Optimizing privacy listening experience with parametric loudspeakers

• Main Author: Zhu, Hongrui
• Other Authors: Chang Joseph
• Format: Thesis-Master by Coursework
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Electrical and electronic engineering::Applications of electronics
• Online Access: https://hdl.handle.net/10356/166488
• Institution: Nanyang Technological University

The basis of a parametric emitter loudspeaker is the emission of an ultrasound beam that demodulates into a beam of acoustical sounds in air and the beam bounces off hard surfaces such as walls. Despite prototypes of the parametric emitter having been reported in the 1980s, the parametric emitter loudspeaker remains largely an emerging acoustical privacy listening. This is in part because the generated acoustical sounds remain a beam that bounces off hard surfaces, i.e., the audio provided thereto is not localized to a small area, i.e., a private audio zone cannot be created. In this dissertation, we realize an optimized method for generating a private audio zone, and investigate its implementation and comparisons against the conventional parametric loudspeaker. The optimized method involves the generation of an private audio zone arising from superimposing ultrasound beams from three parametric loudspeakers. Each ultrasound beam has low equivalent acoustical sound pressure level (SPL), and the overlapped area of the three beams is the private audio zone, i.e., acoustical sounds are largely inaudible beyond the said overlapped area. Our experimental results demonstrate that the private audio zone feature better performance in terms of privacy – arising from the proposed optimization method – compared to the conventional parametric loudspeaker. Specifically, within the overlapped area, the SPL is ~79dB while the SPL beyond the overlapped area is ~59 dB – a very apparent difference of ~20dB between the private audio zone and that beyond the private audio zone. Conversely, the conventional parametric loud-speaker produces a beam of 67dB and as the beam bounces off hard surfaces, there is no private audio zone. The investigation of the optimization includes the influence of the modulated signal frequency. Specifically, with 40kHz ultrasound carrier frequency, the SPL of the acoustical sounds generated (in one beam) reduces with the frequency of the (audio) input signal increased. We attribute this to the fact that the higher the frequency of a sound's component, the greater its rate of attenuation in air. However, this effect is less pronounced within the privacy zone, i.e., the volume provided by the audio zone suffers less decline when the input signal frequency rises.