Volume 23, Issue 1, January 1951
Index of content:
23(1951); http://dx.doi.org/10.1121/1.1906722View Description Hide Description
For a circular plane piston of radius a, producing an ultrasonic beam with propagation constant k (or 2π/λ), an expression is derived for the velocity potential or the acoustic pressure, averaged with respect to magnitude and phase over a “measurement circle” equal in area to the piston and centered in the beam. The expression should be highly accurate for ka ⩾ 100, at distances z from the source governed by . It agrees well with results computed, in another way, by Huntington, Emslie, and Hughes. The assumption that relatively near the source there is a collimated beam of plane waves is shown to be not very accurate; the averaged pressure falls off monotonically over all distances considered. The velocity potential at the rim of the “measurement circle” is also computed, and compared with the plane wave assumption.
23(1951); http://dx.doi.org/10.1121/1.1906733View Description Hide Description
An apparatus is described for measuringacoustic wave fronts in water at frequencies in the vicinity of 1 mc. The sound waves are radiated by a piezoelectric crystal and are picked up by a movable microphone. The voltage output from the microphone is compared in phase with the voltage feeding the piezoelectric source, by means of a phase discriminator. The signal at the microphone is due both to the transmitted acoustic wave and a stray electric signal. These two are combined in the microphone in such a way as to produce a full wavelength modulation in the resultant electric signal. A wave front in the sound wave is then traced by moving the microphone along such a line or surface as will cause the phase discriminator to indicate constant phase difference. A mathematical analysis of the problem is given and the method of measuring the shape of acoustic wave fronts in water is described. The apparatus and method which are described reduce the time required for measuringwave fronts to about one‐third that previously needed. Typical experimental results are given.
23(1951); http://dx.doi.org/10.1121/1.1906718View Description Hide Description
A phenomenological theory of volume viscoelasticity formulated, resulting in an equation recently used by Hall. Application of this equation to the dispersion and absorption of sound in fluids is extended to the whole range of frequencies. Results of calculations of the volume viscosity and the instantaneous and relaxational compressibilities for gases and liquids from certain available absorption data are given. The bearing of this theory on the classical theory of hydrodynamics is pointed out.
23(1951); http://dx.doi.org/10.1121/1.1906720View Description Hide Description
A technique is described which makes use of Bragg reflection of x‐rays to give a “picture” of the vibration pattern on the face of a crystalline plate. The dependence of this pattern on the direction of the x‐ray beam is illustrated, and a comparison between dust patterns and x‐ray patterns for a vibrating bar fairly well establishes that the local curvature of the crystal is responsible for the observed increase in x‐ray reflection. Reference is made to quantitative measurements on statically bent quartz and to an approximate theory treating the effect.
The Coarse Pattern of the Electrical Resistance in the Cochlea of the Guinea Pig (Electroanatomy of the Cochlea)23(1951); http://dx.doi.org/10.1121/1.1906721View Description Hide Description
With the knowledge now available concerning the mechanical properties of the cochlea, it is possible to understand how movements of the stapes footplate are transmitted to and along the cochlear partition. But we know little about the electrical constants inside the cochlea (the resistances and capacitances). It is difficult therefore to draw conclusions about how voltages at a given point are transmitted to other parts of the cochlea, for instance, how microphonic voltages are transmitted to the round window. This paper tries to show that for parts of the cochlea near the two windows the cochlear tube (consisting of the two scalae and the cochlear duct) can be considered electrically as a transmission line. The characteristic values of this transmission line are measured for different distances from the windows. In this way, a start has been made toward the development of a type of electroanatomy. The impedance values of the cochlear partition are important for determining the electrical energy losses in the cochlea.
23(1951); http://dx.doi.org/10.1121/1.1906723View Description Hide Description
A vibrating electrode has been developed, with which the microphonics produced at a single point on the cochlear partition can be measured. With this electrode, which can be made to vibrate in any of three orthogonal directions, different places on the cochlear partition were investigated. The basilar membrane is more sensitive to displacement in the radial direction than in the direction perpendicular to its plane. The important question is whether microphonics are produced by a constant stationary displacement or whether they are produced only during the movement of the cochlear partition. With the aid of trapezoidal waves it was possible to show that in a fresh cochlear preparation only the static displacement produces the microphonic voltage. A single displacement of the basilar membrane produces astatic voltage lasting for several seconds. This fact has an important bearing on the energetics of the microphonics.
Preliminary experiments suggest that the same technique may be used to investigate the inner parts of the cochlear partition.
