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

Acoustical characteristics of porous materials

Measurement of the scattering-matrix of acoustical two-ports

Active Control of the Noise from a Rijke Tube

The effectiveness of a cylindrical perforated liner with mean bias flow in its absorption of planar acoustic waves in a duct is investigated. The liner converts acoustic energy into flow energy through the excitation of vorticity fluctuations at the rims of the liner apertures. A one-dimensional model that embodies this absorption mechanism is developed. It utilizes a homogeneous liner compliance adapted from the Rayleigh conductivity of a single aperture with mean flow. The model is evaluated by comparing with experimental results, with excellent agreement. We show that such a system can absorb a large fraction of incoming energy, and can prevent all of the energy produced by an upstream source in certain frequency ranges from reflecting back. Moreover, the bandwidth of this strong absorption can be increased by appropriate placement of the liner system in the duct. An analysis of the acoustic energy flux is performed, revealing that local differences in fluctuating stagnation enthalpy, distributed over a finite length of duct, are responsible for absorption, and that both liners in a double-liner system are absorbant. A reduction of the model equations in the limit of long wavelength compared to liner length reveals an important parameter grouping, enabling the optimal design of liner systems.

The absorption of axial acoustic waves by a perforated liner with bias flow

An experimental technique is presented for the determination of normal acoustic properties in a tube, including the effect of mean flow. An acoustic source is driven by Gaussian white noise to produce a randomly fluctuating sound field in a tube terminated by the system under investigation. Two stationary, wall‐mounted microphones measure the sound pressure at arbitrary but known positions in the tube. Theory is developed, including the effect of mean flow, showing that the incident‐ and reflected‐wave spectra, and the phase angle between the incident and reflected waves, can be determined from measurement of the auto‐ and cross‐spectra of the two microphone signals. Expressions for the normal specific acoustic impedance and the reflection coefficient of the tube termination are developed for a random sound field in the tube. Three no‐flow test cases are evaluated using the two‐microphone random‐excitation technique: a closed tube of specified length, an open, unbaffled tube of specified length, and a pro...

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D. F. Ross

Papers

Experimental determination of acoustic properties using a two‐microphone random‐excitation technique