摘要:

Two elementary problems of guided elastic wave propagation—a point source of SH waves in a free solid plate, and an arbitrary distribution of torsional stress across a normal section of a solid free cylinder—are considered, and exact transient solutions derived.

ISSN:0001-4966    

DOI:10.1121/1.1908744

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

The flexural vibration of an infinite elastic beam which is excited by a localized harmonic driving force and attached over its entire length to a thin elastic plate of infinite extent is investigated theoretically. The results are expressed in terms of two parameters,kbLandr. The parameterkbLdescribes the frequency dependence and degree of beam‐plate coupling; and the parameterris the ratio of flexural wave velocity on the plate to that on the beam. Forr1, plate excitation is localized near the beam. In geometries of practical interest, the strong radiation case prevails. Small deviations in the input admittance of the beam from that of a free beam are initially obtained by employing a perturbation procedure which is valid in the limitkbL≫1, the relevant range for geometries of practical concern. In addition, a variational principle for the admittance is constructed and shown to yield the results of the first‐order perturbation solution when the displacement of a similarly excited free beam is used as a trial function. At very high frequencies the input admittance is found to approach that of a free beam, while at lower frequencies it is found that the plate adds an additional stiffness susceptance to the input admittance. For a typical beam‐plate geometry it is found that the beam is affected by the plate for all forcing frequencies below about 5000 cps and may be estimated by the results of the present calculation down to a few hundred cps.

ISSN:0001-4966    

DOI:10.1121/1.1908746

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

An infinite plate to which a narrow infinitely long beam is attached rigidly and continuously may be considered as an idealization of some typical flight vehicle structures. Reflection, transmission, and near‐field effects, associated with a straight‐crested flexural wave impinging on one side of the beam, are presented as functions of the properties of the plate and beam and interpreted in terms of trace matching of the flexural waves in the plate with flexural and torsional waves in the beam. The concepts of dynamic stress and strain concentration factors are discussed, and upper bounds on these factors are given. Some implications pertaining to the design of beam reinforced panel structures with favorable fatigue properties are pointed out.

ISSN:0001-4966    

DOI:10.1121/1.1908748

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

Formulas for the attenuation of bending waves that travel over a beam‐plate system, containing one or several beams, are derived and compared with measurements. It is found that one beam gives an attenuation which is very small near the resonances of the beam and rather high elsewhere, but in the average over wide frequency bands the attenuation appears to go nearly like the square root of frequency. The addition of beams of the same material and dimensions does not give an appreciable increase in attenuation. Formulas for the force impedance and mean‐square velocity of beam‐plate systems are also given. Finally, the damping of a beam by a plate attached to it is discussed.

ISSN:0001-4966    

DOI:10.1121/1.1908750

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

Sinusoidal vibration has practical value as a substitute for random vibration in equipment qualification testing. How and when to employ a sinusoidal substitution is discussed in this paper. In particular, a damage criterion is discussed, which relates the number of sinusoidal peaks occurring while passing through a resonance to the number of random peaks exceeding a specified amplitude value.Formulas are given for the sine‐random response relationship, with formulas and a diagram for the random test duration to be used with the amplitude relationship.

ISSN:0001-4966    

DOI:10.1121/1.1908752

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

The response of a coupled two‐degree‐of‐freedom system to white noise excitation is obtained analytically for a wide variation of the dimensionless parameters of the system. The interrelations of these parameters on the response are illustrated and shown to be very significant when the uncoupled natural frequencies of the system are of the same order. The limitations of approximation methods based on single‐degree‐of‐freedom system or uncoupled two‐degree‐of‐freedom system models are indicated. The applicability of the study to dynamic vibration absorber principles is evident. The responses are compared to previously published results.

ISSN:0001-4966    

DOI:10.1121/1.1908754

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

An experimental study of the scattering of acoustic energy from solid metal spheres in water has been carried out in the frequency range 50–150 kc and with a range of sphere sizes from 1 to 7 in. in diam, giving values ofka(acoustic radius) from 4.1 to 57. Data are presented which show the scattered pulse formation for pulses which are short compared to the sphere and for pulses which are long compared to the sphere as a function of frequency, of scattering angle, and of sphere composition. The study shows that the spheres cannot be treated as rigid bodies since an appreciable amount of energy penetrates the surface and results in a complicated echo structure for the scattered signal. The back‐scattered target strength of spheres for short pulses (and considering only the surface‐reflected pulse) is essentially constant with frequency for largeka, and is slightly less than the theoretical value for rigid spheres. For pulses which are long compared to the transit time across the sphere, the target strength fluctuates as much as 30 db for small changes in frequency. For short pulses the angular distribution of the scattered energy is fairly uniform over the back 180°, but this is not true for long pulses.

ISSN:0001-4966    

DOI:10.1121/1.1908756

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

The physical properties and dimensions that determine the equivalent noise pressures of several different shapes of piezoelectric transducers are examined. Expressions for the equivalent noise pressure, free‐field voltage response, and electroacoustical efficiency at low frequencies—those frequencies for which the free‐field voltage response is flat—are obtained in terms of generally known or easily measured parameters. The results indicate that at low frequencies a well designed hydrophone should have an equivalent noise pressure well below zero sea‐state ambient. It is emphasized that the free‐field voltage response is not an indication of the ability of a hydrophone to measure low‐level signals, but that either the efficiency or the equivalent noise pressure is an essential indication of this ability.

ISSN:0001-4966    

DOI:10.1121/1.1908758

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

A sound intensity field that is influenced by refraction of high frequency sound in a high‐frequency internal wave is compared with the intensity field in the same medium with no internal wave on the thermocline. Ideal ocean mediums composed of three distinct layers are assumed for both cases. The surface layers have no sound velocity gradient, and the other two layers in both mediums are characterized by linear sound velocity gradients. Properties of the internal waves are similar to those observed off the coast at San Diego, California, in the summer months. The results show that narrow zones of high intensity and broad zones of low intensity alternately occur over a range as predicted by an earlier theory. In the first case (no internal wave in the medium 1, contrasts in the intensity of about 5 db occur over distances of one internal wavelength or less. In the second case (an internal wave in the medium), intensity contrasts of as much as 22 db occur over the same distance.

ISSN:0001-4966    

DOI:10.1121/1.1908760

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

Experiments on propagation and compression of pulses in an acoustic waveguide are described. A water filled channel of 32 ft length with pressure release surfaces is used as an acoustic waveguide with a cutoff frequencyfc=13.5 kc. Measurements on transient waveforms and on the dispersion curve of the guide in the frequency range between 13.5 and 26 kc are reported. In the pulse compression experiments the wave‐guide is used as a “matched filter” with a frequency‐dependent delay time. A frequency modulated input pulse of 12.6 msec length is compressed into an output pulse of shorter duration and of higher peak amplitude. The frequencies in the input pulse vary from 14 to 22 kc. A time compression ratio of 52.5:1 and a peak amplitude gain of 18.6 db are achieved experimentally. The detection of a pulse in the presence of noise for an input signal to noise ratio of −20 db is illustrated.

ISSN:0001-4966    

DOI:10.1121/1.1908762

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP