The HD-Audio Challenge II

[David A]

 Are my ears up to it? Probably not. 

Hi res music has the technical capacity to produce frequencies above red book CD quality, which is about 22 kHz. That's the one and only difference between CD and Hi res. Anything above 20 kHz or so is beyond the scope of human hearing so to answer your question; nope, and neither are anyone else's.

i have read this claim many times and it seems reasonable.

However I, and quite a few people I have spoken to, can definitely hear ultrasonic catstoppers that, as far as i could check are typically specced at 23 khz.

i am not sure what to make out of this.

Cheers,
Bernard

Unless you're under 20 years of age I can pretty well assure you that you can't hear 23 kHz. I don't know what kind of a signal catstoppers put out but something may be audible.

One of our common audio specifications is for intermodulation distortion. If you have 2 tones at different frequencies, they generate distortion at frequencies which are multiples of the difference in frequency of the 2 tones. The standard test for intermodulation distortion is to use a test signal comprised of tones at 19 kHz and 20 kHz respectively and both of those frequencies are beyond the hearing range of most people out of their teens, probably out of the range of most teenagers as well unless they've managed to avoid listening to music at loud levels. The distortion products occur at 1 kHz, the difference between the 2 tones, and at multiples of 1 kHz. While we can't hear the 19 kHz and 20 kHz tones, if they are loud enough we can hear the distortion tones at 1 kHz and it's multiples that lie within our hearing range. If your catstopper uses two or more tones to deter cats then while you may not be able to hear the tones it uses, you may well be able to hear the intermodulation distortion tones that are generated.

Such tones actually sound quite strange. I have a recording by an avant grade performance artist named Meredith Monk. One track consists of her creating a high frequency tone by running her wet finger around the rim of a crystal glass, generating a high pitched tone caused by exciting the resonance of the glass. At the same time she starts singing a note just above the frequency of the tone generated by the glass and slowly drops the pitch of her sung note to below that of the tone generated by the glass, then brings her note back up to above that of the glass and then down again, and simply keeps repeating this process for a couple of minutes. What you hear while listening to this track are the sounds of the glass and of the voice where you would expect to hear them in the soundstage but at the same time you hear a slowly changing rather piercing tone in your ears, the sound of the varying difference in frequencies between Monk's voice and the resonance of the glass, an intermodulation product. It's actually surprisingly loud and it tends to "drive people crazy" as the saying goes. Any time I play that track to someone it's almost guaranteed to generate a "take that damn thing off!" response because the intermodulation tone which sounds like it's located inside your ears is so annoying. Strangely, it's not a low pitched sound which you would expect because the difference in frequency between the voice and the wine glass resonance is quite small, but a rather high pitched sound, a multiple of the difference between the 2 actual sounds on the recording. The reason it appears to come from a location within your ears is probably because one of the higher intermodulation products is actually exciting a standing wave resonance within your ear canal and the pitch is high because the physical length of the air column being excited in your ear canal is very short. I think part of the annoyance also comes because the resonance actually causes a slight physical vibration within the ear which is also unpleasant.

As I said, i don't know what the signal catstoppers put out is like but just if it's above the range of your hearing that doesn't necessarily mean that you won't hear anything while it's working. If it's emitting 2 or more tones simultaneously you may quite possibly be hearing the sound of the intermodulation products created by those tones because the intermodulation products are within your hearing range while the actual tones generating them are not.