Thermoacoustic engines (TAEs) can be used to pump heat using a sound wave or pump a sound wave using a temperature gradient. The basic arrangement is a gas-filled acoustic resonator with appropriately positioned thermoacoustic elements. Two types of thermoacoustic elements are used in these engines: (1) heat exchangers used to communicate heat between the gas and external heat reservoirs; and (2) the TAE, also known as a stack. The TAEs are sections of porous media that support the temperature gradient, transport heat on the acoustic wave between the exchangers, and produce or absorb acoustic power. Previous results have been developed for TAEs with circular or parallel slit pore geometries. The theory is extended for gas-filled TAEs to include pores of arbitrary cross-sectional geometry. An approximate analysis of energy flow and acoustical measurements of a thermoacoustic prime mover are given. >

http://ieeexplore.ieee.org/iel2/886/6033/00234265.pdf

Thermoacoustic engines

Annual Energy Outlook 2008 With Projections to 2030

The need exists for high-sensitivity, low-noise vibration sensors for various applications, such as geophysical data collection, tracking vehicles, intrusion detectors, and underwater pressure gradient detection. In general, these sensors differ from classical accelerometers in that they require no direct current response, but must have a very low noise floor over a required bandwidth. Theory indicates a capacitive micromachined silicon vibration sensor can have a noise floor on the order of 100 ng//spl radic/Hz over 1 kHz bandwidth, while reducing size and weight tenfold compared to existing magnetic geophones. With early prototypes, we have demonstrated Brownian-limited noise floor at 1.0 /spl mu/g/Hz, orders of magnitude more sensitive than surface micromachined devices such as the industry standard ADXL05.

https://www.uta.edu/rfmems/Paper_for_teralum/MEMS%20vibration%20sensor/00809058.pdf

Low-noise MEMS vibration sensor for geophysical applications

The dominating method of measuring sound intensity in air is based on the combination of two pressure microphones. However, a sound intensity probe that combines an acoustic particle velocity transducer with a pressure microphone has recently become available. This paper examines, discusses, and compares the two measurement principles with particular regard to the sources of error in sound power determination. It is shown that the phase calibration of intensity probes that combine different transducers is very critical below 500 Hz if the measurement surface is very close to the source under test. The problem is reduced if the measurement surface is moved further away from the source. The calibration can be carried out in an anechoic room.

/pdf/a-comparison-of-two-different-sound-intensity-measurement-1r2xcegj4w.pdf

A comparison of two different sound intensity measurement principles

Design, fabrication, and preliminary characterization of a novel MEMS bionic vector hydrophone

To address the question of the role of nonlinear effects in the propagation of noise radiated by high-power jet aircraft, extensive measurements were made of the F-22A Raptor during static engine run-ups. Data were acquired at low-, intermediate-, and high-thrust engine settings with microphones located 23–305m from the aircraft along several angles. Comparisons between the results of a generalized-Burgers-equation-based nonlinear propagation model and the measurements yield favorable agreement, whereas application of a linear propagation model results in spectral predictions that are much too low at high frequencies. The results and analysis show that significant nonlinear propagation effects occur for even intermediate-thrust engine conditions and at angles well away from the peak radiation angle. This suggests that these effects are likely to be common in the propagation of noise radiated by high-power aircraft.

The role of nonlinear effects in the propagation of noise from high-power jet aircraft.

DOI: 10.2514/1.26484 Crackle is a phenomenon sometimes found in supersonic jet noise and can comprise an annoying and dominant part of the overall perceived noise. In the past, crackle has been commonly quantified by the skewness of the time waveform. In this investigation, a simulated waveform with a virtually identical probability density function and power spectrum as an actual F/A-18E afterburner recording has been created by nonlinearly transforming a statistically Gaussian waveform. Although the afterburner waveform crackles noticeably, playback of the nonGaussian simulated waveform yields no perception of crackle at all, despite its relatively high skewness. Closer examinationofthetwowaveformsrevealsthatalthoughtheyhavevirtuallyidenticalstatistics,thereareconsiderable differences in their time rates of change in the intense compressive portions of the waveforms. The afterburner waveform is much more shocklike with its more rapid variations in pressure than the non-Gaussian simulated waveform. This results in a significant difference in the probability distributions of the time derivatives of the actual and simulated data and suggests that the perception of crackle in jet noise waveforms may be better quantified with statistics of the time derivative of the waveform, rather than by the skewness of the time waveform itself.

On the Perception of Crackle in High-Amplitude Jet Noise

Acoustic particle velocity is commonly inferred from measurement of pressure or pressure gradient; however, in water, direct measurement is simple. A moving‐coil sensor embedded in a neutrally buoyant package produces a voltage directly proportional to the particle velocity in the surrounding fluid for frequencies above the mass‐spring resonance. Leslie et al. [J. Acoust. Soc. Am. 28, 711–715 (1956)] built such a sensor by mounting a moving‐coil element inside a hollow brass sphere. The sensor described in this paper is identical in principle but is considerably easier to fabricate. Useful from tens of hertz to several kilohertz, this sensor consists of a glass‐ microballoon‐and‐epoxy composite cast around a small, commercial geophone. The sensor is inexpensive, rugged, and has good immunity to interference. In conjunction with a pressure hydrophone, acoustic intensity can be measured without the errors associated with subtraction of nearly equal signals (as in the two‐hydrophone method).

A simple neutrally buoyant sensor for direct measurement of particle velocity and intensity in water

Preliminary analysis of nonlinearity in military jet aircraft noise propagation

An underwater acoustic intensity sensor is described. This sensor derives acoustic intensity from simultaneous, co-located measurement of the acoustic pressure and one component of the acoustic particle acceleration vector. The sensor consists of a pressure transducer in the form of a hollow piezoceramic cylinder and a pair of miniature accelerometers mounted inside the cylinder. Since this sensor derives acoustic intensity from measurement of acoustic pressure and acoustic particle acceleration, it is called a p-a intensity probe. The sensor is ballasted to be nearly neutrally buoyant. It is desirable for the accelerometers to measure only the rigid body motion of the assembled probe and for the effective centers of the pressure sensor and accelerometer to be coincident. This is achieved by symmetric disposition of a pair of accelerometers inside the ceramic cylinder. The response of the intensity probe is determined by comparison with a reference hydrophone in a predominantly reactive acoustic field.

Thomas B. Gabrielson

Papers

The role of nonlinear effects in the propagation of noise from high-power jet aircraft.

On the Perception of Crackle in High-Amplitude Jet Noise

A simple neutrally buoyant sensor for direct measurement of particle velocity and intensity in water

Preliminary analysis of nonlinearity in military jet aircraft noise propagation

Development of an accelerometer-based underwater acoustic intensity sensor.