What are Thermoacoustic speakers?
Thermoacoustic speakers are speakers that generate sound waves from temperature fluctuations. They rapidly heat and cool 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 keep complicated mechanical parts for sound production. They also generate good quality sound in all directions. For that reason, you can place them on any surface, including curved ones, without canceling out sounds made from opposite sides.
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 interaction of the oscillating gas with the surface of the stack generates a heat transfer from one end of the stack to the other end. This thesis reports on the design, development, and optimization of thermoacoustic refrigerators. 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 has a negative effect on 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.
The use of a lightweight aluminum resonator instead of a heavy stainless-steel resonator resulted in a thermal time constant, which is a factor two and half times smaller. They manage to achieve a low temperature of -67 0C, the lowest reported temperature up 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 performance of the refrigerator is systematically studied by using helium noble-gas mixtures. Specifically, the coefficient of performance improves as the Prandtl number decreases, as expected. But the cooling power decreases when the mole fraction of the heavy noble gas component increases in the mixture.