Thermoacoustic Speakers

What Are Thermoacoustic Speakers?

Thermoacoustic speakers generate sound waves from temperature fluctuations by rapidly heating and cooling conducting materials. Unlike conventional voice-coil speakers, thermoacoustic speakers do not rely on vibrations to produce sound. Therefore, they do not need bulky acoustic boxes to house complicated mechanical parts. They also generate good-quality sound in all directions, so they can be placed on any surface, including curved ones, without canceling sounds from opposite sides.

Thermoacoustic Refrigerators

Loudspeaker-driven thermoacoustic refrigerators are devices that are driven by sound to generate cooling. These coolers use a loudspeaker to sustain a sound wave in a resonance tube. The gas-filled resonator houses a stack of plates and two heat exchangers. Moreover, the oscillating gas's interaction with the stack's surface generates a heat transfer from one end of the stack to the other. This thesis reports on thermoacoustic refrigerators' design, development, and optimization. The influence of some fundamental thermoacoustic parameters, such as the Prandtl number of the gas and the sheet spacing in the stack on the performance, have been studied systematically. Studies show that the viscosity of the gas hurts performance.

What's more, minimizing the Prandtl number enables performance optimization. The measurements show that the gas-spring system works very well. By varying the spring constant of the driver, the mechanical resonance frequency can be adjusted. An electroacoustic efficiency of 35 % is obtained when the mechanical resonance of the driver and the acoustic resonance are equal. Additionally, the efficiency is constant over a wide frequency range. This is advantageous from the point of view of thermoacoustic refrigeration because the operating acoustic frequency is temperature-dependent. A flat efficiency peak provides the refrigerator with a temperature-independent performance.

Lightweight Aluminum

Using a lightweight aluminum resonator instead of a heavy stainless-steel resonator resulted in a thermal time constant that is two and a half times smaller. They achieved a low temperature of -67 degrees Celsius, the lowest reported temperature today. One experimental study used spirally wounded stacks and parallel plate stacks. The effects of the dynamic pressure and some properties of the spiral stack on the performance are discussed briefly. The influence of the Prandtl number on the refrigerator's performance is systematically studied using helium noble-gas mixtures. Specifically, the coefficient of performance improves as the Prandtl number decreases, as expected. However, the cooling power decreases when the mole fraction of the heavy noble gas component increases in the mixture.

Source Texts

Graphene Thermoacoustic Speakers Sound Like The Future

Loudspeaker-Driven Thermo-Acoustic Refrigeration