Nathanael K. Mayo

Alternative nanostructures for thermophones.

Thermophones are highly promising for applications such as high-power SONAR arrays, flexible loudspeakers, and noise cancellation devices. So far, freestanding carbon nanotube aerogel sheets provide the most attractive performance as a thermoacoustic heat source. However, the limited accessibility of large-size freestanding carbon nanotube aerogel sheets and other even more exotic materials recently investigated hampers the field. We describe alternative materials for a thermoacoustic heat source with high-energy conversion efficiency, additional functionalities, environmentally friendly, and cost-effective production technologies. We discuss the thermoacoustic performance of alternative nanostructured materials and compare their spectral and power dependencies of sound pressure in air. We demonstrate that the heat capacity of aerogel-like nanostructures can be extracted by a thorough analysis of the sound pressure spectra. The study presented here focuses on engineering thermal gradients in the vicinity of nanostructures and subsequent heat dissipation processes from the interior of encapsulated thermoacoustic projectors. Applications of thermoacoustic projectors for high-power SONAR arrays, sound cancellation, and optimal thermal design, regarding enhanced energy conversion efficiency, are discussed.

Thermal management of thermoacoustic sound projectors using a free-standing carbon nanotube aerogel sheet as a heat source

Carbon nanotube (CNT) aerogel sheets produce smooth-spectra sound over a wide frequency range (1-10(5) Hz) by means of thermoacoustic (TA) sound generation. Protective encapsulation of CNT sheets in inert gases between rigid vibrating plates provides resonant features for the TA sound projector and attractive performance at needed low frequencies. Energy conversion efficiencies in air of 2% and 10% underwater, which can be enhanced by further increasing the modulation temperature. Using a developed method for accurate temperature measurements for the thin aerogel CNT sheets, heat dissipation processes, failure mechanisms, and associated power densities are investigated for encapsulated multilayered CNT TA heaters and related to the thermal diffusivity distance when sheet layers are separated. Resulting thermal management methods for high applied power are discussed and deployed to construct efficient and tunable underwater sound projector for operation at relatively low frequencies, 10 Hz-10 kHz. The optimal design of these TA projectors for high-power SONAR arrays is discussed.

Thermophones for sonar applications

Thermophones are electrically driven thermoacoustic sound projectors which have historically been used as a primary source of sound. Thermophones have a sound spectra that are largely determined by their housing and support and efficiencies that are determined by the device’s ability to maintain a dynamic temperature gradient across a gaseous layer surrounding the thin heating element. A number of factors make thermophones an attractive technology for underwater use including the relative ease and low cost of production, the large thermal reservoir provided by the surrounding aquatic environment, and the ability to tune the spectra over a broad range of frequencies. We present calibrated acoustic underwater tests performed on thermophones which demonstrate the potential for a new class of sonar transducers. Small, 6.35 cm diameter, inert gas filled laminate pouch thermophones were fabricated which provide a low frequency resonance. Additionally, a liquid filled thermophone demonstrates a smooth response o...

Liquid filled encapsulation for thermoacoustic sonar projectors

Thermoacoustic projectors produce sound by rapidly heating and cooling a material with low heat capacity. These “thermophones” were originally demonstrated in 1917 using thin platinum filaments [Arnold, H., I.B Crandall, (1917) Phys. Rev. 10(1):22-38], but were very limited in their efficiencies and bandwidth until the much more recent discovery of new nanomaterials. The first underwater thermophones were made by Aliev et al. in 2010 [Aliev, A. E et al, (2010) Nano letters 10 (7), 2374-80] which utilized a carbon nanotube (CNT) sheet submerged in deionized water. These devices worked well for demonstrational purposes, but the CNT sheet would become damaged when retracted from the water, which made characterization difficult. More recently, new methods for enhancing the robustness of freestanding sheets have been developed which allow few layer CNT sheets to be repeatedly dipped and withdrawn from a water bath without damage. Such methods have enabled revisiting the study of submerged thermoacoustic projectors. Our studies of CNT thermophones in various liquid baths give evidence to the mechanism for acoustic wave generation in these systems. The potential to improve the impedance matching between the thermoacoustic source and surrounding fluid media suggest enhanced designs for compact sonar transducers.Thermoacoustic projectors produce sound by rapidly heating and cooling a material with low heat capacity. These “thermophones” were originally demonstrated in 1917 using thin platinum filaments [Arnold, H., I.B Crandall, (1917) Phys. Rev. 10(1):22-38], but were very limited in their efficiencies and bandwidth until the much more recent discovery of new nanomaterials. The first underwater thermophones were made by Aliev et al. in 2010 [Aliev, A. E et al, (2010) Nano letters 10 (7), 2374-80] which utilized a carbon nanotube (CNT) sheet submerged in deionized water. These devices worked well for demonstrational purposes, but the CNT sheet would become damaged when retracted from the water, which made characterization difficult. More recently, new methods for enhancing the robustness of freestanding sheets have been developed which allow few layer CNT sheets to be repeatedly dipped and withdrawn from a water bath without damage. Such methods have enabled revisiting the study of submerged thermoacoustic projec...

Investigation of carbon nanotubes for acoustic transduction applications

Traditional acoustic transduction sources typically begin with the generation of an electrical excitation pulsed through an amplifier into an electroacoustic material to create a mechanical vibration which is then converted into an acoustic wave to produce sound. The lower the preferred transmitting frequency (and hence, longer acoustic range) desired, the larger the required size of the source. Often this means that for acoustic projectors producing sound at frequencies below a few kHz, that the electroacoustic device will need to be very large in order to produce very long sound waves. This has a limitation for incorporating low frequency sonars on smaller autonomous underwater vehicles (AUVs). The topic of our presentation is an acoustic source technology that relies on the conversion of thermal energy to acoustic energy. Though the concept was first demonstrated in 1917, the recent advent of carbon nanotubes (CNT) now makes it possible to transmit acoustic waves in small and affordable packages using ...