Hi!
The Original Piezo diaphragm was a standard 2" in a 3" Element,........
I tried a standard 4/28 (12Vdc) Piezo Horn alarm/siren at a store (Ralph's...) to test, and I got about what would sound like 2-->3 volts going through it,...... not loud, hardly audible for an alarm.
I tried a 4/28 (12Vdc) Metal Buzzer Alarm at home with 40ohm resistance, and the Alarm Board emitted 40VAConly, and the Alarm was not audible. The VOMs are a factor in the alarm. The more ohms the more VAC output.
Do you think 12volt transducer using VOMs at a "4 volt VDC capacity" would sound loud since it would drop from Ultrasonic 40Khz to Sonic 5Khz...?
Thanx
Bob