Auditory filtering at low frequencies

This paper is concerned with the comparison of the critical
bandwidth (CB) and the equivalent rectangular bandwidth (ERB) of the auditory
filters at low frequencies. The method of critical bandwidth determination based
on the critical modulation frequency (CMF) has been questioned, particularly for
frequencies less than 500 Hz. The CMF, which is the modulation frequency at
which amplitude modulation (AM) and frequency modulation (FM) detection
thresholds become identical, is confunded as a proper measure of the auditory
filter's width. It refers to the modulation rate for which one of the sidebands
is most detectable. For low carrier frequencies the higher sideband is mnost
detectable whereas for higher carrier frequencies the lower sideband becomes
most detectable. Thus, at least for low carrier frequencies (i.e. less than 200
Hz), the CMF does not reflect the auditory system's sensitivity for detecting
the phase differences of the spectral components of the signal. These findings
can account for the fact that the critical bandwidth flattens off at low carrier
frequencies, whereas the equivalent rectangular bandwidth of the auditory
filters continues to decrease down to very low centre frequencies. It was also
shown that, at least for very low frequencies, critical bands do not reflect
directly the auditory filtering that takes place in the peripheral auditory
system.