Volume 25, Issue 6, November 1953
Index of content:
25(1953); http://dx.doi.org/10.1121/1.1907235View Description Hide Description
Absorption and scattering from resonators in a free field as well as in walls are discussed. The effect of different aperture geometries on the resonance frequency of resonators is considered and illustrated by examples. Considering losses due to viscosity,heat conduction, and radiation, the optimum design for maximum resonanceabsorption is analyzed, and the results are expressed in terms of design charts. Nonlinear effects on the absorption and resonance frequency are also included, and a discussion of the onset of turbulence is presented.
25(1953); http://dx.doi.org/10.1121/1.1907236View Description Hide Description
The near field of a spherical Helmholtz resonator exposed to a plane wave is computed and shown to be in good agreement with measurements. Comparison is made with the near field of a resonator terminating a cylindrical tube. A discussion of the calculation of the dissipation in the resonators is included.
25(1953); http://dx.doi.org/10.1121/1.1907237View Description Hide Description
This paper presents a systematic study of experimentally significant losses contributing to the damping of an acoustical cavityresonator.Absorptionmeasurements in air on the 0, 1 wave of a round cavity were made by a resonance response method employing a transverse particle velocity pickup device. This study reveals that mechanical losses due to the vibrations of the walls are experimentally significant. Empirical relaxation time formulas are established for air at 24 and 27°C. The thermal and viscous boundary‐layer loss formula is fitted to these data by choosing the thermodynamic ratio νρ0/μ equal to 1.31.
25(1953); http://dx.doi.org/10.1121/1.1907238View Description Hide Description
An experimental investigation of the attenuation of sound in two air‐filled tubes of 0.238 and 0.0292 cm inside radius in the frequency range from 3.8 to 20 kc was conducted, and the results were compared with values of attenuation calculated from the Kirchhoff theory for rigid walled tubes. Agreement of experimental with theoretical values was found within the accuracy of the experiment, provided that the comparison made only with the portion of attenuation proportional to square root of frequency, as suggested by Fay. The attenuation was determined by a method which was different, in some respects, from those previously employed. The variable length, closed tube was driven with a high‐impedance source, and the input pressure was sampled with a high‐impedance microphone probe. The input pressure was recorded as a function of closed tube length. Since the input pressure can be assumed to be proportional to input impedance for these conditions, the attenuation can readily be determined from the ratio of the pressure maxima or minima.
25(1953); http://dx.doi.org/10.1121/1.1907240View Description Hide Description
This paper is a report of a theoretical investigation of sound scattering from liquid cylindrical obstacles immersed in water following closely the conditions prevailing in the previous experimental work of Tamarkin, Bauer, and others. An incident plane wave being assumed, series solution was found for the total scatteredwave. Numerical calculations were carried out using parameters approximating those of Tamarkin's experiments (radius of obstacle, 0.635 cm; sound frequency, 1.145 mc/sec; wavelength, 0.13 cm). A series of scattering patterns, confined principally to the forward direction for an angular spread of about 14° (with the obstacle at the vertex) were calculated for eight liquids, chosen to give a spread of kia values (ki the wave parameter inside the liquid and a the radius) between 26.0 and 41.0. The results of this theoretical investigation confirm the experimental findings that for the liquids examined the scattering is diffractive and lies in the zone between the geometrical region and the region of Rayleigh scattering. Quantitative comparisons are made between the theoretical and experimentally plotted results with special emphasis on the experimental resonance curve and its theoretically computed counterpart.
25(1953); http://dx.doi.org/10.1121/1.1907241View Description Hide Description
The scattering of a sound wave in a medium undergoing shear flow confined to a finite region is investigated under the assumption that the total velocity field is everywhere small compared to the velocity of sound. Formulas are obtained for the angular distribution and frequency distribution of the scatteredwave in terms of the four‐dimensional Fourier transform of the shear velocity field.
Under typical conditions, the cross sections for the scattering of a plane wave of frequency ω by a shear flow of given scale and spatial structure are approximately of the form ω4 M 2, where M is a characteristic Mach number of the flow. The coupling between the shear and longitudinal velocity fields has a tensor character such the the scattering vanishes at 180 degrees and at 90 degrees. The spectrum of the scattered sound wave is very sharp in the forward direction and becomes broader at larger scattering angles.
Explicit expressions for the cross sections are obtained for the case of scattering from a region of isotropic turbulence. When the frequencies of importance in the turbulence are small compared to the frequency of the incident sound wave, the average differential scattering cross section can be expressed directly in terms of the energy spectrum of the turbulence.
25(1953); http://dx.doi.org/10.1121/1.1907242View Description Hide Description
The reflection coefficient for plane waves incident obliquely on a medium in which the velocity decreases exponentially is computed approximately using the WKB method and compared with a computation from the rigorous solution. The approximate reflection coefficient where ω is the angular frequency, g is the velocity gradient at the start of the exponential decrease, and θ the angle of incidence is within 0.05 percent of the rigorous value for ω/g ⩾ 5 at normal incidence and within 5 percent of the rigorous value for ω/g ⩾ 20 for angles of incidence up to about 45°.
25(1953); http://dx.doi.org/10.1121/1.1907243View Description Hide Description
The region of validity of the single‐scattering approximation used in Part I is found by considering the next higher approximation; it is valid for , where k 0 is the wave number of the incident sound, r is the range from source to observer, a is the mean size of the inhomogeneities, and α is the rms value of the refractive index variations. Some approximate results are given for the case of wavelength large compared with inhomogeneity size. By consideration of the intensity variations, it is found that ray theory is valid for (k 0 a 2/r)≫1, as has been found for plane waves.
25(1953); http://dx.doi.org/10.1121/1.1907244View Description Hide Description
An apparatus for measuring the absorption of sound in gas mixtures has been developed and tested. The apparatus consists of a sound tube, a movable ribbon‐type speaker, and a fixed condensor microphone. The absorption is determined by recording the sound pressure as the path length between the speaker and the microphone is increased. The measured attenuation coefficient is corrected for the effect of the tube and also for the attenuation due to the viscosity and heat conduction of the gas. Both frequency and pressure are variable and range from 2 to 10 kc and from 3 to 300 mm of Hg, respectively.
25(1953); http://dx.doi.org/10.1121/1.1907245View Description Hide Description
The absorption of sound has been measured in , , , and in mixtures of the latter two gases with for frequencies ranging from 2 to 10 kc and for pressures ranging from 3 to 300 mm of Hg. The tube method was employed. The absorption was determined by observing the decrease in sound pressure as the path length between the speaker and the microphone was increased. A fixed condensor microphone and a movable ribbon‐type speaker were used.
The measuredabsorption coefficient was corrected for the effect of the tube and also for the absorption due to viscosity and heat conduction of the gas. The corrected absorption coefficient was then plotted as the attenuation coefficient per wavelength against the logarithm of the ratio of frequency over pressure. These curves were then compared to those determined from Bourgin's Theory for mixtures of absorbing gases. The agreement between the observed attenuation and that predicted by the theory was within 5 percent.
25(1953); http://dx.doi.org/10.1121/1.1907246View Description Hide Description
Steady rotational flow is known to occur in the vicinity of a smooth rigid object oscillating to and fro in a viscous incompressible fluid. For a small oscillation amplitude the steady flow is shown theoretically to be invariant to the coordinate transformation which renders the object stationary.
25(1953); http://dx.doi.org/10.1121/1.1907247View Description Hide Description
A new technique for the measurement of sound velocity in liquids has been developed, which is especially suited to frequencies in the range from 0.5 to 5 mc. It employs an approximation to the boundary conditions in Guptill's closed form solution for the radiation field between two infinite walls with a piston source in one wall.
Measurements have been made in distilled water at 0.6 and 7.6 mc with an error of less than 0.2 percent in the low‐frequency measurements. No dispersion is indicated to the order of the experimental error, in the temperature range from 10 to 20°C.
A description of the apparatus is given.
25(1953); http://dx.doi.org/10.1121/1.1907248View Description Hide Description
The cavitation threshold of water supersaturated with air at hydrostaticpressure is utilized as a pressure reference level against which the output of a transducer may be referred. The threshold may be determined within 0.2 decibel in a purely radial standing‐wave field. A simple system is described for performing the measurements.
25(1953); http://dx.doi.org/10.1121/1.1907249View Description Hide Description
The electronic sound absorber is so named because it absorbs sound or reduces the sound level by means of an electronic transducing system, as contrasted to conventional sound absorption by direct conversion from acoustical to heat energy. The electronic sound absorber consists of a microphone,amplifier, and loudspeaker connected so that, for an incident sound,wave and sound pressure at the microphone is reduced. Thus it will be seen that the electronic sound absorber is a feedback system which operates to reduce the sound pressure in the vicinity of the microphone. The sound pressure in the neighborhood of the microphone can be reduced 10 to 25 decibels over a frequency range of three octaves in the low‐frequency portion of the audio‐range. The electronic sound absorber may be used to reduce the noise over a small volume, that is, spot type noise reducer or it may be used with an acoustical resistance to obtain a high order of sound absorption in the low‐frequency range.
25(1953); http://dx.doi.org/10.1121/1.1907250View Description Hide Description
Relations for an electromechanical network that contains both electrostatic and electromagnetic coupling and has the antireciprocal transfer characteristic described by McMillan are developed. The effect of hysteresis and eddycurrents is included. An equivalent electrical circuit that makes use of the “ideal gyrator” to give the antireciprocal characteristics presented. Several sets of experimental measurements are described which clearly show the nonreciprocal characteristics of the system.
25(1953); http://dx.doi.org/10.1121/1.1907251View Description Hide Description
Three distortion methods, the harmonic, the SMPE intermodulation, and the CCIF (a difference frequency), have been studied, mainly on a theoretical basis, as a means of analyzing distortion encountered in phonograph reproduction. The harmonic method is somewhat difficult to apply and some of the harmonics may be beyond the range of the system. The IM has been used in the past with considerable success, and appears to be a good method of analyzing tracing distortion. The difference frequency methods appears to be somewhat insensitive to tracing distortion but offers theoretical advantages in that the analysis may be made right up to the cut‐off frequency of the system, practically it is somewhat difficult to apply.
25(1953); http://dx.doi.org/10.1121/1.1907252View Description Hide Description
The electro‐optic effect in the crystal ammonium dihydrogen phosphate provides a high‐speed light valve with ruggedness, long life, and ease of handling not possessed by the similar Kerr effect in liquids. In this paper the design of a practical sound reproduction unit for making variable density 35‐mm tracks will be described. Frequency response of the light valve is flat to over 20 000 cycles, the over‐all system being limited to about 10 000 by the slit‐width film speed effect. Tests of distortion and sound quality in the sound tracks obtained are described.
25(1953); http://dx.doi.org/10.1121/1.1907253View Description Hide Description
Experimental data are submitted indicating the existence of a direct action of ultrasonic waves on a photographic emulsion, contrary to the views of some other workers. All of the evidence obtained here indicates a closer correlation with the softness of the emulsion than with the known behavior of ultrasonically produced luminescence. Information is presented as to temperature dependence of the image, effect of ambient pressure, light shielding, and the behavior of various types of commercial emulsions.
25(1953); http://dx.doi.org/10.1121/1.1907254View Description Hide Description
An acoustic lens combined with a conical horn can be used to obtain a highly directional microphone without some of the disadvantages of the parabolic microphone. The directional characteristics can be calculated satisfactorily if one assumes that the horn provides uniform flooding of the lens aperture.