IMPULSE ALIGNMENT OF LOUDSPEAKERS AND MICROPHONES

8/12/2019 IMPULSE ALIGNMENT OF LOUDSPEAKERS AND MICROPHONES

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IMPULSE ALIGNMENT OFLOUDSPEAKERS AND MICROPHONES

by Tom Lubin and Don Pearson

PART ONE

When the earliest recordings were done there was little if anyattention given to the acoustic phase or electrical polarity of themechancial devices used to record and reproduce sound. Phaseand polarity have little significance as long as only onemicrophone picks up the sound and one speaker plays. Whenrecording left the experimental stage it became possible to mixtogether more than one microphone. This allowed for bettercontrol and balance among the instruments.

With advancements in technology, multi-microphonetechniques developed. In not too long a time the recordingengineer discovered that occasionally when the outputs of twomicrophones were combined their summed output level wouldbe less than the output of each one separately In some cases the

explanation of polarity and phase should be given.Polarity and phase are relative terms. Polarity refers to the

property that physical quantities have of being greater or lessthan some reference value that we may arbitrarily designate asthe point of reference or zero. A point on a line may be thoughtof as being closer to an observer than another point thought of asa reference or farther away than that same reference point. Itsposition may be described as corresponding to a positivenumber in one case and to a negative number in another. Avoltage may be thought of as being positive with respect to onereference potential and another voltage may be observed to benegative referenced to that same potential. Both voltages may beeither positive or negative when referenced to the potential of the


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positive pulse (Figure 1). Most manufacturers indicate with should be the same for all units, however with the use of

colored binding posts a difference between the speaker specialized speakers which reproduce only one area of the audio

terminals. Unfortunately, inconsistant or incorrect polarity range, a crossover of some type must be introduced into the

distinction is quite common, partly due to erratic quality control audio path. Almost all crossovers introduce some degree o

and the fact that some manufacturers wire their elements phase shift in order to achieve a sufficiently steep roll off to both

opposite to other manufacturers. The polarity of a woofer can be sides of the pass band.

easily determined with a 1% volt battery. When the speaker leads For example, lets say we have a three-way network tha

touch the battery terminal, the cone will move in or out. If it crosses at 500 Hz and 2,000 Hz. At 500 Hz both the woofer and

moves out the terminal touching the positive side of the battery is the mid-range are reproducing a signal that is 3 dB down from

positive in that it condenses the air. If the cone moves in then the their respective full power passband levels. At that frequency

terminals are reversed. The negative side of the battery is both of them are theoretically reproducing an equal acoustic

connected to the positive side of the speaker causing the cone to power level, so their on-axis response will sum by 3 dB. If the

rarify. sum by 3 dB, and they are both down by 3 dB at the crossove

Unfortunately, high-frequency speaker elements cannot be point then the system should have a flat response providing al

checked in this manner because the diaphram movement is so the phases are correct. But what is the correct phase? Thats th

slight and is usually difficult to see since it is usually deep inside crunch.

the horn. The degree of phase rotation introduced by the crossover wilvary from unit-to-unit, but can be considerable. Almost alway

Crossovers the phase of the roll-off/roll-on of adjacent bands will rotate in

In systems that use a number of full range speakers the phase opposite directions at the crossover frequency. The net resul

will have the mid-range acoustically out of phase with the woofe

IMPULSE at the crossover frequency because of the phase shift in theGENERATOR SPEAKER crossover.

In order for the entire system to be acoustically in phase at the

POWER crossover point, it may be necessary to alternate the speake

AUDIO

AMP polarity of each adjacent band. Manufacturers of crossovers fa

_ OUT to meet this need as phase reversal switches are seldom

SYNC OUTprovided. The fact that the speakers of two adjacent passband

OSCILLOSCOPEare electrically out of phase is of no consequence since it is only

at the crossover points that they share common information

and must be acoustically in phase with one another.

EXT TRIGGERFinally, a speaker is theoretically a single point source o

INPUTFigure 2 sound. With the addition of each speaker to a system, the


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number of point sources increases. If each point source is not

exactly the same distance from the listener then what occurs is

an auditory double image. This is particularly apparent at thecrossover points. Basically, a single moment in time is generated

by all the speakers at the same instant, but arrives at the listener

at a number of different times. This causes the intelligibility of the

entire system to be lowered.

Clarity is also affected by out of band distortion generated bythe enclosure. If a cabinet is not adequately braced it will

resonate or ring substantially. The box can put out almost as

much sound as the speaker itself. Likewise metal horns, if not

properly dampened, can contribute undesired vibrations.

The solution to many of these problems is fairly simple once an

accurate method of measurement is provided. Operating as an

independent testing service, Don Pearson and Gary Leo, of

Ultra Sound, located in Larkspur, California, has developed

such a system. With the aid of their computer they provideinformation on all of the previously mentioned problems as well

as a number of other acoustic and electronic parameters relating

to the sound equipment used by their clients. A permanent

record is kept, in the form of a printout, for these clients who

number several very prominent sound reinforcement

companies.

A Simpler Method

Ultra Sound has allowed the publication of a simple circuitwhich when constructed can be used with a microphone and an

oscilloscope to measure polarity, phase of a wave, and ringing.

Figure 2 is the schematic of an impulse generator with variable

frequency and repetition rate. Figure 3 illustrates the prop

hook-up of the system. Any oscilloscope with an external trigg

will do as will any conventional microphone. Ultra Sound prefe

a Nakamichi CM 300 fitted with a CP 3 super-omni -in

element. The microphone should be placed a few feet from th

speaker (the larger the speaker, the greater the distanc

between the mike and speaker). The speakers impulse outp

should be loud enough to mask any room noise.Before testing can be done, the polarity of the microphone

be used as the standard must be determined. First connect th


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with identical polarity. Figure 9 is the response of the pair. Figure10 has the polarity of the two speakers opposite one another.Figure 11 is the resulting frequency response. Figure 12 is anoverlay of Figure 9 and Figure 11. The efficiency of the twocurves was maintained so that a direct comparison could bemade. The top trace is of the speakers with identical polarity.The bottom trace has them opposite.

Now that the basic procedure and how to read the impulse isunderstood, lets take a look at a two-way system. Adjust thefrequency control of the generator to a low frequency andobserve the polarity of the bass speaker. Re-adjust the frequencyon the generator to the crossover frequency and observe thepolarity of the woofer and tweeter (Figure 13). The small firstpeak is the tweeter and the second larger one is the woofer. Both

are in the same polarity. Figure 14 shows theresponse. Figure is the same system but with the tweeter polarity opposite that the woofer. Figure 16 is the resulting response showingsubstantial dip at the crossover frequency. Figure 17 agshows a comparison between the system in phase and outphase response.

The same procedure can be followed with three- and four-wsystems. The frequency of the generator is changed to eachthe crossover frequencies until the polarity of all the transduchas been determined. If there is more than one driver in any o

band range it will be necessary to move the microphone clo(one inch) to each driver to eliminate interference from the otdrivers in that range.

The effect of band-to-band polarity considerations will result


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either a peak or a notch in the frequency response at thosecrossover points.

Another use of the impulse is to check the polarity of

microphones. Once the polarity of the test microphone has beendetermined, other microphones can be compared to it byconnecting each microphone in turn to the oscilloscope. If any ofthe mikes are out of phase with the majority, then reverse thesignal wires on that mikes connector. It should be noted that

most European microphones are opposite in polarity toAmerican ones.


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IMPULSE ALIGNMENT OF

LOUDSPEAKERS AND MICROPHONES

PART TWO

by Gary Leo and Don Pearson

Part one of this article (R-e/p,

would suffer if the drivers wereconnected inproperly.

December, 1978) described a There are a couple of additions andmethod for observing the electri- corrections to Part One of Impulsecal polarity of speakers and Alignment. First, picture 4A had nomicrophones with respect to a explanation. It shows the signals generatedknown source. I t included a by the circuit shown. The top trace is the

circuit and a test setup for making signal for the oscilloscope sync input, whilethese observations. It then went the bottom trace shows the signal after iton to show how to determine the has gone through the bandpass filter (pulsepolarity between drivers in a output). When using this circuit, adjust themultiband speaker system and oscilloscope so that it is being triggered onhow the frequency response the negative edge of the pulse.

