Hmm, that's a good point. I always assumed the membrane was already mechanically tensioned by default.
I initially said I was going to generate the ultrasonic chirp with a DAC- but my hope was to have an 8-bit parallel input DAC with an update rate that was 16x the frequency of the highest pulse. That would have meant 1/(60000x16) = 1.04us settling time. All the DACs I have found that have that settling time are serial- and although they react that fast enough to changes, they cannot accept data fast enough to define 16 points per cycle in a 60kHz sinusoid.
So...I was looking at what some other people had mentioned earlier about filtering square waves. For some reason I crossed this off my list early on since I didn't think I could get a multi-frequency bandpass filter that was accurate enough...but then I remembered about switched-cap filters. And it also seems that I do not need a bandpass filter either, just a lowpass filter right? Since there are no subharmonics in a square wave signal. Actually, I don't think I even need a switched cap filter. If my sinusoid is only ever going to be 40kHz-60kHz, I could just get a lowpass filter that has sufficient attenuation above 120kHz, and any square wave sent into the filter would exit as a sinusoid with the fundamental of the input, right?
The thing I am having the most trouble with now is figuring out hwo to bias the transducer. it recommends a max AC signal of 200Vp and a DC bias of 200V. I can see how to get 200VAC from regular logic with a transformer (assuming I can find a high frequency one that works 40kHz-60kHz). But I don't understand where you would get 200VDC from, and apply it to the transducer, especially if there was a transformer also connected to the transducer leads (wouldn't that destroy the transformer?)