  We report on the use of an optically-trapped microsphere as an acoustic
transducer. A model for the hydrodynamic coupling between the microsphere and
the surrounding acoustic fluid flow is combined with thermo-mechanical
calibration of the microsphere's position detection to enable quantitative
acoustic measurements. We describe our technique in detail, including the
self-noise, sensitivity, and minimum detectable signals, using a model
appropriate for both liquid and gas environments. We then test our approach in
an air-based experiment and compare our measurements with two state-of-the-art
commercially-available acoustic sensors. Piezoelectrically-driven bursts of
pure tones and laser ablation provide two classes of test sounds. We find
accurate measurements with a bandwidth of 1 MHz are possible using our
technique, improving by several orders of magnitude the bandwidth of previous
flow measurements based on optically-trapped microspheres.
