Auditory filters

In auditory perception and psychophysicas the shape and organisation of the basilar membrane (BM) (in the cochlear of the ear) means that different auditory frequencies resonate particularly strongly at different points along the BM. This leads to a tonotopic organisation of the sensitivity to frequency ranges along the BM, which can be modelled as being an array of overlapping band-pass filters known as "auditory filters". The auditory filters occur along the basilar membrane (BM) and increase the frequency selectivity of the cochlea (for anatomy and physiology see later) and therefore the listener’s discrimination between different sounds. They are non-linear, level-dependent and the bandwidth increases from the apex to base of the cochlea as the tuning on the BM changes from low to high frequency. The bandwidth of the auditory filter is called the critical bandwidth. This was first suggested by Fletcher (1940) and is the band of frequencies which are passed by the filter. If a signal and masker are presented simultaneously then only the masker frequencies falling within the critical bandwidth contribute to masking of the signal. The larger the critical bandwidth the lower the signal-to-noise ratio (SNR) and the more the signal is masked.

Filters are used in many aspects of audiology and psychoacoustics including the peripheral auditory system. A filter is a device which boosts certain frequencies whilst attenuating others. In particular, a band-pass filter allows a range of frequencies within the bandwidth to pass through whilst stopping those which are outside the cut-off frequencies.



Another concept associated with the auditory filter is the Equivalent rectangular bandwidth (ERB). The ERB shows the relationship between the auditory filter, frequency and the critical bandwidth. An ERB passes the same amount of energy as the auditory filter it corresponds to and shows how it changes with input frequency. The ERB is calculated using the following equation:




 * ERB = 24.7*(4.37F + 1)

Where the ERB is in Hz and F is the centre frequency in kHz.

equivalent of around 0.9mm on the BM. The ERB can be converted into a scale that relates to frequency and shows the position of the auditory filter along the BM. For example an ERB number of 3.36 corresponds to a frequency at the apical end of the BM whereas an ERB number of 38.9 corresponds to the base and a value of 19.5 falls half-way between the two.

One filter type used to model the auditory filters is the gammatone filter. It provides a simple linear filter, which is therefore easy to implement, but cannot by itself account for nonlinear aspects of the auditory system; it is nevertheless used in a variety of models of the auditory system.