This project is dead because I'm not sure that it's safe.
If you recall - I don't blame you if you don't - this speaker works by pumping out a 40kHz square wave, then varying the duty cycle. The human ear would respond to the varying duty cycle. This means that the ultrasonic transducers are continuously emitting full power.
Unfortunately, there have been very few studies on how loud ultrasonic sounds affect people. I'm just not sure about connecting up a 400W power supply and pumping out a continuous, full power 40kHz sound. Because of the directivity of the wave, the volume might even exceed 140dB.
I don't even know how to measure the output.
Finally, we now have a cat. Cats' hearing ranges up to 64kHz, and I woudn't want to frighten or deafen her.
Thanks to everyone who gave me ideas and encouragement while this project was alive!