Perceptual binding of harmonics - and pop out

This is a little demo that we found described in the book "Vergleichende Tierphysiologie" by Neuweiler. It nicely illustrates that multiple harmonic components in a tone complex are usually "perceptually bound" and not individually perceived (unless you force them to "pop out"). The sounds in this demo are two simple "tone complexes". The first tone complex, let us call it tone A, comprises the first ten harmonics of 200 Hz (i.e. 200, 400, 600, ... , 2000 Hz and so on). The second tone complex, B, is identical to A, only that the 6th harmonic (1200 Hz) is missing in B. This spectrogram illustrates the spectrum of

harmonic complex sounds

First listen to sounds A and B. You will note that they sound like a single pitchy buzz, not like something made up of different "components". You will also notice that A and B sound almost identical. The difference between them, the missing 6th harmonic, contributes so little to shaping our percept of the tones that most of us are unable to detect whether it is present or not.

A:


B:

Now listen to A and B played in quick alternation. The thing to watch out for is that the 6th harmonic of the A tone, the 1.2 kHz component, seems to take on a life of it's own. What you are likely to hear is a continuous buzz with a high frequency "beep" switching on and off four times a second. This "beep" has become "unbound" in your perception from the A tone. ABABABAB:

Interestingly, the "perceptual grouping" of the frequency components performed by your hearing is not unlike that performed by your visual system if you look at the figure above. The line at 1.2 kHz "belongs" to the A tone shown on it's own on the right, but "does not seem to belong" to the alternating ABABAB sequence on the right.