摘要:

The study of ultrasonic propagation in liquid crystals, greatly intensified in recent years, has provided a useful contribution to the knowledge we currently have about these systems. A previous paper (Part I of a two‐part series) contains a review and analysis of the ultrasonic work done on the nematic mesophase. In the present paper, most ultrasonic experimental investigations performed to date in smectic and cholesteric compounds, and some liquid‐crystalline mixtures, are presented in a systematic manner; the results are discussed and compared, whenever possible, to the predictions of several theories (outlined in the paper) for the various mesophases and the many possible phase transitions. In the course of this analysis, several areas which need further ultrasonic studies are identified; these areas are summarized in the concluding section of the paper.

ISSN:0001-4966    

DOI:10.1121/1.381906

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Scattering of acoustic waves from silicone rubber cylinders and spheres is considered. The effect of shear is found to be negligible. Special consideration is given to cases where the specific impedance of the rubber equals that of water.

ISSN:0001-4966    

DOI:10.1121/1.381907

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Plane incidentp‐waves which propagate through continuous (possibly absorptive) media are scattered by a fluid‐filled spherical cavity contained in it. Using an approach familiar in nuclear scattering theory but novel to acoustics and elastodynamics, it is possible to express the scattering amplitudes (or the partial waves contained in them) for the scatteredp‐ands‐waves in a form which clearly exhibits their dependence on two interacting contributions, one being the broad background of an ideally soft cavity, and the other the superimposed narrow spikes due to resonances excited in the cavity fluid. The cavity appears as a perfectly soft obstacle to the incident waves at all frequencies except in the near vicinity of the cavity eigenfrequencies where there is wave penetration into the filler fluid. When this happens, the interference of this wave with the ’’potential scattering’’ of the background is seen to cause the fluctuating character of the amplitudes (or cross section). We have numerically computed these isolated contributions for a variety of material combinations. The isolated resonances are traced (as Regge pole trajectories) as they reappear at the higher frequencies in subsequent partial waves. Due to its application in the analysis ofacoustic‐coatingperformance we have further studied the case of air‐filled cavities in lossy rubber. The following findings have emerged: (1) The resonances are comparatively narrow, (2) their locations are apparently independent of the amount of absorption present, (3) absorption only affects the background, (4) shear absorptionF1only affects the mode‐converted amplitudefps, and (5) the dilatational absorption, controlled by the parameterF, only influences the nonmode converted amplitudefpp.

ISSN:0001-4966    

DOI:10.1121/1.381908

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Recent checks of published seawater sound‐speed measurements indicate that the Del Grosso–Mader data provide the best relationships with temperature and salinity, while the Wilson data have the preferred pressure dependence. Possible error sources in the original measurements are discussed. Anderson’s pressure‐dependence, based on the Wilson data, is applied to Del Grosso’s equation and to the Del Grosso‐Mader data, resulting in three new equations. The third and simplest equation seems preferable for most applications. The new equations agree well with Anderson in comparison with acoustically determined convergence‐zone ranges at sea.

ISSN:0001-4966    

DOI:10.1121/1.381909

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

A numerical method using Kirchhoff’s retarded potential integral equation to determine the time‐dependent pressure distribution on the surface of an arbitrary smooth, convex, and axisymmetric rigid scattering body due to the impact of a plane acoustic wave having discontinuous wave front incident from any direction is developed and presented. This numerical technique is developed for general purposes, and it is applicable not only to single structures of same shape but also to composite structures of different shapes such as circular cylindrical, spherical, conical, and toroidal segments as well as flat plate. Typical numerical solutions for a hemisphere‐capped cylinder impinged upon by a unit‐step‐pressure pulse from three different directions, i.e., axial incidence, side‐on incidence, or oblique incidence of 30° off the longitudinal axis of the cylindrical section are presented. Numerical solutions of transient scattering of acoustic waves of normal incidence by a sphere are also given, and results agree very well with the analytical solutions obtained by other investigators.

ISSN:0001-4966    

DOI:10.1121/1.381910

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

We have demonstrated that coherence theory can be applied to the practical computation problem of the propagation of a beam in an infinite half‐space with stratified but arbitrarily varying speed of sound variations. The intensity distributions found in this manner are the same as those of an equivalent point source in regions removed from the caustics of the geometrical ray family, but the intensity formula is continuous everywhere in the field and may be used in the vicinity of caustics. The computer time and storage requirements are equivalent to those of a ray program.

