摘要:

A free field method is reported for the measurement of the normal impedance and the absorption coefficient of a material. The pressure and phase of an incident “plane” wave in steady state are measured at a point in the absorptive surface and compared with the pressure and phase measured at the same point in space at the surface of a perfectly reflecting material, or at the same point in free space without any panel. From these data are determined the phase and magnitude of both the incident and reflected waves, and this information in turn yields both the normal impedance and the free wave absorption coefficient for the particular angle of incidence involved. Tests have been made with an 8‐ft square sample of material mounted in an anechoic chamber and rotated about a vertical axis in the presence of a fixed horizontal beam of an approximately plane wave. As a check of the validity of the method, measurements have been made using two types of material with known properties, one with the normal impedance essentially independent of the angle of incidence and the other with a considerable angular dependence. The experimental values are shown to be in good agreement with calculations.

ISSN:0001-4966    

DOI:10.1121/1.1906796

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

The growth and decay of sound in a one‐dimensional room was investigated by using a very special room, namely, a helical spring. Compressional waves in the audiofrequency range were sent down the spring by an electromechanical driver and received at the far end by an electromechanical pickup. Reflection occurred at each terminal, allowing standing waves to be built up. The length of the spring was 30 in. and the transit time was 53.6 milliseconds, giving a propagation velocity of about4612 ft/sec. The 30‐in. spring is thus equivalent to a room about 58 ft long.Oscillograms were obtained for frequencies below 100 cps, showing the discontinuous nature of the reverberation produced in the one‐dimensional room.During the analysis of the oscillograms, Eyring's mechanism of image sources was found to fail to explain some of the oscillograms. A new mechanism was therefore created, called the Two‐Pipe Mechanism, which seems to explain all the oscillograms.The Two‐Pipe Mechanism clarifies: (1) the behavior of a normal mode when decay is started before steady state has been reached; (2) the growth and decay of standing waves on a violin string, by demonstrating the quantized envelope‐levels which control this growth and decay; (3) the growth and decay of non‐normal modes, by showing how decay can consist of the steady‐state frequency alone, even though this is a “not‐allowed” frequency.

ISSN:0001-4966    

DOI:10.1121/1.1906797

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

The absorption coefficients of flat, splayed, and cylindrical panel assemblies made from one‐quarter‐inch three‐ply fir plywood have been measured by the reverberation chamber method under three different conditions. All panels have greater absorption at the lower frequencies (128 and 256) than at higher frequencies. The effective absorption of each type can be increased by using a half‐inch kapok felt in combination with it, either mounted on the wall behind the panel or spot‐cemented to the plywood. Each type of panel has desirable characteristics under certain conditions. The absorption of all types indicates that such panels are important acoustical materials in the lower frequency range.

ISSN:0001-4966    

DOI:10.1121/1.1906798

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

This paper extends and unifies the analysis of sound absorptive structures consisting of (1) a layer of porous material separated from (2) a rigid wall by (3) an air cavity, which may or may not contain (4) separating partitions; and the porous material may or may not be covered with (5) a perforated facing. The normal impedance of the porous layer is expressed in a series form in which the first term suffices for a very thin layer. Equations and design charts are given for the impedance and the absorption coefficient, as functions of angle of incidence, for the complete structure with and without partitions. The statistical coefficient, averaged over angle, is plotted for each case over the commonly encountered ranges of all design variables. Calculated coefficients for eight different designs are compared with those measured by the reverberation method and found to be in good agreement.

ISSN:0001-4966    

DOI:10.1121/1.1906799

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

The sound field of a point source located at a plane boundary was measured both with respect to its amplitude and phase characteristics. This was done for a very high impedance boundary and one composed of material which is relatively absorbing. The fields were calculated, the calculations for the latter material being based on measured values of impedance and propagation constant, and found to be in good agreement with those measured.

ISSN:0001-4966    

DOI:10.1121/1.1906800

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

An expression is obtained for the amplitude and phase of an acoustic wave above a boundary due to a point source on or near a boundary which exhibits a constant normal specific acoustic impedance. This is shown to be a special case of the solution for an isotropic medium with constant characteristic acoustic impedance; specifically, the case is one in which the ratio of the propagation constant in the upper medium to that in the lower approaches 0.A material which obeyed a constant normal impedance boundary condition was constructed from ordinary drinking straws. Measurements of amplitude and phase as a function of receiver position along and above the boundary showed good agreement with theory.The nature of the approximations involved in the solution are discussed and it is shown to be an adequate representation of the sound field for distances greater than one wavelength from the source.

ISSN:0001-4966    

DOI:10.1121/1.1906801

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

The mechanism by which noise is transmitted in liquid‐filled piping systems was studied by tests on a straight free‐free 2‐inch pipe 53 feet long. The vibrations of the system below 1000 cps were characterized by highQresonant modes consisting of longitudinal vibrations in the water, and both longitudinal and flexural vibrations in the pipe wall. Attenuation for all types of vibrations was very low, ranging from 0.001 to 0.09 db/ft. Large coupling between water vibrations and flexural pipe‐wall vibrations was found to exist, so that longitudinal excitation of the water column resulted in flexural pipe‐wall vibrations of large amplitude. Because of the large number of highQmodes in a practical piping system, it appears that the modes are the means of transmission of noise. The flexural modes are more numerous than other modes at low frequencies and are also important because of their coupling to radiating surfaces.

ISSN:0001-4966    

DOI:10.1121/1.1906802

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

This paper reports measurements of the elasticities of various Weimarn gels by the use of an ultrasonic pulse reflection method. Considerations of porosity and composite density of the gel lead, theoretically, to an expression for the bulk modulus which agrees remarkably well with the experimental values. Furthermore, the aging of the gel samples gives ground to conclude that the formation of a gel resembles the separation of a solid phase from a supersaturated solution. The variation of the elastic moduli with temperature has been explained by the postulated structure of these gelatinous substances.

ISSN:0001-4966    

DOI:10.1121/1.1906803

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

The technique and tests herein described were devised to study possible improvement in rating the acoustical excellence of rooms, by using higher pen speeds than had hitherto been available. Recordings of reverberation decay characteristics, made with the new ultra speed technique, are compared with results from older methods of recording reverberation; and it is found that the new method provides a much more adequate diagnosis of recognized acoustical faults.The new technique makes possible ink recording of acoustical levels on a db basis at pen speeds of 5000 to 10,000 db per second, or ten to twenty times that afforded by the high speed level recorder. It uses a new stabilized logarithmic Varistor system for instantaneous conversion to a db basis, followed by a direct writing oscillograph. Hitherto unrecognized requirements for the electronic system are indicated as necessary if adequate stability, linearity, and scale range are to be achieved.

ISSN:0001-4966    

DOI:10.1121/1.1906804

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

Horizontal small‐scale temperature inhomogeneities in the ocean have been measured with the aid of a highly sensitive, rapid‐response thermometer mounted on a submarine. The mean size of these inhomogeneities is of the order of 60 cm; a typical average temperature variation is 0.05°C. The physical features of the inhomogeneities which determine acoustical effects are shown to be described by the autocorrelation function; a specially constructed computing machine was used to calculate this function from the temperature records. The calculated backward scattering (reverberation) resulting from these inhomogeneities is given as a function of frequency. In the region 10 to 100 kc the calculated scattering is, in general, less than that which is actually observed in the ocean, suggesting that biological material is the major source of scattering in the ocean. A calculation of the attenuation of transmitted sound in the ocean resulting from scattering shows that this effect is small compared with absorption at the higher frequencies. Refractive effects of the inhomogeneities result in another acoustical phenomenon: local warping of the wave fronts produces intensity enhancement or diminution at the receiver. These effects quantitatively explain rapid acoustic fluctuations (scintillations) which are observed in the ocean.

ISSN:0001-4966    

DOI:10.1121/1.1906805

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP