摘要:

A set of analytical expressions has been developed which will predict the absorption of sound in air at 20°C. The results agree within 3% of previous work in the frequency range of 100 Hz 1 MHz for all humidities. Below 100 Hz accuracy cannot be judged because of lack of knowledge about the vibration‐vibration energy transfer betweenCO2andO2.

ISSN:0001-4966    

DOI:10.1121/1.1913183

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

We develop a theoretical treatment of the effect of pressure fluctuations on the phase of an electromagnetic wave propagating through them. The pressure variations may be either acoustic or turbulent in nature. We then describe an experiment in which acoustic noise excitation of a weakly resonant system is measured by laser interferometry. Wide‐band acoustic noise energy is generated by a “free jet” emptying into a long pyrex pipe which is continuously pumped on by a large‐capacity mechanical vacuum pump. The acoustic noise propagates down the pipe into a region traversed axially by a laser beam in which acoustic resonances occur. The phase of the coherent light is measured relative to a stable path and is related to the density variations produced by the random acoustic field. The spectrum of the phase fluctuations in the path gives explicit information about the acoustic noise. Theory and experiment are compared.

ISSN:0001-4966    

DOI:10.1121/1.1913184

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

Ultrasonic absorption measurements have been made in aqueous acetic solutions at 15 to 85 MHz using pulse‐echo and pulse send‐receive techniques. A weighted nonlinear regression method has been developed for the computation of the relaxation parameters. A single relaxation has been found with a relaxation frequency at 13.2 MHz in 0.1Macetic acid and 66.1 MHz in 1.0Macetic acid at 25°C. These values and the maximum excess relaxational absorption per wavelength are in good agreement with the values calculated from equilibrium and kinetic parameters for the ionization of acetic acid as determined by non‐acoustical techniques (density, conductance, electrode kinetics, and Wien effect II dispersion). The rate constants and reaction volume change of the ionization‐association process, calculated from the ultrasonic absorption measurements in acetic acid, arekdissoc = 7.8 × 105 sec−1, kassoc = 4.4 × 1010 M−1 sec−1, andΔVdissoc = −11.2 cm3/moleat 25°C.

ISSN:0001-4966    

DOI:10.1121/1.1913185

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The scattering strength of air bubbles in water has been determined experimentally at frequencies of 1 and 5 MHz. The bubble size and sound frequencies represent the short wavelength limit, whereka>1. By scanning the sound fields with a PZT transducer of 1 mm radius, angular distributions have been obtained out to an angle of 35° with and without the scatterers present. The results agree well with recent scattering measurements in suspensions and also with scattering theory.

ISSN:0001-4966    

DOI:10.1121/1.1913186

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

It is shown that the errors in the absolute values of the measured delay times (after correction for diffraction phase and bond thickness) in the pulse‐echo overlap and the pulse‐superposition methods can be less than 0.015τ and 0.027τ, respectively, where τ is the period of the fundamental radio frequency of the transducer used. If broad‐band pulses are used with the pulse‐echo overlap method, then the error is larger but still less than 0.10τcat the center frequency. Translated into fractional errors in the single round‐trip delay time, the errors may be as small as a few parts per million. It is emphasized that the diffraction phase can introduce an error up to 0.25τ if the diffraction phase correction is not applied. Empirical methods for finding some contributions to the total error are given.

ISSN:0001-4966    

DOI:10.1121/1.1913187

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The effective diffraction loss and phase change in the field of broad‐band transducers is computed in terms of the normalized distanceSc=zλc/a2at the center frequency of the pulse. Diffraction corrections for attenuation and velocity are explained, and their limitations stated for broad‐band pulses. It is shown that the corrections are functions of bandwidth as well as ofSc.

ISSN:0001-4966    

DOI:10.1121/1.1913188

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The momentary contact pulse‐echo method with pressure coupling has been extended to the measurement of ultrasonic attenuation in hot specimens. Previously, only velocity was measured. In the present method, the specimen is contacted momentarily by one end of a cool buffer rod about three times as long as the specimen. A piezoelectric transducer on the other end of the buffer rod transmits an ultrasonic signal and receives echoes from the buffer/specimen interface and from the free end of the specimen. The echoes are displayed on an oscilloscope and recorded by a camera. Three echoes are required to define both the attenuation coefficient in the specimen and the reflection coefficient at the buffer/specimen interface. Our results in steel show that the solution and precipitation of carbides affect the velocity drastically, while the attenuation is affected by at least one of the following: (1) magnetism, (2) recrystallization, or (3) carbon solubility, and also by grain growth at elevated temperatures.

ISSN:0001-4966    

DOI:10.1121/1.1913189

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The forced harmonic and random vibrations of an elastic cylindrical shell surrounded by an inviscid fluid and concentrically contained by another thin elastic shell which itself is immersed in another inviscid fluid of infinite extent is considered. The motion of the shells and fluids is assumed independent of the axial coordinate. The motion of the shells is described by a theory which accounts for transverse shear and rotatory inertia. The motion of the fluids is described by the classical wave equation. Expressions for the acoustic pressure at the outer surface of the inner shell and the inner and outer surfaces of the outer shell are obtained along with the displacements (velocity) at these surfaces. Numerical results for the near‐ and farfield acoustic pressure are given for the case wherein the interior of the cylinder is subjected to diametrically opposed point forces which vary either harmonically or randomly with time. The random excitation is assumed to be spatially uncorrelated, broad‐band white noise.

ISSN:0001-4966    

DOI:10.1121/1.1913190

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The acoustic beam of a circular plane piston (radiusa) is analyzed as it approaches either the short‐wavelength limit (SWL) or the large‐radius limit (LRL). Using a new expression for the beam, we show that with no absorption the SWL would be a cylindrically collimated beam, with radiusa, of plane waves. If absorptive effects are minor, the chief deviations during approach to the SWL occur in two decreasing volumes, a horn‐shaped paraxial region and a cylindrical shell of mean radiusa. One consequence is that any second harmonic, self‐generated in the beam, approaches its own SWL more rapidly than does the first harmonic. Approach to the LRL yields a known paradox: Even the piston's nearfield need not approach a uniform plane wave. We show that the presence of absorption does make the LRL a plane wave and that in the absence of absorption there is a valid reason for the paradox: The piston beam must have a unique axis of cylindrical symmetry; an unbounded plane wave cannot.

ISSN:0001-4966    

DOI:10.1121/1.1913191

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The moiré fringe wave visualization technique produces instantaneous records of the wave field, from which the propagation characteristics of the surface may be deduced. This is particularly useful for surfaces whose properties are time variant. Although the wave damping properties of the polymer monolayers examined were considerably more complicated than those of monolayers of simpler surface active agents, existing theory was shown to be adequate in explaining the observed effects on the basis of the surface tension and surface dilational elasticity of the monolayer. This agreement of theoretical prediction with observation supports the evidence for the existence of longitudinal surface waves. Using the moiré technique to investigate the time variation of solution‐adsorbed monolayers of poly (ethylene oxide) on water, it was found that this polymer produces from solution the same type of stable monolayer as is produced by spreading the material in a monolayer on a pure water substrate.

ISSN:0001-4966    

DOI:10.1121/1.1913192

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP