periodicity and perception
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Why speakers with identical specs sound different?TRANSCRIPT
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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.
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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~.
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.--.'~
~- .. ~ ~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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