摘要:

It was shown in a paper by Morgan [J. Appl. Phys.29, 1358–1368 (1958)] that there exist an infinite number of spherically symmetric, perfect‐focusing lenses of which the Luneburg lens is but one example. Using the Helmholtz formulation of diffraction theory, we examine the farfield radiation characteristics of these lenses for both the case of an omnidirectional source and a particular directional source.

ISSN:0001-4966    

DOI:10.1121/1.1911380

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

It was shown in Part I [J. Acoust. Soc. Amer.43, 709–715 (1968)] that the wave equation for an acoustic Luneburg lens is∇2p − (1/ρ)∇ρ⋅∇p = (nL2/c02)∂2p/∂f2, wherepis the pressure, ρ the density,c0the constant speed of sound in water andnL2 = 2−r2/a2Luneburg's index of refraction. Hereris the radial distance measured from the center of the lens andais the radius of the lens. Also, we have in general thatnL2(r) = c02ρ(r)κ(r), where κ(r) is the compressibility. In Part I, the solution of the wave equation was obtained for plane‐wave irradiation when ρ is constant. In this paper, we first obtain the solution to the wave equation for plane‐wave irradiation whenρ(r) = nL2(r)/c02κ, κ being constant. Then the general case is examined when ρ(r) is an arbitrary function of position. It is found that at low frequencies, the speed of sound is affected by the presence of ∇ρ. while at high frequencies, this effect is negligible. Consequently, a completely arbitrary choice of ρ(r), even though κ(r) is chosen so thatρ(r)κ(r) − nL2/c02, could destroy the perfect focusing property (no spherical aberration) of the lens at low frequency. Therefore, conditions are established for ρ(r) so that the lens will retain its perfect focusing property at all frequencies.

ISSN:0001-4966    

DOI:10.1121/1.1911381

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

A solution, valid at high frequencies, for the sound field in shallow water having a negative velocity gradient is obtained by normal‐mode theory. Using this solution, relatively simple expressions for the decay of intensity over intermediate and long ranges can be obtained if it is assumed that propagation is basically governed by the mode with phase velocity equal to the surface sound velocity. The transmission loss as a function of range follows a three‐halves‐power law at intermediate ranges and a cylindrical spreading law at long ranges. In both regions, attenuation due to bottom losses is assumed to be present, and it is shown that the value of this attenuation is approximately the same as that derived by a ray‐theory model. In addition, the results of this analysis are in accord with other semiempirical predictions for the transmission loss in shallow water.

ISSN:0001-4966    

DOI:10.1121/1.1911382

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

Two parallel infinite plates are proposed for the dome study. The first plate, simulating the dome plate, is in general, arbitrarily reinforced. The second plate, simulating the hydrophone‐array plane, is assumed to have a given spectral impedance. The received pressure at a hydrophone and an array of hydrophones is obtained for an incident plane wave going through the dome plate that is also excited by the turbulent pressure. Noises produced by the dome plate and supports are found to be controllable by (a) radiation, (b) amplitude, (c) frequency, and (d) directivity separately or jointly.

ISSN:0001-4966    

DOI:10.1121/1.1911383

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

Measurements have been made on the critical pressurepcrequired to cause gas flow from a capillary into liquid in the presence of sound. It is found thatpcincreases as the square of the sonic amplitude. Results are explained in terms of acoustic radiation pressure exerted on the meniscus. Specifically, it is deduced that radial vibration of an emerging hemispherical meniscus with average (peak) displacement amplitude ξ0causes it to be subjected to the excess time‐averaged pressure(ρ0ω2ξ02/4); here, ω is the angular frequency and ρ0the liquid density.

ISSN:0001-4966    

DOI:10.1121/1.1911384

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

As a random process, the temperature microstructure in water exhibits both time and spatial correlation characteristics. Thus, acoustic signals propagated along diverging or converging paths should indicate some degree of cross correlation due to the temperature microstructure. This dependence is investigated in a water tank with a controlled temperature microstructure. The angular separation between paths is varied and cross‐correlation functions are computed for the various separations. The measurements were made using pulsed signals at 500 kHz. The correlation between the received signals decreases as the angular separation between the paths is increased.

ISSN:0001-4966    

DOI:10.1121/1.1911385

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

The sound generated aerodynamically by a relatively smooth disk rotating in its own plane was measured in an anechoic room. The sound field was broad band owing to random, acoustic, dipole sources caused by the turbulent‐boundary‐layer flow on the disk. The low‐frequency part of the spectrum was due to a uniform distribution of pressure dipoles over the disk surface. The separated flow from the disk periphery had a well‐defined periodic vortex development and was the cause of the low‐frequency sound. The high‐frequency part of the spectrum was due to a distribution of pressure dipoles very near the disk periphery confirming Powell's [J. Acoust. Soc. Amer.32, 982–990 (1960)] prediction of edge noise. When the surface roughness was one‐tenth the boundary‐layer thickness, it was found that significant high‐frequency sound was generated at the point of roughness, indicating that apparent shear‐stress dipoles can be important sound sources. The sound‐power level was found to be 70.5 Db for the relatively smooth disk.

ISSN:0001-4966    

DOI:10.1121/1.1911386

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

DICANNE(Digital Interference Cancelling Adaptive Null Network Equipment) is a real‐time digital processor that is designed to reject plane‐wave interference from an operator‐controlled direction prior to normal beam forming. The description includes computed characteristics of the specific array geometry selected and a functional outline of the digital processing equipment.

ISSN:0001-4966    

DOI:10.1121/1.1911387

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

Analysis is presented for the enhancement of a plane‐wave signal at an acoustic array relative to noise of which a part is also a plane wave from another direction. The process considered (described in the preceding paper) employs conventional beam forming to estimate the interfering plane‐wave form, which is then removed at each element by subtraction. Removal of interference by more than one plane wave, and the relation to optimum beam forming, are briefly considered.

ISSN:0001-4966    

DOI:10.1121/1.1911388

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

Masked thresholds for spondees were measured during 37 binaural listening conditions covering homophasic, antiphasic, parallel time‐delayed, and opposed time‐delayed listening in the presence of one to four competing maskers at three nominal masker levels (65, 75, and 85 dB SPL). Two of the maskers used were white noise modulated 4 times per second by 10 dB with a 50% duty cycle; the modulation patterns were such that when the noises were combined they produced a continuous white noise. The other two were whole sentences spoken by different male talkers. Masking‐level differences (MLD's)rehomophasic performance in the same masker complex emerged for every one of the 28 conditions of dichotic listening. MLD's were slightly smaller in the presence of a single competing talker (with or without noise also present) than with two talkers, and MLD's were always largest for the antiphasic conditions (range 4.4–6.8 dB). MLD's for parallel delayed listening (entire masking complex given 0.8‐msec lead to a single ear) averaged about 1 dB less than antiphasic conditions, ranging from 3.5 to 6.6 dB. MLD's for opposed time‐delay conditions (0.8‐msec delays given to all portions of the masker complex but not all masker signals delayed to the same ear) reduced the MLD another 0.6 dB from the antiphasic conditions on the average (range 2.6–6.4 dB). Since subjective separation of competing signals was more definitive during opposed time delays than during either parallel time delays or antiphasic presentation, but since MLD's were smaller during opposed time delays than in the companion dichotic conditions, this study indicates anew that escape from masking and lateralization of signal origin, while depending on common input information, are separate capacities involving two different neurological mechanisms. These findings also suggest that listening in interaurally complex conditions brings into play phenomena that are not covered by contemporary theories of masking.

ISSN:0001-4966    

DOI:10.1121/1.1911389

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP