Carrier frequency influence on the audible and ultrasonic fields generated by an omnidirectional parametric loudspeaker excited with exponential sine sweeps

While the parametric acoustic array (PAA) phenomenon has been traditionally exploited for generating focused sound spots, an omnidirectional parametric loudspeaker (OPL) is a sound source that uses the PAA for producing omnidirectional sound fields. An OPL is made of hundreds of ultrasound transducers set on a sphere, each one emitting an ultrasonic carrier wave modulated by an audio signal. Thanks to non-linear propagation effects in air, the air itself demodulates the emitted ultrasonic signal bringing it back to the audible range. The carrier frequency f_c is a critical parameter to the efficiency of the OPL and it is investigated in this work. To that goal, the performance of an OPL prototype consisting of 750 ultrasound transducers set on a sphere is evaluated for a large set of f_c values, which are selected to be close to the resonance frequency of the transducers. Measurements are carried out in free-field conditions using (modulated) exponential sine sweeps as the excitation signal, which have recently proved useful to obtain a good frequency response, sound pressure level and directivity of the OPL in the audible range. That methodology is extended in this work to not only analyse the audible field but also the ultrasonic one. The measurements reveal that the OPL behaviour is very sensitive to f_c, yielding to large variations in the ultrasonic and audible range. Therefore, when selecting an f_c value, a balance must be found between maximizing the sound pressure levels in the low frequency range and preserving the sound field omnidirectionality, which is important for room acoustic applications.