By means of a listening test, an investigation was made to see whether low-pass filters with cut-off frequencies between 15 and 20 kHz give rise to audible signal changes. The test signal used was a 500-Hz tone whose harmonics were very pronounced up to 25 kHz. That signal was reproduced through two loudspeakers (standard stereo arrangement) in a room having a reverberation of about 1 s. It was evident that the precise shape of the transfer function of the filters beyond 15 kHz has only an extremely slight influence on the audibility of the resulting signal distortion. It appears that only a few subjects, whose hearing ability covers a particularly wide frequency band, are able to detect a considerable attenuation of the signal components beyond 15 kHz. It is very probable that the phase or group delay distortion associated with the band limitation is not audible even when the filter cut-off is very steep. It is deduced from the measured results that a bandwidth of 15 kHz is sufficient in all practical cases to guarantee the best possible sound quality. The demands on a listening-adapted low-pass filter are specified as amplitude frequency response and group delay time throughout the transmission band f-15 kHz as uniform as possible, attenuation a<1 dB and group delay time (symbols)<1 ms at f=15 kHz.
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