scholarly journals On the theory of the inertia and diffraction corrections for the Rayleigh disc

1935 ◽  
Vol 153 (878) ◽  
pp. 17-40 ◽  
Keyword(s):  
Wave Front ◽  
Acoustic Radiation ◽  
Stationary Wave ◽  
Negative Sign ◽  
Circular Disc ◽  
Sound Waves ◽  
Velocity Amplitude ◽  
Radiation Fields ◽  
Quartz Fibre

A thin circular disc suspended by a quartz fibre tends to set itself broadside on to the direction of the propagation of incident sound waves, and its use in combination with resonators is well known as a means of measuring relative intensities of acoustic radiation fields. In a progressive or stationary wave in which the velocity amplitude is │ξ│, the average couple on a circular disc of radius a is usually given by the well-known formula L͞ = - ⅔ρ 0 a 3 │ξ│ 2 sin 2α, (1) where ρ 0 is the density of the medium and α is the angle between the direction of propagation of the wave-front and the normal to the disc, while the negative sign indicates that the couple tends to diminish α.

scholarly journals On the acoustic radiation pressure on spheres

1934 ◽  
Vol 147 (861) ◽  
pp. 212-240 ◽  
Keyword(s):  
Radiation Pressure ◽  
Acoustic Radiation ◽  
Wave Length ◽  
Theoretical Development ◽  
Order Effects ◽  
Pressure Amplitude ◽  
Sound Waves ◽  
Radiation Fields ◽  
Acoustic Radiation Pressure ◽  
Relative Measurements

Although frequent reference is made to acoustic radiation pressure in treatises and memoirs on sound, there appears to be no systematic theoretical development of the subject enabling actual pressures on obstacles of simple geometrical form to be calculated. In the audible range of acoustic frequencies, it is possible to devise, in a number of ways, means of measuring pressure amplitudes in sound waves as first order effects. At supersonic frequencies, however, these methods are no longer serviceable. When the dimensions of resonators of diaphragms become comparable with the wave-length, the physical effects which enable the pressure amplitude to be measured involve intractable diffraction problems, while the extremely high frequencies and small amplitudes involved make the employment of stroboscopic methods of observation extremely difficult. It has been shown, however, that at supersonic frequencies the acoustic radiation pressures on spheres and discs become sufficiently large to be measured easily, at any rate, in liquids. The mean pressure is generally assumed to be proportional to the energy density in the neighbourhood of the obstacle, and on this basis relative measurements can be made, for instance, in the radiation field of a supersonic oscillator. Such formulæ may be obtained without restriction as to wave-length, for spheres in plane progressive and stationary radiation fields, and the magnitude of the pressure is found to be of entirely different orders of magnitude in the two cases.

scholarly journals Measurement of absorbing power of materials by the stationary wave method

1930 ◽  
Vol 127 (804) ◽  
pp. 89-110 ◽  
Keyword(s):  
Plane Waves ◽  
Stationary Wave ◽  
Small Samples ◽  
Wave System ◽  
Pressure Amplitude ◽  
Wave Method ◽  
Sound Waves ◽  
Reflected Waves ◽  
Velocity Amplitude ◽  
The Difference

The stationary wave method of determining the absorption coefficient of a material employs plane waves of sound at perpendicular incidence. It requires the use of only small samples of material and provides a rapid and convenient means of obtaining useful information. The principle of the method has been previously described, so that a brief outline is sufficient. A long pipe is provided with a source of sound at one end and is closed at the other by the test specimen. Sound waves from the source travel down the pipe and are reflected by the specimen to an extent depending on its absorbing power. The superposition of the incident and reflected waves gives rise to a stationary wave system, and the pressure amplitude varies continuously along the pipe, going through a series of maximum and minimum values. The same description applies to the velocity amplitude, with the difference that the pressure maxima coincide in position with the velocity minima and vice versa .

scholarly journals On the acoustic radiation pressure on circular discs: Inertia and diffraction corrections

1935 ◽  
Vol 153 (878) ◽  
pp. 1-16 ◽  
Keyword(s):  
Radiation Pressure ◽  
Complex Analysis ◽  
Acoustic Radiation ◽  
Complete Solution ◽  
Diffraction Problem ◽  
Wave Length ◽  
Stationary Waves ◽  
Sound Waves ◽  
Radiation Fields ◽  
Acoustic Radiation Pressure

