Characterization of an omnidirectional parametric loudspeaker with exponential sine sweeps

An omnidirectional parametric loudspeaker (OPL) is a sound source that is made of hundreds of ultrasonic piezoelectric transducers set on a sphere. Relying on the parametric acoustic array (PAA) phenomenon, each transducer emits an ultrasonic carrier beam modulated by an audible signal. Thanks to non-linear propagation in air, natural demodulation takes place and focused audible beams are obtained. An OPL thus emits hundreds of audible sound beams in all directions. In this work, the performance of an OPL prototype containing 750 transducers set on a sphere of radius 0.125 cm is examined in free-field conditions. The frequency response, sound pressure level (SPL) and directivity are measured using exponential sine sweeps (ESS). Compared to broad band signals such as white or pink noise, the OPL can produce higher SPLs with ESS because the acoustic power concentrates on a single frequency only. This makes the OPL suitable for room acoustic measurements. A conventional dodechaedral sound source is also measured for comparison. Results show that the OPL produces a better omnidirectional sound field above ∼1 kHz, but it performs worse at lower frequencies. Moreover, it presents a flatter frequency response in all directions, although it generates lower SPL values. The Directivity Index (DI) required by the ISO 3382-2 and ISO 16283-1 standards has also been computed for both sources. The OPL cannot fulfill the requirements of the standard at low frequencies. This paper also demonstrates and discusses DI measurements of high resolution both for the new OPL and the traditional onmidirectional speakers.