ark@rabbit.UUCP (Andrew Koenig) (01/14/84)
Quote from Phil Rastocny: Now if there is a set of technical parameters that will assist me in relating what can be heard to what can be measured, fine. Specs are supposed to steer us in the direction of what equipment we want by correlating them to what we hear. AND we should be able to measure literally anything that we hear. Great! No argument. But if I can hear something that any of the specifications supplied with a piece of equipment cannot be correlated to the observation, then where do we go? We talk about what we hear (like the soundstage size or inner detail) and then poke around in the circuit until we realize exactly what causes the observation spec-wise. Amplifiers that have similar distortion, damping, power, bandwidth, etc., should all sound about the same. But when comparing two equivalent amplifiers (like an Acoustat TNT-200 to a Kenwood Basic M-2) on a suitably refined reference system, they still sound different. (Both amps are class B about 200W, < 0.01% THD and IM distortion, both slew > 100V/uS, and both are finely engineered.) All of the specs are orders-of-magnitudes greater than the ear should be able to detect and essentially identical. But yet they still do not sound insignificantly different. I'll mention this again. A number of years ago, Audio magazine did a carefully controlled double-blind test. Very briefly, they concluded: 1. If two amplifiers with reasonably low distortion figures are made to match VERY accurately in frequency response, and they are not driven into clipping, it is not possible to tell the difference between them by listening, even if the listener is a "golden ear." 2. Two amplifiers that differ in frequency response by a little tiny bit will be perceived by a careful listener as different, but not in any way subjectively related to frequency response. Rather, the listener will hear differences in "depth," "imaging," and so on. 3. We are talking about frequency response variations of the order of 0.3 dB -- small enough that two samples of the same make and model will be that far apart. In fact, a SINGLE amplifier will vary that much from one day to the next, from changes in temperature and humidity. One of these days, when I have the time, I will scare up the exact reference. I posted it the first time I mentioned this test. In any event, the description of the test conditions was detailed enough that unless the author was lying outright there was no doubt in my mind that the test was fair. I have NEVER seen any documented evidence that it IS possible to hear differences in amplifier behavior that cannot be accounted for by objectively measurable parameters.
rentsch@unc.UUCP (Tim Rentsch) (01/14/84)
It seems worth pointing out that the testing method may have had more profound impacts on the results than what the experimenters were trying to test. In particular, if the amplifiers were frequency matched by some kind of frequency euqalization circuit, i (for one) would believe the effects of the F.E. circuit to swamp the effects of distortion (of the extremely low variety) in the amplifier. You have to be VERY careful when doing experients of this kind that you are measuring what you intend to measure. (The other results, i.e., that freq. resp. difference was not percieved as such, was very interesting. This is, however, exactly what I would expect if a frequency equalization method was used.)
ark@rabbit.UUCP (Andrew Koenig) (01/15/84)
It has been suggested that equalization might introduce enough distortion that it would mask the differences between the amplifiers. The experimenters considered this objection, and offered the following counter-argument: the amount of signal processing done in the experiment was far less than the amount done during mastering of the commercial LPs used as a signal source. Even direct-to-disc has to go through RIAA equalization, and 'conventional' records are made with much more signal processing than that. I do not think the people who claim to be able to hear the difference between power amps insist that they can only do so if you are playing a live 30 IPS master tape without any equalization...
gregr@tekig1.UUCP (Greg Rogers) (01/16/84)
Well controlled double-blind testing should prevent the problem that you site. Very simply, the frequency equalization device is only applied to one of the units under test at a time. That is one component is equalized to the other, not both components to some arbitrary flatness. There are two distinct cases. If the objective of the test is to disprove a particular component superior to another, the equalization device is applied only to the presumed inferior device which can only result in making it even more inferior except with regard to frequency response of course. If the test then shows no detectable difference between components, the equalization device certainly cannot be blamed for degrading the "superior" component. The assumed superior component is hence proved equivalent except for frequency response variations which can be simply corrected as in the test. If the purpose of the test is to prove no differences exist between components (neither being claimed as superior) then the test should be run twice with the equalization device first applied to one component and then the other. If both tests show no differences between components, then it is proved that no differences exist other than frequency response which again can be simply corrected as in the test. It is of course more complicated if the results of the test indicate that differences do exist. It is then necessary to isolate the cause as either the equalizer or components. There are numerous ways of doing this but it would then be up to the OPPONENTS of component differences to prove the equalizer at fault. In other words, the use of an equalizer could incorrectly influence the result in favor of those that claim differences in components, but CANNOT favor those that claim no differences when done in the correct manner that I've outlined. I hope this helps you understand how really objective these objective tests are when designed within the rules of scientific method. Greg Rogers
mmt@dciem.UUCP (Martin Taylor) (01/17/84)
It's a natural presumption that any subjectively detectable variation in sound quality can be correlated with some variation in objective measurements, but the question often is "what should we measure?" The ear is a funny beast. Sometimes it doesn't hear what you might think was the most awful mess, and other times its sensitivity is exquisite. Training matters enormously, although the training may have to be very specific to the listening task. For example, one classic old experiment used a specially invented signal whose total energy was constant during the entire pulse of some tens of msec. But the energy distribution across the ears was a function of time. It started at 100% in one ear and wound up up at 100% in the other ear, with a linear shift over the pulse duration. So it was heard as a left-to-righ or right-to-left sweep. In one of the four quarters of the sweep, the energy split reverted for about 10 msec (if I remember properly) to 100% in one ear. The observer had to tell which quarter had the blip in it. People were trained for days and weeks, and still could score only chance results, until suddenly they found out how to listen, and then over the space of a day or two went up to essentially perfect scores. Another example: Take someone off the street and get them to listen for lateralization of sound caused by an interaural time delay. Typically, an inter-aural shift of some 100+ microseconds is required for perfect lateralization (telling which side of centre the sound is). After prolonged training, subjects may suddenly start being able to do the task with only around 10 usec shifts. It is quite likely that the standard measurement specs do not take into account some signal parameters that are used by some listeners and not by other *equally good* listeners. I would trust subjective discrimination tests to tell me that there was something different, and would then look to objective measures to find out what those differences might be. After finding out, I would tend to look for those differences in any later tests on new equipment. There is still art in psychoacoustics. -- Martin Taylor {allegra,linus,ihnp4,uw-beaver,floyd,ubc-vision}!utzoo!dciem!mmt