Date of Award

2016

Document Type

Open Access Master's Report

Degree Name

Master of Science in Mechanical Engineering (MS)

Administrative Home Department

Department of Mechanical Engineering-Engineering Mechanics

Advisor 1

Andrew R. Barnard

Committee Member 1

Jason R. Blough

Committee Member 2

Christopher Plummer

Committee Member 3

Charles D. Van Karsen

Abstract

Carbon nanotube thermophones can create acoustic waves from 1 Hz to 100 kHz. The thermoacoustic effect that allows for this non-vibrating sound source is naturally inefficient. Prior efforts have not explored their true efficiency (i.e. the ratio of the total acoustic power to the electrical input power). All previous works have used the ratio of sound pressure to input electrical power. A method for true power efficiency measurement is shown using a fully anechoic technique. True efficiency data are presented for five different drive signal processing techniques: standard alternating current (AC), direct current added to AC (DCAC), amplitude modulation of an AC signal (AMAC), spectral envelope decimation of an AC Signal (FCAC), and Dynamic Linear Frequency Compression of an AC signal (TCAC). These signal processing techniques are needed to limit the frequency doubling non-linear effects inherent to carbon nanotube thermophones. Each type of processing affects the true efficiency differently. Using a 72 Wrms input signal, the measured efficiency ranges were 4.3 E-6 - 319 E-6, 1.7 E-6 - 308 E-6, 1.2 E-6 - 228 E-6, 1.01 – 1083 E-6, and 1.26 – 388 E-6 percent for AC, DCAC, AMAC, FCAC, & TCAC, respectively. These data were measured in the frequency range of 100 Hz to 10 kHz. In addition, the effects of these processing techniques relative to sound quality are presented in terms of total harmonic distortion. It is shown that although the different signal processing techniques do affect the true efficiency, none of them will increase the efficiency of the CNT thermophone to the level of current moving coil loudspeakers. Future work optimizing the efficiency and ruggedness are needed.

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