Psychophysics of Pitch Perception

Superimposition of Pure Tones: First Order Beats and the Critical Band

In this section we will analyse the fundamental characteristics of the superimposition of two sine tones. In doing so we will come across some very fundamental concepts of the physics and psychophysics of music.

There are two sorts of superimposition effects, depending on where they are processed in the listeners auditory system:

1 First order superimposition effects: in which the processing is mechanical, ie in the cochlear fluid and along the basilar membrane.

2 Second order superimposition effects are the result of neural processing. These are much more difficult to detect, describe and measure unambiguously. We shall focus on first order effects.

The Physical Meaning of "Superposition of Sound"

The Eardrum moves in and out in response to pressure variations of the air in the auditory canal. If it oscillates in pure simple harmonic motion of a given frequency and amplitude, we hear a sine tone of a certain pitch and loudness. If two such tones of different characteristics are sounded together the auditory system reacts as if the eardrum was receiving two independent commands simultaneously, one given by each sine tone.

Provided that the amplitudes are not too large, the resulting motion of the eardrum (and all the other vibrating components) is the sum of the individual motions that would occur if each tone were presented alone, in the absence of the other. This characteristic is called a linear superposition of two vibrations.