Binaural Question 4

Question 4: Why would presenting two tones with slightly different frequencies create a perception of motion?

Consider the behavior of the two tones over the course of one full second.

Assume that on the first cycle (i.e. t = 0), the two tones start at the same time.

The frequencies of the two tones are very similar. But over the course of one full second, the 250-Hz tone will complete 250 full cycles while the 251-Hz tone will complete 251 full cycles. The 250-Hz tone has a duration of 4 ms while the 251-Hz tone has a duration of 3.98 ms.

On the last cycle (i.e. t = 1 s), the two tones finish their cycle at the same time.

If the two tones finish at the same time on the last cycle, the 251-Hz must have started 0.02 ms later than the 250-Hz tone.

Thus, the ITD was 0.02 ms at the start of the last cycle, but decreased to zero by the end of the last cycle. So the ITD changed over the course of the last cycle (as well as every other cycle) giving rise to the perception of motion.