ABOUT ULTRA SOUND:In addition to consulting and

design engineering, Ultra Soundoffers custom construction ofone-of-a-kind electronic projectsfor sound reinforcement applica-tions and rental of electronicequipment.

The circuit itself needs one minorcorrection. The jumper, connecting theoutput of Op-Amp #2to the wiper of the 5kilohm potentiometer going to the pulse

output should be omitted. The final additionis the calibrations on all the frequencyresponse measurement photos and they areidentical to those in Figure 3 of this article.

A recent project was construc-tion of the six-way stereo elec-tronic crossover used during thefinal shows at San Franciscos

To begin this, the final article on implusealignment, a few points are in order. If youroscilloscope does not have sufficient

polarity, and optimize the alignment. Thimay be omitted if nothing has changed sincthe last use or alignment.

2 - Use pink noise and a real timespectrum analyzer to adjust the system foflat response by adjusting crossover outpulevels and polarity. Then adjust the systemequalizer.

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The system is then adjusted to soundgood by the person mixing. This is moscritical and someone with proven experience will be able to please the greatesnumber of people. It is not uncommon for sound system to be hired because of thability of the personnel operating it rathethan its hardware.

Part two will discuss the observation oand problems associated with enclosur

resonances and some of the correctivmeasures that can be taken using the same

basic test set-up. It will also demonstrate method for observing and adjusting tharrival time of the signal from each of thspeakers in a multi-driver system.

Common Applications


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Figure 7. Frequency response of the systemshown in Figure 3. after bracing.

Figure 8. Arrival time difference -showingboth woofer and mid-range pulse. (Differ-ence = 1.6 divisions x0.5 milliseconds/div= .8ms or 10.8 inches)

Figure 9. Same as Figure 8 with speakersadjusted for equal arrival time.

dependent upon the actual size of thecabinet, larger cabinets need thicker walls.

In all cases, high quality wood should beused. If plywood is used, only accept woodwith plys made of solid laminations with nogaps or filler. Filler, consisting of sawdustand glue, is added to the gaps in cheapplywood. The price of proper materials ismuch more expensive than ordinary woods.If there are any gaps in the layers, they willstart vibrating and breaking up the filler

between the plys causing buzzs and other

sounds. Hard Truckers only uses imported14-ply Finnish birch three-quarter-inchmarine plywood in their construction and

bracing. All of their products makeextensive use of these materials andtechniques. High quality particle board maybe substituted, but it will not be as durable.

l An enclosure made with cheap plywoodmay superficially resemble a high quality

cabinet, but after a short period of time it willsound like a rattle trap.l It is necessary to isolate the speaker fromthe cabinet with a gasket. The gasket servestwo purposes; one is to insure an airtightseal and the other is to provide some shockisolation.l The impulse response can be dampened

by changing the amount of stuffing insidethe cabinet. Increasing stuffing should stop

the high frequency reflections within theenclosure from radiating out through theport or through the cone material.l Ringing in horns can also be suppressed tosome degree. One approach to the probleminvolves the utilization of some carpetunder-padding (waffled horse hair covered

with jute). This material also works well ascabinet stuffing. Hotels and theaters usually

throw it away when they replace theircarpets. Coat the outer surfaces of the hornwith plastic resin or similar adhesive, andpress the padding into the adhesive andallow it to harden. Make sure that all fittingsand connections are airtight and that anymounting plates or adapters cannot rattleagainst any other surface.

Figure 6 shows the impulse response ofthe same system as depicted in Figure 2,

except that bracing and stuffing techniqueshave been applied. Figure 7 is the frequencyresponse of the same system show in Figure3 after bracing.

Background InformationIn the 1920s, researchers at Bell Labs


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SPEAKERS

MIKE \ A

\

L

CROSS-OVER

I

discovered that the arrival time of the soundfrom speakers reproducing differentfrequency components of the same signalwas important if high intelligibility was to bemaintained. Until recently, these conceptshave not been applied in the design ofcommercially available systems. UR Ioffers Time Aligned Studio Monitors

TEST SETUP

(Time Aligned is a trademark of E. M. Longand Associates). Other products are alsomarketed under various tradenames.

Using the same impulse system setuppreviously mentioned, it is possible toobserve the arrival of the impulse withrespect to time. For test purposes, considera four-way sound system containing an

Vert In

active crossover and a separate amplifier foreach speaker component (see diagram).

Preparation Before TestingIn the following discussion, it is important

that the crossover be included in the testsfor two reasons: first, the crossoverprevents out of range signals from damagingthe speakers and, second, the crossoverexhibits a frequency dependent time delaywhich needs to be considered in the


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In the past, many people have lined up thevoice coils in a vertical plane. This is a step inthe right direction, but not a completesolution. The problem of misalignment may

be observed fairly easily. If all of thecomponents are mounted in separate

enclosures, moving them forewards orbackwards relative to each other will helpbring the system into closer alignment.

Most large portable reinforcementsystems are built in a modular fashion, somoving individual sections is not toodifficult. In the studio, however, the systemspeakers are usually fixed into a particularplace so they are not easily moved.

It should be noted that the speakersshould not be tipped or tilted as this

movement will alter the polar response.Also, too much movement could put areflective surface in the path.

Testing ProcessThe procedure for observing alignment is

similar to that used for checking polarity.First, set the generators output filter to alow frequency setting and adjust theoscilloscope so that the knee or breakingpoint of the received impulse lines up withone of the vertical lines on the scopegraticule (continue to trigger on the negativeedge of the waveform).

Now slowly move the generatorsfrequency control to a higher setting. Theamplitude of the low frequency speaker will

decrease and the mid-range speakersimpulse will appear and increase inamplitude. This impulse knee will then beeither to the right or left of the notedgraticule line. If it is to the left, then thespeaker is too close to the microphone and

needs to be moved back. Conversly, if it is tothe right then it needs to be moved closer.

Again move the generator to a higherfrequency and reposition the speakeraccordingly. Continue until all speakershave been observed and adjusted. If youmove any speaker, then you may have toadjust its level to compensate for theincrease or decrease in SPL since it is nowcloser or farther from the microphone. The aforementioned procedure can also


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be used to calculate the misalignment. Thisis accomplished by observing the relativetime interval between break points, thenconvert the measured time delay to

distance. The time delay between thedrivers may be found by multiplying thescope horizontal sweep switch setting inmilliseconds times the number of divisionsseparating the impulse break points. Then,since sound travels at 1130 ft/sec @ 70degrees F, multiply 1130 times the time

interval in seconds. The result is thedistance of separation in feet.

D = CxT = 1130 x.00l=11.13 ft/msWhere:

D = distance in feetC =sound velocity (1130 ft/sec @70F)T =time in seconds

The idea is to get all of the knees to line uph i l i l li

approximately 14feet. This was a portabreinforcement system whose componenconsisted of RCA type W horns, a closebox array of 16 twelve-inch speakercompression drivers on radial and lonthrow horns, and ESS Heil Blue Ox AMotion Transformers. The misalignmewas so great, that no correctiveaction coube taken.

In some commercially-available speaksystems, the alignment is achieved throug

a combination of driver placement anspecial delay networks added to the passicrossover. On a multi-amp system, delawould be required at each of the crossovbands prior to the power amplifier. At thpresent time, Ultra Sound has been unab

to locate a commercial delay which operatsatisfactorily while achieving the desiralignment. Some of the available delatested could be used in the lower frequenranges, but none were acceptable in thhi h f h bi di


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which to properly adjust the impulse

response of a sound system. The authors

suggestions here are offered as a relativelysimple means of observing speaker andmicrophone connections and placement.Such studies are otherwise limited to thosewith access to complex and expensiveequipment such as spectrum analyzers,

wave analyzers and Fourier analyzers.Many subjects in this discussion were just

briefly mentioned since the purpose of thisarticle was to offer a basic measurementtechnique. It is hoped that interestedreaders will investigate the references listedin the bibliography that follows. Copies ofthese references are available at most public

libraries.

The important information to be seenfrom the received waveform is the polarityof the first arrival and its time relationship toa similar first arrival from another speaker.The ripples following the first impulsesometimes can be seen to have a regular,

per iodic structure. This may be anindication of resonances within the speakeror cabinet. The waveform also containssignals from reflections of other objects nearthe speaker, microphone, or in the pathbetween them, so an exact interpretation isdifficult. This interpretation of the receivedwaveform could be accomplished withFourier analysis to derive a frequency vs.amplitude plot, for example. q

BIBLIOGRAPHY:

1 - The Application of Impulse Techniquesto the Detection of Linear Distortion -Alfred Schaumberger, JARS, September, 1971,p. 664.

2 -A Catalog of Frequency and TransientResponses -D. Pries, JARS, December, 1977,

p. 990.3 -Linear Distortion -D. Pries, JAES, June,

1976, p 346.4 Acoustical Measurements by Time

Delay Spectrometry -Richard C. Heyser,JAES, October 1967, p. 370.

5 -Loudspeaker Phase Characteristics andTime Delay Distortion, Part One -RichardC. Heyser, JARS, January, 1969, p. 30; Part Two-April, 1969, p. 130-137.

6 -Determination of Loudspeaker SignalArrival Times, Part One

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Richard C. Heyser,JAES,October, 1971. D. 734-743: Part Two -

November, 1971, p. 829-834;Part Three -December, 1971, p.902-905.

7 -The Response of Loudspeakers to ToneBursts -V. Kaminsky, JAES, April, 1965, p.119-123.

8 -Delay Effects in Loudspeakers -PaulKiipsch, JAES, October, 1972, p. 634.

9 -A Shuffleboard Experiment of Time-Delay Effects in Loudspeakers - GaryGilluum,JAES, May, 1978, p. 361.

10 - Active and Passive Filters asLoudspeaker Crossover Networks -Ashleyand A. L. Kaminsky, JAES, June, 1971, p. 494.

11 -On the Transient Response of IdealCrossover Networks -J. R. Ashley, JARS,July 1962, p. 241.

12 - Op-Amp Implimentation of IdCrossover Networks -J. R. Ashley and L. Henne, JAES,January, 1971, p.7-11:

13 -Electronic Crossover Network aTheir Contribution to Improve LoudspeakTransient Response -A. P. Smith, JARSeptember, 1971. P. 674-679.

14-Network withMaximally Flat DelayW. E. Thompson, Wireless Engineer, Octob1952, p. 256-263.

15 - Butterworth Filters as LoudspeakFrequency Dividing Networks -J. R. AshlProc. IEEE, 1970, vol. 58, p. 959.

16 - Notes on How Phase and DelDistortions Affect the Quality of SpeecMusic and Sound Effects -J. K. Hilliard, IETrans. Audio, vol. AU-12, p. 23-25, March-Ap1964).

17 -Phase Distortion -P. Tappan, IEETrans. Audio, vol. AU-12, p.22 (March-Ap1964).

18 -Plain and Fancy Test Signals for MusReproduction Systems -Ashley, Matson, aSaponas, JAES,April, 1971, p. 294-305.

19 - Time Alignment in Loudspeakers -M. Long, Audio, August, 1977.

20 -Crossover Network Design -E. MLong, Audio, March, 1972, p. 34-36.

21-Loudspeaker Damping - Nelson PaAudio, March, 1973, p. 52-54.

22 -Dividing Networks for LoudspeakSystems

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John K. Hilliard and H. R. Kimbareprinted JAES, November, 1978, p. 850-855.

23 -Audio Cyclopedia - Howard TremaiHoward Sams & Company, 1959.

24 How to Build Speaker EnclosuresAlexis Badmaeiff and Don Davis, Howard Sam Company, 1966.

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IMPULSE ALIGNMENT OF LOUDSPEAKERS AND MICROPHONES