, we cannot assume that the relatively small effort so far expended in the wide-band siren development could have resulted in the selection of the most

advantageous over-all configurations and in a perfect design for a specific type of siren. The influence of too many parameters is still unknown, and the theory is not yet completely developed. The development effort on the wide-band siren principle over the last few years

has been much less than that expended by various organizations over the same time period to improve the conventional discrete frequency siren. This discrete frequency siren has been studied for more than 130 years, as evidenced by many experimental and theoretical papers. Therefore, the wide-band siren cannot yet be expected to be perfect, but the practical results obtained to date are so convincing that this new principle certainly deserves further study and development. Furthermore, the performance of the siren models in their present state qualifies them as some of the best tools available for high-intensity acoustical-environmental testing.

VI. SUMMARY AND CONCLUSIONS

The program to develop the new wide-band siren principle confirmed the results of the preliminary investigations which show that the siren is a suitably efficient broad-band noise generator for high-intensity environmental acoustic testing. The sound pressures (150 to 175 dB re 0.0002 microbar) and the spectral character (20 to 20 000 cps) of the noise fields acting on aerospace systems can be generated, and the random character of jet and rocket noise can be simulated to a degree probably sufficient for most practical purposes. Instantaneous peaks, exceeding the rms value of the noise by 20 dB, are achieved. It appears possible that future designs should be able to produce such broad- band noise fields with a mechanical-acoustical efficiency (from 15 to 30%) not much different from the efficiency of sirens producing sinusoidal waveforms. Several basic wide-band noise-siren configurations have been considered, but only two designs have been built and tested to date. The four-rotor, coaxial, multiple-port siren appears thus far to be the most efficient and economical approach. Based on results of earlier phases of this program, several large-scale sirens having an acoustic power output of 10-20 kW are used in industry for environmental testing. A 50-kW siren for the Aero- nautical Systems I)ivision's Sonic Fatigue Facility is presently under development. In the investigations to date, the basic parameters involved in the wide-band noise siren principle have been studied only crudely, and considerably more work has to be done to arrive at an optimum design with respect to efficiency and economy.


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Acoustic Siren for Generating Wide-Band Noise