Construction and characterization of an omnidirectional parametric loudspeaker consisting of ultrasound transducers set on a sphere

Standard omnidirectional sound sources typically consist of conventional loudspeakers arranged in a dodecahedral configuration. For such devices, a microphone in a circular trajectory around the source should measure a constant sound pressure level. However, the directivity of loudspeakers sharpens with increasing frequency. The omnidirectionality of the source therefore wanes at the high range of the spectrum, the effect being very noticeable at the vertices of the dodecahedron. In this work it is proposed to circumvent this problem by exploiting the parametric acoustic array (PAA) phenomenon. An omnidirectional parametric loudspeaker (OPL) is built by setting hundreds of small piezo-electric transducers (PZTs) on a sphere. Each PZT emits a collimated primary beam consisting of a carrier ultrasonic wave modulated by an audible signal. Thanks to non-linear propagation of sound in air, the air itself demodulates the primary beam giving rise to an emission cone of audible acoustic pressure (secondary field). For an appropriate distribution of PZTs on the sphere, the emission cones are such that an omnidirectional acoustic pressure field is recovered. In practice, a balance is required between an optimal PZT distribution and the practical needs for a prototype construction. Our choice has been to follow an equal-area partitioning strategy on the sphere surface, which aligns the PZTs in parallels and performs almost as well as an optimal PZT distribution, as shown by theoretical prediction models. The parallel PZT alignment strongly facilitates the PZT welding process and setting the cable circuitry. An equal-area distributed OPL prototype has been built resorting to 3D printing for the spherical casing. The prototype has been tested in an anechoic chamber showing a remarkable omnidirectional character for all frequencies.