摘要:

On the assumption that the forced modes of vibration of a structure, subjected to pressure fluctuations random in time and space, can be approximated by the composition of the motions of the uncoupled natural modes, a general analysis is made using the ideas of vibration theory and spectrum analysis. The power spectrum, and hence the rms value, of any quantity depending linearly upon structural distortions is derived and it involves a quantity (called the “joint acceptance”) concerning the spacewise structure of the pressure field and of the geometry of the modes of vibration. It is shown how this result may be used (on assuming “normal” randomness) to estimate the fatigue life on the hypothesis of cumulative damage.

ISSN:0001-4966    

DOI:10.1121/1.1909481

出版商:Acoustical Society of America    

年代:1958

数据来源: AIP

摘要:

If a plane sheet (membrane of plate), which is excited by homogeneous random pressures, is assumed to be infinite in extent then a very direct method of analysis is possible. However, several important applications concern structures which are far from infinite in extent (e.g., the excitation of panels by boundary layer pressure fluctuations, and the problem of structural vibrations due to jet noise). This restriction may be overcome by using the method of normal modes, which is more cumbersome by its very nature. It turns out that if it is the power spectrum of displacement which is required, then the “infinite” approach can be used only when the sheet is large enough for there to be only negligible reflections from its edges. However, if one is concerned only with the average mean square value of displacement (or related quantities), and if the detailed shape of the spectrum is of no consequence, then the infinite solution may be used, regardless of the magnitude of the damping, provided that the gravest modes are not excited.

ISSN:0001-4966    

DOI:10.1121/1.1909483

出版商:Acoustical Society of America    

年代:1958

数据来源: AIP

摘要:

The study is based on the general differential equation of a mechanical system. Driving force and solution are expressed as a series of natural functions of the dissipationless system. Dissipation is taken into account by introducing complex elastic constants. Each Fourier coefficient of the driving force is written as the product of an excitation constant and the total driving force. The excitation constants can then be included in the mode parameters of the system and the solution, written as the product of the total driving force and the “mechanical impedance” of the system. The solution is exact and useful at the lower frequencies. A simple solution for higher frequencies can be derived by replacing the series by an integral and evaluating this integral. Three important parameters can thus be obtained: the driving‐point impedance that describes the velocity of the driving point and the reaction of the system to the driving force, the effective impedance that represents the average velocity amplitude over the system, and the effective dissipation resistance with respect to this average‐velocity amplitude.The expression derived for the high‐frequency driving‐point impedance then turns out to represent, also, the general background level at lower frequencies. The solution for the driving‐point impedance in the intermediate frequency range is obtained by adding to this background the contribution of the natural mode that has its resonant frequency closest to the frequency of the force; the effective impedance, on the other hand, is given with good approximation as the impedance of the mode whose resonance frequency is closest to the frequency of the force; and the effective dissipation resistance is given by the resistive component of this mode impedance.The mode parameters are computed for rods, membranes, plates, and shells for three situations—a point force, a linear force distribution, and a force distribution given by a Fourier integral. The results make it possible to handle complicated mechanical systems with almost the same ease as simple mass points, and to compute their sound radiation.

ISSN:0001-4966    

DOI:10.1121/1.1909485

出版商:Acoustical Society of America    

年代:1958

数据来源: AIP

摘要:

The sound field generated by a composite vibrator consists of a wattless near field that decreases very rapidly with distance and an energy‐carrying radiation field. For radiators more than half a wavelength apart, the sound fields are spatially incoherent. The radiation field of a complex sound generator can therefore be computed by adding up the energy contributions of its various radiating elements. The radiating elements can be grouped as vibrators that are small in comparison to the wavelength, vibrators without nodal lines that are large in comparison to the wavelength, and vibrators with a nodal line pattern.The radiation resistance of vibrators without nodal lines can, for most practical purposes, be stated with sufficient accuracy in terms of that of the equivalent sphere. The sound radiation of vibrators with nodal lines can be attributed to two causes. For the low‐order modes of vibration, the contributions of the zone of positive and negative amplitude do not compensate one another entirely, and the radiation resistance, though small, is different from zero. Such modes may therefore be expected to contribute to the sound pressure if they are excited in their resonance range. For the high‐order modes compensation is complete, but a point force or a force distribution along a line or over a finite area also excites to forced vibrations many low‐order modes with very few nodal lines. Since the distance between the nodal lines is greater than the acoustic wavelength in the surrounding medium, the radiation impedance for these modes is very nearly equal to ρc. Since these modes are forced to vibrate at frequencies above their resonant frequency, the resistive component of their impedance being negligible in comparison to the reactive component, this sound pressure is independent of the damping of the system. The radiation impedance is computed for the cases mentioned.

ISSN:0001-4966    

DOI:10.1121/1.1909487

出版商:Acoustical Society of America    

年代:1958

数据来源: AIP

摘要:

An approximate theory based on an energy principle has been applied for contour vibrations of rectangular plates and compared with experimental data of ADPZ‐45° cuts with a satisfactory agreement.The existence of a special mode, which has attracted little attention heretofore, is shown and its connection with the complex branches of the dispersion equation is discussed.

ISSN:0001-4966    

DOI:10.1121/1.1909489

出版商:Acoustical Society of America    

年代:1958

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1909491

出版商:Acoustical Society of America    

年代:1958

数据来源: AIP

摘要:

In an attempt to develop a method for measuring auditory acuity for high‐frequency tones, the authors discuss pilot experiments in progress using an electromagnetic method for setting the eardrum into vibration (alternating magnetic fields acting on a permanent magnet fixed to the eardrum).

ISSN:0001-4966    

DOI:10.1121/1.1909493

出版商:Acoustical Society of America    

年代:1958

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1909495

出版商:Acoustical Society of America    

年代:1958

数据来源: AIP