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Dictionary of Acoustics
No full textThe science and technology of acoustics embraces an unusually wide range of disciplines, from aircraft noise reduction to ultrasonics in medicine, from psychoacoustics to signal processing. The student of acoustics has to become familiar with a corresponding range of specialist terms in order to communicate with others and to understand the literature. Here, in one informative dictionary, for the first time, are listed accurate and helpful definitions to provide the student - or the specialist from another discipline - with a point of entry into the world of acoustics. The dictionary's 2,800 entries cover most of the essential concepts and terminology that the practicing acoustician needs to understand, outside the subfields of music and speech communication. The author has drawn on experience gained during a long career spent mostly at Southampton University's multidisciplinary Institute of Sound and Vibration Research, supplemented by the expertise and perspective of a team of subject specialists
The role of viscosity in aerodynamic sound generation
No full textThe interpretation of unsteady surface shear stresses as dipole sound sources is discussed. Provided omega nu c2 <<1, it is legitimate to calculate the radiated pressure using viscous surface dipoles in combination with an inviscid-medium Green function. This is demonstrated via comparisons of acoustic-analogy predictions with exact solutions, for two plane-boundary problems. A Green function that is tailored to the linear relation between pressure and normal velocity on the boundary, as used by Powell [J. Acoust. Soc. Am. Vol. 32, 1960, pp. 982-990], highlights the role of viscous stresses in aeroacoustic calculations
Shear layer refraction corrections for off-axis sources in flight simulation: theoretical analysis
No full textTheory of sound generation in ducted compressible flows with applications to turbomachinery
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Aeroacoustics, moving boundaries, and bursting balloons: acoustic sources revisited
No full textThe use of equivalent acoustic sources to describe scattering in a nonuniform medium dates back to Rayleigh's theory of sound. The idea of equivalent sources in a uniform medium at rest was later developed by Lighthill into his "acoustic analogy," capable of describing the generation of sound by turbulence and other vortical flows. In the present paper Lighthill's acoustic analogy formulation is generalized to encompass initial-value problems; the initial conditions are represented by impulsive sources and dipoles distributed over the domain, and boundary conditions are represented in the usual manner by surface sources and dipoles. David Blackstock's bursting balloon example, discussed in Chapter 3 of Fundamentals of Physical Acoustics, can be solved by this method. However, in situations where the medium is of nonuniform density (for example, a gas with a specified temperature distribution at the initial time), the impulsive source distribution obtained by a direct application of time windowing to the acoustic analogy is nonphysical. The apparent paradox is resolved by introducing the energy conservation equation, and reformulating the acoustic analogy with pressure, rather than density, as the wave variable
Shear layer refraction corrections for off-axis sources in a jet flow
No full textA set of equations is derived for converting acoustic measurements taken in a free-jet flight simulation facility, such as the U.K. Noise Test Facility at Pyestock or the French CEPRA-19 wind tunnel at Saclay, to equivalent farfield flight conditions. The equations are based on the high-frequency geometrical acoustics approximation, whose application in the present context was justified in early studies by Morfey and Tester in 1977 and by Amiet in 1978. However, the present work differs by allowing the source to be positioned off the jet centreline, anywhere within the flight stream. The flight stream jet is modelled as an axisymmetric parallel shear flow, with a shear layer thickness which is small compared with the jet diameter. The model also permits the microphone to be located anywhere outside the flow, arbitrarily close to the open jet. The consequences of off-axis source location are illustrated by numerical calculations
In-duct pressure measurements for sound power determination in flow-ducts (presented at Institute of Acoustics Spring Conference 2002, Past, Present and Future Acoustics and EPSRC Theme Day in Acoustics, Salford, UK, 25-27 March 2002)
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