11.3 Frequency selectivity

In preceding sections we examined two ways in which the auditory system may code frequency information: the place theory and phase locking. In this section we will look at the psychophysical evidence for place coding on the basilar membrane by examining the ability of the auditory system to resolve the components of sinusoidal waves in a complex sound – a phenomenon known as frequency selectivity.

The perception of a sound depends not only on its own frequency and intensity but also on other sounds present at the same time. You will all be familiar with the experience of one sound ‘drowning out’ another sound. For example, typical classroom sounds, created by movement, coughing, rustling of papers, make the instructor's voice difficult to hear. This phenomenon is called masking. Technically speaking, masking is defined as the rise in threshold of one tone (test tone) due to the presence of another (masker) tone.

It has been known for many years, that a signal is most easily masked by a sound having frequency components close to those of the signal. This led to the idea that our ability to separate the components of a complex sound depends on the frequency-resolving power of the basilar membrane. It also led to the idea that masking reflects the limits of frequency selectivity and provides a way to quantify it.