To verify that the piezos I took from the misting devices can be used as ultrasonic transducer, I setup a test environment for a basic ultrasonic transmission. I simply took two gummy bears to simulate a human body because I did not want to use my finger in the test setup all the time.

I setup a Signal Generator to produce a slow triangle pattern, which I fed into a Voltage Controlled Oscillator and into the X deflection of an oscilloscope. I hooked the output of the VCO to the upper piezo, so it generates an ultrasonic wave, which gets picked up by the gummy bears and is transmitted to the other piezo, which is connected via a high pass filter to the Y deflection of the oscilloscope.

Since I don't own a modern digital oscilloscope to store the data, I took long time exposure pictures of the screen. I combined three picture to create the spectrum below. I also marked some interesting frequencies.