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Are these biases?

Posted by David Aiken (A) on April 24, 2007 at 19:00:57

In Reply to: Where are the data to prove listeners could hear a difference with and w/o the ultra high frequencies? posted by Richard BassNut Greene on April 24, 2007 at 09:38:35:

That's a serious question.

IF (note: big "if") you want to find out whether HF content can be perceived by listeners in some way (note: I did not say 'heard' for reasons I will state a little later), then I think as an initial test it is more than reasonable to start with subjects who are under 35 and largely female, and with music with a high HF content, because if you can't get a positive result under those circumstances you would seriously need to consider giving up on the hypothesis. If you get a positive result under those circumstances, then you could be encouraged to continue with a wider range of test subjects including strong representation from older people and males, and with music with less HF content to see just how perceivable the HF content is for a wider and more representative set of subjects and with a wider range of music if, of course, you were interested in the question from a musical enjoyment angle rather than a straight perception angle which I'm not certain the researchers are. I think it's actually worth while starting out by seeing whether people who are most sensitive to higher frequencies in sound show a response than by starting out with a sample of people who are less sensitive to higher frequencies or even with a group of people representative of the population as a whole. I'd rather establish that some "more sensitive" people show a response and then see just how representative their response is than come up with a negative result from a less sensitive group that leads to the question being dropped and ending up not knowing that some people do show responses.

Now, as to "hearing". Our various senses register a wide range of stimuli including, in total, a much wider band of the radiation spectrum than any single sense registers. We can feel low frequency sound if the volume is high enough, and we can feel subsonic frequencies that we can't 'hear'. Does that feeling aspect of low frequency perception affect our experience of the low frequency sound that we hear, and does it affect our enjoyment of that sound? Under some conditions, I think it does. I like the vibratory effect I've perceived with bowed double bass sounds at time, and it adds to my enjoyment of the auditory aspect of the sound. Testing has failed to show that people can hear pure tones much above 20 kHz, and that we lose the ability to hear the last 5 kHz or so of that with increasing age. Does that mean that frequencies above 20 kHz don't contribute to our perception of sound? Not necessarily. What if frequencies above that are perceived by one of our other senses, as subsonic frequencies can be perceived? If that is the case, does such perception modify our overall perception of sound containing HF content in its overtone structure? That's a hard set of questions to answer but they are worth asking.

What this study attempts to do is to ascertain whether there is a difference in perceptual response to sounds containing HF content to similar sounds with the HF content filtered out. The brain measures are a good, objective means of determining whether there is a difference in response and this study indicates that there were differences. That's a good starting point but there are a lot of things that need to be sorted out before one can know what that really means. Amongst those things that need to be sorted out are:

- whether this study can be replicated with different subjects from the same demographic (ie under 35, largely female) with similar test signals. The reason for that is simply that this is the starting point and if there was some problem with this study that threw up a false positive result, it would be nice to find that out early;

- next, if replication is possible, what happens when the demographics of the test group are changed to a more representative sample? How do age and gender affect the results?

- next, vary the test signals with different sounds and music to determine how the proportion and nature of the HF content affects the results.

- and somewhere in the process, start trying to find out just what mechanisms are involved in the perception of the HF content. Is it hearing or some other sense.

- finally, at least as far as science is probably concerned but hopefully a little earlier for those of us with more personal interests in this particular question, is resolving the issue of whether such perception—once we're genuinely satisfied that it is present—modifies our enjoyment of music and in which ways does it do so.

I reckon this is a damn good study for an initial foray into an extremely interesting area. It doesn't answer all of the questions, and it raises a few of its own, but at least it gives some good strong support to a push for further investigation into the area and I think that's a very good thing. I think it was worth doing the study and I think it was worth publishing the results as they are because that may well encourage others to look at the topic and that's essential to the process of finding out a bit more about it.

As for your final comments on FM quality and sound quality in general. I don't know whether the best sound quality I've heard is from CD or from vinyl because I've heard damn good sound from both and we know vinyl does contain higher frequencies than CDs do. I quite enjoy FM radio from time to time. I'm also 60 this year and, though I haven't had my hearing tested in years, I do know that I can't hear a 15 kHz test tone on my system at the levels at which I normally listen to music. I also know that I prefer the sound of CDs on my system (no vinyl source in my system) to the sound of FM radio. Is that because CDs contain higher frequency content than FM—content at frequencies that I know I can't "hear", because my CD player is of better quality than my FM receiver, for some other reason, or for a combination of 2 or more of those reasons? I don't know the answer to those questions and I'd venture to suggest that neither do you. At least this study is a first step to answering some of those questions and it's nice to see that step taken.

I'm not unaware of problems with this study but I'd rather appreciate it for what it does than rubbish it because I don't like its shortcomings and/or have severe doubts about what it sets out to prove. I think the researchers deserve a fair bit of credit for their work on this one.

David Aiken

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