摘要:

Phase reversal sound barriers, composed of periodic slotted waveguides, have shown promising results in low‐frequency noise control for plane waves normal to the obstacle, when compared with solid barriers. The phase reversal effect results in destructive interferences in the shadow zone of the barrier. In order to introduce this new phase‐controlling device in real conditions (obliquely incident ground reflections, variable source position, etc.), it was necessary to investigate its performance at any angle of incidence. This complementary experimental study, performed in an anechoic chamber, is a verification of the previous theoretical [Mongeauetal., J. Acoust. Soc. Am.80, 665–671 (1986)] and experimental [Chvojkaetal., J. Acoust. Can.13(1985)] studies. The results show that, in a measuring plane normal to the slots (waveguide entrances), the performance of this new type of barrier remains quite unchanged at any oblique angle of incidence within −60 ° to +60 °. On the other hand, the studies have clearly shown that this performance is much more angle dependent in a measuring plane parallel to the slots.

ISSN:0001-4966    

DOI:10.1121/1.394939

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

The effect of refraction caused by a sound‐speed gradient at the surface of a diffracting obstacle is investigated by examining both exact and asymptotic solutions for the sound field behind a rigid cylinder. Depending on the sign of the gradient, the curvature due to refraction over the surface of the obstacle either adds or subtracts from the geometric surface curvature and thereby alters the shadow zone of the obstacle. Results of exact and asymptotic solutions are found to display similar trends. Implications regarding improved acoustic performance of barriers are discussed.

ISSN:0001-4966    

DOI:10.1121/1.394940

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

Diffraction patterns characteristic of transverse cusps are known to be observable in light reflected from curved surfaces or scattered from liquid drops. It is anticipated that transverse cusps may be produced when high‐frequency sound is reflected from (or radiated by) certain curved surfaces or is refracted by inhomogeneities. An explicit description is given of a wave which propagates to produce a transverse cusp; the amplitude in thexyplane is exp[ik(g−ct)] withg=a1x2+a2y2x+a3y2,a2≠0. Propagation of this wave in a homogeneous medium is shown to yield a shear‐free transverse cusped caustic which locates a transition in the number of rays which contribute to the amplitude. The Fresnel approximation of the two‐dimensional diffraction integral is evaluated. The diffracted wave field is proportional to the Pearcey functionP(X,Y) or toP*(X,Y), depending on the sign ofa1+(2z)−1, wherezis the distance from thexyplane. The real parametersX,Ydepend on theaj,z,k, and the transverse coordinates in the observation plane. The stationary‐phase points for the diffraction integral are discussed. The problem considered is distinct from that of acoustical longitudinal cusps which unfold along the propagation direction.

ISSN:0001-4966    

DOI:10.1121/1.394941

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

The propagation of a one‐dimensional sound wave within a nondiffusive, chemically reacting, ideal gas is considered. The particular case of a dissociating diatomic gas is analyzed, though the general procedure may be applied to more complex gas mixtures. The appropriate dispersion equation is derived from fundamental considerations and expressions for the sound speed and the absorption coefficient as a function of frequency are presented. The low‐frequency (equilibrium) and high‐frequency (frozen) limits are evaluated. Results are presented for oxygen for temperatures of 2000–6000 K and pressures of 10, 100, and 1000 kPa. The calculations for oxygen indicate that absorption due to chemical reaction can be quite strong, particularly in the high‐frequency limit at high temperatures and pressures.

ISSN:0001-4966    

DOI:10.1121/1.394942

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

A uniform asymptotic expansion in the frequency domain is derived for the Green’s function of the two‐dimensional acoustic equation. The expansion is uniform in that it is valid near the source region. It is not valid for caustics, which can arise due to rapid changes in the gradients of the material parameters, the density, or the bulk modulus. The Green’s function which is obtained describes only the body wave acoustic arrivals in a smoothly varying whole space. Other wave types, such as surface waves or critically refracted (head) waves, are not included in this expansion.

