Index of content:
Volume 125, Issue 4, April 2009
- TRANSDUCTION 
Fundamental constraints on the performance of broadband ultrasonic matching structures and absorbers125(2009); http://dx.doi.org/10.1121/1.3081529View Description Hide Description
Recent fundamental results concerning the ultimate performance of electromagnetic absorbers were adapted and extrapolated to the field of sound waves. It was possible to deduce some appropriate figures of merit indicating whether a particular structure was close to the best possible matching properties. These figures of merit had simple expressions and were easy to compute in practical cases. Numerical examples illustrated that conventional state-of-the-art matching structures had an overall efficiency of approximately 50% of the fundamental limit. However, if the bandwidth at −6 dB was retained as a benchmark, the achieved bandwidth would be, at most, 12% of the fundamental limit associated with the same mass for the matching structure. Consequently, both encouragement for future improvements and accurate estimates of the surface mass required to obtain certain desired broadband properties could be provided. The results presented here can be used to investigate the broadband sound absorption and to benchmark passive and active noise control systems.
125(2009); http://dx.doi.org/10.1121/1.3082115View Description Hide Description
The rotary subwoofer is a novel acoustic transducer capable of projecting infrasonic signals at high sound pressure levels. The projector produces higher acoustic particle velocities than conventional transducers which translate into higher radiated sound pressure levels. This paper characterizes measured performance of a rotary subwoofer and presents a model to predict sound pressure levels.
125(2009); http://dx.doi.org/10.1121/1.3082118View Description Hide Description
A miniature differential microphone is described having a low-noise floor. The sensitivity of a differential microphone suffers as the distance between the two pressure sensing locations decreases, resulting in an increase in the input sound pressure-referred noise floor. In the microphone described here, both the diaphragm thermal noise and the electronic noise are minimized by a combination of novel diaphragm design and the use of low-noise optical sensing that has been integrated into the microphone package. The differential microphone diaphragm measures and is fabricated out of polycrystalline silicon. The diaphragm design is based on the coupled directionally sensitive ears of the fly Ormia ochracea. The sound pressure input-referred noise floor of this miniature differential microphone has been measured to be less than .