Critical Band - Relationship To Masking

Relationship To Masking

Auditory filters are closely associated with masking in the way they are measured and also the way they work in the auditory system. As described previously the critical bandwidth of the filter increases in size with increasing frequency, along with this the filter becomes more asymmetrical with increasing level.

These two properties of the auditory filter are thought to contribute to the upward spread of masking, that is low frequencies mask high frequencies better than the reverse. As increasing the level makes the low frequency slope shallower, by increasing its amplitude, low frequencies mask high frequencies more than at a lower input level.

The auditory filter can reduce the effects of a masker when listening to a signal in background noise using off-frequency listening. This is possible when the centre frequency of the masker is different from that of the signal. In most situations the listener chooses to listen ‘through’ the auditory filter that is centred on the signal however if there is a masker present this may not be appropriate. The auditory filter centred on the signal may also contain a large amount of masker causing the SNR of the filter to be low and decreasing the listeners ability to detect the signal. However, if the listener listened through a slightly different filter that still contained a substantial amount of signal but less masker, the SNR is increased, allowing the listener to detect the signal.

The first diagram above shows the auditory filter centred on the signal and how some of the masker falls within that filter. This results in a low SNR. The second diagram shows the next filter along the basilar membrane, which is not centred on the signal but contains a substantial amount of that signal and less masker. This reduces the effect of the masker by increasing the SNR.

The above applies to the power-spectrum model of masking. In general this model relies on the auditory system containing the array of auditory filters and choosing the filter with the signal at its centre or with the best SNR. Only masker that falls into the auditory filter contributes to masking and the person’s threshold for hearing the signal is determined by that masker.