The nonlinear distortion of directional loudspeaker

A directional audible sound could be generated from the nonlinear interaction of ultrasound in the propagation medium. The demodulated audible signal is proportional to the second time derivative of the square of the ultrasound envelope, described in Berktay’s Far-field solution, which is nonlinear to the process of sound reproduction. Double Side-Band modulation (DSB) is proposed to amplitude-modulate the ultrasound carrier with audio signal. Total harmonic distortion (THD) is calculated for signal with single frequency firstly, and it increases with the increase of the modulation index. However, it’s more complex for input with two frequencies or multi-frequency. Except for the interaction between the modulated ultrasound and the carrier, the intermodulation distortion (IMD) is generated from the interaction of modulated signal with multi-frequencies. Hence, Single Side Band modulation (SSB) is proposed for reducing the THD and IMD. There is no 2nd harmonic frequency or higher frequencies for input with single frequency. And the amplitudes of harmonics are only related to the modulation index for input with two frequencies or wideband. SSB is recommended to reduce THD or IMD.