摘要:

This paper shows how to process backscattered acoustic data to directly generate the image of an anomaly in a plane such as an anticline, syncline, or fault. The result is based on a physical optics farfield inverse scattering (POFFIS) identity. This identity states that a phase‐ and range‐normalized scattering amplitude is proportional to a function which is unity in the region of the anomaly and zero outside that region.Subject Classification: [43]20.30, [43]20.15; [43]40.50.

ISSN:0001-4966    

DOI:10.1121/1.381012

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

In a homogeneous, isotropic, perfectly elastic medium a scattering obstacle of finite extent is present. When a plane, time‐harmonic, elastic compressional (P) or shear (S) wave is incident on it, the extinction cross section (i.e., the sum of the scattering and the absorption cross section) of the obstacle is directly related to the far‐zone amplitude of the particle displacement of the scattered wave, observed exactly behind the obstacle. This relationship is discussed. In spite of the fact that in elastodynamic scattering the scatteredPwave as well as the scatteredswave contribute to the time‐averaged scattered power, the extinction cross section appears to depend only on the value of that farfield scattered‐wave amplitude which is of the same type (PorS) as the incident wave.Subject Classification: [43]20.15, [43]20.30.

ISSN:0001-4966    

DOI:10.1121/1.381013

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

An account on the development of a directional microphone system for jet‐noise‐source strength distribution measurements is given. The system selected is based on the imaging principle of a spherical concave reflector. As expected, the system is diffraction limited; a loss of resolution results. A special technique is discussed to recover the axial‐source strength distribution from the diffracted data. Sample experiments are presented to show the successful application of such a system for jet noise measurements.Subject Classification: [43]28.65; [43]50.55; [43]85.62.

ISSN:0001-4966    

DOI:10.1121/1.381014

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Sound propagation measurements at 10, 20, 30, 40, and 60 kHz were made in April 1974 under the ice pack near Pt. Barrow, Alaska. Absorption coefficients were calculated for each frequency and have been reported in detail [G. R. Garrisonetal., J. Acoust. Soc. Am. 58, 608–619 (1975)]. In April 1975, an attempt was made to furnish more accurate results at the lower frequencies by measuring the absorption at 7, 13, 20, 30, and 60 kHz. The 1975 measurements gave the following results for the absorption coefficients dB/kyd at a temperature of −1.6 °C and a salinity of 32%: 0.2±0.4 at 7.1 kHz; no data at 13 kHz because of transducer failure; 3.9±0.2 at 20 kHz; 7.2±0.3 at 30 kHz; and 13.1±0.5 at 60 kHz. These values indicate a relaxation frequency, due to MgSO4, of 36±3 kHz, which is slightly above our previous results, but still much lower than that predicted by the Schulkin–Marsh equation for a temperature of −1.6 °C.Subject Classification: [43]30.20.

ISSN:0001-4966    

DOI:10.1121/1.381015

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The effect of spatial separation on the cross correlation between the system response functions for a two‐receiver surface‐scatter channel is investigated. The system response at the two receivers is determined for all possible pairs of frequencies. The analysis assumes relatively close separation of receivers and uses the Fresnel‐corrected model for surface scatter in the specular direction. The surface is assumed to be Gaussian, with typical size of boundary deformations small compared to typical Fresnel zones.The analysis shows that the correlation is approximately maximum for zero‐frequency separation and drops off at a rate determined by the reverberation time. For small reverberation times and small grazing angles correlation may extend over fairly large values of frequency difference. The effect of spatial separation is generally a reduction in correlation and a more rapid decrease of correlation with frequency, but this can be compensated to some extent by time shift between the two system responses. Experimental evidence is included to support the theoretical predictions presented.Subject Classification: [43]30.20; [43]20.15; [43]60.20.

ISSN:0001-4966    

DOI:10.1121/1.381016

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Acoustic measurements of insertion loss and echo reduction at normal incidence were made for thin plates of eight materials for evaluation of their use in underwater sound transducers in the frequency range 50–500 kHz. Materials tested included Lucite; silicone rubber RTV– 560; the commercial polyurethane products PRC–1527, CPC–19, Scotchcast 8, and Scotchcast 221; and two of these materials with talc added during curing to vary the properties. Values of longitudinal sound velocity, attenuation constant, and characteristic impedance are derived from comparison of theoretical and measured characteristics.Subject Classification: [43]30.30; [43]40.55; [43]55.75; [43]85.40.

ISSN:0001-4966    

DOI:10.1121/1.381017

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Acoustic wavefront fluctuations and time‐base distortions were measured on a long linear receiving array with a 2000 ft vertical aperture. The receiving hydrophones were buoyed slightly off the ocean bottom in a roughly uniform distribution over the aperture length. Continuous acoustic transmission up to 17.6 h of a hyperbolic FM signal with a frequency sweep from 350 to 450 Hz was used in each of seven experiments at ranges of 145, 250, 495, and 635 NM to measure signal time‐base distortions. Simultaneously, wavefront fluctuations were determined from changes in arrival time across the aperture of a 10‐msec 400‐Hz pulse from the same projector. Wavefront fluctuations, rotations, and distortions were calculated and compared to signal time‐base distortions measured on a narrow beam directed toward the incoming arrivals. Both RSR (define) and completely refracted path were insonified. In the total wavefront fluctuation, the rotation part was dominant. However, remarkably small standard deviations ranging from 5 to 15 ft over the entire aperture were found for wavefront rotations and distortions. The spectra of the measured time fluctuations and wavefront rotations and distortions reveal common causes, such as tides, overtides, and internal waves.Subject Classification:[43]30.20.

ISSN:0001-4966    

DOI:10.1121/1.381018

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Wavefront fluctuations, rotations, and distortions, measured for a particular arrival in the multipath set for long‐range propagation, were cross correlated with acoustic‐signal time‐base distortions for the same arrival. Although the wavefront perturbations contain higher frequency components than for the signal time‐base fluctuations, significant cross correlation amplitudes were found for time and rotation, time and distortion, and distortion and rotation cross correlations. The results demonstrate that common causes, such as tides, overtides, and internal waves, underlie both the temporal and spatial perturbations of the acoustic signal. Although these perturbations were significantly correlated in a high percentage of the tests, the degree of correlation between them was not stable over periods of days or weeks.Subject Classification:[43]30.20, [43]60.20.

ISSN:0001-4966    

DOI:10.1121/1.381008

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

A detailed review of the experimental data for the rotational collision numberZrotfor air, O2and N2has been made to finalize a temperature correction for the absorption due to rotational relaxation for incorporation in a draft standard on air absorption being prepared by the Sl−57 Committee on Propagation.Subject Classification: [43]35.20, [43]35.35.

ISSN:0001-4966    

DOI:10.1121/1.381009

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

An approximate model for determining the response functions of reflectors of known shape has been used to provide theoretical support for experimental investigations of flaw characterization. The reflection of broad‐band ultrasonic pulses from an object can result in modifications to both the frequency spectrum and phase information in the pulses. These changes are largely controlled by the parameters of the reflector, and relating these factors has been the subject of many experimental investigations. Within the fundamental approximations, the model can readily be applied to any reflector of known shape and provides a simple interpretation of what is physically occurring. It has the advantage that additional experimental factors can be included to determine their effect on the results.Subject Classification: [43]35.26; [43]20.30; [43]35.80.

ISSN:0001-4966    

DOI:10.1121/1.381010

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP