Volume 29, Issue 2, February 1957
Index of content:
29(1957); http://dx.doi.org/10.1121/1.1908824View Description Hide Description
Dispersion of elastic waves in a cylindrical rod, determined experimentally by means of a wide‐band short‐duration pulse technique, compares well with theoretical curves of the classical theory. This method, compared to previous methods, extends to smaller values the range of the wavelength to diameter ratios which may be studied conveniently.
One antisymmetric and two symmetric modes were studied. In addition, waves of a resonant nature associated with the ends of the rod were identified.
29(1957); http://dx.doi.org/10.1121/1.1908826View Description Hide Description
Equations are derived for the propagation in a liquid, in a state of fine scale turbulence, of a type of so‐called transverse wave in which the elasticity is associated with the rotation of an element rather than with its shearing as in a solid. This elasticity is proportional to the density of the kinetic energy of turbulent motion. The equations are identical with those for Kelvin's gyrostatic ether model and, for small displacements, are the analog of those of Maxwell for free space.
29(1957); http://dx.doi.org/10.1121/1.1908828View Description Hide Description
This paper examines the calculation of the coefficient of variation of intensity for acoustic transmission in a randomly inhomogeneous medium. An equation for this coefficient is derived which is valid over the range of frequencies between the wave limit and the ray limit previously reported by other authors.
29(1957); http://dx.doi.org/10.1121/1.1908830View Description Hide Description
Owing to the inherent nonlinearity of the equations of motion for a perfect fluid, two or more sound waves passing through a common region will, in general, interact with one another and give rise to scatteredwaves. In this paper, a source function is obtained for the lowest order scattering process which is quadratic in the primary field variables. This function is rewritten in a form that demonstrates that no scatteredwaves exist outside the region of interaction of two sound beams intersecting each other at right angles. Furthermore, it is demonstrated that no such scatteredwaves exist when the two beams intersect at the angle , where ρ, c, and p stand for the density, sound velocity and pressure, respectively, and the subscript 0 means ambient values of these quantities. The interfering effect of pseudo‐sound, induced by radiation pressure, is suggested as an explanation for the results of Ingard and Pridmore‐Brown, who have reported recently what they believe to be scatteredwaves from the interaction region of two sound beams intersecting each other at right angles.
29(1957); http://dx.doi.org/10.1121/1.1908832View Description Hide Description
In an isotropic, thermoelastic solid shear waves are unaltered by thermal effects. However, two distinct dilatational waves exist, both of which are dispersed and attenuated by the medium. One of the waves (denoted the Ewave) is close in character to the pure elastic wave, the other wave (denoted the Twave) is similar in nature to the pure thermal wave.
The properties of the two dilatational motions are studied and relations are given expressing the variation, in each, of phase velocity, amplitude attenuation, and specific loss with impressed frequency. For the Ewave the result is verified that this disturbance propagates at the adiabatic velocity at low frequencies and at the isothermal velocity at very high frequencies. An explanation based on physical considerations is offered to account for this generally overlooked phenomenon. It is further found that the amplitude is attenuated exponentially as the square of the frequency at relatively low frequencies, but approaches finite value as the frequency increases without limit. The specific loss reaches maximum for the E wave, a minimum for the Twave, near the frequency whose period is equal to the relaxation time due to thermal currents.
Finally, the ratios are computed of the amplitude of the temperature to the amplitude of the displacement in each of the two modes of motion. Numerical work indicates that, for metals at room temperature, the effect of coupling between elastic and thermal motions is very small.
29(1957); http://dx.doi.org/10.1121/1.1908834View Description Hide Description
We consider random distributions of arbitrary identical protuberances on free or rigid base planes, and derive the approximate reflection coefficients
29(1957); http://dx.doi.org/10.1121/1.1908836View Description Hide Description
The reflection of a plane wave of sound at an interface between two perfect fluids is considered. Previous analyses of Rudnick, Keller, and Franken and Ingard are found in error as a result of improper boundary conditions. It is found that, in addition to the possibility of total reflection in some range of angles of incidence at all finite, relative speeds, there exists the possibility of a reflection coefficient exceeding unity for sufficiently high, supersonic speeds; in particular, resonance may occur at one or more angles of incidence. The question of stability of the vortex sheet separating the two fluids also is discussed.
29(1957); http://dx.doi.org/10.1121/1.1908838View Description Hide Description
The characteristic frequencies of a circular membrane carrying a rigidly attached concentric circular mass of finite area and vibrating symmetrically are studied. The main problem is that of determining the ranges of the parameters for which the frequencies will always be above or always be below those of the unloaded case. One of the chief tools is the minimum principle appropriate to the problem. The first term in the asymptotic expansion of the higher frequencies is obtained.
