摘要:

A parametric acoustic receiving array (PARRAY) uses nonlinear acoustic effects to achieve directional receiving array characteristics. The response of the PARRAY to transducer motion is significantly different from the response of conventional hydrophone arrays. In this paper, a theory relating motion of the tranducers to effects on the PARRAY output signal is developed and several models are derived. The models are useful both for evaluating the potential of the PARRAY for various applications and for specific PARRAY system design. Experiments with 5‐ and 340‐m‐long PARRAY’s are described and data which verify the models are presented. Electronic techniques of counteracting the effects of transducer motion are discussed.

ISSN:0001-4966    

DOI:10.1121/1.384311

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Simultaneous measurements have been made of sound pressure above the ground and seismic velocity below the ground surface resulting from a source suspended in the air a variable distance from the surface. The ratio of seismic velocity to acoustic sound pressure has been determined; there are peaks in the ratio in the vicinity of 45 and 90 Hz depending on the height of the speaker. The source–receiver distance was 10, 30, and 60 m; the source height was varied between 1 and 10 m. The frequency of maximum acoustic coupling was found to agree well with theory; the first and third shear modes appear to be excited. Results for vertical, horizontal, and radial motion indicate the coupled seismic signal is greatest for vertical, next greatest for radial, and least for transverse, though the difference between radial and vertical displacement velocities was not great and depended on the speaker altitude.

ISSN:0001-4966    

DOI:10.1121/1.384312

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A model is proposed to predict the amplitude of sound generated by an edgetone. The edgetone configuration under consideration consists of a flat plate located in the medial plane of a fully developed two‐dimensional jet. The flow is modeled as follows. A periodic disturbance at the jet origin leads to the formation of an asymmetric vortex street which propagates downstream with a fixed convection velocity and wavelength. The vortex strength, convection velocity, and wavelength are determined as functions of the Strouhal number by applying conservation laws and kinematic relationships. The farfield acoustic pressure is calculated from a potential flow estimate of the periodic force on the edge. Predicted sound pressures are compared with experimental results and found to be in reasonable agreement.

ISSN:0001-4966    

DOI:10.1121/1.384313

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A high vertical/lateral resolution subbottom survey using a 3‐m paraboloidal reflector operating at 12 kHz in a 1/2‐ms pulsed mode was taken at the University of New Hampshire Ocean Modeling Facility at Lake Winnepeasaukee, New Hampshire. Echograms show a vertical time series composed of several low amplitude single pulsed width signals and several high amplitude unresolvable interacting pulses. The low amplitude pulses are assumed to be reflections from discrete boundaries and the high amplitude unresolvable signal is assumed to indicate a continuous impedance function over depth. A combined discrete and continuous vertical impedance model has been used to simulate the experimental vertical time series. Facsimile graphs taken with the narrow beam show semirandomly positioned reflectors in the sediment column with high definition of subbottom features, while those taken simultaneously over the same cruise track with a wide beam show layers and loss of images. A lateral diffraction model was used to demonstrate that the narrow beam indication of semirandomly positioned reflectors is consistent with the wide beam indication of layers.

ISSN:0001-4966    

DOI:10.1121/1.384323

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The strength and diffraction parameters, Φ and Λ, are of importance in understanding fluctuations in oceanic acoustic transmission. A method for calculating Φ and Λ is described. Floquet’s theorem is shown to be of use in the calculation of Λ for a range‐independent sound‐speed profile. The sensitivity of Λ and Φ to changes in the sound‐speed profile and the buoyancy‐frequency profile is investigated. It is shown that rays with large intensity tend to have small Φ and small Λ except very near caustics.

ISSN:0001-4966    

DOI:10.1121/1.384324

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The effects of source motion on an acoustic signal received by an underwater sensor or array of sensors are examined using ray theory and time series analysis. The signal was modeled for the passage of a constant depth source over several kilometers in range to include propagation both inside an outside of a convergence zone. The effects of radial and skew (nonradial) source motions are determined and selective fading is observed when the reception of a narrow‐band multiline signal of 200 ±1/2 Hz is investigated. The frequency, temporal and spatial coherences, and power spectra are calculated for the computed time series. Results show the effect of source placement in range and depth with a deeper source (450 m) having a broader spectrum and a much smaller coherence time than a shallower (100 m) source. Coherence bandwidths were only slightly affected by source position while spatial apertures were extremely large due to the simplified propagation model. Although the results for specific transducer separations are included, arguments based on multipath behavior with range generalize those results to the 500‐to 700‐km region.

ISSN:0001-4966    

DOI:10.1121/1.384325

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The effects of periodic source micromotion on an acoustic signal are examined with micromotion in the vertical plane being emphasized. Single phone radial and skew source motions are investigated utilizing cw and narrow‐band equispaced, equal amplitude, multitonal signals of 200 ±1/2 Hz. Brief comments on array reception are included. The study shows that source micromotion does not appreciably affect the coherence bandwidth of the received signal, but introduces a fluctuation having the period of the micromotion,Tmm, which can reduce coherence times appreciably. Spatial coherence lengths are extremely large due to the simplified ray model. Micromotion transfers power from the central peak of the power spectrum to side bands separated from that central peak by ±1/TmmHz. The power in the spectral side band of the received signal is dependent on the signal level and the magnitude of the source micromotion.

ISSN:0001-4966    

DOI:10.1121/1.384326

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Geometric construction by straight edge and compass of the multipaths is developed for the symmetric (i.e., both source and receiver at the same depth) first‐convergence‐zone case for a given range with a bilinear sound‐velocity profile.

ISSN:0001-4966    

DOI:10.1121/1.384327

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The use of the Helmholtz resonator in low‐frequency underwater acoustic applications is discussed. Specific design parameters and their effect on acoustic performance are addressed. A design example is used in the analysis.

ISSN:0001-4966    

DOI:10.1121/1.384328

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The Biot theory for sound propagation in porous media has been specialized to suspensions of particles and compared with an earlier model based on scattering theory, for sound velocity and viscous attenuation in suspensions. It is shown that the Biot theory gives fairly correct predictions of attenuation as a function of concentration, whereas the suspension model fails when the concentration exceeds a few percent. The reason for this is discussed and a modification to the suspension theory is proposed. This theory has been applied to study viscous attenuation as a function of frequency and it is shown that viscous attenuation may have an almost linear dependence on frequency when account is taken of distributed grain sizes.

ISSN:0001-4966    

DOI:10.1121/1.384329

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP