摘要:

It is shown that the mathematical solution of problems involving the propagation of noise is materially aided by the introduction of the space correlation function ψ(x1,x2, τ), defined as the average, overt, ofp(x1t)p(x2t−τ),pbeing the acoustic pressure in the noise field. The differential equations satisfied by ψ are derived. Its relation to Ψ(α, β, γ) is discussed, α, β, γ being the propagation vector of a sinusoidal wave, and Ψ/2ρc2being the density of potential energy in the α, β, γ space.The theory of uniform noise fields, both isotropic and anisotropic, is developed in detail. The anisotropy caused by a reflecting plane is discussed. The radiation of noise by a vibrating plane is discussed, neglecting the reaction of the radiation on the motion of the surface. In particular, it is shown that to this approximation the sea surface cannot radiate subsonic energy into the high atmosphere because the velocity of the surface gravity waves is less than the velocity of sound in air.The theory of noise, as here developed, is analogous to the theory of turbulence, and the relation between noise and turbulence is briefly discussed.

ISSN:0001-4966    

DOI:10.1121/1.1907018

出版商:Acoustical Society of America    

年代:1953

数据来源: AIP

摘要:

The force‐time history required to produce a step in the velocity of a circular piston mounted in an infinite baffle is determined with the aid of the Laplace transform. Working from this result, the inverse problem of the velocity response to a step force is calculated approximately. These two results suffice to treat arbitrary distributions of applied force or velocity by Duhamel superposition. The application of the results to a loudspeaker model shows that a system designed for critical damping on the “steady‐state” approximation for the air loading will actually be slightly overdamped in its initial motion, but that the characteristic time during which the design assumption is inadequate is of the order of 10−3second.

ISSN:0001-4966    

DOI:10.1121/1.1907019

出版商:Acoustical Society of America    

年代:1953

数据来源: AIP

摘要:

The force‐time history required to produce a unit step in the velocity of a two‐dimensional plate mounted in an infinite baffle is determined and applied to more general velocity variations by Duhamel superposition.

ISSN:0001-4966    

DOI:10.1121/1.1907020

出版商:Acoustical Society of America    

年代:1953

数据来源: AIP

摘要:

Diffraction of an acoustic wave by an aperture in a plane reflecting screen has been investigated experimentally at a wavelength of about 3 cm. The pressure amplitude distribution within a circular aperture illuminated by a plane, progressive sound wave exhibits oscillations which do not conform to theoretical requirements. A possible explanation for the disagreement offered. Diffraction measurements made in the near‐field of the circular hole were found to be in fair agreement with the Fresnel‐Kirchhoff theory. A modified theoretical expression, valid for apertures large as compared to the wavelength, is also presented. It is concluded that this theory is capable of predicting the diffraction effects to a greater degree of accuracy and over a larger region of space than Kirchhoff's classical formula.

ISSN:0001-4966    

DOI:10.1121/1.1907021

出版商:Acoustical Society of America    

年代:1953

数据来源: AIP

摘要:

Exact solutions of the one‐dimensional gas dynamic equations, representing periodic sound waves of finite amplitude, are obtained for a particular medium. The progressive wave from a vibrating piston and the standing wave in a closed tube are examined in detail. Limits on the amplitude of the sound wave are found in order to avoid shocks or cavitation.

ISSN:0001-4966    

DOI:10.1121/1.1907022

出版商:Acoustical Society of America    

年代:1953

数据来源: AIP

摘要:

Well‐known laws which govern the reflection of elastic waves that strike free surfaces obliquely are used to deduce particle motion at the free surface of a body. The surface particle motion is presented in the form of diagrams and graphs for the case of an incident longitudinal wave. Considerations indicate that, for oblique incidence, the particle motion at the surface will not, in general, be perpendicular to the surface but will depend on the angle of incidence and the Poisson's ratio of the material. The data are expected to be of value in the solution of problems connected with impulsively loaded bodies such as metal‐explosive systems.

ISSN:0001-4966    

DOI:10.1121/1.1907023

出版商:Acoustical Society of America    

年代:1953

数据来源: AIP

摘要:

A relatively tractable expression for the transmittivity of steel plates immersed in a fluid medium is formulated. Criteria for total transmission and total reflection are inherent in the expression.An investigation of the discrepancies between calculated and observed transmittivity indicates a probability that the assumption of negligible losses associated with shear waves in steel is not tenable.

ISSN:0001-4966    

DOI:10.1121/1.1907024

出版商:Acoustical Society of America    

年代:1953

数据来源: AIP

摘要:

The (1 0) and (2 0) modes of acoustic waves in rectangular wave guides have been excited to the virtual exclusion of plane waves. The experimental techniques depend on the use of acoustic sources equivalent to two or more pistons with appropriate relative phases and amplitudes, precise adjustment of which is accomplished with the aid of an accurately located probe microphone. The standing wave pattern which arises when the wave guide is terminated by a partially absorbing surface may be used to determine the specific normal impedance of the surface, the value of which is characteristic of waves having a particular oblique angle of incidence and may be compared with normal incidence values obtained from plane wave measurements in the same wave guide and at the same frequency; the angle of oblique incidence may be varied by changing the frequency. The apparatus described operates in the 1–3 kc/sec frequency region and the accuracy of impedance measurement with (1 0) and (2 0) waves is comparable with that attainable with the usual plane wave tube techniques. Equivalent angles of incidence at the absorbing surface up to 84° have been used. The principles underlying the design of the apparatus and some of the distinctive problems which arise with higher order mode waves are discussed.

ISSN:0001-4966    

DOI:10.1121/1.1907025

出版商:Acoustical Society of America    

年代:1953

数据来源: AIP

摘要:

The experimental techniques described in Part I have been applied to the measurement of some acoustic properties of specimens of rockwool, hairfelt and acoustic tile, backed by a rigid wall. The more porous surfaces show substantial increases resistance at oblique angles of incidence, an increase of nearly 100 percent at 80° a incidence being observed in one case; smaller changes in reactance also occur. The absorption coefficients corresponding to impedance measurements at oblique incidence are compared with those “predicted” from normal incidence impedance measurements and significant differences are found. The measurement of surfaces having structure of finite dimensions is discussed with particular reference to an acoustic tile specimen, the impedance of which varies only slightly with angle of incidence. In some cases slight scattering of acoustic energy from one mode to another provides evidence of lack of uniformity of impedance over the surface of the specimen.

ISSN:0001-4966    

DOI:10.1121/1.1907026

出版商:Acoustical Society of America    

年代:1953

数据来源: AIP

摘要:

Depending upon the kind of electrode system employed, a sinusoidal electric current gives rise to at least five phenomena. (1) With a moving electrode on the skin or on the roof of the mouth (fricative effect), the subject receives the strongest sensation of hearing. He may hear either the first harmonic or the first and second harmonics, or the second harmonic alone, depending upon the applied voltage and upon the placement and properties of the electrode. (2) With a large‐area electrode any place on the skin of the head, if the skin is dry the subject hears the second harmonic; if it is wet, he hears nothing. (3) When the electrode is immersed in salt solution in the ear (the most common method), the subject hears a complex tone containing mostly second harmonic, suggesting the action of a square‐law transducer. (4) When the electrode is on the mucous tissue inside the middle ear (with eardrum removed), the subject hears the first harmonic and/or a noise. (5) When the electrode is in contact with the epidermis of the meatus, the subject hears the first harmonic, and at low frequencies he may also hear a noise. The experimental results seem to indicate that the hearing of a tone under any of these five conditions is probably due to vibrations set up outside the cochlea, although there appear to be at least four different transducing mechanisms. Contrary to earlier hypotheses, the tympanic membrane is apparently not involved in the conversion of the electrical energy into mechanical vibration.

ISSN:0001-4966    

DOI:10.1121/1.1907027

出版商:Acoustical Society of America    

年代:1953

数据来源: AIP