A Theoretical and Experimental Study for Speakers with Anisotropic Beam Pattern
Date Issued
2009
Date
2009
Author(s)
Cheng, Chih-Chiang
Abstract
Based on the fundamental theory of acoustics, we investigated the anisotropic beam pattern induced by a cylindrical pot-like ultrasonic sensor. As the sensor size is small due to its application constraint, the design parameters are thus highly coupled to one another. In this dissertation, we focused on the analysis of parameters related to the four main key performance indices: anisotropic beam pattern, resonance frequency, reverberation time, and source level. Then, we defined the corresponding parameters on the structural design. Furthermore, a finite element analysis was introduced in the development of the structural design. In order to validate the finite element model, an automatic detecting platform was established to measure the spatially anisotropic beam pattern. In addition, a Laser Doppler Interferometer was utilized to determine the displacement of the vibrating plate surface. After validating the finite element model, it was introduced to study the parameters which alter the far-field beam pattern of an ultrasonic sensor. We found that the increase of the resonance frequency of an ultrasonic sensor is an effective method to narrow its far-field beam width, which results in a poor anisotropy of beam pattern. Besides that, the shape of the vibrating plate is an insignificant parameter for narrowing the far-field beam width under a specific frequency. However, if the shape of the vibrating plate was designed unsuitably, it could broaden the far-field beam widths inadvertently. Finally, the thickness discontinuity of the vibrating plate in the two opposite vertical sides is also an important parameter in narrowing the far-field beam width of an ultrasonic sensor. Moreover, by detecting the impedance of the piezoelectric plate stuck on the inner surface of the vibrating plate of an ultrasonic sensor and then calculating its corresponding equivalent circuit parameters, the reverberation time was predicted through the electronic circuit simulation. Finally, we integrated our design which possesses a highly anisotropic beam pattern into the obstacle detection system to determine the detectable region for a standard obstacle. The design thinking and design platform can provide a guideline for the design of an ultrasonic sensor in automotive industry. Due to the highly mechanical quality factor of a piezoelectric based ultrasonic sensor, it can radiate a high-intensity ultrasound beam at its resonance frequency from a small size emitting surface. An amplitude-modulated ultrasound whose carrier is the resonance frequency of the ultrasonic sensor was radiated through air. Using the non-linearity of air, the demodulated audible beam was thus induced at a modest strength. To ensure these sound pressure signals are within a reasonable dynamic range of measuring instruments, a low-pass filter was adopted to condition the detecting signals. Then, using a sound power spectrum analysis through a 1/3 octave band and reliability analysis of the filtered signals, the demodulated audible sound was determined precisely. We found an ultrasonic sensor possessing a spatially anisotropic beam pattern provides a demodulated audible sound with the same beam pattern. In addition, the frequency response of the demodulated audible sound has a plateau in the frequency range below 1kHz. Furthermore, we studied the possibility of the localized sound cancelling through a highly directional sound beam provided by ultrasonic emitter array. Finally, the acoustic dome integrated into the ultrasonic emitter based directional loudspeaker to improve its directivity of demodulated beam was also studied.
Subjects
anisotropic beam
piezoelectric unit
ultrasonic transducer
directional speaker
amplitude modulation
parametric array
self-demodulation
Type
thesis
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