I think we're going slightly off-topic here. I still don't understand why a 15mm piezo disk is inaudible unless I put it at the entrance to my ear canal (or right next to my hearing aid mic) whereas the 27mm disk I can hear clearly from 2m away.

 

Maybe the piezo rear sounds cancel the front sounds when near your ear. Maybe when the piezo is 2m away then it is put down so there is no sound from its rear.

Did you sweep the frequencies so that the piezo is playing its loudest resonant frequency?

 

Once such Princess Diana was living alive. She got the Law internationally forbidding the production and use of pedestrian mines. Yet she (probably) paid for this Law with her life as revenge, the military plants accumulated milliards of piezo elements for ignition those bombs. How to sell it now?! Then all the 90-ies averywhere one may buy the very cheap propane gas lighters for 10 cents, later 13 cents, later 22 cents, what used not a stone roller but the round shaped piezo stake, about 2 mm in diameter and 20 mm in length. If to bend it, the rod produced about few kilovolts. If to feed on the few kilovolts, the rod bended. Thus, let make the sond oscillations, apply it over serial resonance, and set the rod parallel to inductance or capacitance who cares what. And everything will beep with just detonating power.

 

You have been told multiple times that the piezo disk frequency response likely is way down at 1KHz, but I see evidence you have tried other frequencies.

 

You have been told multiple times that the piezo disk frequency response likely is way down at 1KHz, but I see evidence you have tried other frequencies.

I programmed the Arduino to produce a 3 octave chromatic scale starting at C6 (1047Hz). It was maybe a little louder in the 2 - 3 KHz range but not usefully. By 4KHz both my hearing and pitch discrimination were failing fast. Holding the piezo sounder next to my hearing aid mic didn't make a dramatic difference.

 

Here is the frequency response of another piezo sounder: