Why Humans Can Hear the Inter-Aural Delay

I’ve given this subject some attention in the past. It seems to be a fact, that as long as a sound stream is temporally complex, Humans can use the Inter-Aural Delay, as one of several hints as to the direction, which the sound came from. But as soon as the sound is temporally uniform, we cannot.

The way I’d explain this is without physical controversy. The way neurons fire, they can either be seen to carry binary or analog information. In short, one firing of a neuron could be like a ‘1’ as opposed to a ‘0’. Or, the steady rate at which a different type of neuron is firing, could encode an analog level. Well, some neurons seem to operate in both modes. At the onset of a signal, they could fire a short burst, after which a steady rate indicates a sustained amplitude.

The length of the path, which signals from the left auditory nerve need to take, to reach the left auditory cortex, may be exactly the same, as the length of the path, with which signals from the right auditory nerve take, to reach the left auditory cortex.

Therefore, the auditory cortex should be in a good position to discern in which order pulses of sound, or onsets of sound, reach it, as part of its information to determine direction, hence, to perceive the IAD.

But AFAICT, if the amplitude of a sine-wave is constant, then there is no real way in which our cortex can discern in what relative phase position it has reached our two ears.

(Updated 11/22/2018, 18h55 … )

(As of 11/22/2018, 15h25 : )

Now I suppose that a valid question the reader could ask next would be:

‘I know that if the listener is sitting in front of two actual speakers, which are outputting a sine-wave of equal amplitude, but with the left speaker’s signal phase-advanced 90⁰ to the phase of the right speaker’s signal, and with the listener’s distance equal to both speakers, the sound will seem to be coming directly from the left speaker, not the right speaker. Why would that still happen?’

And the short answer to that question would be the fact, that speaker-output is not the same thing as cochlear intensity.

The slightly longer answer would be, that if there are 2 speakers and 2 ears, the sound is traveling along at least 4 paths:

  1. From the left speaker to the left ear,
  2. From the left speaker to the right ear,
  3. From the right speaker to the left ear,
  4. From the right speaker to the right ear.

Not only that, but depending on the acoustics of the room, there could be more paths, due to reflections.

And, even if the reader is not aware of this consciously, his or her brain – its auditory centers specifically – have spent decades figuring this out, when the brain interprets sound. Further, while it might be tempting to assume that the paths crossed over, are additive, in fact they could be subtractive. I.e., sound which has traveled from the left speaker to the right ear, is affected by the fact that the head casts an acoustical shadow. And the aerodynamics of the head, as well as of the (outer) earlobes might be such, that the effect of the crossed path is subtractive.

And, if the listener cannot discern the phase-position of the sound that reaches each cochlea in fact, then this would also never have given the listener a conscious hint to what was happening. I.e., we wouldn’t notice the path that crosses the head being subtractive, consciously.

And so, in such a case, what started out as a 90⁰ phase shift at the speakers, can end up as a blunt difference in amplitudes, in the cochlea.


 

(Update 11/22/2018, 18h55 : )

I can speculate a bit, about a greatly simplified example.

The speakers I wrote about above, could be at 45⁰ in front of the listener. Assuming that the distance to either is equal but long, this means that the delay with which the crossed-over waves reach the opposite ear, after they have reached the closer ear, will follow as the square root of (1/2) times ~0.5 milliseconds, or 0.35 milliseconds.

Let’s say that the waves crossing from left to right are delayed by 0.35 milliseconds, and then inverted, and that the frequency from the speakers is 700Hz. As the phase-position of the left speaker’s output advances, the phase-delay with which its waves reach the right ear decreases. By the time the left-speaker’s signal is advanced 90⁰ in phase, its waves which reach the right ear are no longer delayed, but merely inverted.

This could lead to nearly complete cancellation, on the right ear, while the opposite effect should happen on the left ear, where partial cancellation from before, leads to no cancellation…

Dirk

 

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