23(1951); http://dx.doi.org/10.1121/1.1906724View Description Hide Description
Ear wardens in form of ear plugs acting as acoustic low pass filters are described. They allow a part of speech frequencies to pass through into the ear and in this way improve the speech intelligibility in comparison with other ear plugs. Nevertheless their protective action against noise was proved to be sufficient.
23(1951); http://dx.doi.org/10.1121/1.1906725View Description Hide Description
“Tone‐pips” were produced by brief rectangular electrical pulses being delivered through two sound‐effects filters in cascade with both high and low cut‐offs set at 2000 cps. Nearly all of the acoustic energy of the final signal was found to be concentrated in a band about an octave wide and centering at 2000 cps. The pulsing frequency was varied independently between 90 and 150 pips per second.
Listeners describe the resulting sound as a “metallic buzz.” Listeners vary greatly in their ability to identify the two “pitches” present in this sound and in the accuracy with which they match with a pure tone either the pulsing frequency (about 130 per second) or the band‐pass frequency (2000 cps in the present series). Errors of exactly one octave are particularly common.
In the theoretical discussion we argue that the “pitch” of a pure tone is a double attribute compounded of “buzz” (correlated with frequency of volleys of nerve impulses) and “body” (correlated with position of maximum stimulation on the basilar membrane).
23(1951); http://dx.doi.org/10.1121/1.1906726View Description Hide Description
The vowel sounds are produced by vibration of the vocal cords and are modified by the filtering effect of the resonant cavities. Thus, in the case of voiced speech sounds and of the sounds of singing, two factors play a part in the final shaping of the sound picture: the cord‐tones and the resonators. The present paper deals with the cord‐tones and with the peculiarities of vibration of the vocal cords.
23(1951); http://dx.doi.org/10.1121/1.1906727View Description Hide Description
The art of “voicing” an organ pipe, that is, treating it in order to produce a satisfactory musical tone, is highly developed, but on an entirely empirical foundation. Flue pipes only are dealt with here, chiefly the open diapason, an open metal cylindrical pipe. The several tone‐producing mechanisms in it are described, and some two dozen different variables, each of which can affect the tone, are described with reference to the stage in manufacture in which they occur. Some account is given of the interrelationship of the various adjustments. Finally mention is made of the place of any one pipe in the tonal structure of a complete organ.
23(1951); http://dx.doi.org/10.1121/1.1906728View Description Hide Description
As one phase of a noise survey of the Chicago area, measurements were made of the acoustic output of commonly used automobile horns. Over‐all sound levels on the axis at three feet ranged from 108 to 125 db. Loudness levels ranged from 125 to 140 phons. Fundamental frequencies of all horns ranged between 160 and 380 cps. Two types had approximately harmonic overtones, with large amplitudes below about 2000 cps and smoothly decreasing amplitudes at higher frequencies. A third type, with the most unpleasant sound, had inharmonic overtones, groups of which were greater in amplitude than the fundamental. Sound from a pair of horns at various distances was measured both inside and outside a closed automobile. The over‐all level outside was 88 db at 50 feet and 74 db at 300 feet, with corresponding loudness levels of 105 and 82 phons. The over‐all level inside was 60 db (loudness level, 72 phons) at 50 feet and 50 db (loudness level, 54 phons) at 300 feet. Filtering out overtones above 1200 cps improved the quality of horn sounds markedly, reducing the loudness level inside the automobile by four phons at 50 feet, but only one phon at 300 feet. Since the typical horn is louder than necessary at close range, use of a low pass acoustic filter on automobile horns appears desirable.
23(1951); http://dx.doi.org/10.1121/1.1906729View Description Hide Description
A fixed path ultrasonicinterferometer for use in vapors is described, in which the stationary wave system between the quartz source and the reflector is explored by a traveling hot‐wire. The hot‐wire is calibrated for its change of resistance in terms of velocity‐amplitude in the ultrasonic wave system. A correction is made for diffraction of the ultrasonic beam.
In this apparatus wavelength and absorption measurements are made in ether, ethylene, benzene, carbon disulphide, and acetaldehyde vapors. All show an absorption considerably greater than the theoretical values based on the Stokes‐Kirchhoff equations.
23(1951); http://dx.doi.org/10.1121/1.1906730View Description Hide Description
A reverberation method has been used to measure the absorption of sound at frequencies down to 140 kc in a number of liquids of widely different properties. For glycerol the experimental result agrees well with that predicted by classical theory. For benzene and water good agreement is found with the values calculated using the modified theory employing the values of second viscosity determined by Liebermann. The absorption in carbon disulfide agrees well with Ouang's result. For these four liquids the absorption is found to be proportional to the square of the frequency, but results indicate that toluene has a relaxation frequency of the order of 105 sec−1.
The reverberation method is especially useful at the lower ultrasonic frequencies, where loner wave‐lengths and lower absorption render methods used at higher frequencies less reliable or unusable.
23(1951); http://dx.doi.org/10.1121/1.1906731View Description Hide Description
Experimental values are reported for the sound absorption coefficients in aqueous solutions of magnesium sulfate over the frequency range 3 to 43 megacycles. Measurements were made by an electrical method at the lowest frequency and by the pressure balance method at higher frequencies (12–43 mc). Absorption coefficients are reported as a function of frequency, concentration, and temperature. An excess absorption (relative to water) is found at the higher frequencies which cannot be interpreted by the present theory.
23(1951); http://dx.doi.org/10.1121/1.1906732View Description Hide Description
The ultrasonic absorption of an associated liquid, glycerin, has been measured, in the liquid and vitreous state, at 31 Mc. The value of absorption found exceeds the theoretical value at high temperatures by a factor of 2.1. This indicates that both compressional and shear viscosity contribute to the absorption process. When the liquid was cooled, a peak in the absorption was found at −6.5°C; and at still lower temperatures the absorption fell to considerably lower than classical values. The appearance of a peak in the absorption experimentally verifies the fact that both compressive and shear viscosity are associated with a relaxation phenomenon. The relaxation times for the two processes are of the same order of magnitude, and the same is true of the activation energies.
23(1951); http://dx.doi.org/10.1121/1.1906734View Description Hide Description
It is shown that with or even without ultrasoniclenses or other devices for the concentration of ultrasonic energy, by ultrasonics,images can be made on photo‐sensitive emulsions. Several of the first ultra‐sonographs, as these images are called, are reproduced and some potential fields of application of the new technique are mentioned.
23(1951); http://dx.doi.org/10.1121/1.1906735View Description Hide Description
Some improved sound wave interference methods for measuring the longitudinal and transverse ultrasonic velocity in opaque as well as transparent solids may be simply carried out by using the ultrasonic light‐diffraction system (as arranged for making sound beams visible on a screen). The sonic unit of the system is arranged to produce two individual traveling‐wave sound beams, by use of two generators by splitting a single beam. Three simple arrangements are described in detail. In Case A one beam travels entirely in a reference liquid, while the other beam travels a parallel path in an immersed transparent test specimen. In Case B one beam travels entirely in a reference liquid, while the other beam travels an adjacent course through an immersed, transparent or opaque test prism, and on into the liquid at an angle to the first beam. In Case C the two beams are generated at the equal edge faces of a transparent or opaque isosceles test prism (only the base edge face contacting the liquid). The two beams traverse the prism to the base, where they are refracted into the test liquid as confluent beams.
In Cases A and B, simulated interference (by optical integration), and in Case C true interference, each give a light and dark band interference pattern on the screen, whose band spacing is used in calculating the velocity of sound in the test solid. The other required factors are the optical image magnification, the frequency of the sound, the angular disposition of the one or more acoustic surfaces of the test solid relative to the incident sound beams, and in Cases A and B the velocity of sound in the reference medium. Other variations of arrangement are suggested.
Advantages of the improved methods are simple preparation of test specimen, directness and simplicity of measurement and calculation, good accuracy, low sonic power requirements. A table of measuredvelocities (and attenuations) in two metals and in numerous plastics and polymers show the wide range of materials that may be measured by the new interference methods.
23(1951); http://dx.doi.org/10.1121/1.1906736View Description Hide Description
A general analysis of variable resonant frequency crystal systems which utilize liquid media as backing is presented. Formulas are obtained for evaluating the effect of different geometries of backing on the resonant frequencies of the system. The various loss factors associated with such a system are related to the measurable quantity, the quality factor.
The observed operating characteristics of a variable resonant system which covers a two‐to‐one frequency band (40 kc to 80 kc) are presented. The results of an experimental comparison of such a system with fixed resonant frequency systems for generation of ultrasound in liquid media are given. The theory is used to correlate the experimental observations.
- LETTERS TO THE EDITOR
23(1951); http://dx.doi.org/10.1121/1.1906714View Description Hide Description
23(1951); http://dx.doi.org/10.1121/1.1906715View Description Hide Description