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I don't know much about engineering.
I think the first step in your mission is to measure whatever it is that you think you are trying to nullify. Start with a broadband ultrasound receiver and a spectrum analyser.
The problem with active noise reduction over a large area is the short wavelength of ultrasound. At 20 kHz the wavelength is around 1.7 cm so the peak of the incoming pressure wave at any point on, say, a 20 cm sphere (physicist's model of a head) is going to be quite different from the measured value a centimeter away. You could produce a fairly soundproof planar wall with an array of independent transducers. This works OK for low frequency deadening of large rooms but you will need a vary large number of transducers. If you are to intercept and cancel, say, 50 MHz transmissions over a 30 cm square, you need 3600 independent elements, each consisting of a signal generator, an amplifier and a receiver.I think the first step in your mission is to measure whatever it is that you think you are trying to nullify. Start with a broadband ultrasound receiver and a spectrum analyser. I recommend the Picoscope range of USB plug-in analysers - good value and the software is ridiculously easy to use. Not sure what transducer to use but ...sorry, you cannot view external links. To see them, please
REGISTER or LOGIN may be helpful. The problem is that most commercial transducers are tuned to send and receive at a sharp resonant frequency. However as you are looking for damage-inducing pressure levels, you might find that an old television remote control or a parking sensor does the job adequately.