Design Characteristics for Noise Control of Jet Engine Test Cells

THE JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA VOLUME 24, NUMBER 2 MARCH, 1952

Design Characteristics for Noise Control of Jet Engine Test Cells How^•) C. H^m)Y

Armour Research Foundation of Illinois Institute of Technology, Chicago, Illinois (Received December 6, 1951)

An example is shown of the steps in the design of the structure for noise control of a jet engine test cell from the determination of the design goal to the final acoustic treatment of the cell. An arbitrary design goal was set from a previous survey of city noise. With the expected octave band levels at a distance of 3000 feet, for 10 jet engines with afterburners the quieting necessary is 50 and 55 db, respectively, in the 150-300 and the 300-600 cps bands, these two bands being the most difficult to control. An analysis of the required volume and dependent cost of the structure is given as the function of the amount of quieting needed, the quantity of heated gases exhausted, the temperature of the gases, and the velocity of flow permitted. Two unconventional but economical systems of noise control, (1) a series of acoustically treated plenums and (2) a series of acoustical lined 180-degree turns, are recommended. Design equations and confirming data from model studies are presented.

INTRODUCTION

F the many noise problems in the aircraft industry, the one which has received most recent attention

is that facing the manufacturer of jet engines, the problem of testing simultaneously many engines in production test cells without hearing damage to operating personnel and without disturbing nearby residents. The object of this paper will be to discuss a comparatively economical approach to this problem, particularly in the testing of engines in jet engine production plants. This also provides an excellent opportunity to describe how a design grows out of the various aerodynamic, mechanical, architectural, and acoustical requirements. The design proposed is in some ways hypothetical, but in many respects it is very similar to the one being constructed at the present time in what will be one of the largest jet engine production plants in the United States.

I. DESIGN GOAL

The first step in the design is to delineate the problem by establishing a realistic design goal. This particular step would be easy if it were possible to establish a universal design goal or specification, but this is im- possible. The variables are'

(1) Ambient noise which already exists in the neighborhood. This may vary by 30 to 40 db.

(2) Level of acceptance at residential locations, which in turn involves previous indoctrination, or sometimes passive acceptance, and sometimes even local politics. This variable may be 20 db.

(3) Terrain features. Presence of uneven terrain, rivers, woods, may make differences of the order of 10 db.

(4) Industrial zoning, which may prohibit the presence of residential areas close to the factory site. This may cause a difference of 5 to 10 db.

An unsophisticated recommendation would be to establish the plant as far as possible from civilization, but the truth of the matter is that any obscure site which is at all suitable will be invariably inhabited by a few people, often those who are used to extreme quiet.

The presence of the factory usually causes the development of homesites in the area, and the protests against noise are only postponed. The best formula is to recom- mend the choice of a site which is as close to noisy civilization as possible, in an area which is already ex- posed to the noise of commerce and industry, and where the amount of noise reduction required may be rela- tively small.

Figure 1, curve A, shows the expected noise at five feet of a large jet engine with afterburner, which is also approximately the noise level in a concrete test cell. The level without afterburner is approximately 10.•db lower than this. Curve C is the approximate spectrum at 3000 ft, which we will take in this problem as the hypothetical distance at which the sound must be con- trolled. Curve B is the level which would occur if ten

engines with afterburners might happen to be running at the same time, a reasonable possibility in•"a plant which has 50 or more test cells. Curve D•is a design goal spectrum which has been set up for a rather'quiet locality at night. The construction described in the

160

-"- ENOI'NE WITH AFTER BURNER AT 5 FT•:...

-'--' T[•N E•NGIN•s AT O0

O CTAVE I saNOs 5½ PS

140

120

I00

.o 60

2:0

o 57.5 75 150 300 600 1200 2:400 4800 75 150 300 600 1200 2400 4800 9600

Fro. 1. Spectra of jet engine with afterburner at various locations. Desigr/goal is given in curve D. The construction proposed in this paper will lower noise given in curve B to that of curve E. The short horizontal lines at the left of the figure represent wide band levels; otherwise octave band levels are plotted. The reference pressure is the usual 0.0002 microbar.

185

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23:50:25

186 HOWARD C. HARDY

iooo

ioo

6o

4o

Ol

006

004

O02

OOl TOTAL

LOUDNESS 375 75 150 300 600 1200 2400 4800 75 150 300 600 1200 2400 4800 9600

130

I0

7o •. o

io

FtG. 2. Loudness curve' for data from Fig. 1. C' is the proposed design goal.

following discussion will reduce the noise level to that of curve E when ten engines with afterburners are being run.

The design goal given in Fig. 1 was determined by considering the average spectrum which occurs when there is no traffic in the immediate vicinity of homes in the average quiet neighborhood in a typical American city, as reported by Bonvallet2 This spectrum should be weighted for loudness and the loudness curve for it is shown as curve C in Fig. 2. The graph paper s presented is one which weights octave band data for loudness by the equivalent tone method originally presented by Beranek and co-workers? The total loudness is about 7 sones. Actually the low frequencies in this type of spectrum would not be heard at all and the loudness would be raised only slightly if the spectrum was changed to that of the dashed line (curve C'). This curve would have a loudness of 10 sones. This is the

design goal plotted in the previous figure. While viewing this figure one can compare the loud-

ness if no quieting were used. For one engine this would be curve B, and for ten engines with afterburners it would be curve A. The total loudnesses are 270 and 590

z G. L. Bonvallet, J. Acoust. Soc. Am. 23, 435-439 (1951). • F. Mintz and F. G. Tyzzer, J. Acoust. Soc. Am. 24, 80-82

(1952). a Beranek, Marshall, Cudworth, and Peterson, J. Acoust. Soc.

Am. 23, 261-269 (1951).

sones, respectively, which is a good deal louder than even the accelerating noise of.the loudest diesel trucks at a distance of a few feet.

The proposed construction which is discussed here results in a calculated loudness curve given by D, which has a total loudness of about 4 sones, about the loudness of the refrigerator in the average kitchen. All of the curves are for a distance of 3000 ft.

If one subtracts curves A and C' of Fig. 2, the amount of noise reduction needed varies from 25 to 65 db. The

frequency bands most difficult to treat are the 150-300 and 300-600 cps bands. For the 150-300 cps band, 50 db is needed, and 55 db for the 300-600 cps band.' These two bands become the fulcrum in the design. It will usually result that the rest of the design will be a. dequate as a matter of course if adequate silencing i.s provided in the 150-600 cycle region. This principle has been known empirically for many years by heating and ventilating engineers, who usually use acoustic data at 250 cps for design considerations. In the following, reference will often be made to data in the 300-600 cps band, and, where it is necessary to abbreviate the discussion, reference will be made only to data from this band.

II. ARCHITECTURAL REQUIREMENTS

It might be well at this point to describe the problem from the other end and point out the architecture one starts with for which a quieting design must be given. Figure 3 shows the elements of a test cell. The turbo jet engine is the oversimplified piece of plumbing in the center of the figure. The jet engine must be placed in a structure, usually concrete, which will prevent wide- spread damage from fire and such accidents as compressor explosions which scatter shrapnel over the sur- roundings. There must be a door to take the engine in and an opening to bring in a large amount of air for combustion and cooling. The exhaust gases are usually vented through a pipe which not only provides a means for exhausting them but also a region in which the gases can be cooled by either water spray rings or augmented air or both. A convenient cross section for such a cell

is 12X 18 feet, in order to accommodate all the instru- mentation needed for the tests. In this structure ade-

quate noise attenuation must be provided by the doors and walls, and for the passage'•of the intake air and the exhaust gases.

In what follows, we will be concerned with the aerodynamic requirements of a much larger engine than is

Fro. 3. Construction necessary for a jet engine test cell before adding noise control.


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JET ENGINE TEST CELLS 187

currently in production, because any practical construction must be adequate for the higher capacity engines of the near future. For purposes of discussion, this futur• engine is thought of as one which will use 225 lbs of air per second (about twice the consumption of the current models). Aerodynamic designs are now being made to augment this air about 2.5 to 1 for cooling. The total intake air is about 700,000 cubic feet per minute. Because of the net expansion by heat and added water spray, the total exhaust air is of the order of 1,600,000 cubic feet per minute. The total volume of gases and the amount of attenuation required are far above the practical economic limit for structures made of steel alone, and one must think instead in terms of masonry or, more particularly, reinforced concrete enclosures. The quieting structure will then generally be an exhaust tunnel of approximately square construction.

III. DESIGN PARAMETERS AFFECTING VOLUME OF STRUCTURE

Considering first the quieting of the exhaust system, the volume and cost of such a structure is a function of

various parameters such as velocity of flow and the allowed temperature. The volume will be

V=œS, (1)

where L is the total length of the exhaust system, which will be the width d times the number of diameters

n necessary to obtain adequate noise reduction. The cross section

S=d2=O/v, (2)

where Q is the quantity of gases (cubic feet per second) flowing in the duct and v is the linear flow velocity (feet per second). We see, therefore, that the total volume is

V=nda=n(Q/v)L (3)

The quantity of gases flowing will depend not only on the amount of combustion air but on the quantity of added air for cooling, which depends on the degree of cooling required. To a fairly good approximation (see Fig. 4) the quantity of exhaust gases is inversely pro- portional to the final cooled temperature in degrees Fahrenheit (øF) (if a simplified engineering rather than a scientific thermodynamic function can' be used), so that

V o: n(1/øFv)l between 400 and 900øF. (4)

If one wishes to make the structure as small as possible, the exhaust structure should have the maximum at-

tenuation per diameter length in order to keep n as small as possible, and the gases should have the maximum velocity and temperature permissible. These two criteria are mutually inconsistent, and a reasonable design is a compromise between them. At high temperatures, the volume of gases Q is not very sensitive to temperature, which can be seen in Fig. 4. Here the quan-

.00055 w

tity of gas flow is plotted as a function of the temperature in degrees Fahrenheit. The dotted line is plotted from the approximation that Q• (1/ø.F). We can see from this that a fairly optimum temperature might be 700 or 800øF. However, even at 450øF, the quantity of gas passed is only the ratio of 3 to 2 over that at 700øF. The ratio of volumes'of structure needed is 1.8 to 1.

It should be pointed out at this point, however, that what might be gained in allowing the temperature of the exhaust to be high might be lost in the efficiency of the acoustical materials. For instance, a loss of acoustical attenuation of 2 to 1 per diameter will more than make up for the size factor between 450øF and 700øF. When this is added to the economic factors involved in de-

terioration in structures which undergo excessive changes of temperature and temperature cycling, the advantage is weighted toward the lower temperatures.

The most efficient noncombustible acoustical materials are, of course, the mineral wool fibers. The highest temperature now recommended for them and for concrete is 450øF and, therefore, this temperature has been used in this design.

The velocity of gases passed will generally be limited by two factors:

(1) The back pressure on the exhaust stream. Large back pressures are tolerated by some manufacturers on the theory that the thrust of the engine must be expended in the atmosphere and back pressure will not affect the performance. Since large back pressures affect the burning controls, the trend in the air- frame manufacturers is toward tests with less back

pressure, and these specifications will probably reach the engine manufacturers in time. Another point is that with larger engines it is becoming more difficult to augment air for cooling with large back pressures.

(2) The erosion of the acoustical material. This erosion, which is also affected by temperature, appears to be the chief criterion for limiting velocities at the present time. Tests of erosion have been made on mineral wool with particular surface treatments and, for the treatment recommended in this paper, velocity


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188 HOWARD C. HARDY

l• S i Ia IzS E = Ii S•: db loss = !0 log •

A

dbloss = I0 log •(I-a)

il ' For a square duct with 13 180 ø turns

izsz, II d2: 4end•+ d2 I 2 A =4aLd+

B Ii Si S = d z L db loss = I0 log 4an + !

I-a

FIG. 5. Two suggested silencing structures, (a) a serivs of acoustically lined plenums, (b) a series of acoustically lined 180- degree turns.

in excess of 125 feet per second (approximately 85 miles per hour) is not recommended at the present time.

Using the capacity given above of 1,600,000 cubic feet per minute for the largest jet engine anticipated in the immediate futare, the required cross section is 210 square feet which is the approximate cross section (12 ft by 18 ft) of the test cell shown in Fig. 3.

IV. SILENCING STRUCTURE

Consider now possible geometries which will keep the number of diameters, the volume of structure, and the cost at a minimum and yet obtain the 50 to 55 db attenuation needed according to Fig. 2.

It is obvious that extreme and impractical lengths would be needed, if only a straight duct with acoustical lining were used. In ten diameters with 100 percent absorption at the walls, the inverse square drop would be only 31 db. The usual solution is to divide the duct into parallel ducts with baffles (horizontally, vertically, or both). The economic disadvantages of such a treatment are:

(1) In order to attenuate the low frequencies, the thickness of such baffles must be great, which means

OPEN SOURCE END

ø•kk I OCrAVE B•,NOS ('CPS) I •1 I MOOEL I ':•(• i SCALE SCALE

Xxl 400-800 •o 50- I00 10 • xx;•"• 1600-3200 x----x 200-40C) /I x,',, F"-- 4eoo-seoo o----. eoo-

0 P. 4 6 SECOND SECTION FIRST SECTION 0 Z 4 6 THIRD SECTION

• DiAMETER•O F/G. 6. Data from an early model study of the attenuation as a

function of distance. The average absorption coefficient of the lining was 0.6 db above 1600 cps and approximately 0.4 at 400 cps.

that the effective cross section must be materially re- duced, resulting in high velocities or very much larger total cross sections. Most treatments which exist today do not adequately reduce the low frequencies because the baffles are not thick enough.

(2) Very substantial baffles have to be built in order to withstand the very large dynamic forces on the baffle. These forces are of the order of 15 to 20 lbs per sq ft and tend to excite the baffle into vibration like any other airfoil, causing mechanical failure. When mineral wool is used in the baffles, this excess vibration is very likely to disintegrate the acoustical material. To overcome this handicap, practical baffle constructions often cost three to five times as much per square foot as surface wall treatments.

This economic problem has been analyzed carefully by the Acoustics and Vibrations Section of the Armour Research Foundation, and two other geometries, which are more economical and efficient at low frequencies, are recommended. The two methods are the use of (1) a series of acoustically lined plenums, or (2) a series of

FIo. 7. One form of exhaust silencing structure recommended. Six to twelve decibels are gained by the upward beaming of the exhaust exit.

lined 180 ø turns. The _two methods are illustrated in

Fig. 5. In the first case, the exhaust gases are brought into

and expansion plenum where there is a great reduction of acoustic energy density. The lowered intensity and the exhaust gases flow through a second opening into a second plenum and so on until adequate reduction is obtained. An approximate theory for this case is as follows: The energy flux entering the first plenum is I• times the area of cross section S. For reverberant rooms

this would be equal to the energy flux leaving the room I2A, where A is the total absorption in the room. How- ever, for very absorbing rooms, one must take into ac- count the energy which has not yet reached a wall. Following Hopkins and Stryker, 4 this term must be divided by l-a, where a is the average absorption coefficient of all surfaces in the room. The term involv-

ing the direct energy has been dropped, since it will usually be negligible near the opening of the next plenum.

4 H. F. Hopkins and N. R. Stryker, Proc. Inst. Radio Engrs. 36, 315-335 (1948).


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JET ENGINE TEST CELLS 189

The attenuation is, therefore, ?-'"'-.-'"'--.ji•;•i::•:•i:i::::;?•i•i;it:::•?i::ii•';'; •!:•i:; ....... ?::'???• ........... ;-•: ................. ::œ.;• ...... -'"'-'

For instance, if one has an entrance duct of 100 sq ft and a room with 3000 sq ft of surface, all lines with a material of 0.8 absorption coefficient, the attenuation would be

db loss'-10 log(0.ax3000)/(100X0.2)"-21 db. One can, however, see the futility of trying to increase this very mi•ch. An increase of only 3 db would be ob- :•'""• ....... ...- .... tained by decreasing the intake area 2 to 1 or doubling " .............. :' '- ............ ß ........

the absorbing area. It should be noted, also, that this type of noise reducing structure is not very sensitive to absorption coefficient. A coefficient of 0.5 will give 15 db (or 0.3, 1! db) which means that even at rather low frequencies this type of treatment can be made rather effective.

The plenum construction is especially suitable when one plenum can be used for multiple test cells where only one cell is used at a time, as in development laboratories. For production test cells, the method of 180 ø turns is more applicable.

In the second method the gases are brought around a series of !80 ø turns as indicated in Fig. 5. In large cross section ducts such as we are considering here, where the dimensions are large compared to a wavelength, each separate section of the duct can be considered as a separate room with an opening at each end, and the same theory applies as above. In each section of duct has a length nd and a cross section d •, the resulting approximate equation is

db loss-' 10 log[(4naq-1)/(I--a)-!. (6)

To demonstrate the validity of this analysis, model tests to a scale of ! to 8 or ! to 10 have been made on several

different constructions. The results of an early model test are shown in Fig. 6. Each section of this model was six diameters long, and the absorption coefficient of the lining was 0.4-to 0.6. The resulting attenuations at various frequencies are shown. In the first section where the source is small, there is an attenuation due to spherical spreading; in the second section the level is approximately constant. In the last section the level drops slightly because of the open end. The drop per turn is 16 db at high frequencies where the coefficient is approximately 0.6, and about 12 db at low frequencies where it is only 0.4, fitting very closely the theory outlined above. Notice that attenuations of 45 db or more are obtained.

V. MODEL STUDIES

By this approach one can see how a silencing structure for the exhaust can be designed very precisely. Referring back to our original objective, one form of silencing structure might look like that in Fig. 7. This structure has the same cross section as the test cell

FIO. 8. Photograph of a model (with side removed) constructed according to Fig. 7. The model is lined with thin Fiberglas. An 8-inch loudspeaker is used for the source.

12 ft by 18 ft and is 36 ft high. A model study was made of it on a scale of ! to 8 and using a frequency scale factor of 8 to !. A view of the model is shown in

Fig. 8. On this scale an eight-inch loudspeaker simulates very well a five-foot exhaust pipe. This arrangement takes advantage of the upward beaming of the noise which adds 6 to 12 db to the design.

Sound pressure level measurements in the 300-600 cps band (full scale) are shown in Fig. 9. The absorption of the material 0.6 was somewhat less than that pro- jected in the full scale. One can see that the total drop was 52 db. The expected levels in the full scale can be obtained by adding 50 to all of the readings giving 98 db at the top of the stack.

It is well at this time to point out that the sound transmission path through the walls of the structure is a very important path for transmission of energy. In fact, in the !50-300 cps band the path through the walls is the most important path. The level at this point is 144 db and just outside of 12-inch reinforced concrete walls will be approximately 95 db. Fortunately, these structures are placed beside each other so that any sound energy emitted is beamed upward somewhat.

2400-4800 CP$

{300-600 CP$ FULL SCALE) OCTAVE BAND

DATA DB IN ACOUSTIC MODEL

1oo

48

50

52

5•

o 54

FIG. 9. 2400-4800 cps octave band data (300-600 cps full scale) on model with average absorption coefficient of 0.6. The probable full scale octave band data of engine with afterburner can be obtained by adding 50 db to each reading.


