A 250 Hz tone is prented to the left ear while a 251 HZ tone is presented to the right ear.
An important issue in the study of sound localization concerns the ear's ability to process phase differences at the two ears. One way to study this phenomenon is to present two sinusoids of slightly different frequencies, one to each ear. At low frequency the sound may appear to fluctuate or beat slowly at a rate equal to the frequency difference between the two tones.
Note that these binaural beats are quite unlike the physical beats that can be heard by a single ear (Demonstration 32). There, the small difference in the two frequencies caused the physical stimulus to wax and wane in intensity; if this fluctuation was slow enough, it was experienced as a beating sensation. With binaural beats, however, interaction between the two tones occurs because of some kind of interaction in nervous system of the inputs from each ear.
One might expect that these binaural beats would occur only at low frequencies since at higher frequencies it is difficult to imagine that the nervous system can preserve the temporal structure of the waveform at each ear - a condition that must met for their interaction to be noticeable. This conjecture is supported by quantitative measurements (Licklider, Webster, and Hedlun, 1950). They found that the best binaural beats occurred at frequency separations of about 30 Hz near 400 Hz and much smaller frequency separations at the higher frequencies. No binaural beats are evident above about 1500 Hz. Tobias (1965) found that men appear to perceive binaural be at a higher frequency than women, but this needs more exploration.