In supersonic radiation fields the acoustic radiation pressure on a circular disc may be measured by means of a suitably designed torsion balance. In order to interpret such measurements it is necessary to have available an exact formula for this pressure in various types of radiation fields. It is found that the radiation pressure on a disc, however small compared with the wave-length, depends on the nature of the field as a whole as related to the made of generation of the sound-waves. In a progressive plane wave the magnitude of the radiation pressure on a small disc is very much less than that in a stationary wave. The disc shares these peculiarities as regards pressure with the sphere, for which exact formulae may be obtained in certain types of radiation fields including plane progressive and stationary waves. For discs, the exact evaluation of the radiation pressure involves the complete solution of the associated diffraction problem, To avoid unduly complex analysis, the problem is here dealt with by means of cylindrical wave functions, although the solution arrived at is limited to discs of circumference considerable less than the wave-length.

scholarly journals On the problem of the electrified disc

1947 ◽  
Vol 8 (1) ◽  
pp. 14-19 ◽  
Author(s):  
E. T. Copson
Keyword(s):  
External Field ◽  
Gravitational Potential ◽  
Electrostatic Field ◽  
Surface Density ◽  
Wave Length ◽  
Circular Disc ◽  
Sound Waves ◽  
Total Potential ◽  
Induced Charge ◽  
Lord Kelvin

When a perfectly conducting uniform thin circular disc is kept at a potential V0 in an external electrostatic field of potential Φ, electric charge is induced on the surface of the disc; the problem is to find the surface-density σ of this induced charge and its potential V so that the total potential V + Φ has the constant value V0 on the surface of the disc. This problem was first discussed by Green in 1832, and the solution in the case when there is no external field was deduced by Lord Kelvin from the known formula for the gravitational potential of an elliptic homoeoid. The problem is still of interest since similar ideas occur in the theory of diffraction by a circular disc and in the theory of the generation of sound waves by a vibrating disc when the wave-length is large compared with the radius of the disc.

scholarly journals Influence of span-wise coherence on the acoustic radiation in a cylinder wake

2021 ◽  
Vol 1 (1) ◽  
Author(s):  
Lars Siegel ◽  
Guosheng He ◽  
Arne Henning ◽  
Karen Mulleners
Keyword(s):  
Particle Image Velocimetry ◽  
Particle Image ◽  
Acoustic Radiation ◽  
Maximum Flow ◽  
Flow Speed ◽  
Circular Cylinders ◽  
Cylinder Wake ◽  
Sound Waves ◽  
Image Velocimetry ◽  
Nozzle Size

The aim of this study is to detect and visualise the influence of span-wise coherence on propagating sound waves emanating from a flow around circular cylinders with span-wise variations of the local radius. Synchronous particle image velocimetry (PIV) and microphone measurements are performed in a circular wind tunnel with a nozzle size of 0.4 m×0.4 m at a maximum flow speed of U∞ = 43m s−1 . The test section is surrounded by a full anechoic chamber of approximately 9 m×9 m×5 m.

One-Dimensional Nonlinear and Nonlocal Oscillations of Plasma in Solid Body

2006 ◽  
Vol 113 ◽  
pp. 521-525
Author(s):  
Paulius Miškinis
Keyword(s):  
Wave Front ◽  
Solid Body ◽  
Nonlinear Waves ◽  
Traveling Wave ◽  
Field Potential ◽  
Electron Plasma ◽  
Dimensional Model ◽  
One Dimensional ◽  
Velocity Amplitude

A simple one-dimensional model describing nonlinear and nonlocal oscillations of electron plasma is considered. Due to regard of the nonlocality of the field potential, it allows one to smooth down the peaks of nonlinear waves, to steep up the wave front, and to modify the velocity amplitude and phase of the traveling wave.

scholarly journals Studies of excitation modes of thermoacoustic emitters of sound – of thermophones

2020 ◽  
pp. 164-168
Author(s):  
Ф.Ф. Легуша ◽  
Н.С. Григорьева ◽  
В.Д. Лукьянов ◽  
К.В. Разрезова ◽  
А.В. Троицкий
Keyword(s):  
Electric Current ◽  
Active Element ◽  
Acoustic Radiation ◽  
Radiation Power ◽  
Electrical Power ◽  
Sound Waves ◽  
Radiation Level ◽  
Direct Electric Current ◽  
Excitation Mode ◽  
Alternating Electric Current

В работе проведён анализ влияния режимов возбуждения термофона на его акустическую эффективность. В настоящее время для возбуждения современных термофонов используют два режима возбуждения соответствующих случаям, когда в активном элементе термофона текут: 1) постоянный электрический ток I0 и переменный электрический ток i(f) = Imsin(ωt); 2) только переменный ток i(f) = Imsin(ωt). В этих случаях термофон излучает звуковые волны, амплитуды колебательных скоростей которых um1 и um2 соответствуют номерам режимов. При этом показано, если выполняется неравенство I0 >> Im, то отношение колебательных скоростей um1 / um2 ≥ 28. Как следствие этого, уровень излучения звука при 1 режиме возбуждения более чем на 29 дБ выше, а мощность акустического излучения в 860 раз выше по сравнению со вторым режимом возбуждения. Таким образом, для создания мобильных эхолокационных систем, работающих в газах, могут быть использованы термофоны, в которых реализован первый режим возбуждения, имеющий более сложную схему электрического питания. The paper analyzes the influence of thermophone excitation modes on its acoustic efficiency. Currently, to excite modern thermophones, two excitation modes are used corresponding to cases when in the active element of the thermophone flows: 1) direct electric current I0 and alternating electric current i(f) = Imsin(ωt); 2) only alternating current i(f) = Imsin(ωt). In these cases, the thermophone emits sound waves whose vibrational speed amplitudes um1 and um2 correspond to the mode numbers. It is shown that if the inequality I0 >> Im, is satisfied, then the ratio of vibrational speeds um1 / um2 ≥ 28. As a result, the sound radiation level at 1 excitation mode is more than 29 dB higher, and the acoustic radiation power is 860 times higher compared to the second excitation mode. Thus, to create mobile echolocation systems operating in gases, thermophones can be used, in which the first excitation mode is implemented, which has a more complex electrical power supply scheme.

scholarly journals First-principle description of acoustic radiation of shear flows

2019 ◽  
Vol 377 (2159) ◽  
pp. 20190077 ◽  
Author(s):  
Xuesong Wu ◽  
Zhongyu Zhang
Keyword(s):  
Acoustic Waves ◽  
Shear Flows ◽  
Acoustic Radiation ◽  
Higher Order ◽  
Far Field ◽  
Sound Waves ◽  
Leading Order ◽  
Mean Flow ◽  
Asymptotic Approach ◽  
Flow Distortion

As a methodology complementary to acoustic analogy, the asymptotic approach to aeroacoustics seeks to predict aerodynamical noise on the basis of first principles by probing into the physical processes of acoustic radiation. The present paper highlights the principal ideas and recent developments of this approach, which have shed light on some of the fundamental issues in sound generation in shear flows. The theoretical work on sound wave emission by nonlinearly modulated wavepackets of supersonic and subsonic instability modes in free shear flows identifies the respective physical sources or emitters. A wavepacket of supersonic modes is itself an efficient emitter, radiating directly intensive sound in the form of a Mach wave beam, the frequencies of which are in the same band as those of the modes in the packet. By contrast, a wavepacket of subsonic modes radiates very weak sound directly. However, the nonlinear self-interaction of such a wavepacket generates a slowly modulated mean-flow distortion, which then emits sound waves with low frequencies and long wavelengths on the scale of the wavepacket envelope. In both cases, the acoustic waves emitted to the far field are explicitly expressed in terms of the amplitude function of the wavepacket. The asymptotic approach has also been applied to analyse generation of sound waves in wall-bounded shear flows on the triple-deck scale. Several subtleties have been found. The near-field approximation has to be worked out to a sufficiently higher order in order just to calculate the far-field sound at leading order. The back action of the radiated sound on the flow in the viscous sublayer and the main shear layer is accounted for by an impedance coefficient. This effect is of higher order in the subsonic regime, but becomes a leading order in the transonic and supersonic regimes. This article is part of the theme issue ‘Frontiers of aeroacoustics research: theory, computation and experiment’.

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