Equal loudness is mainly to increase the high and low tones, and it is better to turn it on when the speed is fast or the sound is low. Equal loudness adjustment should be based on loudness, but the power amplifier circuit can only be relatively fixed according to the position of the volume potentiometer, and there are some differences between speakers with different sensitivities.

Extended data:

The function of equal loudness control is to increase the volume of high-frequency and low-frequency components at low volume, so that the loudness ratio of low, middle and high parts remains the same as that at high volume. The actual loudness of sound is not completely linear with the loudness actually felt by human ears. When the volume is small, the human ear will have a physiological attenuation to the hearing of middle and high frequencies, and the smaller the volume is, the more obvious this attenuation is.

Because the human ear has poor auditory sensitivity to high-frequency sounds, especially low-frequency sounds, it is required to make auditory compensation for high-frequency and low-frequency sounds at low volume, that is, it is required to improve the low-frequency sounds greatly and also improve the high-frequency sounds to some extent. Some compensation at low loudness is better than no compensation at all. Moreover, the frequency, level and slope of a relatively elegant equal loudness circuit can be adjusted.

References:

Baidu encyclopedia-equal loudness