Generation of a focused nonlinear difference frequency using high-amplitude time reversal focusing of airborne ultrasound
Time reversal (TR) focusing of high-amplitude airborne ultrasound can be used to create a difference frequency. Methods are employed to increase the amplitude of the focus. These methods include creating a miniature reverberation chamber, using multiple sources, and using the clipping processing method. The use of a beam blocker to make the sources more omnidirectional is also examined, and it is found that for most source/microphone orientations, the use of a beam blocker increases the amplitude of the focus. A high-amplitude focus of 134 dB peak re 20 μPa sound pressure level with a center frequency of about 38 kHz is generated using eight sources. Then, using four sources centered at 36.1 kHz and another four sources centered a 39.6 kHz, nonlinear difference frequency content centered at 3.5kHz is observed in the focus signal. The difference frequency amplitude grows quadratically with increasing primary frequency amplitude. When using beam blockers, the difference frequency content propagates away from the focal location with higher amplitude than when beam blockers are not used. This is likely due to the differences in the directionality of the converging waves during TR focusing.