摘要:

The dependence of spectral variations within a reflected broad‐band ultrasonic pulse on the size and orientation of the reflector was determined experimentally. An analytical model is developed on the assumption that interference of the waves received from the edges of the reflecting surface is responsible for the variations of the received frequency spectra. It explains the experimental results very well. The feasibility of determining the size and orientation of hidden flaws in metals by this method is demonstrated.

ISSN:0001-4966    

DOI:10.1121/1.1912935

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

A theory is developed for the farfield diffraction of a Gaussian light beam that passes axially through an ultrasonic cylindrical standing wave. The approach is applicable only in the range of parameters for which the ultrasonic perturbation may be characterized as an axially symmetric phase grating. Although the diffraction integral is not completely integrated in terms of known functions, some useful expressions for the average and time‐dependent light intensities are obtained. This acousto‐optic interaction geometry, with the light‐beam waist diameter equal to the first nodal diameter in the ultrasonic field, comprises an effective light modulator. Calculations of the on‐axis time‐dependent light intensity are presented and are confirmed by experimental results.

ISSN:0001-4966    

DOI:10.1121/1.1912936

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

High‐power ultrasonics has been a useful tool in studying fatigue in metals, since a large number of cycles can be obtained in a small time. In the case of brass and iron the mechanisms appear to differ with frequency, since at the high frequencies fatigue occurs in isolated regions whereas at the low frequencies the slip is generally spread over the grains. The same mechanism can be produced at high frequencies by going to higher temperatures. In iron the stress required for fatigue at higher frequencies is considerably less than for low frequencies. Recent measurements in titanium have shown that the required stress is about the same for high and low frequencies. The crack propagation velocity, however, is much faster at the ultrasonic frequencies. This was proved by striation measurements with a scanning electron microscope. Thus, the high‐frequency fracture is not only localized, it is virtually catastropic in the localized area.

ISSN:0001-4966    

DOI:10.1121/1.1912937

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The problem of combined translational and internal relaxation of sound waves in polyatomic gases is studied using the 17‐moment approximation. When applied to the sound problem, the moment equations, which are valid for any number of internal degrees of freedom, yield a generalized complex matrix eigenvalue equation which is solved numerically for the absorption coefficient and propagation speed as a function of the rarefaction parameter (the approximate ratio of collision frequency to sound frequency). The physical parameters in the equations are the self‐diffusion coefficient (assumed the same for each internal mode), the internal relaxation times, and the internal specific heats. Our solutions for the combined absorption for the case of a single internal mode are compared to those obtained assuming the total absorption is the sum of that due to the translational and internal effects separately, and to that obtained by using the combination rule of Greenspan. Comparison is also made between the combined dispersion calculated by the 17‐moment method and the Greenspan rule. In addition, absorption curves are calculated for the case of two internal degrees of freedom for various combinations of internal collision numbers. When applied to rotational relaxation inN2our theory yields, for a rotational collision number of 5.26, good agreement with experiment over the range of values of the rarefaction parameter for which a continuum theory is expected to be valid.

ISSN:0001-4966    

DOI:10.1121/1.1912938

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

An automatic gain control (AGC) for maintaining constant gain of the i.f. amplifiers in an ultrasonic pulse‐echo system is described. The AGC stabilizes the receiver against long‐term drifts. It is applicable if the duty cycle of the sonic‐echo train is small (5% or less) and the rest of the cycle is available for introduction of a reference signal. The AGC samples the amplified reference signal and keeps its level constant, thereby maintaining constant gain in the i.f. amplifiers without the necessity for using an additional gated integrating amplifier. The AGC improved the reliability of measurements of sonic‐echo voltage from ±3% without the AGC to ±0.5% with the AGC over a long period of time. The total ultrasonic system for attenuation measurements in the range 20–500 MHz, including the AGC, is described.

ISSN:0001-4966    

DOI:10.1121/1.1912939

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

A wave equation for the propagation of sound in suspensions or emulsions has been formulated from the simple considerations of mass and momentum balances on a sufficiently small homogeneous and compressible volume element of the suspension in the sound field. The induced masses of the non‐neutrally buoyant oscillating particles and the adiabatic compressibilities of suspension components have been shown to modify the equations of continuity and momentum and thereby the wave equation, with respect to those equations for the single‐phase suspending fluid. A formula for the velocity of sound in suspensions has been deduced, which agrees with that derived from the scattering theory.

ISSN:0001-4966    

DOI:10.1121/1.1912940

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

Schlieren visualization of the radiated field in water above and below aluminum plates, illuminated by short acoustic pulses, is presented. At certain angles of incidence of the incoming pulse, radiated wavefronts are seen to progress along the plates with specific angles of inclination and associated speeds of progression. In particular, strong effects are observed near the longitudinal and shear critical angles. The measured angles of incidence, at which these effects were generated, are in good agreement with predictions of Lamb theory for plates whose thicknesses are 0.58 wavelengths and larger. For thinner plates, the effects of water loading appear to become significant. A path trace analysis gives good agreement with the group velocity of observed effects when obvious trapped propagation paths are suggested by the reflection coefficient curves for a water‐aluminum boundary. Specific radiated wavefronts on plates are related to waves which were observed on cylindrical shells. Relationship of the shell results to Lamb modes is discussed. Plates with thicknesses from 0.002 to 0.241 cm, and frequencies of 5 and 7.2 MHz, are used.

ISSN:0001-4966    

DOI:10.1121/1.1912941

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The radiation resistance of a beam is theoretically determined from the total acoustic power radiated into the farfield. The beam is supported in an infinite baffle, with both hinged and clamped supports considered. Asymptotic solutions are derived for frequencies well below the critical frequency. Curves, covering the entire frequency range between the low‐ and high‐frequency asymptotic solutions, are obtained through numerical integration for the first ten modes of beams with various width‐to‐length ratios. For frequencies well below the critical frequency, the ratio of the radiation resistance of a beam clamped at each end to that of a beam hinged at each end is 0.851 for the first mode, 0.711 for the second mode, and asymptotically approaches 2 as the mode number tends to infinity. For both hinged and clamped supports, the radiation resistance of all modes increases with the beam width‐to‐length ratio.

ISSN:0001-4966    

DOI:10.1121/1.1912942

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The radiation resistance corresponding to the natural modes of a finite rectangular panel is theoretically determined from the total energy radiated to the farfield. The panel is assumed to be simply supported in an infinite baffle. Asymptotic solutions for the low‐frequency region are derived, and curves covering the entire frequency range for various mode shapes and aspect ratios are obtained through numerical integration. When the ratio of the acoustic wavenumber to the panel wavenumber is a constant much less than unity, the radiation resistance for all modes is a minimum if the intranodal area (the area between adjacent node lines) is square, and increases with the aspect ratio of the intranodal area.

ISSN:0001-4966    

DOI:10.1121/1.1912943

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

A transducer for the generation of high directional ultrasonic radiation in fluids is studied. It consists of a plate of particular shape set in oscillation by an adequate vibrator. Since the directional pattern depends substantially on the phase with which the single elements of the plate vibrate, the radiating surface is shaped into steps covering areas vibrating with the same phase, the height of the steps being equal to a half‐wavelength of the radiated sound, in order to obtain a coherent acoustic radiation. To facilitate the design of such plates, an approximate theory of their vibration is shown, and the experimental proofs, whose results confirm the full validity of the adopted method, are described.

ISSN:0001-4966    

DOI:10.1121/1.1912944

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP