I wanted to have a look at the frequency dependence of the ultrasound transducers, separately from the ultrasonic Fabry-Perot (UFP). Here, I subject the transducer to a single 2 us pulse generated by Timer 1 on the UNO and look at the time domain response with a Rigol scope. Immediately after applying the pulse it is essential to send the drive pin to a high-impedance state with pinMode(X,INPUT), otherwise the transducer response is damped out almost immediately.
The Fourier transform is probably more useful and shows at least a pair of modes at around 200 kHz, where the transducer is specified for operation as a transceiver. There is also a much more significant mode at about 270 kHz, which will be worth looking at with the UFP.
It will be interesting to discuss this behaviour in terms of the equivalent circuit of the transducers but I will have to leave that for another time.