摘要:

Time‐averaged holography has been used to quantitatively investigate sound radiation from an edge‐clamped circular flat plate mounted in an infinite rigid baffle. For a particular mode of vibration, the plate response is measured using holography and the sound power radiated is measured in a reverberant room with the plate mounted in one of the room walls. From these measurements a radiation efficiency is determined. The theoretical plate responce is calculated using both classical and Mindlin–Timoshneko plate theory and is shown to agree well with experimental measurements. Radiated sound power is calculated for each mode of interest by solving the wave equation in oblate spheroidal coordinates at the plate surface. These calculations are verified by direct evaluation of the Rayleigh integral in the farfield. Good agreement is obtained between experimentally measured radiation efficiencies and theoretical predictions. Small discrepancies between theory and experiment are discussed.Subject Classification: [43]20.55, [43]40.24, [43]35.65.

ISSN:0001-4966    

DOI:10.1121/1.381128

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

A matrix theory is developed for investigating the scattering of elastic waves in solids by an obstacle of arbitrary shape. The scattering matrix which depends only on the shape and nature of the obstacle relates the scattered field to any type of harmonic incident field. Expressions are obtained for the elements of the scattering matrix in the form of surface integrals around the boundary of the obstacle, which can be evaluated numerically. Using the principle of reciprocity and the conservation of energy, the scattering matrix is shown to be symmetric and unitary. These properties are essential to assure the accuracy of numerical calculations. Both two‐ and three‐dimensional problems are discussed, and the obstacle may be an elastic inclusion, a fluid inclusion, a cavity, or a rigid inclusion of arbitrary shape.Subject Classification: [43]20.15, [43]20.30.

ISSN:0001-4966    

DOI:10.1121/1.381129

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Upon invoking Huygen’s principle, matrix equations are obtained describing the scattering of waves by an obstacle of arbitrary shape immersed in an elastic medium. New relations are found connecting surface tractions with the divergence and curl of the displacement, and conservation laws are discussed. When mode conversion effects are arbitrarily suppressed by resetting appropriate matrix elements to zero, the equations reduce to a simultaneous description of acoustic and electromagnetic scattering by the obstacle at hand. Unification with acoustic/electromagnetics should provide useful guidelines in elasticity. Approximate numerical equality is shown to exist between certain of the scattering coefficients for hard and soft spheres. For penetrable spheres, explicit analytical results are found for the first time.Subject Classification: [43]20.15, [43]20.30.

ISSN:0001-4966    

DOI:10.1121/1.381130

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

It is well known that the beam pattern of a parametric acoustic source whose difference frequency is generated largely within the nearfield of the primary beam includes a multiplicative aperture factor that can be important at large angles when ka≳1 (kis the difference frequency wave number,ais the aperture radius). Not so well known, however, is the fact that the same aperture factor arises in the case of a spherically spreading, conical primary beam of finite initial aperture. The importance of the aperture size in determining the off‐axis behavior of parametric sources is discussed.Subject Classification: [43]25.35; [43]20.30; [43]30.75.

ISSN:0001-4966    

DOI:10.1121/1.381131

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Three sound recordings of tornadoes have been spectrally analyzed over the frequency interval between 100 and 2000 Hz. The low‐frequency analysis was limited by the response of the microphones used for the recordings; the upper frequency limit was imposed both the microphone response and the low‐signal level. One recording was made by James Cramer of a tornado which passed through Clay Center, Kansas on 25 September 1973. A second recording was made by Richard Allen Lindley of a tornado that passed through Guin, Alabama on 3 April 1974, and a third recording was made by Tom Bittman of a tornado which damaged his home in Tulsa, Oklahoma on 8 June 1974. All of the recordings are of low quality and the audio information required major spectral corrections, but the data analysis does indicate that the audio emissions from the tornadoes decrease in intensity as a function of increasing frequency. An attempt has been made to correlate the data with real (and conjectured) physical characteristics of the tornadoes and attendent atmospheric phenomena. It is clear to us from an objective analysis (and the more difficult subjective evaluation of the recordings) that identification of tornadoes based on acoustic emissions is possible . A study of tornadic sounds can not only provide a new tool for gaining insight into electrical and mechanical disturbances within a tornadic storm, but will also allow acoustic detection of a tornadic storm. A study of the change in intensity of the sounds emitted by the approaching Guin storm at both high and low frequencies suggests that noises generated by the high‐speed winds of the principal tornado vortex as its base scoured the ground might be discerned from noises generated by the winds of one or more smaller vortices moving around the tornado, and/or electrical discharges aloft.Subject Classification: [43]28.45, [43]28.65.

