Life beyond 20kHz

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“At least one member of each instrument family (strings, woodwinds, brass and percussion) produces energy to 40 kHz or above, and the spectra of some instruments extend beyond 100 kHz. Harmonics of muted trumpet extend to 80 kHz; violin and oboe, to above 40 kHz; and a cymbal crash was still strong at 100 kHz.”

Studies show we cannot hear beyond 20 kHz, most of us less than that. Yet we recognize higher sample rates sound better – and higher means higher than we can hear – which the measurementists claim is poppycock. But, what if we can hear above 20kHz? Might that explain some of why we like extended bandwidth equipment?

James Boyk, of the Caltech Music Lab (yeah – I thought they only did spaceships too) wrote a fascinating paper entitled There’s Life Above 20 kHz .

Given the existence of musical-instrument energy above 20 kilohertz, it is natural to ask whether the energy matters to human perception or music recording. The common view is that energy above 20 kHz does not matter, but AES preprint 3207 by Oohashi et al. claims that reproduced sound above 26 kHz “induces activation of alpha-EEG (electroencephalogram) rhythms that persist in the absence of high frequency stimulation, and can affect perception of sound quality.”

Oohashi and his colleagues recorded gamelan to a bandwidth of 60 kHz, and played back the recording to listeners through a speaker system with an extra tweeter for the range above 26 kHz. This tweeter was driven by its own amplifier, and the 26 kHz electronic crossover before the amplifier used steep filters. The experimenters found that the listeners’ EEGs and their subjective ratings of the sound quality were affected by whether this “ultra-tweeter” was on or off, even though the listeners explicitly denied that the reproduced sound was affected by the ultra-tweeter, and also denied, when presented with the ultrasonics alone, that any sound at all was being played.

From the fact that changes in subjects’ EEGs “persist in the absence of high frequency stimulation,” Oohashi and his colleagues infer that in audio comparisons, a substantial silent period is required between successive samples to avoid the second evaluation’s being corrupted by “hangover” of reaction to the first.

Boyk’s own conclusion suggest that if true, and there seems ample evidence it might be, then hard filtering everything above 20 kHz, as in a CD, might just be the worst thing we can do – and explain much about why higher sample rates makes sense, even though we can’t technically hear above them.

It’s just one more possible nail in the coffin of the measurementists who steadfastly refuse to recognize what many of us perceive just might be right.