Speaker
Description
Parametric array loudspeakers use arrays of ultrasonic transducers to generate highly directive audible fields through the non-linear properties of air in the ultrasonic range. They require very high ultrasonic pressure levels to generate the audible field, but these levels, in turn, lead to measurement inaccuracies produced by the microphone. Known as spurious sound, this measurement error can be circumvented by incorporating a filter in front of the microphone to reduce the ultrasonic levels. However, its effects have been studied mainly in the far field and at a few fixed measurement points. This paper explores this phenomenon by mapping the sound field using an automated robotic system, composed of a 6-degree-of-freedom robot mounted on a linear track. By recording exponential sine sweeps and deconvolving with the corresponding inverse filters, sound field maps for the audible and ultrasonic fields are obtained. Measurements were performed with and without the microphone filter to isolate the spurious sound. Results confirm a correlation between the ultrasonic field and the spurious sound, and show that spurious sound levels increase with the audible difference frequency.