摘要:

Analysis of the sonar emissions produced by five big brown bats (Eptesicus fuscus) during detection and range-discrimination tests show that each bat has a distinctive, “personal” emission. This may help bats distinguish their own emissions from those of others and thereby prevent jamming. Emissions usually contain three harmonics and are strongly frequency swept. Modeling of the sweep shape showed it to be more nearly linear in log time than hyperbolic (linear period modulation), which is optimal for Doppler tolerance. However, logarithmic time and linear period modulation are similar in their Doppler properties, and so bats may suffer little penalty for using a slightly nonoptimal sweep shape. We suggest that bats use logarithmic time modulation rather than linear period modulation in order to simplify signal processing during approach to prey, as this type of modulation stabilizes the filtering of echoes resulting from the bat’s automatic gain control. Likewise, the use of several harmonics in the emission may simplify signal processing of Doppler-shifted echoes by reducing the number of matched filters needed for reception.

ISSN:0001-4966    

DOI:10.1121/1.400660

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The effects of an adjacent fluid’s viscosity and density on the characteristics of torsional stress waves transmitted in a waveguide with a circular cross section are studied theoretically and experimentally. Expressions for the torsional wave speed, dispersion relations, and attenuation are obtained as functions of the adjacent fluid’s viscosity and density. The theoretical results are compared with experimental observations. It is demonstrated that a device similar to the one described herein can be used as a rugged, real-time, on-line sensor for measuring the viscosity of a fluid with a known density. Such a sensor can measure the viscosity of fluids with a density viscosity product(ρfμ)greater than100 kg2/m4 sto a precision of 1% or better.

ISSN:0001-4966    

DOI:10.1121/1.400541

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The planar near-field calibration array (NFCA) was developed by Trott [W. J. Trott, Underwater Sound Ref. Rep. No. 55 (1961); also J. Acoust. Soc. Am.36, 1557–1568 (1964)] for use in determining the far-field acoustic radiation from underwater transducers by measurements in the near field. As a projector, the NFCA produces a nearly uniform plane wave over a large volume in its near field and over a large frequency range. As a receiver the NFCA acts like a plane-wave filter for acoustic radiation (or target scattering) originating from within the plane-wave volume. Thus the NFCA can be used to determine the far-field performance of both receiving and projecting transducers. In addition, it can be used to insonify nearby targets with plane waves and determine the resulting scattered far-field pressure. Earlier papers concentrated on the geometrical design of NFCA’s and the computation of their relative element responses (or sensitivities) for a specified array configuration, near-field volume, and frequency range. This paper provides analytical expressions for the near-field transmitting voltage and current responses and near-field receiving voltage and current sensitivities of a planar NFCA. The paper also provides information to aid in the NFCA element selection process, especially with regard to shading of the NFCA.

ISSN:0001-4966    

DOI:10.1121/1.400542

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

To calculate the transmitting characteristicsof sonar arrays, taking into account the acoustic interactions between the transducers and the finite size of the array, a computer model has been developed. The method applies to arbitrary-shaped arrays with piston transducers. The acoustic mutual impedance matrix between the transducers is first computed, and comparison with analytical results in a special case is presented. Then, some results on arrays containing 8 and 32 transducers are shown. For a densely packed array of circular pistons with diameter equal to λ/4 at the resonance, the acoustic interactions may have large effects on the pressure and velocity distributions on the surface of the array.

ISSN:0001-4966    

DOI:10.1121/1.400543

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Tapered-thickness piezoelectric ceramics have been used in novel devices and imaging techniques, such as wideband transducers and frequency-controlled acoustic-beam translation. However, further application requires an adequate theoretical model of these tapered ceramics. In this work a staircase model is developed in which the thickness gradient is represented as discrete steps. Each step is modeled as an independent, constant-thickness section which contributes to the net response. The theoretical electric input impedance, spectral content, pulse, and continuous-wave pressure profile responses obtained with the model are shown to be in close agreement with the measured response of a tapered piezoelectric transducer.

ISSN:0001-4966    

DOI:10.1121/1.400544

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

An ultrasonic Gaussian transducer with a curved back electrode has been designed and fabricated. A theoretical principle of the design based on electromagnetic theory is developed. The experimental results show that the sound fields generated by the Gaussian transducers fabricated based on the design agree well with the theoretical predictions.

ISSN:0001-4966    

DOI:10.1121/1.400661

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Measurements were made of the piezomagneticd33coefficient, the free permeability,μ33T,and the open-circuit elastic compliance coefficient,s33H,of grain-oriented Terfenol-D,Tb0.3Dy0.7Fe1.93,produced by a modified Bridgman technique. Prestress levels to 9500 psi (65 MPa) and magnetic bias fields up to 2200 Oe (175 kA/m) were applied with a laboratory electromagnetic modified so that one pole piece served as a hydraulically actuated piston. The results indicate thatd33, μ33T,ands33Hare dependent on stress and magnetic field, so that proper mechanical prestress and magnetic bias conditions are critical to the successful use of Terfenol-D in transducers and actuators. Radiated-power limits for underwater acoustic projectors are estimated and compared with those for projectors having lead zirconate titanate (PZT-4) piezoelectric drivers. Terfenol-D based transducers can be as much as 8 dB superior to PZT-4 under low-Q, field-limited conditions (Q=mechanical quality factor).

ISSN:0001-4966    

DOI:10.1121/1.400678

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Recently, a set of high eigenfrequencies of small aluminum blocks have been measured and analyzed quantitatively by using methods initially developed in nuclear physics on the basis of an analogy with random matrix theory [R. L. Weaver, J. Acoust. Soc. Am.85, 1001 (1989)]. At the foundation of the application of random matrix theory is the (classical motion) ⇔ (finite frequency vibration) correspondence according to which the nature of classical geometrical acoustic trajectories determine the correspondence with random matrix theory and thereby the structure of the spectrum (classical chaos→GOE spectrum; regular motion→Poissonian spectrum). Although the classical geometrical acoustics trajectories are not chaotic in the system studied by Weaver, he finds a good agreement with the GOE prediction, in contradiction to the usual wisdom concerning the classical-vibration correspondence. It is suggested that this paradox stems from finite wavelength effects that introduce a coarse graining in the relevant classical dynamics. Interpreted in this way, Weaver’s results seem to confirm that the GOE spectrum initially studied for simple scalar Helmholtz equation may also be valid in the more complicated case of elastodynamics.

ISSN:0001-4966    

DOI:10.1121/1.400662

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Acoustic pulse distortion in liquids exhibiting a continuous distribution of Maxwell relaxation processes is analyzed by means of linear systems theory. The causality condition for the phenomenological model is noted. Distortion quantities relating the pulse shape to the absorption and dispersion are presented. It is shown that these quantities can be expressed in terms of moments of the relaxation time spectrum.

ISSN:0001-4966    

DOI:10.1121/1.400663

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Some clarifying remarks regarding earlier work cited in the subject paper are given.

ISSN:0001-4966    

DOI:10.1121/1.400966

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP