FDR Pingers transmit at 37.5 kHz while most pilots can't hear much beyond 16 KHz (maybe 20 if you are young).
In an earlier life I had some experience finding underwater equipment which was "marked" by a pinger. Pinger receivers usually relay the signal at a frequency that the user (pinger receiver operator) can hear, and some receivers are directional allowing an operator to "home" in on the signal by moving in the direction from which the signal appears to be coming. Having said this, this is not a very accurate way of finding a pinger, and even in shallow water ( 30 m) it is difficult to get much closer than 100-150m from a pinger. Hence the need for divers.
They use 37.5 kHz since there is not so much background sea noise at that frequency.
Seabreeze