Wrap-up - Sound & Communications

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Impulsive Thinking Part 2Things that go ‘Bang,’ ‘Crack’, ‘Hiss’ and ‘Chirp.’By Peter Mapp, FASA, FAESLast month, I briefly reviewedsome of the uses of impulse responsemeasurements and why theyhave become so important in acousticsand audio work. This time, I willtake a look at how impulse signalsare generated and some of the relatedapplications.Theoretically, an ideal impulsewould last for only an infinitely smallperiod of time. Its short durationensures that it has a wide bandwidthand thus covers the frequency rangeof interest. Unfortunately, such asignal contains little energy andwouldn’t be very loud. The pioneersof audio impulse measurements,therefore, had to devise ways of successfullycapturing and processingthese relatively delicate signals.Electronic testing, where highsignal-to-noise ratios are readilyachieved, even with normal circuitry,is one thing, but working in thereal acoustic domain, with typicalambient noise levels, is quite another.Early IR (impulse response)measurements of loudspeakers, forexample, generally were made indedicated quiet rooms, often constructedspecially for the purpose.The possibility of measuring a workingsound system, in a real-worldenvironment, was virtually out of thequestion.Various techniques were tried inorder to improve the signal-to-noiseratio. These included turning up thesignal level and signal averaging.Turning the level up would seem tobe a simple and fairly obvious thingto do, but it is not without its complications,because increasing thelevel too much can cause the systemor item under test to either becomenonlinear in its operation or, indeed,to burn out completely!Signal averaging is a powerfultechnique and can significantlyFigure 1. Note that the actual “bang” is far frominstantaneous, lasting some 10 to 20 mS.Figure 2. Spectrum of Balloon burst, obtained fromcarrying out Fast Fourier Transform (FFT) of theimpulse in Figure 1.improve the signal-to-noise ratio ofalmost any measurement, regardlessof the signal stimulus employed. Theimprovement (in dB) can be calculatedby using the formula 10 log N,where N is the number of averagesperformed. For example, doublingthe number of averages (e.g., goingfrom 1 to 2) provides a 3dB improvement,while quadrupling the averages(e.g., going from 1 to 4) providesa 6dB improvement.However, the law of diminishingUnited Kingdom-based Peter Mapp, FAES, FASA, is principal of Peter Mapp Associates. Achartered engineer and physicist, he also is a Fellow of the Acoustical Society of America andFellow of the Audio Engineering Society. Send comments to him at pmapp@testa.com.16 Sound & Communicationswww.soundandcommunications.com
Figure 3a. Log Sweep Signal.Figure 3b. Zoomed Log Sweep Signal.Figure 4. Sweep signal when picked up in test room.Note how different this is to the input signal.realized that these signals are notimpulsive, but are either steady stateor pseudo-steady state sounds thatwould appear to bear no resemblancewhatsoever to an impulse. Deadright! But this is where the power ofthe modern PC and some nifty mathematicsstep in and, by the processof convolution or, actually, de-convolution,transform the test signal intoits equivalent impulse. The beautyof these signals is that, not only arethey highly repeatable, but they arereasonably noise immune—particularlywhen averaged a few times.The most robust of these, by far,is the log sine sweep. I have madehighly successful acoustic measurementsusing this form of sweep withvacuum cleaners operating in theaged.(See Figure 1 for the anechoictime response of a balloon burst andcorresponding energy content as afunction of frequency).Precision measurements requirethe use of a repeatable electronicsignal. Spark generators are usedfor some specialist applications suchas microphone testing and acousticscale model work, though, again,there can be a degree of variationbetween “zaps” or “cracks.” Themost commonly used test signalsemployed for impulse response measurementstoday are sine sweeps, logsweeps, Maximal Length Sequences(MLS) and pseudo-random noise,i.e., pink and white noise.Now, faster thinkers among youreading this will have immediatelyreturns soon sets in, so, going from,say, 10 averages (a 10dB improvement)to 20 averages would only producean additional 3dB improvement(i.e., 13dB). To gain a further 10dBof improvement would require afurther tenfold increase, or a total of100 averages, in this case to achievean overall improvement of 20dB.Now, although this may well bepractical for the laboratory testing ofloudspeakers, it is not a viable propositionwhen testing a sound systemor for measuring room acousticparameters. Not only might the testduration become excessive, but localvariations and fluctuations in theacoustic environment that occur inalmost any large room or space mayaffect the resultant answer. Clearly,a different technique/test signal isrequired. Enter bangs, chirps, andhisses and buzzes.Simple acoustic measurements ofreverberation time, sound decay oreven echo reflection structures canbe made using a relatively crudemechanical impulse such as that obtainedfrom a balloon burst, gunshotor mechanical impact. A significantproblem with this type of stimulus isthat every individual impulse can be(and usually is) quite different fromthe next. Therefore, such testingcannot be used to look at the finestructure of reflection sequences,and a number of impulses must beused and the processed results aver-September 2008 17
Page 1: Vol. 54 No. 9 September 22, 2008AV
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