摘要:

The transient acoustic near field in air from impacted cantilever plates of two different geometrical shapes and materials is studied using two‐reference‐beam double‐pulsed holographic interferometry. The acoustic waves, observed in two directions simultaneously, are generated by traveling flexural waves in the plates and by the impact process itself. A subsequent phase‐stepping procedure is used for quantitative evaluation of the registered two‐dimensional projections of the acoustic fields. An interesting observation is the acoustic radiation at the free end of the plate, where sound waves propagate into the air in oblique directions from the plate. At the region right above the free end no acoustic waves are observed. Supersonic flexural waves in the plates generate trace‐matched acoustic waves in the surrounding air. Calculations of the corresponding two‐dimensional transient acoustic fields are performed. Those are based on an analytical solution of the Kirchhoff plate equation. Calculated results reproduce the character of the observed transient acoustic waves quite well.

ISSN:0001-4966    

DOI:10.1121/1.414646

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

A viscoelastic model is established for the coupled motion in a thin solid rod and an infinite viscous medium surrounding it, starting from the basic laws of elastic and fluid mechanics. The effect of various material parameters of the surrounding medium on the propagation of acoustic waves along the rod is investigated using numerical calculations. It is demonstrated that, at low frequencies, the fundamental radial–axial mode (R01) responds most to changes in the material parameters of the surrounding medium related to shear motion in a manner similar to that of the fundamental torsional mode (T00). Based on this effect, a model miniature viscosity and density sensor is proposed and tested in viscous liquids. The attenuation in the device agrees well with the theoretical model and exhibits an approximately linear dependence on the square root of the product of the density and the shear viscosity of the surrounding medium. This work shows that simple traditional acoustic technology can be used to develop small size, low‐cost sensors for liquid viscosity and density measurement with the possibility of on‐line applications.

ISSN:0001-4966    

DOI:10.1121/1.414647

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The general formulation for the radiation force on a spherical object in an axisymmetric acoustic field is provided. The sphere is described in general by three parameters: its density, compressional wave speed, and shear wave speed. Other types of spheres, including the rigid and immovable sphere and the infinitely soft sphere (void), are treated as limiting cases. Specialized formulations of the radiation force function are provided for several types of incident waves of common interest. A low‐frequency expansion for each case is provided for comparison with results from the literature. Among the solutions provided are the scattering function of an elastic sphere in a focused acoustic field and the radiation force on the sphere. The radiation force function is used to calibrate high‐frequency transducers. Experimental data are provided for a focused transducer for frequencies up to 10 MHz, where the size of the elastic sphere is comparable to or larger than the −3‐dB beamwidth of the sound field.

ISSN:0001-4966    

DOI:10.1121/1.414648

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

High power drives for thermoacoustic refrigerators are being investigated by several groups. Theoretical calculations have been performed of the feasibility of parametrically driving a longitudinal resonance tube by uniformly modulating the temperature of the gas with a high‐power laser. Parametric drives are promising since the drive mechanism is distributed over the entire volume of gas and because the response may become large before saturation occurs. Although it is demonstrated that laser drive is not a practical means of high‐power drive for longitudinal resonators, much interesting physics has been considered in the analysis of the proposed system and is discussed.

ISSN:0001-4966    

DOI:10.1121/1.414672

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

A new simple scheme of an acoustical laser with mechanical pumping is suggested. In accordance with this scheme the pumping is created by a piezoelectric emitter of the piston type. The liquid with dispersed particles (gas bubbles) plays the role of an active medium. Phase bunching of the particles is realized by acoustical radiation forces. Generation conditions for this acoustical laser are evaluated. It is shown that two types of losses must be overcome for the beginning of the generation. The first type results from the energy dissipation in the active medium and the second one is caused by radiation losses at the boundaries of the resonator.