ISSN:0001-4966    

DOI:10.1121/1.394943

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

Signal transmission loss and spatial coherence data for source–receiver separations between 100 and 250 km were acquired in the Gulf of Mexico with a calibrated seismic‐streamer measurement system at 400‐m depth, a towed projector at 100‐m depth which emitted 67‐ and 173‐Hz tones, and a moored Webb sound source at 988‐m depth driven at 175 Hz. Range‐dependent bathymetry and sound velocity profiles and other environmental data were measured. The 67‐Hz data showed a persistent sound transmission with a mean of measured range‐averaged loss values (corrected for cylindrical spreading) of 41 dB ranging between 37 and 45 dB; the 173‐Hz data showed several pronounced transmission loss minima with a mean measured range‐averaged loss value of 51 dB ranging between 41 and 60 dB as well as a rapid increase in loss over the slope at ranges greater than 225 km and water depths less than 1.2 km. Slope enhancements were found to be on the order 2–4 dB at 67 Hz and 6 dB at 173 Hz when compared to flat bottom calculations. Pair wise coherence data showed the effect of signal‐to‐noise ratio variations due to multipath interference. Estimates of signal coherence length from the coherent summation of streamer hydrophones yielded coherence lengths ranging between 70 m (8λ) and 300 m (35λ) with an average of 181 m (20λ) at a frequency of 173 Hz (λ=8.67 m). Fast asymptotic coherent and normal mode transmission loss calculations produced results in qualitative agreement with measured data for the deep flat portion of the measurement track when measured geoacoustic profiles or the derived bottom loss curves were used. The results of implicit finite difference parabolic equation calculations were consistent with range‐averaged data for the flat portion of the track as well as on the slope. These results show that if proper descriptions of the subbottom velocity profiles are used, then computations employing either parabolic equation or normal mode techniques provide qualitative agreement with experimental results.

ISSN:0001-4966    

DOI:10.1121/1.394944

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

Measurements of propagation loss were obtained in an experiment to study downslope acoustic propagation over the continental slope off the Canadian west coast. The propagation was strongly influenced by the bathymetry and a downslope enhancement was observed at a receiver in the deep sound channel for shallow explosive charges deployed over the continental slope. The maximum enhancement was observed for sources near the edge of the continental shelf at a range of about 110 km, where conversion to low‐loss water column propagation paths by bottom interaction occurred at a depth approximately equal to the sound channel axis. The propagation loss measured for these sources was as much as 15 dB less than that estimated for propagation over a flat ocean bottom, and was equivalent to levels recorded for sources at only 25–30 km. The data were interpreted by examining the structure of the signals received from the charges deployed over the slope, and by using ray theory to determine the propagation paths. The range dependence of the propagation loss was modeled using a wide‐angle parabolic equation method with a realistic geoacoustic model of the environment which included sound speed, density, and attenuation profiles. The model results were in excellent agreement with the measured values over the entire frequency band of the measurements.

ISSN:0001-4966    

DOI:10.1121/1.394945

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

Some effects of shear wave processes in a multilayered ocean bottom are examined. Normal mode theory is used to calculate the stress and displacement components at receivers in and on the bottom. The effects of the seafloor on normal modes are contained in a bottom boundary condition involving the impedance derived from the plane‐wave reflection coefficient. Shear wave processes are included when the reflectivity is calculated using a solid description of the bottom. Comparisons of the range dependence of stresses and displacements calculated using fluid and solid descriptions of the seafloor indicate that shear wave processes significantly affect the phase of all acoustic field components, both on and in the bottom. Within the bottom, all acoustic component amplitudes may be 10 dB greater if the solid, instead of the fluid, seafloor description is used. On the seafloor, the only acoustic component amplitude substantially affected by shear wave processes is the vertical displacement, which may be 10 dB smaller for a solid bottom than for a fluid one.

ISSN:0001-4966    

DOI:10.1121/1.394946

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

Experimental results are presented for the downslope propagation of acoustic normal modes in a shallow water wedge with a penetrable bottom and a slope of up to 9°. A 7‐element line source was used to generate individual low‐order normal modes. To obtain single modes in a wedge it was necessary to align the line source to form an arc of a circle centered on the wedge apex. The resulting normal modes propagate with wave fronts which are also curved into arcs of circles centered on the wedge apex. These ‘‘wedge modes’’ propagate without coupling. The amplitudes, mode functions, and group velocities of the wedge modes are described by simple adiabatic normal mode theory, but the wave front curvature is not.

ISSN:0001-4966    

DOI:10.1121/1.394947

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

Experiments to measure downslope sound propagation in a shallow water wedge with a penetrable bottom have been conducted in an indoor tank. Mode filtering has been applied to the signals received on a vertical array to extract the waveforms of individual normal modes. Results show that the normal modes of the wedge propagate with wave fronts which are curved into arcs of circles centered on the wedge apex. These wedge modes propagate downslope without coupling and are therefore the true normal modes of the wedge. Conventional normal modes with vertical wave fronts show coupling effects for slopes of 2° and higher. The group velocities of the wedge modes are strongly dependent on the slope and water depth and are in agreement with adiabatic normal mode theory. Modes which are not trapped in the water depth at the source can become trapped when the water depth is sufficient to support them.

ISSN:0001-4966    

DOI:10.1121/1.394948

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP