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Re: Didn't have to read much to recognize trash -- and here's why ---

Posted by David Aiken (A) on April 21, 2007 at 13:53:27

In Reply to: Didn't have to read much to recognize trash -- and here's why --- posted by Richard BassNut Greene on April 21, 2007 at 07:26:28:

"If you want to test for audibility of sound above 20kHz. then you use test signals ONLY above 20kHz."

Do you? There are 2 issues here. The first is whether or not we can hear pure toned above 20 kHz. For that, you test with signals only above that frequency. The second is whether HF content above 20 kHz affects what we hear below 20 kHz. For that you test with broad band signals as they did in this test.

"At best the study might prove that human hearing does not have a "brick wall filter" at 20kHz. But we already knew this."

Do we know this? How many people do you see here and elsewhere saying that we don't hear anything above 20 kHz and suggesting that supertweeters should therefore be inaudible. Many people believe "can't hear anything above 20 kHz" equates to saying we have a brick wall filter there. There's quite a deal of variation in what audiophiles appear to believe about this point based on what gets said here and elsewhere.

"If someone here wants to use a study to prove super tweeters are good components (for people who visit here), then he should cite a study using supertweeters playing ordinary Western music, not some bizarre "music" that contains an unusual amount of high frequencies, and then using brain measurements as a proxy for audibility."

I think you're wrong. We use test signals for determining what people can hear in the accepted audible range rather than "ordinary Western music" or speech, so why not use signals other than those for tests like this. What you want for test material is sounds with a lot of HF content and you'd probably want to vary the content below 20 kHz in order to determine whether frequency content below there affected how we respond to frequency content above there. The frequency distribution of sounds in Western music and in other musics such as gamelan music varies and there's no reason why Western music should be a more suitable 'test signal' than gamelan music. Since the study was done by Asian researchers, it's probably not surprising that they chose non-Western music for their tests. Western researchers are quite free to duplicate the study using Western music and that would probably be a good idea.

As for using brain measurements as a proxy, this does produce 2 useful results. First it removes the subjective response issue that would be present if the researchers only asked the subjects questions about what they thought they heard. You criticised such approaches quite strongly in your original "Dumb Study" post when you said "People will choose A or B because they are trying to please the experiment leader -- rather than saying "I don't know", or "I can't hear a difference". True of high end audio too!". Second, it relies on an objective physical measurement that the subject can't control so it produces quantifiable data which is always a plus in any study.

It does leave us with a problem about whether or not we wish to say that the person "hears" the hf content above 20 kHz if they don't report hearing anything when presented only with sounds above that frequency.

What we are left to deal with, if the results are repeatable and accepted, are a number of questions like:

- does the fact that we respond to hf content extending above 20 kHz affect our perception of that portion of the sound below 20 kHz? About all we can do hear is question the subjects and see whether they have any sort of repeatable, consistent response to sounds with extended hf content that is not present when they are presented with sounds with the above 20 kHz content filtered out.

- do people prefer sounds with or without such extended hf content.

To answer both those questions we're going to be forced to rely on the subjects' subjective responses to questions about their experience, but at least there can be available the objective data of each individual's brain responses as well and it may prove possible to correlate some of the subjective responses to particular aspects of the brain response.

There are probably some very good grounds for criticising this study, but I don't think your points here are among them.

David Aiken

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