ISSN:0001-4966    

DOI:10.1121/1.414649

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

A theoretical analysis of radial wave thermoacoustic engines in cylindrical resonators is developed. Impedance and pressure translation equations are presented for open sections of the resonator and for heat exchangers. Coupled first‐order differential equations are given for pressure and impedance in the temperature gradient supporting engine section (stack). These quantities are used to calculate heat and work flows and to predict engine performance. Theory and design of a variable quality factor resonator for enhanced photoacoustic spectroscopy are presented. The short stack approximation is developed for the radial geometry and is used along with plane‐wave equations to compare refrigerator performance for these two geometries. Results of the comparison are that engines in the plane‐wave geometry are better overall refrigerators when maximizing the coefficient of performance and cooling capacity together.

ISSN:0001-4966    

DOI:10.1121/1.414650

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

An asymptotic analysis has been carried out to study the propagation of sound at long ranges in an arbitrary downward refracting sound‐speed profile. It has been shown that the solution can be expressed in a form of the Weyl–Van der Pol formula and the present theory offers an extension for the established theory to the case of a vertically stratified medium. The analysis starts from the full wave equation and the derivation assumes that the ambient properties only depend on the height above an impedance ground and that there is no wind or turbulence in the medium. In addition, the theory also assumes that the sound speed of the atmosphere increases monotonically with height. For such an atmospheric condition, the medium forms a waveguide that enhances the sound levels at long ranges along the ground surface. It is demonstrated that the acoustical path length derived by the asymptotic method is identical to that derived by the analytical ray trace model. Furthermore, the asymptotic method also allows a proper inclusion of the surface wave term rigorously.

ISSN:0001-4966    

DOI:10.1121/1.414651

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

In this paper, a very simple method is introduced for the efficient computation of the sound field in the near region above an impedance ground or an extended reaction ground. The procedure is based on complex image theory. Using Prony’s method, the slowly varying factor in the integrand of the Sommerfeld integral is approximated by a short series in terms of complex exponential functions. The result can be exactly evaluated by the Sommerfeld identity. A new closed‐form approximate expression of the sound field is obtained, which is represented as the original source, its quasistatic image, and several ‘‘images’’ with complex amplitudes and positions. Avoiding numerical evaluation of the Sommerfeld integral makes these closed‐form expressions computationally very efficient. The representation of the field with complex images makes the formula useful in measurement of ground impedance or in other cases requiring repeated mapping of the sound field. Numerical results show that these closed‐form Green’s functions are very accurate for the computation of the near field.

ISSN:0001-4966    

DOI:10.1121/1.414652

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Long‐range, low‐frequency sound propagation over varying terrain conditions is an important problem with both civilian and military applications. This work considers the modeling of cw acoustic signals as they propagate over variable‐impedance topography. The NASA implicit finite‐difference (NIFD) implementation of the parabolic approximation sound propagation model that incorporates variable‐impedance ground surfaces was used. The accuracy of this numerical model is demonstrated with a pair of test problems. An application of the results is used to predict excess attenuation of pure‐tone signals whose propagation path includes portions of a flat lake surface.

ISSN:0001-4966    

DOI:10.1121/1.414653

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

In 1987 the United States Air Force (USAF) collected the lateral distribution of sonic booms produced by 43 supersonic flights of military aircraft near Edwards Air Force Base (EAFB). These flights were accomplished using eight different aircraft at several Mach number and altitude combinations. The flights were planned for steady and level operation, but unsteadiness and maneuvering in some of the flight profiles caused variability and focusing of the propagated booms along the lateral measurement array. The full sonic boom waveforms collected during this study are included in a digital database, BoomFile, which is accessible to researchers. The database also includes the aircraft tracking and local weather profiles. A comparison of the measured to predicted peak overpressures along the lateral spread of the boom carpet demonstrates the results from this study agree as expected with previous sonic boom studies. The measurements are compared with predictions calculated by Carlson’s simplified sonic boom model and a sonic boom ray tracing model. The analysis demonstrates that, in general, the predictions are in good agreement with the measured data, but the models overestimate the overpressures as the lateral cutoff is approached. Moreover, the comparison reveals that Carlson’s method is a robust tool for predicting sonic booms produced by nonmaneuvering flights.

ISSN:0001-4966    

DOI:10.1121/1.414654

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP