ISSN:0001-4966    

DOI:10.1121/1.1916066

出版商:Acoustical Society of America    

年代:1940

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1916067

出版商:Acoustical Society of America    

年代:1940

数据来源: AIP

摘要:

An indirect experimental check is now obtained for the calculated “shape factor” for an absorbing strip of material in a reflecting wall, reported previously The effect of the strip on each of the component “partial waves” in the incident plane wave is studied by placing the strip in a special enclosure, the shape of which allows the partial waves to be studied individually as isolated normal modes of vibration. A semicylindrical room is employed; its concave wall tends to “focus” sound upon the center of the opposite, flat wall on which the absorbing panel is centered. If the wave equation is set up in elliptic cylinder coordinates with the edges of the panel as foci, the boundary conditions are readily satisfied. The theory is strictly applicable to a semielliptical room only, but holds well in the semicircular for strip widths up to one‐third the diameter. The experimental enclosure has a 14‐inch radius, a “thickness” of 4 inches between the parallel walls, and is constructed with highly reflective metal surfaces. The selective damping of various normal modes is investigated for strip widths from about one to ten inches, for several values of normal acoustic impedance. The results substantiate the calculated values of “shape factor” which were given in the previous paper.

ISSN:0001-4966    

DOI:10.1121/1.1916068

出版商:Acoustical Society of America    

年代:1940

数据来源: AIP

摘要:

A method has been developed for obtaining a wide range of impedance values, both as to magnitude and phase angle. In one form, a variable impedance unit consists of (1) a steel face with perforations of variable area (obtained by overlapping identically drilled plates), (2) a “resistance” element—a layer of porous absorbing material with appropriate flow resistance, (3) a “mass” element—a solid board through which small channels are drilled at uniform spacing over the whole surface, and (4) a “stiffness” element—a cavity of variable depth backed by a rigid steel plate. The behavior can be predicted approximately by analysis of an equivalent electrical circuit. A unit with face 8×8″ has been built and calibrated, at several frequencies, using a straight acoustic impedance line of the same sectional area. The useful range appears to include phase angles from −60° to +45° and magnitudes ofz/ρcfrom 0.4 to 10, for frequencies between 250 and 800 c.p.s. Normal incidence “absorption coefficients” of 99 percent have been obtained. In using this type of impedance structure with non‐normally incident sound it is found necessary to break up the backing cavity into cells of transverse section small compared with a wave‐length. This modification minimizes tangential components of particle velocity and renders the specific acoustic impedance at the face essentially independent of angle of incidence and of the area of the surface, and yields a marked increase in “average absorption coefficient.”

ISSN:0001-4966    

DOI:10.1121/1.1916069

出版商:Acoustical Society of America    

年代:1940

数据来源: AIP

摘要:

An exact solution is presented for the behavior of sound in a rectangular room whose walls are covered non‐uniformly with acoustical materials. The sound field in the room is first decomposed into normal modes of vibration. By assuming the characteristics of an acoustical material to be representable by its specific acoustic admittance, the normal frequency, decay constant and pressure configuration of each normal mode of vibration are determined as functions of the room dimensions and the distribution of acoustical materials in the room. An approximate theory is also presented, applicable to cases in which the diffracted reflections from the walls can be neglected. It is shown that the effect due to diffraction can be superposed on the result from the approximate theory. Experiments were performed in confirmation of the theoretical results. Applications of the theory to practical problems, such as measurements on acoustical materials, acoustical design of rectangular rooms, etc., are discussed. The possible extension of the present theory to the case of nonrectangular rooms is discussed.

ISSN:0001-4966    

DOI:10.1121/1.1916070

出版商:Acoustical Society of America    

年代:1940

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1916071

出版商:Acoustical Society of America    

年代:1940

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1916072

出版商:Acoustical Society of America    

年代:1940

数据来源: AIP

摘要:

Refinements in method and apparatus have made possible the precise measurement of attenuation of sound in tubes. Preliminary results check the tube wall effect predicted by Kirchhoff very closely. There is also found an unexpectedly large effect depending on the first power of the frequency. Further measurements are required to find whether this loss exists in free air or is due in part to the tube walls.

ISSN:0001-4966    

DOI:10.1121/1.1916073

出版商:Acoustical Society of America    

年代:1940

数据来源: AIP

摘要:

The differential equation governing the propagation of sound in horns and pipes in general with wall absorption is formulated. Expressions for the pressure and power transmission loss in exponential horns and cylindrical pipes are given. Families of curves showing the power transmission loss in horns and pipes with various wall absorption coefficients are computed. Measurements on the power transmission loss in pine wood and fir wood horns, and the absorption coefficients of pine and fir are shown.

ISSN:0001-4966    

DOI:10.1121/1.1916074

出版商:Acoustical Society of America    

年代:1940

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1916075

出版商:Acoustical Society of America    

年代:1940

数据来源: AIP