I have worked with anechoic chambers - and with multi-kilowatt speakers. The first thing I noticed is that even a sealed metal chamber lined with the black absorbing foam does not stop all of the sound. The other thing is that it doesn't take much energy to make something incredibly loud.Thanks for your answer, Scott!
How about small peace of "absorb sound material" (used in record studios) and ultrasound between 20 kHz and 40 kHz.
Do you think it will work ?