Reverberation time and random-incidence sound absorption measured in the audible and ultrasonic ranges with an omnidirectional parametric loudspeaker

An omnidirectional parametric loudspeaker (OPL) is a sound source that relies on the parametric acoustic array (PAA) phenomenon to generate an omnidirectional sound field. It consists of hundreds of ultrasonic piezoelectric sensors placed on a sphere, each of which emits an ultrasonic carrier wave modulated in amplitude by an audible signal. Due to non-linear propagation in air, the audible signal is demodulated, resulting in an omnidirectional sound field consisting of audible and ultrasonic waves. Earlier work has shown that the OPL is more omnidirectional than a standard dodechaedron, although it produces lower sound pressure levels especially at lower frequencies. This sound source was originally designed for room acoustics, but its application to this field remains still unexplored. This paper proposes a method to measure the reverberation time of a room with an OPL using exponential sine sweeps (ESS). In addition, the sound absorption of material samples in a reverberation chamber is obtained. The results show that the OPL can measure these magnitudes with confidence, even though, compared to a standard dodechaedron, it has more difficulties in achieving large signal-to-noise ratios in the decay curves for the lower frequencies. The developed methodology also allows the ultrasonic frequency range to be examined. The results indicate that the ultrasonic waves do not penetrate the sample under test as they are attenuated during propagation in air.