Receive inaudible sound at ~20khz
Most of those very inexpensive "jelly bean" commodity electret microphone capsules have frequency response above 20 KHz. So finding a sensor for 20 KHz should not be a significant issue.  They typically don't publish specifications for frequency response above 20 KHZ because most applications don't require it.

If you used an amplifier stage and a resonant circuit detector (or PLL, etc.), then you could feed the "demodulated" signal directly into your microcontroller (Arduino, etc.) input pin. Making the MC detect 20 KHz would not be anybody's first choice of solutions.  A commonly used chip for tone detection is LM567.