This demo is designed to be listened to over stereo headphones. We therefore added a 5 Hz sinusoidal amplitude modulation to the this demo. Steady pure tones are unpleasant to listen to, and they also produce a great deal of adaptation in the auditory nervous system, making it difficult to perceive them well. So about 2 seconds into the demo, the sound will sound as if it is now suddenly coming from the left, but then it will gradually shift over to the right again over the next 4 seconds, only to then jump again to the left, and so forth. After more than 2 seconds, the right ear phase leads by more than 1 ms, but since the period of the tones is 2 ms long, the brain may interpret this as a less than 1 ms phase lead in the left ear. However, since the frequency in the right ear is ever so slightly higher (the oscillations are ever so slightly faster), over the next 2 seconds the right ear's phase starts to lead by up to 1 ms, giving a changing ITD cue that suggests that the sound is shifted to the right. The left and right ear start in phase, so when listened to over headphones, the sound should start off sounding as if it was in the middle. So the left and right ear go in and out of phase once every 4 seconds.
To the left ear we play a 500 Hz pure tone, to the right ear a 500.25 Hz pure tone. Conequently, tones that are slightly mis-tuned and which are delivered separately to the left and right ear can give the impression of a shifting lateralization from left or right. As discussed in the context of Figure 5-5 of " Auditory Neuroscience" (reproduced here below) ITD cues for sound location are derived from the interaural phase.
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