ISSN:0001-4966    

DOI:10.1121/1.381911

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

The theory of the acoustic radiation pressure on an elastic sphere, placed freely in an inviscid fluid in a field of plane progressive sound waves, is modified to include the effect of a hysteresis type of absorption of shear and compressional waves in the solid material of the sphere. This type of absorption is typical of some types of polymer materials. Calculated results are presented for polymethylmethacrylate lucite and polyethylene spheres in water forkavalues between 0 and 24. The results are of interest in view of the use of radiation pressure measurements to determine sound intensity.

ISSN:0001-4966    

DOI:10.1121/1.381912

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Based on a fully electrodynamic Lagrangian theory of elastic dielectrics, a completely deductive derivation of the dynamical equations and constitutive relations for elastic, electric, and electroelastic phenomena is presented. The equations apply to crystals of arbitrary symmetry, structural complexity, and nonlinearity. These include pyroelectrics as well as dielectrics and piezoelectrics. Emphasis is placed on the lowest order nonlinearities, that is, ones depending either bilinearly or quadratically on the elastic and electric variables. A relation between the electrostriction tensor and the low‐frequency limit of the elasto‐optic tensor, different from any previous relation in the literature, is derived and discussed.

ISSN:0001-4966    

DOI:10.1121/1.381913

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Mach‐2.5 (0.30 caliber) projectiles were used to simulate supersonic aircraft; the acoustic impedance mismatch was provided by either of two rectangular nozzles having exit Mach numbers of 1.00 and 1.82 when delivering correctly expanded helium jets. These nozzles were operated from 40%–150% expansion at a fixed position relative to the presure transducer when the projectile miss distance was 36 or 108 body diameters from the transducer. Schlieren optical photographs demonstrated that the bow shock was always folded or ’’wrinkled’’ after passing through either of the jets. As exit Mach number decreased, the size of the shock‐front folds, as well as the divergent jet spread angle, increase and the bow shock becomes increasingly bent forward by the jet. Pressure signatures measured with a jet operating started 40–80 μs earlier than those measured without a jet. All pressure signatures obtained with a jet operating exhibited similar varieties of waveforms as F‐104, B‐58, XB‐70, SR‐71, and Concorde 002 flight test results. The wide variations in overpressure, rise time, duration, and wave shape are attributed to the velocity and density fluctuations of the turbulent jets.

ISSN:0001-4966    

DOI:10.1121/1.381914

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

The noise‐producing region of a suppressed turbojet exhaust is studied by cross‐correlating static pressure fluctuations within the exhaust with farfield sound for Mach numbers up toMj=0.99, using a 31‐tube nozzle having an area ratioAr=3.1. Measurements made with an unsuppressed turbojet exhaust having an equivalent exit area and operating under effectively equal thrust levels serve as the experimental control for this work. Static pressure‐level measurements, made with a calibrated high‐temperature acoustically damped probe tube, show that noise suppression by multitube nozzles results from reduced turbulence levels. The maximum fluctuating static‐pressure level in the unsuppressed turbojet exhaust is typically 5–6 dB higher than static‐pressure levels in the suppressed exhaust under conditions of effectively equal static thrust. This suggests that the turbulence intensity in the multitube suppressor flow is reduced in excess of 20% compared with the unsuppressed jet exhaust. Static pressure fluctuation spectrums in the suppressor flow are peaked at the eddy passage frequency and a double spectral peak is found in the region where the suppressor jets coalesce. Estimates of the eddy‐scale‐length to jet‐diameter ratio made from measurements of local jet velocity and eddy passage frequency indicate that a reduction in eddy size occurs when neighboring suppressor tube flows are brought into close proximity to one another. Maximum normalized cross‐correlation coefficients in the unsuppressed turbojet exhaust of approximatelycPF?0.2 are found forMj=0.99 at an angle of 30° from the jet axis. Normalized cross‐correlation coefficients are significantly lower in the interior of the multitube suppressor flow. Measurements of the partially normalized cross‐correlation function show that sound radiated from within the hot, high‐speed suppressor flow is refracted by the flow from neighboring jets. This work demonstrates that the high‐frequency sound radiated directly to the farfield by a multitube suppressor nozzle is primarily from the exterior tubes and that the overall suppression of noise can be attributed to a reduction in turbulence intensity.

ISSN:0001-4966    

DOI:10.1121/1.381915

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP