manufacturing a hydrophone

i want to make a hydrophone that listen to two pingers of 45 KHz and 37.5 KHz.
what i got from searching till now, that i can make it from an electret condenser microphone or i can get the transducer from a fish finder. But what i want to know is what specification should i consider while selecting the microphone to match these specific frequencies.
moreover, i want to pass the signal to a preamplifier , amplifer and bandpass filter. i found a bunch of circuits for these elements. but i am not sure which one i should use, because i am still sophomore with less backgrounds on these.

if there is any tips or hints about which mic i should choose, and the circuits attached.

thanks alot

look for ultra sonic motion (and distance) sensors. Most of the ones I played with are in the 20k to 30khz range.