I'll admit it is possible to say in 10 or less words why brushed escs beyond human audible range are rare. Read the text in bold and ignore the rest.
One bogus explanation of the noise source being town to motor quality has to be properly slayed before it gets resurrected. It may also help to explain there are design and cost constraints that influence why noisy, 3 to 4 kHz escs are made at all. The noise comes from the motor windings vibration in tune with their own magnetic field, particularly noticeable when the brushed esc switches above 2kHz and the below the electrical resonance of the motor. If you were persuaded that the that noise follows around poorer quality motors then cover all the permutations by simple substitution. You would find annoying whine follows wherever the esc is around the 1-5 kHz mark. Young people may hear 100 to 20kHz but old, LoFi telephone amplifiers limit to the 1kHz to 5kHz band for a reason. That band is what you hear best unless you have cyber enhancement.
Now to why the highest frequency (tens of kHz) brushed escs are rarely seen in boats. 1kHz is good enough for decent low throttle response and at 1kHz, they won't howl anything as bad as above the 3 kHz mark.
I estimated the component cost for some 2kHz Chinese 20 amp brushed escs being discussed in the submarine section of an American RC forum and it is only a couple of quid all-in. To make a quiet, 10+ kHz esc would call for a bulky, expensive, low ESR reservoir capacitor (cap) to drive up to a 380 motor. This cap (on the esc at the power supply) would multiply the size and weight of that little controller as well as bust the component budget.
Plenty mayhem members understand cable inductance, impedance, stored energy & damaging transient voltages but, in short, a low ESR cap is needed to sink energy coming back out the motor on a 10kHz esc but not a 1 or 2kHz esc.
Mechanical resonance
The compromise of 1kHz is sufficient for an esc to get above the mechanical resonance of the motor. That resonance point is why a 1kHz brushed esc allows better performance at the lowest end of throttle than the older, juddering 50Hz escs do.
You loose the low-throttle stickiness of 50Hz controllers but still get substantial brush arcing on 1kHz as the motor's stall current is being switching or interrupted. That is possibly why some escs, noted for the motor howling they cause, have been pushed to design limits of 3to 4kHz, or as high before adding cost and complexity.
Electrical Resonance Point
Heavy iron-cored motors don't strictly need a frequency above human hearing for quiet operation. The huge inductance reduces the electrical resonant frequency (or forces the current to near d.c.) when the voltage is switching below the 20kHzr audible limit of humans. The opposite is true for coreless motors where precious metal brushes won't tolerate arcing and the low inductance needs a frequency beyond 60kHz.
So if you don't like motor whine then choose an esc that can pulse well below 3kHz. Stay above 500Hz to avoid low throttle juddering or go for one above 10kHz if you can get one.
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Topic: Motor whine (Read 3859 times)