2
$\begingroup$

If the ear is phase insensitive (Ohm’s Law of acoustics), and you listen on headphones, how can the brain hear a phase inversion on one channel of a stereo signal?

I play bass guitar and practice on stereo headphones. With a split mono signal, it centred exactly between my ears. Experimenting, I inverted one channel, and it turned into surround sound from ear to ear! Of course, on loudspeakers the sound waves would cancel, but not in the brain. It’s not given phase information by the ears. Puzzling. As far as I know, Ohm’s Law is based on the resonance theory of hearing - each hair in the cochlea produces an output at its resonant frequency.

$\endgroup$
2
$\begingroup$

Stereo hearing in the brain depends on timing delays and phase difference. There is also the phenomenon of binaural beats: the sensation when one ear is exposed to a pure tone with slightly different frequency than the other ear (when the frequency is not too high for that).

This is possible because the nerve impulses are more likely in one half of the period of the hair cells. The pulses are far fewer than the number of cycles per second, but below about 0.5 kHz they are linked to the phase. The central nervous system is able to process that in some way.

(And it is not the resonant frequencies of the cilia themselves. It is caused by how the basal membrane moves in the tapered cylinder that is rolled up as the cochlea.)

$\endgroup$
5
  • 1
    $\begingroup$ As a recovering ex-bass guitarist (there's a 12-step program for that but it is notoriously ineffective) I can attest to this effect as well. This also tells you whether or not the pickups in a 2-pickup bass are properly phased as well! $\endgroup$ – niels nielsen Oct 24 '20 at 17:59
  • $\begingroup$ I agree, Pieter, there must be more to it than resonance. As for timing delays, a continuous sine wave doesn’t have any delays in the magnitude of its envelope, which is what I assume the cilia are responding to. If the cilia can actually detect the phase of a signal, doesn’t that mean they will respond to waveform changes due to changing phase of the 3rd harmonic? The ear/brain doesn’t detect that, does it? However, binaural beating as you describe it, does mean the brain is somehow responding to the instantaneous magnitude of the waveform - I think. Or maybe the zero crossing points. $\endgroup$ – Brian F Oct 25 '20 at 12:10
  • $\begingroup$ I now realise that binaural beating on headphones must the same effect as the one I describe. So I expect this to happen: - When the different tones are in phase, the sound should appear in the middle of my head. - When the tones are anti phase, the sound should appear as surround sound. So the beating effect on headphones should be an oscillation between these two states. However, using loudspeakers, the sound waves will cancel for a central observer. Is this so? $\endgroup$ – Brian F Oct 27 '20 at 9:10
  • $\begingroup$ @BrianF Binaural beats are a weird sensation. Yes, it is different from ordinary beats which can be described as an amplitude that varies in time (the beat frequency). $\endgroup$ – user137289 Oct 27 '20 at 16:29
  • 1
    $\begingroup$ Thanks for clarifying, Pieter, an interesting subject. Pity it hasn’t attracted more interest. I’m now wondering whether Niels comment is a joke about 12 semi-tones in an octave. Bye everyone. $\endgroup$ – Brian F Oct 28 '20 at 17:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.