摘要:

We studied the free‐field of external ear canal acoustic impulse responses. The fine structure of the frequency domain characteristics was correlated with the time domain profile of the pinna reflections previously [Hiranaka and Yamasaki, J. Acoust. Soc. Am.73,291–296 (1983)]. A recent measurement with wide band facilities (up to 50 kHz) revealed the fact that the global structure of the frequency response can be correlated with such effects as the head shadow and the pinna direction in a very simple form: the logarithmic amplitude declines almost linearly with linear frequency for all directions of a sound source, and the degree of the declination varies with the direction. In the experiment, impulsive sound source was positioned at nine elevation angles (30°/step) in each of six vertical planes, and 1.5 m from a subject's head. The directional map of the declination (ranging from − 1 dB/10 kHz to − 10 dB/10 kHz) indicated the opening pattern of the pinna. The patterns of such maps from several subjects showed their physical individuality.

ISSN:0001-4966    

DOI:10.1121/1.2022098

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

In describing the effects of internal waves on acoustic normal modes excited by a time‐harmonic point source, all previous descriptions have neglected the existence of azimuthal fluctuations in the acoustic field. This study compares this azimuthally symmetric case with the case where azimuthal fluctuations in the acoustic field are taken into account. Although azimuthal variations enhance the randomization of interference effects so that different depth modes decorrelate more quickly in range, this enhancement is generally small. The mean energy flux vector (j) becomes radial at long ranges; the vertical components of (j) decay somewhat more quickly with range when azimuthal fluctuations are considered than when they are neglected. [Work supported by Office of Naval Research.]

ISSN:0001-4966    

DOI:10.1121/1.2022105

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Theoretical and computer simulation results recently obtained by Frisk and Lynch [G. V. Frisk and J. F. Lynch, J. Acoust. Soc. Am.76,205–216 (1984)] indicate that one can perform a relatively simple cw experiment which would, after processing of the data using the Hankel transform, yield a depth‐dependent Green's function which contains information about the nature of the discrete and continuous modal spectra, as well as the plane wave reflection coefficients of the waveguide boundaries. Specifically, the method consists of measuring the magnitude and phase versus range of the pressure field due to a cw point source and then numerically Hankel transforming these data to obtain the Green's function versus horizontal wavenumber. In order to test this technique in shallow water, a series of experiments was performed in May 1984 at selected sites in Buzzards Bay and Nantucket Sound. The data collected at these sites appear to be of extremely high quality, and preliminary analysis using both forward modeling of the pressure field and the inverse Hankel transform (to obtain the Green's function and thus the modal structure) indicates that the method has considerable promise. Modal interference patterns in the pressure field and modal peaks in the Green's function are clearly observed. Of particular interest is the observation of a so‐called “virtual” mode or “pseudoresonance” in the modal continuum, which we believe to be the first clear measurement of continuum structure in ocean acoustics. [Work supported by ONR.]

ISSN:0001-4966    

DOI:10.1121/1.2022110

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

An inverse procedure for complex shear modulus profile was developed based on the Yamamoto theory of poro‐elastic bed response to surface water waves. Laboratory wave tank experiments were conducted to measure the wave‐induced bed motion of sand beds and clay beds. Seismometers were used to measure the small motion of sand beds while movies were used to measure the large motion of clay beds. The complex shear modulus (rigidity and frame loss) profiles of beds were determined from the bed motion data using the inverse theory. Good agreements were observed between the inverted complex shear moduli and the complex shear moduli determined directly from the torsional resonance column tests. It is concluded that the proposed inverse method is powerful and useful for determining the bottom sediment properties in relation to geoacoustic modeling. [Work supported by ONR.]

ISSN:0001-4966    

DOI:10.1121/1.2022112

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP