Volume 123, Issue 4, April 2008
Index of content:
- GENERAL LINEAR ACOUSTICS 
123(2008); http://dx.doi.org/10.1121/1.2874631View Description Hide Description
Typically, sound speed in gases is smaller and mass density is much smaller than in liquids, resulting in a very strong acoustic impedance contrast at a gas–liquid interface.Sound transmission through a boundary with a strong impedance contrast is normally very weak. This paper studies the power output of localized sound sources and acoustic power fluxes through a plane gas–liquid interface in a layered medium. It is shown that, for low-frequency sound, a phenomenon of anomalous transparency can occur where most of the acoustic power generated by a source in a liquid half-space can be radiated into a gas half-space. The main physical mechanism responsible for anomalous transparency is found to be an acoustic power transfer by inhomogeneous (evanescent) waves in the plane-wave decomposition of the acoustic field in the liquid. The effects of a liquid’s stratification and of guided sound propagation in the liquid on the anomalous transparency of the gas–liquid interface are considered. Geophysical and biological implications of anomalous transparency of water–air interface to infrasound are indicated.
123(2008); http://dx.doi.org/10.1121/1.2839891View Description Hide Description
Radiation characteristics are calculated for a circular planar sound source in free space with a uniform surface pressure distribution, which can be regarded as a freely suspended membrane with zero mass and stiffness. This idealized dipole source is shown to have closed form solutions for its far-field pressure response and radiation admittance. The latter is found to have a simple mathematical relationship with the radiation impedance of a rigid piston in an infinite baffle. Also, a single expansion is derived for the near-field pressure field, which degenerates to a closed form solution on the axis of symmetry. From the normal gradient of the surface pressure, the surface velocity is calculated. The near-field expression is then generalized to an arbitrary surface pressure distribution. It is shown how this can be used as a simplified solution for a rigid disk in free space or a more realistic sound source such as pre-tensioned membrane in free space with non-zero mass and a clamped rim.
123(2008); http://dx.doi.org/10.1121/1.2875046View Description Hide Description
An acoustic intensity-based method is proposed for the reconstruction of acoustic radiation pressure. Unlike the traditional inverse acoustic methods, the proposed method includes the acoustic pressure gradient as an input in addition to its simultaneous, co-located acoustic pressure in a radiated field. As a result, the reconstruction of acoustic radiation pressure from the input acoustic data over a portion of a surface enclosing all the acoustic sources, i.e., an open surface, becomes unique due to the unique continuation theory of elliptic equations. Hence the method is more stable and the reconstructedacoustic pressure is less dependable on the locations of the input acoustic data. Furthermore, the proposed method can be applied for both inverse and forward problems up to the minimum sphere enclosing the sources of interest. The effectiveness of the method is demonstrated by the results of several acoustic radiation examples with single or multi-frequency source in a two-dimensional configuration. The results from the method also show a measurable improvement in accuracy and consistency of reconstructedacoustic radiation pressure, in particular when the effect of the signal-to-noise ratio is included.