ISSN:0001-4966    

DOI:10.1121/1.381132

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Periodic vortex streets are formed in the wakes of blunted trailing edges on airfoils and struts. The pressures generated on the shedding struts by the vortices in these wakes are periodic in time with a frequency that is set by the shedding rate for the vortices. A simple analytical formulation is derived to relate wake‐induced pressures to the characteristics of the wake near the edge. The chordwise distribution and magnitude of the pressure is shown as a function of the circulation of shed vortices, as well as the formation distance and the spacing of the vortices in the street. Predictions from the theory are compared to some recent measurements which were obtained in the wakes downstream of different trailing edges. These measurements were made at Reynolds numbers, based on trailing edge thickness, on the order of 104to 105.Subject Classification: [43]28.65; [43]50.55.

ISSN:0001-4966    

DOI:10.1121/1.381133

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The theory of sound radiation from cylinders vibrating in resonance with vortex shedding is extended to consider the effects of vibration amplitude and mode shape. Farfield intensity and total radiated power are expressed as functions of given structural and flow parameters. Closed form solutions for intensity are obtained when cylinder vibration velocity is either much smaller than or comparable to mean flow velocity.Subject Classification: [43]28.65, [43]40.26, [43]50.55.

ISSN:0001-4966    

DOI:10.1121/1.381134

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Ultrasonic Doppler flowmeters can simultaneously obtain spatial and velocity distribution patterns of flow in a blood vessel. The factors fixing spatial and velocity resolution, however, must be determined if optimum utilization is to be realized. The parameters influencing spatial resolution have been described in the literature, but velocity resolution has received relatively little attention. This paper demonstrates analytically how optimum velocity resolution can be derived with a simple mathematical model and presents experimental data to verify theory. In addition, a ’’resolution product’’ is offered which characterizes pulsed Doppler flowmeters. This product shows explicitly the necessary compromise between position and velocity resolution for a given instrument.Subject Classification: [43]28.60, [43]28.20; [43]80.70; [43]35.80.

ISSN:0001-4966    

DOI:10.1121/1.381121

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

An experiment designed to measure normal mode amplitude functions and attenuation coefficients was conducted in shallow water on Campeche Bank off the Yucatan Peninsula. Measurements were made at two locations on the bank in water of about 30 m in depth over a bottom consisting of consolidated limestone having a measured and sound velocity of 1900 m/sec. Pulsed cw signals with frequencies of 400, 750, and 1500 Hz were used. Theoretical calculations of the mode amplitude functions using a fluid model of the bottom were found to agree well with the measurements. In order to reconcile the measured mode attenuation coefficients with theory, it was necessary to assume that the shear velocity of the bottom was 1000 m/sec. The latter is lower than the minimum sound velocity in the water column so that the generation of propagating shear waves in the bottom was the dominant attenuation mechanism. Significant differences in the measured mode attenuation coefficients at the two stations were explained by the deepening of the low velocity channel at the bottom of the water column.Subject Classification: [43]30.20, [43]30.50.

ISSN:0001-4966    

DOI:10.1121/1.381122

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The amplitude of a sonar echo from a fish depends upon the species and size of the fish, acoustic wavelength, aspect, position of the fish in the sonar beam, range and backscattering cross section. We simplify the problem to a single species and size of fish, vertically downward echo sounding, single aspect, and nonoverlapping echoes. After removal of attenuation due to range and absorption two random functions remain. The position of the fish in the sonar beam is random and the scattering cross section for each trail is random. We assume that the fish have a uniform density (number/m3) and calculate the probability density function (PDF) for insonification and reception. We assume that the PDF of the envelope of the echo (excluding the variability of insonification and reception) has a Rayleigh PDF. Assuming two PDF’s are independent, we calculate the PDF of the echo envelopeswE(e).wE(e) depends upon the beamwidth of the sonar and the mean backscattering cross section. The theoretical PDF has the same shape as the measured PDF of echoes from alewife in Lake Michigan. We use the fit of the PDF’s to estimate the backscattering cross section and fish density. This calibrates the echo‐integration processing system. A profile of the density of alewife in Lake Michigan is shown.Subject Classification: [43]30.40; [43]80.40.

ISSN:0001-4966    

DOI:10.1121/1.381123

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP