In audiology and psychoacoustics the concept of critical bands, introduced by Harvey Fletcher in 1933 and refined in 1940, describes the frequency bandwidth of the "auditory filter" created by the cochlea, the sense organ of hearing within the inner ear. Roughly, the critical band is the band of audio frequencies within which a second tone will interfere with the perception of the first tone by auditory masking.
Psychophysiologically, beating and auditory roughness sensations can be linked to the inability of the auditory frequency-analysis mechanism to resolve inputs whose frequency difference is smaller than the critical bandwidth and to the resulting irregular "tickling" of the mechanical system (basilar membrane) that resonates in response to such inputs. Critical bands are also closely related to auditory masking phenomena – reduced audibility of a sound signal when in the presence of a second signal of higher intensity within the same critical band. Masking phenomena have wide implications, ranging from a complex relationship between loudness (perceptual frame of reference) and intensity (physical frame of reference) to sound compression algorithms.
Auditory filters
Filters are used in many aspects of audiology and psychoacoustics including the peripheral auditory system. A filter is a device that boosts certain frequencies and attenuates others. In particular, a band-pass filter allows a range of frequencies within the bandwidth to pass through while stopping those outside the cut-off frequencies.
thumb|400px|right|A Band-pass filter showing the centre frequency(Fc), the lower(F1) and upper(F2) cut off frequencies and the bandwidth. The upper and lower cut-off frequencies are defined as the point where the amplitude falls to 3 [[decibel|dB below the peak amplitude. The bandwidth is the distance between the upper and lower cut-off frequencies, and is the range of frequencies passed by the filter.]]
The shape and organization of the basilar membrane means that different frequencies resonate particularly strongly at different points along the membrane. This leads to a tonotopic organisation of the sensitivity to frequency ranges along the membrane, which can be modeled as being an array of overlapping band-pass filters known as "auditory filters".
The auditory filters are associated with points along the basilar membrane and determine the frequency selectivity of the cochlea, and therefore the listener's discrimination between different sounds.
They are non-linear, level-dependent and the bandwidth decreases from the base to apex of the cochlea as the tuning on the basilar membrane changes from high to low frequency.
The bandwidth of the auditory filter is called the critical bandwidth, as first suggested by . 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.
[[File:ERBBandwidth.svg|thumb|450px|right| ERB related to centre frequency: The diagram shows the ERB versus centre frequency according to the formula by .
Psychoacoustic tuning curves
The shapes of auditory filters are found by analysis of psychoacoustic tuning, which are graphs that show a subject's threshold for detection of a tone as a function of masker parameters.
Psychoacoustic tuning curves can be measured using the notched-noise method. This form of measurement can take a considerable amount of time and can take around 30 minutes to find each masked threshold. In the notched-noise method the subject is presented with a notched noise as the masker and a sinusoid (pure tone) as the signal. Notched noise is used as a masker to prevent the subject hearing beats that occur if a sinusoidal masker is used. The cochlea is a complex structure, consisting of three layers of fluid. The scala vestibuli and scala media are separated by Reissner's Membrane whereas the scala media and scala tympani are divided by the basilar membrane.
frame|centre|Simplified schematic of the basilar membrane, showing the change in characteristic frequency from base to apex
The basilar membrane supports the organ of Corti, which sits within the scala media.
which showed rounded (roex) filter shapes.]]
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.
frame|centre|The auditory filter of an impaired cochlea
The auditory filter of an impaired ear is flatter and broader compared to a normal ear. This is because the frequency selectivity and the tuning of the basilar membrane is reduced as the outer hair cells are damaged. When only the outer hair cells are damaged the filter is broader on the low frequency side. When both the outer and inner hair cells are damaged the filter is broader on both sides. This is less common. The broadening of the auditory filter is mainly on the low frequency side of the filter. This increases susceptibility to low frequency masking i.e. upward spread of masking as described above.