Frequency selectivity for frequencies below 100 Hz: comparisons with mid-frequencies

Auditory filter shapes were derived for signal frequencies (fs) between 50 and 1000 Hz, using the notched-noise method. The masker spectrum level (N0) was 50 dB (re 20 μPa). For fs = 63 and 50 Hz, measurements were also made with N0 = 62 dB for the lower band. The data were fitted using a rounded-exponential filter model, with special consideration of the filtering effects of the middle-ear transfer function (METF) at low frequencies. The results showed: (1) For very low values of fs, the lower skirts of the filters were only well defined when N0 = 62 dB for the lower band; (2) The sharpness of both sides of the filters decreased with decreasing fs; (3) The dynamic range of the filters decreased with decreasing fs; (4) The equivalent rectangular bandwidth of the filters decreased with decreasing fs down to fs = 80 Hz, but increased for fs below that; (5) The assumed METF, which includes the shunt effect of the helicotrema for frequencies below 50 Hz, increasingly influenced the low-frequency skirt of the filters as fs decreased; (6) Detection efficiency worsened with decreasing fs for fs between 100 and 500 Hz, but improved slightly below that.