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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 http://parsonicscorp.com/ 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.