12.3 Interaural time delays: continuous tones
Coincidence detectors and delay lines cannot be used to localise a continuous tone.
Because, a continuous tone is always present at both ears and if we don't hear the onset of a sound then our auditory system cannot determine the initial difference in arrival times at the two ears.
So, in order to localizs a continuous tone the auditory system uses another kind of temporal information: the time at which the same phase of the sound wave reaches the ear.
Recall that neurons are capable of phase locking to a sound stimulus: they fire at characteristic points or phase angles along the sound wave. A neuron tuned to one frequency would tend to fire, for example when the wave is at baseline (0 degrees), although it may not fire every time the wave reaches this position. A neuron tuned to a different frequency will tend to fire at a different phase angle, such as when the wave is cresting (90 degrees). In both ears impulses produced by neurons tuned to the same frequency will lock to the same phase angle. But, depending when the signals reach the ears, the train of impulses generated in one ear may be delayed relative to the impulse train generated in the other ear.
Imagine you are exposed to a 400 Hz sound coming from the right (Figure 44a). At this frequency, one cycle of sound covers about 85 cm, which is more than the 20 cm distance between your ears. After the peak in sound wave passes the right ear, you must wait 0.6 ms, the time it takes the sound to travel 20 cm, before detecting the same peak in your left ear. Because the wavelength of the sound wave is much longer than the distance between your ears (85 cm versus 20 cm) you can reliably use the interaural delay in peaks in the wave to determine sound location. What about wavelengths that are shorter than the distance between the ears?
Continuous tones of frequencies above about 1500 Hz produce what are known as phase ambiguities. This is because a sinusoidal wave of 1500 Hz has a wavelength about equal to the width of the head. You can see in Figure 44b, that both ears will detect a peak in the sound wave at the same time. Clearly, the peaks detected at the ears are different (labelled 1 and 2 in the figure) but as far as the brain is concerned, there would be no phase difference and the sound would be perceived as coming from the front. Head movements may resolve this ambiguity to some extent. However when the wavelength is less than the path difference between the two ears, ambiguities increase; the same phase difference could be produced by a number of different source locations. Phase differences therefore only produce useful cues for frequencies below about 1500 Hz.