摘要:

The local variations of acoustical‐field quantities, in the nearfield of an ultrasonic transducer, influence most calibration techniques including the optical methods. A mathematical formulation is presented which accounts for these local variations and, in fact, allows the mapping of these quantities from conventional ultrasonic light‐diffraction measurements. This analysis requires acquiring considerable amount of data and reducing it in the spatial Fourier domain for mapping pressure, particle velocity, and intensity distributions. However, simpler procedures are developed for the determination of radiated power and intensity along the primary beam. An important attribute of this technique is that modeling of the field is not necessary; optical measurements can be taken as close to the transducer as is convenient, and the method is not restricted to transducers with simple geometries. However, theory is developed within the constraints of linear acoustics.

ISSN:0001-4966    

DOI:10.1121/1.381447

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Phased‐array echosonographic systems, currently in use in medical diagnosis, employ pulses that are a few wavelengths long at the center frequency. Design of such systems requires that arrays be composed of as few elements as are consistent with acceptable resolution and low side lobes. This paper outlines the development of a model for the field of an idealized phased array. The model is an extension of elementary array theory which includes pulse parameters and beam steering. It is specialized for the generation of simple and useful directivity patterns that are applicable at point focal regions and in the farfield. The directivity‐pattern function provides the basis for a method of designing linear arrays by nonuniform element spacing. Arrays designed by this method offer advantages over uniform linear arrays because of the smaller number of elements required for given azimuthal resolution and side‐lobe level, and because of reduced element interaction.

ISSN:0001-4966    

DOI:10.1121/1.381448

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The relative sound velocities (u−u0) of aqueous HCL, LiCL, NaF, NaCL, NaBr, NaI, NaNO3, NaOH, NaHCO3, Na2SO4, KF, KCL, KBr, KI, KNO3, K2CO3, K2SO4, RbCl, CsCl, NH4Cl, NH4Br, MgCl2, CaCl2, SrCl2, BaCl2and MgSO4solutions were measured from 0.01 to 1.0 at 25°C. The sound velocities were fitted to a function of molality (maximum standard deviation 0.05 m sec−1)u=u0+Am+Bm(3/2)+Cm2, whereu0is the sound velocity in pure water, andA,B, andCare temperature dependent parameters. The adiabatic compressibilities βSwere determined from the sound velocities and were used to calculate the adiabatic apparent molal compressibilities φK(S)of the salts. The φKand βSof seawater were estimated by using Young’s rule. The calculated values of φKand βSagree with those determined from the measurements on seawater solution to ±0.18×10−4cm3mol−1bar−1and ±0.004×10−6bar−1. The sound speeds of seawater calculated from the values of βSestimated from Young’s rule agree on the average to ±0.02m sec−1(maximum standard deviations 0.07 m sec−1) with the direct measurements of Millero and Kubinski.

ISSN:0001-4966    

DOI:10.1121/1.381449

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A circular tube will support axially sysmetric surface acoustic waves at both surfaces, and also a pair of axially symmetric platelike modes. The two pairs of modes are analogous to similar mode pairs on a flat plate. The relationships between surface and plate modes are well known for the flat‐plate geometry. It is shown here that for the tube geometry, the mode relationships are of a different kind. Whereas the high‐frequency surface modes on a flat plate are superpositions of the two lowest Lamb modes, the high‐frequency surface modes on a tube are in general not superpositions at all. On a tube, each surface mode is the high‐frequency representative of a single platelike tube mode.

ISSN:0001-4966    

DOI:10.1121/1.381450

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The vibration of a simply supported rectangular plate with a discretely attached mass and fluid radiation loading is considered. The equations of motion are solved by utilizing theinvacuoeigenfunctions of a homogeneous panel as the basis for Fourier decomposition of both the fluid loading and attached mass loading. A modal matrix equation is established for the interactive system which includes the cross‐modal coupling induced by the discrete mass and acoustic fluid. The solution is exemplified by the numerical investigation of a centrally excited plate with a concentrated mass at center and off‐center locations. The influence of mass attachment upon both the plate response and radiated pressure is illustrated.

ISSN:0001-4966    

DOI:10.1121/1.381451

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The theory describing power flow between two sets of linearly coupled oscillators is re‐examined for the case of arbitrary coupling strength. For simplicity, gyroscopic coupling is assumed and the equations of motion for two linearly coupled oscillators are solved using a Fourier‐transform technique which is found to be useful to increase the ease and elegance of the derivation, as well as to retain an explicit frequency dependence in the solution. Some general assumptions are made on the shape of the Fourier transforms of the mechanical impedances, input forces, and the form of statistical independence. It is found that, as expected, the modal coupling coefficients are functionally dependent upon the strength of the coupling elements. As a limiting case, the result of [Lyon and Maidanik, J. Acoust. Soc. Am. 35, 623–639 (1962)] can be obtained by frequency averaging the coupling coefficient derived in the present analysis. However, it is found tha the radiation resistance can be obtained from the same equation as give by Lyon and Maidanik for weak coupling. Th results of an experiment designed to show the effects of strong coupling are described. The results are in fair agreement with the theoretical predictions from the first part of this paper.

ISSN:0001-4966    

DOI:10.1121/1.381452

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Sound attenuation is usually obtained by means of absorbing materials. This technique, which works quite well for sounds of medium and high frequencies, is very inefficient at low frequencies, because the thickness of absorbing material necessary to produce a constant attenuation increases with decreasing frequency. There is another technique by which destructive interference is used to silence sound fields: it is active sound absorption. Hitherto, this was based on Young’s principle of interference. This paper proposes a general theory, using Huygens’ principle, for three‐dimensional sound‐wave propagation. Some experiments are also reported and discussed.

ISSN:0001-4966    

DOI:10.1121/1.381453

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Some response measures of a reverberant enclosure excited by an impulsive noise source are considered from both experimental and analytical viewpoints. The response measures of interest are the rms sound pressure and the sound‐pressure‐level exceedance probability distribution. A variety of experimental situations are examined for infrequently occurring source impulses — a situation for which an analytical approach can be readily developed. The model involves the use of an energy‐partition method in order to estimate the rms sound pressure and the use of an equivalent linear single‐degree‐of‐freedom system in order to calculate the normalized exceedance probability distribution. The calculated response statistics agree quite well with measured data. Results of these studies have applications in predicting total noise exposure, for example, when dealing with impulse noise‐control problems.

ISSN:0001-4966    

DOI:10.1121/1.381454

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Expressions giving the velocity level difference between bottom and top plates are derived. Calculated and measured values are compared. The frequency dependence of the level difference is found to be between 25 and 40 dB/decade. The slope of the curve depends on the material parameters for the floor. Radiation ratios and reduction indices for some floating‐deck constructions are also discussed.

ISSN:0001-4966    

DOI:10.1121/1.381465

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A linear hydrophone array which is towed in the ocean is subject to snakelike bending. If the array is processed as if it was truly linear, the author has shown that the bending causes a deflection of the measured bearing of a fixed source from its true bearing relative to the array. This deflection results from patterned perturbations in the true sensor positions along the nominal array axis. As the perturbation pattern changes with the flexing of the array, the source appears to move around. A probability model of the perturbations is used in order to develop a theoretical solution to the question of how the space–time information gathered by the array is best used to measure source bearing. The method which is used to reduce the bending perturbation deflection of the bearing is to group the sensors into adjacent subarrays, process these arrays over short time slices, average the subarray bearings for each time period, and then to average the average over time. This averaging method significantly improves the bearing accuracy of the source when the array is bent according to the model.

ISSN:0001-4966    

DOI:10.1121/1.381466

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP