摘要:

This paper presents a review of the status of the theory of sound absorbing materials. The basic concept of absorption coefficient, as well as the relation between absorption coefficient and acoustical impedance are discussed. Other topics considered are, sound propagation in homogeneous isotropic media, perforated panels, suspended absorbers, and resonant absorbers.

ISSN:0001-4966    

DOI:10.1121/1.1906839

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

A precise recording ultrasonic interferometer is presented which is particularly well suited for measuring dispersion in liquids. The device is an interferometer of the two‐crystal type in which the separation of the crystals is continuously varied. The recording process is equivalent to placing a mark on a rotating recording drum whenever the receiving crystal has moved a distance of one wavelength of sound in the liquid. Thus the wavelength of sound may be derived from the known mechanical constants of the system. The precision of the dispersion measurement is about 0.8 part in 10,000; the precision of the absolute determination of sound velocity is 0.2 m/sec. The instrument has been applied also to sound velocity measurements in solids under restricted conditions.Sound velocity in distilled water at 25°C as measured with this instrument is 1496.2±0.2 m/sec, a value somewhat lower than that obtained by earlier investigators.Dispersion in the frequency range 5–25 Mc is shown to be no greater than 0.8 part in 10,000, the limit of experimental error, for all the liquids studied: water, geraniol, xylene, isobutyl alcohol, and solutions of sucrose, NaCl,MgSO4, NaHCO3, CdCl2, CdSO4, Na3PO4, Pb(NO3)2, ZnSO4, andK3Fe(CN)6.

ISSN:0001-4966    

DOI:10.1121/1.1906862

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

The velocity and absorption of ultrasonic waves in oxygen were measured by means of an improved ultrasonic interferometer in the range from one to one hundred Mc per atmosphere. Dispersion of the velocity ranged from 333.14 m/sec, to 357.22 m/sec at 30°C. The ratio (αexper/αclass) dropped from 3.68 to 2.05, and the corresponding value ofCv/Rdropped from 2.50 to 1.61. The increase in velocity and the decrease in (αexper/αclass) is interpreted as caused by the slow exchange of energy between the translational and rotational degrees of freedom. The relaxation time for the rotation was 5.24×10−9second.

ISSN:0001-4966    

DOI:10.1121/1.1906837

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

A general expression is given for the electric input impedance of the acoustic interferometer. From this expression formulas are derived for determining the velocity of sound by varying the frequency, or for determining changes in velocity due to variations of pressure, temperature, etc. In papers dealing with the fixed path interferometer, one commonly finds the suggestion that the actual path length needs some correction when the path ends at a nonperfect reflector. It will be shown that no such correction is indicated by the theory.

ISSN:0001-4966    

DOI:10.1121/1.1906838

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

The first part is the description of a circuit employing rf amplification which converts an ultrasonic interferometer into a sensitive self‐recording instrument whose records can be simply analyzed to yield a rapid determination of the attenuation and reflection coefficients of gases at low pressures. The second part is a summary of the sources of error in ultrasonic interferometry, their detection and their correction. In the third part, some data on the velocity, attenuation, and reflection in helium in the region two to sixty Mc/atmos are presented to illustrate the range, precision, and absolute errors of the instrument.

ISSN:0001-4966    

DOI:10.1121/1.1906840

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

A mechanical shock tester of the free‐fall type has been designed to produce high accelerations in the form of a square wave. The apparatus is described, and the logical considerations which were used as design parameters are explained. Particular attention is paid to the relation between the shocks an equipment is likely to receive in practice and the optimum type of test apparatus.

ISSN:0001-4966    

DOI:10.1121/1.1906841

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

Measurements of the attenuation of sound in water containing air bubbles were undertaken in order to confirm some aspects of a theory due to L. L. Foldy. Bubbles were produced by forcing compressed air through the cloth covering of several trays lying on the bottom of the Black Moshannon Lake. The distribution in size of the bubbles and the number of bubbles per unit volume were determined by photographing the bubbles and by collecting the volume of air which rose over a given area in a given time, the terminal velocity of rise being a known function of bubble diameter. The attenuation of sound was found to be very large at frequencies for which resonant bubbles were present, and much smaller at other frequencies, indicating that resonance absorption was the principal phenomenon observed. For the particular bubble distribution investigated, which contained 0.045 percent air by volume, the attenuation was found to be in satisfactory agreement with the theory and a maximum attenuation of approximately 20 db per inch was measured. Amplitude and phase fluctuations made it impossible to measure the phase velocity, but this quantity was computed from the bubble distribution.

ISSN:0001-4966    

DOI:10.1121/1.1906842

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

A frequency band multiplication or division has been performed by string filters. An exact relationship between the input and output frequencies has been shown. Two different methods have been used.The first multiplies frequencies by rectification with the aid of a push‐pull electrostatic pick‐up. Alternating plus‐minus polarizing techniques are employed to suppress the undesirable sum frequencies. The resolution of such a doubling system, which will not pass the sum frequencies, is approximately 1.5 times the band width of the single filters. However, the band width is limited by the transit time of speech.The second method multiplies and divides by exciting the harmonics or subharmonics of the single strings. This is done by employing light, stiff “fingers” to disturb the free oscillations of the damped strings. The excitation and the resulting frequency band division and multiplication are investigated.It is shown that in order to get time compression or expansion, it is only necessary to record the multiplied or divided frequency bands and play them back with the corresponding diminished or increased speeds.The results given have a general application to similar type filters such as reed, crystal or magnetostrictive.

ISSN:0001-4966    

DOI:10.1121/1.1906843

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

The time constant meter described operates on the principle that differentiation of an exponentially decaying rectified envelope decreases it in inverse proportion to the time constant and reverses its sign. Hence, it is possible to obtain a null by balancing the differentiated signal against a portion of the original. The circuit is simple, requiring only one amplifier tube and a tuning eye.

ISSN:0001-4966    

DOI:10.1121/1.1906844

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

A formal solution in terms of cylindrical wave functions is obtained for the scattering of a plane acoustic or electromagnetic wave by an arbitrary configuration of parallel cylinders which takes into account all possible contributions to the excitation of a particular cylinder by the radiation scattered by the remaining cylinders. The solution, satisfying any of the usual prescribed boundary conditions simultaneously at the surface of each cylinder, is expressed as the incident wave plus a sum of various orders of scattering. The first order of scattering (the usual single scattering approximation) results from the excitation of each cylinder by only the plane wave or primary excitation. The second order results from the excitation of each cylinder by the first order of scattering from the remaining cylinders, and so on to an infinite order of scattering. The first order therefore consists of waves scattered by one cylinder; the second order of waves scattered by two cylinders, etc. The scattering coefficients of them'th order of scattering are expressed recursively in terms of the previous orders, and finally as sums of products ofmscattering coefficients of the single cylinder and Hankel and trigonometric functions depending on the geometry of the configuration.

ISSN:0001-4966    

DOI:10.1121/1.1906845

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP