let's say that (again in the horizontal plane) instead of hearing a sudden sound, you hear a continuous tone. The continuous tone has a very long wavelength, much greater than 20 cm. How would you be able to localize the sound?
in this case, the only thing that can be compared between continuous tones si the time at which the same phase of the sound wave reaches each ear.
Imagine you are exposed to a 200 Hz sound coming from the right. At this frequency, one cycle of the sound covers 172 cm, which is much more than the 20 cm distance between your ears. Afer a peak in the sound pressure wave passes the right ear, you must wait 0.6 msec, the time it takes sound to travel 20cm, before detecting a peak at the left ear.