Measurement of the sound absorption coefficient of materials in a reverberant chamber using an omnidirectional parametric loudspeaker

Abstract

The omnidirectional parametric loudspeaker (OPL) is a sound source that exploits the phenomenon of the parametric acoustic array (PAA) to produce an omnidirectional sound field. It consists of hundreds of ultrasound transducers distributed on the surface of a sphere. Each transducer emits an ultrasonic signal modulated in amplitude by an audible exponential sweep sine (ESS). The signal is demodulated back to the audible range by nonlinear propagation in air. This work investigates the suitability of the OPL for some room acoustics measurements. We focus on measuring the sound absorption coefficient of materials in a reverberant chamber. In fact, the study proposes the first steps towards a methodology to measure the absorption coefficient of materials, similar to ISO 354 standard, but for an OPL. The ESS is used as excitation signal because it allows the OPL to concentrate the emitted sound power on a single frequency, leading to higher ultrasonic power levels and increasing the signal-to-noise ratio of the audible sound pressure levels. The procedure for obtaining the frequency dependent reverberation times is also described. The absorption coefficient and reverberation times measured with the OPL are compared with those of conventional dodecahedral loudspeaker