Do you have a data sheet on the two transducers? Part number?
Sort of:-
40khz one is unmarked open cap 25mm so manufacturer is unknown but
Receiving sensitivity: -65 dB (min)
Capacitance @ 1kHz: 2400 pF says the japenglish 2" square data sheet.
My parallel 6mH (ish) self wound coil (by calculation) within a ferrite core (not tunable) strapped directly on the legs of the transducer.
25khz one stamped ZRX25/3 16mm open cap but I can't find a manufacturer name to match the stamp though.
Receiving sensitivity: -52 dB (min)
Capacitance @ 1kHz: 2500 pF says the japenglish 2" square data sheet as above.
16mH (ish) self wound coil ( by calculation) with a slug tunable core.
Not much to go on really.
If I was clever (and owned a digital camera that worked) I'd try for a screen shot of the response of the 40 khz setup BUT I dropped my wifes one so that's a work/repair in progress.
The 25 khz one won't respond to ANY ultrasound with the coil attached else it works a treat @25khz (ish) without the coil.
Surprisingly bats squeal/k from 14khz to an unbelievable 100khz.
In the UK, 18-50khz I've found is a useful range.
I don't sweep frequency though, I prefer direct frequency division to VCO mixing / hetrodyne.
Bit too complicated for me is hetrodyning.