摘要:

The focusing properties of liquid‐filled cylindrical acoustic lenses are investigated theoretically and experimentally for lens diameter to wavelength (d/λ) ratios greater than 15. Geometrical aberration theory is used to determine the optimum index of refraction of the lens fluid and the maximumd/λ at which acceptable focusing in the Strehl sense will be maintained. A mathematical model that includes the effects of the lens shell is described and used to predict theoretical directivity patterns. A cylindrical lens 40 cm in diameter and 4 cm in height was constructed and tested at frequencies ranging from 25 to 900 kc/sec. Experimental directivity patterns are presented and compared with those predicted theoretically. The results of this investigation indicate that cylindrical lenses are practical atd/λ values up to 138, which is a factor of 10 greater than previously reported.

ISSN:0001-4966    

DOI:10.1121/1.1910866

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

Volume‐reverberation tests were conducted with a vertically directional transducer at a frequency of 15.4 kHz. For each of two source levels, the system was capable of transmitting single frequency (SF) pulses of 10‐, 50‐, 100‐, 300‐, and 1000‐msec duration and 1000‐msec frequency‐modulated (FM) pulses swept 100, 200, and 400 cycles. The relationship between volume reverberation level and pulse length is investigated. The data show a less than linear dependence of volume reverberation on pulse length and a linear dependence on source level. Reverberation levels for FM and SF pulses of 1000‐msec duration are discussed. Although the amplitudes of the deviations from the mean reverberation are lower in the FM mode than the SF mode, no significant difference in mean levels is observed.

ISSN:0001-4966    

DOI:10.1121/1.1910867

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

Decay constants and pulsation frequencies of nitrogen bubbles (2–20‐mm diam) in water have been determined by oscilloscopephotography of the transduced and amplified sound pulses emitted by bubbles formed at submerged jets. The sound decay of single bubbleshas been studied over the pulsation frequency range of 0.3–3.0 kHz, and a technique of time‐exposure photography has been developed wherebydifferences in the decay patterns between individual bubbles can be smoothed by the use of an ensemble of consecutively formed bubbles of the same size. Withinexperimental error, the decay constants and pulsation frequencies have been found to agree with those predicted theoretically by linear pulsationtheory.

ISSN:0001-4966    

DOI:10.1121/1.1910868

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

The large rise in permittivity experienced by transducer ceramics when subjected to stress parallel to the polar axis can reduce the utility of these materials. Measurements are described on PZT‐4 and PZT‐8, representative of ceramics useful for high‐power high‐stress applications, for which a stabilizing heat treatment has reduced the change in permittivity. It is shown that the stabilizing effect is permanent. Several stress cycles to 10 or 20 kpsi also stabilize these ceramics. It is shown that the stabilization with stress for stress cycles to 10 kpsi is not permanent. Measurements were made over an aging range from one day to six months.

ISSN:0001-4966    

DOI:10.1121/1.1910869

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

Measurements are reported of the change in permittivity, loss, and the piezoelectric constants,d31andd32, for piezoelectric ceramic transducer materials subjected to high stress perpendicular to the polar axis (T1) Ceramics useful for high‐power applications (PZT‐4 and PZT‐8) and some donor‐doped ceramics of high sensitivity but with high losses that make such applications unwise (PZT‐5A and PZT‐5H) were tested. Permittivity generally decreased with stress,T1, opposite to results with parallel stress,T3. The piezo‐electric coefficientd31(in the direction of stress application) decreases markedly with stress, whiled32(in the direction perpendicular to the stress and the polarization) generally rises. The effects are less severe for the “hard” ceramics. A permanent change in pseudocrystal symmetry from ∝mto 2mmis observed for the donor‐doped ceramics, even at relatively low stress levels (5 kpsi).

ISSN:0001-4966    

DOI:10.1121/1.1910870

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

Formulas are given that permit the calculation of the radial velocity at the surface of a spherical radiator, such that a prescribed pressure distribution will be achieved over the surface of any sphere, concentric with, and exterior to, the radiator. Formulas valid in the nearfield and the farfield are given. Both symmetrical and unsymmetrical pressure distributions are considered. A special class of symmetrical pressure distributions described by Chebyshev polynomials are discussed in detail. In particular, it is shown how these can be used to independently specify beam width and lobe suppression. Formulas and curves are also given for their directivity indices. A series of numerical cases were selected in which the beam width ranged from 10° through 90° and the side‐lobe suppressions ranged from 10 through 40 dB. The distribution (continuous) of radiator surface velocity required to produce each of these was calculated on a high‐speed digital computer. In addition, one of these continuous velocity distributions was approximated by discrete sets of radiating pistons having different sizes, and the pressure fields due to these arrays were compared to the pressure field that was originally specified. Some of the piston arrays reproduced the specified pressure pattern quite well.

ISSN:0001-4966    

DOI:10.1121/1.1910871

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

A generalized WKB‐type approximation is applied to sound propagation in the split‐beam sound field to investigate diffraction effects associated with turning‐point regions where ordinary WKB or geometrical‐optics approximations fail. Transmission and reflection coefficients are obtained. In some regions where asymptotic expansions for the generalized WKB solutions are valid, ray representations can be developed for some of the diffraction effects. Limitations on the validity of these ray representations and the extent of the regions (including the geometric shadow) in which diffracted rays may be introduced are discussed. Simple geometric schemes can, under certain conditions, be introduced relating particular diffracted rays to corresponding geometrical rays.

ISSN:0001-4966    

DOI:10.1121/1.1910872

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

Ray‐theory expressions for the range, travel time, and intensity are developed for a newly formulated sound‐velocity profile model in which the velocity is expressed as a series in integral and half integral powers of ocean depth. By the process of series inversion, any analytic function for which the first and second depth derivatives do not vanish simultaneously can be cast into this model. The series inversion process is applied to several elementary velocity functions, demonstrating that the general function solutions reduce to ray theory solutions obtainable by direct methods. Procedures and numerical examples for fitting experimental underwater‐sound‐velocity data to the model are presented. Acoustic artifacts, such as artificial layering effects, can be minimized because each layer may contain an arbitrary number of parameters and because the velocity and an arbitrary number of derivatives may be matched at layer interfaces.

ISSN:0001-4966    

DOI:10.1121/1.1910873

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

Less TTS was measured at 1400 Hz following a 2 min exposure to 1000 Hz at 110 dB SPL when, simultaneously with the exposure, the listeners were required to trace threshold for 250 Hz in the nontest ear than when the listeners were passively exposed to the intense signal during this interval.

ISSN:0001-4966    

DOI:10.1121/1.1910874

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

The use of the single dBA level as a hearing‐damage criterion was stated by Flanagan and Guttman [J. Acoust. Soc. Am.36, 1654–1658 1964)] to be inapplicable. It is shown by consideration of a number of industrial noise spectra that dBA levels can be satisfactorily used in practice. A level of 81 dBA or less is safe, and over 88 dBA dangerous; intermediate levels need checking by analysis.

ISSN:0001-4966    

DOI:10.1121/1.1910875

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP