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COVfR ffATURf

Why &paal(ers with idantical &pacitic:atioM sound

be usual methods of speaker testing yield a mystery, one that }Qu're certain to have rome across in the hili press: if two similar speakers from different man-

ufacturers have identical distortion, tone burst and frequency response specifications, why does each speaker have a uni­que sound? Writers struggle to describe these differences, coming up with such terms as stri- . dent, veiled, or muddy in an attempt to capture the subtletiesofsound

One term tbat seems to work is "trans­parent". Tbe accepted meaning is that a trans­pare.nt speaker adds

Bill MARKWICK

nothing of its """ to the Mike Wligllt ill tile RkiU1lood HiU, Gilt lab alld SOl/lid room. sound, producing natural audio that just seems to come out of thin air. In the early years of the hili booni, this was usually described as "an orchestra actually playing in your 1Mng room", an elusive goal for all but the best of systems. A speaker Vthich is not transparent immediately tells you that you're listening to the music through a machine, and this is true whether or not the speaker does well in the standard tests.

Stand.d T .... '" Before the advent of affordable oomputer­controlled test gear, there were a number of metbods used to quantify speaker response, and despite hi-tech advances in equipment, they remain the mainstay. The most popular, and one that gives a great deal of informatioo; is the frequency response test A calibrated micropbone is used to measure the output from the speaker as it is swept over the audio fre­E&1TDlc.,_'.

quency range. Unless you have an anechoic chamber which preven.1s any reflected sound, this test is plagued with the peaks and dips of the room response itself. Some of the ways around this in­clude the averaging of several tests from different directions and the use of rapidly swept frequencies to avoid stimulating room resonances. .

. TeSta. soon realized that steady-state frequency response wasn' teIling the whole story, and the tone burst test was used in an attempt to measure the spcal='s ability to respond quickly without overshoot; the test frequency is switched on and off rapidly, 1ctting through a desired number of Ojdes. The difficulty oomes in trying to interpret the imJJClfect tone burst 1MJich. is picked up by the miCropbone.. Sometimes the. "'l'ults

. have 00. apparent ixmnection with the per­ceived sound.

DiStorfun seems to be an important

parameter, measured with the usual notch fiJ­ter or with a spectrum analyzer that can sum tbc value of tbe har­monies, but again, the difficulty lies in trying to explain wby · a speaker with high distortion sounds better than onc with im peccable specifications.

Adding to the tech­nical difficulty is the processing of the sound by the listener, a subjec­tive variable much we'll come to in a later sec­tion.

P .. tocIcIty In the mid-l97Os, Bell Laboratories published papers on the use of the Fourier transform in sound analysis. The Fourier analysis is a

mathematical tool used to find the various components that make up a complex waveform; a spectrum analyzer displaying the harmonics of;t sound is doing a Fast Fourier Transform (FFT). By doing another transform on the new-found rom­ponents, you can find the periodicity;· such anomalies as reflections or speaker shortoomings show up clearly.

The method of analysis was used to analyze the noise signature from Con­corde jet engines; previously, the tests had been affected by sound reflections from the runway, but the periodicity tests al­lowed engineers to separate pure engine noise from the total sound The method was later used by Bruel & Kjaer in their industrial.failure-prediction equipment to separate undesired machinery noise froin the tOtal sound, allowing detection of im­P"'iding faults without the necessi.tY of shutdown.

9

PariocIcity ani Perc BIJtion

t AMPLITUDE

AMPLITUDE

r TI ME

SQUARE WAVE (TIME DOWJN)

AMPLITUDE

1111111 I I I I

TIM E DOMAIN

t AMPLITUDE COMPONENTS OF SQUARE WAVE AFTER FOURIER ANALYSIS

FREQUENCY

FREQUENCY •

FREQUENCY DOMAIN

FOUJieranalysis tkals MiD, 0", frequency dcJmain flztherthan the.mfNfl Fourier analysis aOows compler wavelQmlS to be separated into Oleir lamiIiI1rlimL domain. freqlll!1lC)' r:cmpotwlts.

Part of a cepsUwn response plot of a DO)1on Wright speake. 2fJ48 samples /uJve been taken lorlimL rorutanJs from .5 to 133 mil­liserauls. The large spikes at the rili,t indicate room response, and the area cirr:Jed in pen indiaztes an anomaly.

1 2 3 4 5 6 7 8 9 10

V I-" \ - I . ""'\ V '-f-'

COUpl EX WA\{ffORM

ffi A · . . . . . . . · . . . . . . . .

\7~ · . . . , . . . . . . .... . · . . . . - . . .

1 OF 1 Q SAMPt INC SINEWAYES

III I I I I I I I I I I RfSU! T IS fREQUENcy SPECTRUM

. the sampIing.silteM<rne is multiplied by die ekmmlln etJCh Mindow 01 thesampkdwavefomr,andisd..,.repemedaahit/ler~.

:'10

.--.'~

~- .. ~ ~lfTtIIIm~

&cause of differmas in 0 .. _ padlkngtllS, the periodicity will /uJve a ccmponmI dlle to the reflection.

Here in Canada. the method was adapted to speaker analysis by Mike Wrigh~ developer of the Dayton 'Wright loudspeakers and Stabilaot 22, a liquid ·semiconductor used as a contact enhancer (see the review in our October issue). P~odicity testillgolIered the posstbility of easy removal of room effects from a respoose plot, as well as the detection of unwanted reflections from the speaker construction itself.

Pawcepdol. One year during • large trade show, Mike noticed that his awareness of speaker quality wasscriously affected by the noise and associated fatigue of maintaiDing the display booth. Speakers . which had pre­viously sounded fine were becoming a chore to listen to, a phenomenon which was easily interpreted as the brain's reJuo­.tance to accePt any morc input

. However, that night hMvenl to a sym­phony concert and discovered that the live sound had none of the expected shortComings. The conclusion he arrived at was that all speakers were introducing small. oddities of Uleir own, anomalies that the brain filtered out. This extremely com­plex filtering allow.; you .to listen to desired sounds in the middle ora noisy party, and lack of it is: why tape recording; of that party will later sound incredibly clattery and jumbled, since the required important information (phase relationships, visual cues, etc) is not present.

The 'periodicity tests seemed like the . bes( way to analyze speaker output and fmd whatever . faults were occupying so mucb of the brain's audio processing.

Tasting The present test setup in the Richmond Hill plant consists of a soundproof room which is finished inside to represent a typi­cal listening environmen~ and even in­cludes easy cbairs. A calibrated' AKG microphone picks up the speaker outpu~ which is .a swept-frequency pulse train. The signals are processed by a Hewlett­Packard spectrum analyzer can be dis­played on its'CRT as a standard frequency responseplo~ or as a spectrum of com­ponents (using the FFl). It also has a 16-bit 9utput whicb is .captured by an HP 6&JOO-based computer.

'. The software, which consists of 17,OOJ lines in HP Basic, can then process the in­fO!1')8tion to plot response, phase, and pe­riodicity (the advantage in the spectrum analyzer lies in its speed ~ the computer takes mucb longer to derive tile FFl).

. '. . E&TT o-l1b .. 1.

When Bell Lab; published their ideas on using Founer anaJysis, someone whimsi­cally labelled Ule various parameters using anagrams of familiar terms, and So the peri­odicity ~ whicb looks something like a frequCIICY spectrum. becomes a cepstrunl. The periodicity is formally defined as the in­verse FFI' of the log power spectrum of the components of the sound, and the cepstrum is a plot of the ripple in'a waveform for eacb time· constant of the components causing

· the ripple. For instance, if the cepstrum reveals a

spike with a time constant of about =, then some two surfaces in the speaker en-· vironment" are causing 3 . reflection, . and · they'll be about 2 feet apart (taking the velocity of sound as 1ft/ms): As to why this information ·is not revealed in siandard speaker testing: the · information is there, but the test format · may not be, ideai for displaying i~just as a ,soope display of a squarewave gives no hint that n's the sum of a long'series of odd harmonics.

. The process ' can be used to detect . small reflections in drivers and cabinets. For example, speakers often sound better with the griI1 cloth removed; it's not just a case of sound absorption by the cloth - reflections from the frame· itself can cause audoble ef­fects. The speaker on the cover is being tested with a fibreglass pad to elirninate sur-

· face effects; in production this would be replaced by acoustical foam, and the

· speaker is constructed so that there's no fr<lIlle protruding past the front surface.

Standard speaker testing· in combina­tion with periodicity plots allows. rapid analysis of the. speaker drivers, enclosure, crossover, and 1!stening environment. nle

, result of investigating and correcting is a speaker that approaches the ideal of transparency, one that never lets you know it's there.

noises behind the curtain indicated that the speakers had been changed, and the test

. was repeated The listeners liked the first ~ 'and said that the second set were in­ferior to it In fa~ the speakers were ncvcr touched What had happened is that the room .aooustics dominated the · sound en­vironment when the people first entered for

. . the first t6st By the time of the second t~ they were used to the room 'and began to judge differently. There's also the fad that novelty affects perception; musicians ollen prefer. someone :else's instrument-for a while. When the novelty wears off, theyre more oojective about deciding.

The curtain in the above tests is also used in other testing because visual cues are so important to sound perceptiOD, par-· ticularIy the 'localizing of a sound Mike said that additional speakers placed at either side of the ·listener will cause them to """"' that the stereo image is much wider, even though the side speakers are not even con-nected. .

Level settings are extremely impor­tant during comparison testing of

· speakers. The usual wisdom is thal one dB · is the minimum.sound level difference we · can detect, but the ear. is much more sensi-· live to chanie in the 'midrange area; if

speakers.are tested with a·level difference of-about 05dB, the higher level gives the

· vague impression. of brighter response. If the speakers arc switcbed using the same

· amplificr; the morc efficient speaker sounds louder and brighter.

There's also evidence that the right ear perceives high frequencies in a dif­ferent marmer than the left, a fact which · may be due to the partitiol)ing of the brain.

The speakers under test .C3?Dof oc-· cupy the same space, so room acoustics

will cause. a different soWld response even if the speakers are identical- If· the test is interrupted and the speakers are inter:

ABlT8II1Ing changed, the delay may not let the listener The above discussion on speaker improve- retain accurate impressions of the sound ments .is somewhat oversimplified; since . . ' To sum up, the NB test must be done there's a great deal more to- speaker · under .e;<tremely well"l controlled cir-analysis than watching 'a plohmd tinkering ' cumstances in order to reveal anything here and there. The tester may . use meaningful Like statistics, they can be periodicity to disoover some small ripple made to prove anything you want. in tJle response, but the decision as to .. .. . .·rAnd .how well did .this. research whether or not this isalIecting the sound. benefit the Dayton' Wright line of depends on the listener, and most listeners. . speakers? They can definitely. have a right are almost . unbelievably flexible in their to the claim of transparency, their sound

· perception of sound.ln mosl.cases, .theire indiCateS meticulous care in design. so · unaware of how·their own mental process,. much);o.that StereQfVidco guide of Oc-ingis fooling them. , . ..';: : . tohei',.1987 rated them as the number-one

Mike Wright held a.speaker Iisterung ' :' speaker.. . test in whicb listeners-carne into the room. ; ._, .:··Special thanks ta Mike Wright for the wbile a set of speakers were playing Then .' . /in,,; -<PetIt explaiJluog<peaker testing.

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