Index of content:
Volume 32, Issue 2, February 1960
32(1960); http://dx.doi.org/10.1121/1.1908004View Description Hide Description
The influence of porosity, gas content, pressure, and temperature on the compressional wave velocity in unconsolidated marine sand sediments is presented. Velocity behavior is explained from theory. The theoretical behavior is compared with velocity data obtained for sediments in situ and velocity data obtained in the laboratory.
The significant conclusions reached in this study are (1) sediment porosity is the most important factor affecting velocity in unconsolidated water‐saturated sand sediments; (2) velocity in a shallow unconsolidated water‐saturated sand sediment is nearly independent of the depth of the water above the sediment (within 1% down to a depth of 1000 ft); (3) velocity in an unconsolidated sand sediment is proportional to the velocity in the sea water saturating the sediment; (4) velocity in an unconsolidated water saturated sand sediment increases with an increase in temperature; and (5) velocity in an unconsolidated sand sediment saturated with a mixture of gas and water is considerably lower than the velocity in a sediment saturated with water.
32(1960); http://dx.doi.org/10.1121/1.1908006View Description Hide Description
Apparatus has been constructed for measuringsound absorption by the tube method in corrosive gases at high temperatures. This apparatus has been used to measuresound absorption in chlorine, bromine, and iodine vapors at temperatures between 25 and 256°C. The measurements revealed vibrational relaxation absorption peaks in all the gases. In the relaxation times varied from 4.90 μsec at 25°C to 1.55 μsec at 255°C. In the variation was from 0.854 μsec at 28°C to 0.460 μsec at 256°C. In the relaxation time was almost independent of temperature, changing only from 0.106 to 0.102 μsec when the temperature changed from 112 to 253°C.
The heights of the absorption peaks were adequately predicted by assuming the relaxing vibrational specific heat to be . Neither the Schwartz‐Herzfeld nor Cottrell‐Ream theoretical equations for the relaxation time accurately predict the measured variation with temperature, molecular mass and vibrational frequency.
32(1960); http://dx.doi.org/10.1121/1.1908008View Description Hide Description
An instrument for measurement of ultrasonic attenuation and velocity in the frequency range from 5–200 mc/sec is described. The unit incorporates pulsed rf oscillator,superheterodyne receiver, exponential wave‐form generator, precision time delay generator (useful in velocity measurements), CRT display circuits, and appropriate synchronization circuits. Ultrasonic attenuationmeasurements made in single crystals of NaCl and KCl during deformation and recovery at several temperatures are reported here.
32(1960); http://dx.doi.org/10.1121/1.1908010View Description Hide Description
Formulas have been derived for determining particle velocity,particle velocity amplification, stresses, length, and mechanical impedance of sections of solid cones—lateral dimensions small compared with the wavelength—in longitudinal half‐wave resonance. Unlike the velocity amplification factor of the exponentially tapered bar, that of the conical section does not approach infinity but approaches a limit as the ratio of end diameters approaches infinity. A sample calculation shows the maximum velocity amplification possible from a conical section of steel half‐wave resonant at 20 kc to be approximately 4.61 as compared with Merkulov's calculated limit of approximately 4.6.
32(1960); http://dx.doi.org/10.1121/1.1908013View Description Hide Description
The amplitude and phase of the pressure in the field of a rectangular piston, assumed vibrating within an infinite, rigid baffle, are examined theoretically for ranges down to the order of the piston length. Various laws of behavior of this field, both on and away from the acoustic axis, are deduced. An indication is given of the errors introduced by the approximations used. Such experimental evidence as is available supports the theory.
Simple means are provided for constructing curves of axial pressure amplitude and phase for any ratio of length to breadth of the radiating surface, and a method is suggested for predicting the long‐range axial pressure of a rectangular piston from measurements taken well within the latter's Fresnel region.
The near fields of a rectangular and a circular piston are compared. That of the rectangular piston produces the more useful approximation to a plane wave.
32(1960); http://dx.doi.org/10.1121/1.1908015View Description Hide Description
The hydrodynamictheory for the axial sinusoidal oscillation of a viscoelastic fluid in a rigid tube of infinite length and circular cross section is given. The fluid is assumed to be incompressible. Its properties in sinusoidal shear are described by a complex coefficient of viscosity. An equation for the velocity profile is developed and typical profile curves are presented. Equations for the acoustic impedance per unit length of tube are also obtained. Functions from which the acoustic resistence and reactance are directly obtainable for a wide range of the physical parameters are determined and are presented in graphical form. Viscoelastic media which range from a perfect viscous fluid to a perfect elasticsolid are treated.
32(1960); http://dx.doi.org/10.1121/1.1908017View Description Hide Description
The method of Ying and Truell for studying the scattering of longitudinal elastic waves by spherical obstacles is applied to the particular case of scattering by a fluid‐filled cavity that is embedded in an isotropic elasticsolid.Exact solutions for the expansion coefficients which describe the scattered elastic wave are obtained. The limiting case of ka≪1 (Rayleigh scattering) is considered in detail. A general expression for the scattering cross section is derived.
32(1960); http://dx.doi.org/10.1121/1.1908019View Description Hide Description
The theory is discussed for the reflection of sound from flat and uniform fluid bottoms. Experimental 1‐kc data obtained over three typical bottoms are presented for a range of grazing angles of incidence from 12° to 90° Experimental data are presented for normal incidence over several bottoms at frequencies of 4, 7.5, and 16 kc. The experimental data are compared with the available theories and the conclusion is reached that the Rayleigh theory modified by assuming complex velocity for the bottom gives the best agreement and that the agreement is best when the bottom attenuation is assumed to vary as the first or higher power of the frequency.
32(1960); http://dx.doi.org/10.1121/1.1908021View Description Hide Description
The Stratford (Ontario) Shakespearean Festival has had a record of continuing success since it was started six years ago in a unique theater tent. Since then, the festival authorities have built a new permanent theater retaining the unusual stage and general seating arrangements which combined the Greek and Elizabethan architectural traditions. The shape of the theater and, in particular, the use of a semicircular rear wall centered in the middle of the acting area, presented certain obvious acoustical problems. This paper describes the means taken to overcome this difficulty together with the results obtained.
32(1960); http://dx.doi.org/10.1121/1.1908023View Description Hide Description
When an individual is exposed first to a high‐level noise H for a time T and then to one of lower level L for time t, his temporary threshold shift (TTS) is equal to the numerical sum of (1) the TTS in db that would be produced by level L acting over a period of time T+t, and (2) the TTS in db that would remain after t min of recovery in quiet from TTS D , where TTS D is the difference between (a) the TTS at the end of the T‐min exposure to H and (b) the TTS that would have been produced by T min of exposure to L. It appears that this result rules out any simple explanation of TTS.
32(1960); http://dx.doi.org/10.1121/1.1908025View Description Hide Description
In 1953, Bekesy first reported the occurrence of shearing motion within the cochlear duct and furthermore the notion that it is this mode of motion which constitutes the adequate stimulus for the hair cells. There were two modes of shear motion: radially directed in the region proximal to the place of maximal amplitude of the traveling‐wave pattern, and longitudinally directed distal to that point.
This phenomenon was studied in cochlear models. The envelopes over both modes of shear motion were found to be much smaller than that over the traveling‐wave pattern. A simple explanation presented itself. The direction of the shearing motion in each domain coincides with the dominant curvature produced by the traveling waves in that section of the cochlear duct.
Earlier, it had been shown that under the effect of Bekesy's eddies, i.e., at higher driving amplitudes, the displacement pattern of the cochlear partition becomes asymmetrical. This asymmetry produces a dc‐shift in both domains of shear motion: in an outward direction within the domain of radial shear and in an apical direction within the domain of longitudinal shear. However, when the round window was driven, both directions of the dc‐components were reversed. The latter finding aided in establishing the causal relationship mentioned in the foregoing.
32(1960); http://dx.doi.org/10.1121/1.1908027View Description Hide Description
The ASA standard “Method for the measurement of the real‐ear attenuation of ear protectors at threshold”—Z24.22/406, specifies the use of threshold shift techniques. It has been observed in our laboratory during extensive evaluation of developmental ear protective devices over the past ten years that large differences in attenuation are obtained when using different attenuation measuring techniques. In addition, these differences vary for the different types of protectors being studied. In the study reported herein, four grossly different types of protectors were measured. These protectors differ in the volume under the protector and in the principal sound transmission path at low frequencies. The test subjects and room environment were the same for all the measurements. The results indicate that threshold shift measurements yield higher values of attenuation than either loudness balance or the microphone method. However, the results vary widely for different types of protectors as a function of frequency. Differences of 3 db or greater between protector types as measured by any of the three measuring techniques were found to be significant at the 95% confidence level using four subjects and three replications.
32(1960); http://dx.doi.org/10.1121/1.1908029View Description Hide Description
An apparatus is described for measurement of the acoustic impedance of the middle ear at the tympanic membrane. The measuring technique offers a possibility of studying the fine structure of the middle ear impedance, which contains not a single but several resonance peaks. The method employed makes use of a constant current sound source mechanically fixed to a microphone probe indicator. Both the sound generator and the sound receiver microphone are coupled to a tube which is inserted into the ear. This unit can be fixed directly to the subject's head, thereby eliminating the effect of the subject's movements on the measurements.
The results of measurements on eight normal subjects are presented, in part as impedance, in part as admittance. The admittance is measured for 50 cps intervals in the frequency range from 200 to 2000 cps. The individual spread of data proved to be very great and appreciably greater than the difference between the two ears of a single individual.
32(1960); http://dx.doi.org/10.1121/1.1908031View Description Hide Description
This is the second of two papers. The first (I) [J. Acoust. Soc. 32, 1 (1960)] dealt with the general magnetic properties of recording tape. The present paper gives an analysis of hf biased recording based upon the anhysteretic properties of the tape as specified in I. Calculations of performance are compared with the results of absolute measurements carried out on a conventional recording machine and on large scale models. Satisfactory agreement is obtained when the theory is corrected to take into account a loss attributed largely to a dynamic form of demagnetization which takes place during recording. The corrected theory can be used to compare tape performance under a variety of operating conditions without reference to a particular test recorder. Attention is confined to recorded wavelengths which are large compared with coating thickness.
32(1960); http://dx.doi.org/10.1121/1.1908033View Description Hide Description
Investigations of the contribution of short‐time delayed echoes to the intelligibility of speech have brought out the fact that reflections arriving within 95 msec after the direct sound in an auditorium have a positive contribution, and, reflections after this period, a negative contribution to the intelligibility of speech.
The instrument described, developed for the analysis of reflection patterns in auditoria, incorporates an electrostatic squaring device and an electronic counter as registration unit. Sound energy can be integrated by means of this instrument over pre determined short periods at any given interval after the direct sound.
32(1960); http://dx.doi.org/10.1121/1.1908035View Description Hide Description
Design considerations are given for a sinusoidal pressure generator (SPG capable of providing sinusoidal pressures up to 26 atmos peak to peak, proportional to input voltage to within 5% over the frequency range from 0 to 10 000 cps. The pressure is generated in a fluid contained in a relatively rigid metal enclosure, by means of a stack of 24 disks of a piezoelectricceramic, connected electrically in parallel, and arranged so that their displacements add. Several applications of the SPG are discussed including several types of evaluation tests for pressure gauges, evaluation of electromechanical drivers, tests of piezoelectric materials near zero frequency, and study of frequency‐dependent effects in liquids. Preliminary relaxation data are shown for two associated liquids, diphenyl metachloride (Aroclor No. 1260) and glycerol.
- LETTERS TO THE EDITOR
32(1960); http://dx.doi.org/10.1121/1.1908037View Description Hide Description
In addition to the so‐called “Reciprocity Methods” for calibratingelectroacoustic transducers, other methods are also based upon the reciprocity principle. It is therefore suggested that, at least in theoretical treatments, the modern methods be quoted as a particular form of the more general class of reciprocity techniques.
32(1960); http://dx.doi.org/10.1121/1.1908039View Description Hide Description
This is a review of the work conducted by the author since 1944 on the development of an instrument which, by means of ultrasonics, identifies a material, examines its microstructure and its homogeneity, detects impurities, and predicts its life before destruction. (“Matterscope” is a new word, coined by the author, to describe this instrument; hence the method “matteroscopy.”)
32(1960); http://dx.doi.org/10.1121/1.1908041View Description Hide Description
Measurements of the yields of some sonochemical reactions show that the number of molecules produced per unit of energy absorbed is several hundred times smaller than the corresponding radiation chemistry (gamma ray) yields. The ratio of the radical yield to the molecular yield in sonochemistry also is smaller than in a gamma‐irradiation reactions, but is approximately the same as in alpha‐particle irradiations.