Acoustic-electric characterisation of loudspeakers for low-cost thermoacoustic electricity generators

Thermoacoustic heat engines convert high temperature heat to acoustic power, the only moving component being the gas undergoing the acoustic motion. The engine's acoustic power can be directly converted to electricity through the electro-dynamic transduction mechanism. Usually, flexure-bearing-supported linear alternators are used for this purpose because they are very robust, reliable and efficient. However, costs of commercially available units are prohibitive. This limits the advantages of the thermoacoustic heat engines for low-cost electricity generators. This paper investigates the feasibility of using commercially available, low-cost loudspeakers to convert the acoustic power to electricity. One of such loudspeakers is selected from the general performance viewpoint taking into account parameters such as: high force factor (Bl), low electrical resistance and low mechanical loss. It is connected to an acoustic resonator which is driven by a pair of bigger size loudspeakers at the other end, to measure the characteristics of acoustic-to-electric energy conversion. The pressure oscillations in front and behind the loudspeaker are measured using two microphones, while the displacement of the loudspeaker's diaphragm is measured by a laser displacement sensor. This allows the measurement of the input acoustic power. The electric power is dissipated by a resistance and measured. The measurement results are presented and discussed in detail.