Firstly, noise generated in an audio channel by heating of resistive components, so called thermal noise, is distributed over the entire output bandwidth of the circuit. Secondly, total output noise levels are proportional to the square root of the system bandwidth, if bandwidth is reduced, the total noise level at the output is also reduced. Thirdly, certain sounds seem to drown the noise and sound clearer, an effect known as auditory masking, and is due to the sound stimulating the same regions of the ear as the noise and in so doing overshadowing the noise. Auditory masking is most prominent with high frequency sounds containing lots of harmonics. The bandwidth is altered by controlling the cut off frequency of the low pass filters with a DC control signal derived from the audio signal's amplitude and frequency content.
Auditory masking is relied upon to drown any noise when the bandwidth is fully open.
The LM1894 takes 0.5ms to adjust to this change. Fortunately the ear has difficulty responding to any distortion caused during this short change. Only one control signal is used to ensure that both left and right channel filters will maintain a stable stereo image. A high pass filter is also included in the control path to prevent low frequency, high amplitude sounds causing a high DC control signal to the filters and unnecessarily raising the cut-off frequency. This would imply the bandwidth has been excessively widened and more noise can get through to the output.