Volume 116, Issue 2, August 2004
Index of content:
- TRANSDUCTION 
116(2004); http://dx.doi.org/10.1121/1.1771617View Description Hide Description
Silicon microphones based on capacitive micromachined ultrasonic transducer membranes and radio frequency detection overcome many of the limitations in bandwidth, uniformity of response, and durability associated with micromachined condenser microphones. These membranes are vacuum-sealed to withstand submersion in water and have a flat mechanical response from dc up to ultrasonic frequencies. However, a sensitive radio frequency detection scheme is necessary to detect the small changes in membrane displacement that result from utilizing small membranes. In this paper we develop a mathematical model for calculating the expected output signal and noise level and verifies the model with measurements on a fabricated microphone. Measurements on a sensor with 1.3 mm2 area demonstrate less than 0.5 dB variation in the output response between 0.1 Hz to 100 kHz under electrostatic actuation and an A-weighted equivalent noise level of 63.6 dB(A) SPL in the audio band. Because the vacuum-sealed membrane structure has a low mechanical noise floor, there is the potential for improved sensitivity using higher carrier frequencies and more sophisticated detection circuitry.
116(2004); http://dx.doi.org/10.1121/1.1768945View Description Hide Description
In this paper, the authors propose a method of enhancing sound in a selected region by controlling multiple sources. The physical variables of enhancing sound have not been well defined, but we may consider basic acoustic variables such as acoustic potential energy, sound power or intensity. A method of maximizing sound potential energy was found to be very straightforward [J.-W. Choi and Y.-H. Kim, J. Acoust. Soc. Am. 111, 1695 (2002)]. In this paper, the authors attempt to control the sound power or intensity of a zone in a desired direction. It is noteworthy that control of the direction and magnitude is needed to enhance the sound intensity. This control requires a new definition of the direction and magnitude of spatially distributed intensity. For this purpose, the authors introduce two different kinds of cost functions, and the theoretical formulation based on the new definitions show the possibility of maximizing the sound intensity of a selected zone in a desired direction.
116(2004); http://dx.doi.org/10.1121/1.1763972View Description Hide Description
Time-varying components are used in some multichannel sound systems designed for the enhancement of room acoustics. Time-variation can usefully reduce the risk of producing ringing tones and improve stability margins, provided that any modulation artefacts are inaudible. Frequency-shifting is one form of time-variation which provides the best case improvement in loop gain, and for which the single channel stability limit has been derived. This paper determines the stability limit for multiple channel systems with frequency-shifting by generalizing the previous single-channel analysis. It is shown that the improvement in stability due to frequency-shifting reduces with the number of channels. Simulations are presented to verify the theory. The stability limits are also compared with those of time-invariant systems, and preliminary subjective assessments are carried out to indicate useable loop gains with frequency-shifting.