As for the ultrasonic sound source (limited frequency range, obviously) could serve a "dog whistle". Or several, from different manufacturers. As I imagine that they would differ in produced frequencies.
Such a whistle (or several) connected to a cheap pressure regulator and a source of compressed air (even a car tyre will suffice or a sprayer mentioned above) is should give relatively consistent results. And be quite easy to put together.
?
Easier than befriending a bat.
