Fundamental efficiency of thermophones

We have designed and fabricated air-bridge arrays comprising hundreds of thousands of aluminum wires. Thermally decoupled from the underlying silicon substrate, the nanometer-thick wires act as thermoacoustic sound sources (thermophones). Periodic heating of wire surfaces is induced by a.c. current. In the surrounding air domain, rapidly attenuating thermal waves excite pressure waves that we understand as sound or ultrasound. Recent experiments with our array loudspeakers and other nanoscale thermophones have revealed properties which are poorly understood theoretically. We have developed analytical Green's function methods that quantitatively explain some features. For example, the effect of a heat-absorbing substrate in the proximity of a suspended source can be studied. We have verified our field-theoretical efforts with finite difference method simulations, and the results are in good agreement with acoustic measurements. We have also found a generic- ultimate limit for the efficiency of thermophones. It is valid in the linear regime of thermoacoustics. The measured efficiency of our array loudspeakers is 1 order of magnitude below the upper bound. At low frequencies this is caused by the presence of the substrate, while at ultrasound frequencies wire heat capacity limits the performance