The audio spotlight: An application of nonlinear interaction of sound waves to a new type of loudspeaker design

TLDR

The loudspeaker’s sound‑reproduction theory relies on nonlinear acoustic interactions of sound waves in air. The study aims to develop a novel loudspeaker design. The design uses a transducer array to emit a finite‑amplitude ultrasound wave modulated by audio, which self‑demodulates in air via nonlinear interaction to generate the audible signal. Experiments show that with an equalizer and improved transducer array the speaker achieves a flat sound‑pressure response and a very sharp directivity pattern, enabling a sound spotlight.

Abstract

This work was done to devise a new type of loudspeaker. The theory for sound reproduction of this loudspeaker is based on nonlinear acoustics of sound wave interaction in air. A finite amplitude ultrasound wave that can be amplitude modulated by any audio signal is radiated from a transducer array into air as the primary wave. As a result, an audio signal is produced in the air because of the self-demodulation effect of the AM sound wave due to the nonlinearity of the air. It is possible to get a flat characteristic of reproduced sound pressure by using an equalizer. In some fundamental experiments the characteristic of the reproduced sound pressure is not quite flat due to an imperfect transducer array. Improvement of the transducer makes it possible to get a flat characteristic. A special feature of this loudspeaker is its very sharp directivity pattern, which makes it possible to realize a sound spotlight.