23:50:25

190 HOWARD C. HARDY

EXIST • I •' •111••l•i•l / RO0• s..•.• • I •1 Ll•iL•k•'-•l• 1 / /

EXIST %• J kX•L ( f• I

•CT•. • III I • I T[ST

AND INTAKE SILENCI•

Fro. 10. Cut-away view of entire design showing intake silencing arrangement. Separate paths are shown for intake and cooling air. Control room is used for pairs of cells placed side by side. Exhaust pipe is covered with earth fill.

VI. INTAKE STRUCTURE

After the exhaust structure is designed, one can proceed in somewhat the same way with the intake structure. Figure 10 shows the entire design. The exhaust pipe has been covered with sand or earth fill. Two separate paths are provided for the augmented and combustion air so that the latter can be measured

accurately. The velocity of the intake air must be kept rather low since a pressure drop will affect performance of the engine. The designed velocity here is 45 ft per sec for the combustion air and 90 ft per sec for the augmented air. Because the exhaust gases in the exhaust pipe-are near sonic speed, somewhat less noise exists in the augmented air chamber. Because of the partition between this and the intake chamber, the latter has correspondingly less of the exhaust noise. The chief noise here is the compressor whine, which is high pitched and approximately 120 db in the ab- sorbent lined room. This simple structure will quiet it adequately.

A large explosion-resistant door protects the outer corridor. The control room is placed at the quiet end of the structure and serves both this and an adjoining cell.

VII. ACOUSTICAL MATERIALS

The material recommended for the lining of the exhaust system is three inches of six-pound density mineral wool with satisfactory high temperature characteristics. This is obtained by using unbonded glass wool or by certain rock or slag wools. Over the material is placed glass-fiber cloth or a thin sheet of Fiberglas industrial mat, which does not affect the absorption of the acous-

TABLE I. Expected octave-band sound pressure levels at various positions for the proposed exhaust silencing structure.

Octave bands (cps) Position 75-150 150-300 300-600 600-1200

Near engine exhaust 150 152 150 148 Top of stack 104 102 98 94 Corrected for beaming effect 99 95 90 85 50 feet horizontally from stack 77 73 68 63 One cell at 3000 feet 41 47 32 25 Ten cells at 3000 feet 51 47 42 35

Outside wall near exhaust 106 102 95 88 Corrected for beaming effect 100 • 94 85 76 Ten cells at 3000 feet 55 49 40 29

Intake stack 95 94 91 90 Ten cells at 3000 feet 42 39 35 29

Total spectrum at 3000 feet 56 51 44 36

Design goal 65 57 49 43

tical materials but reduces the erosion. The fibrous

material is held in place by heavy screen, expanded metal, perforated transite, or perforated metal.

Because only ambient temperatures and low velocities are involved, simple materials for the intake can be used. For this formaldehyde-bonded glass wool, two inches thick and nine-pound density is adequate. This is held in place by hexagonal mesh screen.

VIII. SUMMARY

To recapitulate, the expected sound pressure level in various octave bands is shown in Table I at various

distances. Of interest is the level at 50 feet horizontally from the exhaust stack. Here spherical spreading and the beaming has already furnished a 30 db drop from the level at the top of the stack. It is unfortunate that in many reported data this figure has been added to the reported attenuation of a duct. At least one reported 60 db muffler reduces the noise 24 db by attenuation and 36 db by spherical spreading.

When the total of the exhaust output, the wall trans- misssion output, and the intake'air path are added for ten cells operating simultaneously, the result is the lower line in Table I which gives the curve plotted in Fig. 1.

The writer would like to acknowledge the help of his colleagues, Mr. D. B. Callaway, Mr. D. E. Bishop, and others of the Armour Research Foundation staff for

valuable discussions and cooperation in the designs and measurements.


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Documents

Design Characteristics for Noise Control of Jet Engine Test Cells