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We present a generalized analytical model of thermo-acoustic sound generation based on the analysis of thermally induced energy density fluctuations and their propagation into the adjacent matter. The model provides exact analytical prediction of the sound pressure generated in fluids and solids; consequently, it can be applied to arbitrary thermal power sources such as thermophones, plasma firings, laser beams, and chemical reactions. Unlike existing approaches, our description also includes acoustic near-field effects and sound-field attenuation. Analytical results are compared with measurements of sound pressures generated by thermo-acoustic transducers in air for frequencies up to 1 MHz. The tested transducers consist of titanium and indium tin oxide coatings on quartz glass and polycarbonate substrates. The model reveals that thermo-acoustic efficiency increases linearly with the supplied thermal power and quadratically with thermal excitation frequency. Comparison of the efficiency of our thermo-acoustic transducers with those of piezoelectric-based airborne ultrasound transducers using impulse excitation showed comparable sound pressure values. The present results show that thermo-acoustic transducers can be applied as broadband, non-resonant, high-performance ultrasound sources.
In diesem Beitrag präsentieren wir innovative Methoden für die breitbandige und resonanzfreie Messung und Anregung von Ultraschall. Das Messverfahren verwendet eine Kunststofffolie und ein Laser-Vibrometer als breitbandigen und resonanzfreien Empfänger. Im Allgemeinen ermöglicht dieses Verfahren eine präzise Messung der Schallschnelle und des Schalldruckes in beliebigen, für das Laserlicht transparenten Flüssigkeiten und Gasen mit bekannter Dichte und Schallgeschwindigkeit. Das resonanzfreie Senden von Ultraschall basiert auf einem elektro-thermo-akustischen Wandlerprinzip und ermöglicht, im Gegensatz zu herkömmlichen Ultraschallwandlern, die Erzeugung von beliebig geformten akustischen Signalen ohne Resonanzen und ohne Nachschwingen.
Thermoacoustic generation of airborne ultrasound using carbon materials at the micro- and nanoscale
(2012)
The generation of airborne ultrasound is presented using the thermoacoustic principle applied to carbon materials at the micro- and nanoscale. Such materials are shown to be capable of emitting ultrasound when being fed by an alternating current. We tested the acoustic performance of carbon fibers, bucky papers and electrospun polyacrylonitrile-derived carbon nanofibers and determined the sound pressure for frequencies up to 350 kHz. A comparison between the experimental results and the theoretical prediction showed remarkable agreement for frequencies up to 150 kHz. Beyond 150 kHz, we found slight deviations from the expected sound pressure dependence on the square root of the frequency.
A thermophone is an electrical device for sound generation. The advantages of thermophones over conventional sound transducers such as electromagnetic, electrostatic or piezoelectric transducers are their operational principle which does not require any moving parts, their resonance-free behavior, their simple construction and their low production costs.
In this PhD thesis, a novel theoretical model of thermophonic sound generation in real gases has been developed. The model is experimentally validated in a frequency range from 2 kHz to 1 MHz by testing more than fifty thermophones of different materials, including Carbon nano-wires, Titanium, Indium-Tin-Oxide, different sizes and shapes for sound generation in gases such as air, argon, helium, oxygen, nitrogen and sulfur hexafluoride.
Unlike previous approaches, the presented model can be applied to different kinds of thermophones and various gases, taking into account the thermodynamic properties of thermophone materials and of adjacent gases, degrees of freedom and the volume occupied by the gas atoms and molecules, as well as sound attenuation effects, the shape and size of the thermophone surface and the reduction of the generated acoustic power due to photonic emission. As a result, the model features better prediction accuracy than the existing models by a factor up to 100. Moreover, the new model explains previous experimental findings on thermophones which cannot be explained with the existing models.
The acoustic properties of the thermophones have been tested in several gases using unique, highly precise experimental setups comprising a Laser-Doppler-Vibrometer combined with a thin polyethylene film which acts as a broadband and resonance-free sound-pressure detector. Several outstanding properties of the thermophones have been demonstrated for the first time, including the ability to generate arbitrarily shaped acoustic signals, a greater acoustic efficiency compared to conventional piezoelectric and electrostatic airborne ultrasound transducers, and applicability as powerful and tunable sound sources with a bandwidth up to the megahertz range and beyond.
Additionally, new applications of thermophones such as the study of physical properties of gases, the thermo-acoustic gas spectroscopy, broad-band characterization of transfer functions of sound and ultrasound detection systems, and applications in non-destructive materials testing are discussed and experimentally demonstrated.