Low ultrasonic exposure ( 23.5 khz )

[zenmasterbrian]

I am exploring a number of concepts, all involving switching at lower
frequencies than normally used.

They include high power Class D, and Class H with SMPS for subwoofer
amps, and also the general use of SMPS with analog audio.

I am interested in exploring a lower frequency limit because it does
enable higher current handling per transistor.

But the question arrises, what are the acceptable exposure limits?

Is there precident for how much low ultrasonic can be allowed to bleed
through?