Parametric sound

Parametric sound is created by appropriately pre-distorting and modulating an audio signal onto an ultrasonic carrier (Pompei, JAES, 1999). Non-linearities in air cause a demodulation of the compound signal. The theoretical model of the de-modulation (Berktay, Sound and Vibration, 1965) employs a number of simplifications to the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation that cause performance limitations of existing loudspeakers especially with respect to the maximum output volume. We are going to take the new approach of employing signal processing models such as the Hammerstein model for the demodulating non-linearity of air. The model parameters are identified from measurements and are not restricted to assumptions on operating points. Furthermore, most existing systems use an array of transducers where radiation properties of the entire array are usually static. We intend to electronically steer the ultrasonic beam and thus the audible signal such that it either areflects off the levitating objects or reaches the user’s ears directly.

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