29(1957); http://dx.doi.org/10.1121/1.1908840View Description Hide Description
The dugga or the left‐hand thabala is a companion drum to the right‐hand thabala and the two together are widely used in India. The vibration characteristics of the latter which has a symmetrically loaded membrane were discussed previously where it was shown how this drum produces harmonic overtones.
The left‐hand thabala also has a loaded drumhead, but the loading is here eccentric. The boundary value problem of this drumhead is not separable, and hence there is no readily available method for computing its characteristic frequencies. An approximate theory for computing the frequencies of the lower modes is here developed under the assumption that for these modes the effect of the (unloaded) annular region on the loaded region may be replaced by a suitably determined elastic support around its boundary. This leads to the boundary value problem of a membrane with an elastic boundary support whose stiffness constant per unit length is a function of the polar angle. The problem is still unseparable, but the eigenvalues can be now computed by a boundary perturbation technique from those of a membrane with a boundary support of constant stiffness. Computed and measured frequency ratios agree within seven percent or better.
29(1957); http://dx.doi.org/10.1121/1.1908842View Description Hide Description
Skin of mice has been irradiated with focused ultrasound of one megacycle frequency. Skin in the growing phase of the hair growth cycle becomes ulcerated within two days following treatment while skin in the resting phase of the cycle is relatively unaffected. It is concluded that the response of the skin to ultrasound depends upon the physiological state of the organ at the time of irradiation. Thermal changes which occur during irradiation with ultrasound were measured in resting and growing skin, and no significant difference in absorption of acoustic energy between the systems was found. Since the capacities of the respective systems for absorbing ultrasound are equal this factor can be eliminated as a possible cause for the differential response of the skin. It is also suggested that the differential response of the skin cannot be attributed to structural differences between resting and growing follicles. When skin is irradiated with focused light which produces the same thermal change as that produced by ultrasound,skin in growth is ulcerated and skin at rest is unaffected. The heating component of ultrasound appears to be the physical factor to which the biological response can be attributed.
29(1957); http://dx.doi.org/10.1121/1.1908844View Description Hide Description
Detailed consideration is given to several situations in which two relaxation processes contribute to the absorption of ultrasound. A general but rather involved expression is given for αλ. From the particular cases examined, it appears that when the relaxation frequencies of the two processes are close together, a single experimental relaxational frequency will be observed, with (αλ)max somewhat higher than the value obtained by combining the two separate peaks. The observed relaxation frequency will be approximately equal to the lower of the two individual relaxation frequencies.
29(1957); http://dx.doi.org/10.1121/1.1908846View Description Hide Description
A microphone using the piezoresistive effect of n‐type germanium has been designed and constructed with a cantilever beam arrangement. Frequency response curves were taken in a sound field of 10 microbars with the microphone in a bridge type circuit and biased with 9 and 18 milliamperes.
The microphone produces 10−11 watt in the flat low frequency range and peaks to 10−9 watt at a resonance at 2570 cps with a total bias of 18 ma in a bridge type circuit. Other designs of transducers and different types of semiconductor material are discussed. The performance of a germanium rod microphone is compared with a standard carbon button transmitter.
29(1957); http://dx.doi.org/10.1121/1.1908848View Description Hide Description
A series of small condensermicrophones and accessory equipment has been developed for the measurement of high sound pressure levels. The paper derives expressions leading to the evaluation of the nonlinear distortion of the microphone, describes the construction of the several elements of the microphone system, and presents performance data. Four interchangeable microphones, all employing clamped glass plate diaphragms, provide linear measurements over a pressure range of 1 to 108 dynes/cm2. The microphones are 0.6 in. in diameter and their low directivity is shown by polar response patterns. A cathode follower tube is housed in a printed circuit base on top of which the microphone is mounted. Maximum output is 30 v rms. The equipment can be used over a wide range of temperature and barometric pressure and is sufficiently rugged to withstand severe overload and rough handling.
29(1957); http://dx.doi.org/10.1121/1.1908850View Description Hide Description
It has been observed by several different technical groups that an acoustic transducer placed at the ear can be used as a microphone to pick up the voice of the wearer. This paper presents the results of an investigation to determine the various factors which make possible the use of a microphone in the ear.
The investigation, using a laboratory model dynamic ear‐insert microphone and a protective headset, determined the relative importance of the two principal paths that conduct sound pressure from the mouth to the outer ear canal: (1) The air path around the head, and (2) the path through the head. The voice spectrum in the outer ear canal, contributed by the path through the head, was also obtained.
It appears that the ear‐insert microphone could compete with a conventional microphone in front of the lips only in an application where it is difficult to use the conventional microphone because of its size or its interference with other equipment. The investigation forms a basis for further study to determine whether the ear insert microphone has a future as a practical communications device.
29(1957); http://dx.doi.org/10.1121/1.1908852View Description Hide Description
Studies on the effect of overstimulation upon the ear have been handicapped by lack of an audiometric procedure known to detect subtle changes in the organ of Corti. Fatigue has often been suggested and used but studies of the cochlear potentials associated with low‐intensity vibration of the organ of Corti have never revealed such fatigue, which must arise from more central structures. Fatigue following high levels of sound intensity is probably a sum of “neural” fatigue and an injury to the organ of Corti. On the other hand, such studies have shown aural overload to result from an overdriving of the organ of Corti beginning at an intensity of sound considerably below that which is injurious.
The experiments reported here compare the shift in audibility threshold and in onset of overload following one‐minute stimulations with a 1000‐cps tone at sensation levels of 20, 60, 80, 90, 100, and 110 db. The amount of post‐stimulus fatigue measured 6 seconds after cessation of the fatiguing tone showed little variation with intensity of fatiguing tone whereas the onset of overload, when compensation was made for fatigue of the harmonic frequency, showed a progressive lowering with increase of intensity of fatigue tone.
It is concluded that within the limitations of these experimental conditions the fatigue measured is not a property of the sensory cells whose sensitivity does not change until an injurious level of tone is reached, but that overloading is a property of these cells and the lowering of its onset level reflects an increasing decrement in performance following increasing intensities of stimulation.
29(1957); http://dx.doi.org/10.1121/1.1908854View Description Hide Description
This study was designed to investigate the properties of the first neural response (N 1) to tone pips; the responses were recorded from the round window of the cat. Amplitude, latency, and adaptation (equilibration) curves were studied. The major part of the investigation was concerned with the effect of low‐tone exposure on the amplitude of the N 1 response to tone pips. After a relatively intense exposure, the amplitude of the N 1 response, compared with the pre‐exposure level, shows an initial subnormality (decreased response), a sensitization (increased response), and then a second subnormality. Sensitization was found when the exposure tone was the same, higher and lower in frequency than the basic frequency of the test tone pip. Exposure to low‐frequency noise gives rise to a monotonic recovery process; i.e., a subnormality without subsequent sensitization. Contralateral effects were not detected. This sensitization of the N 1 response is compared with related psychophysical data and may be regarded as an example of post‐tetanic potentiation.
Psychophysical experiments [J. R. Hughes, Ph.D. thesis, Harvard University (1954) and reference 19] have demonstrated the existence of “auditory sensitization” (a lowering of the absolute threshold for pure tones after low‐tone exposure). After a relatively intense exposure, there is a brief rise in threshold, then a sensitization which lasts for approximately 2 min, followed by a long‐lasting second rise in threshold. This psychophysical study also demonstrated that a relatively wide range of exposure frequencies can sensitize the auditory system to a given test tone pip.
In order to obtain comparable evidence for sensitization in the electrical activity of the auditory nervous system, electrophysiological experiments were conducted on cats. The present study was designed to investigate the effect of a pure‐tone stimulus on the properties of the first neural response (N 1) recorded from the vicinity of the round window of the cat's cochlea. The major part of the investigation was concerned with whether, after low‐tone exposure, the N 1 response exceeds in amplitude the pre‐exposure value: this phenomenon, when it occurs, will be labeled “sensitization of N 1.”
These electrophysiological studies on sensitization may be related to post‐tetanic potentiation (PTP) (see references 11 and 21); this possible relationship will be discussed later. PTP refers to the increased responsiveness of the nervous system after application of a tetanizing stimulus.
29(1957); http://dx.doi.org/10.1121/1.1908856View Description Hide Description
Experiments are reported on temporal summation at threshold for thermal noise. Durations from 1 to 850 msec and repetition rates from 1 to 100 per sec were used. Results show that threshold decreases as a function of duration for all repetition rates. Thresholds also decrease as a function of repetition rate, though the effect of repetition rate is not so marked as that of duration, and becomes even less so at long durations.
Influence of Noise upon the Equivalence of Intensity Differences and Small Time Delays in Two‐Loudspeaker Systems29(1957); http://dx.doi.org/10.1121/1.1908858View Description Hide Description
The equivalence of intensity difference and time delay in two‐loudspeaker systems is greatly affected by the presence of extraneous noise. An experiment is described in which this effect is measured with speech and wide‐band noise. It is found that as the noise level is increased the equivalent intensity difference, for a given time delay, decreases, tending to zero for noise levels approaching the masking level.
29(1957); http://dx.doi.org/10.1121/1.1908860View Description Hide Description
Two series of intelligibility tests were conducted. In the tests, speech was presented against a background of interference. The line‐spectrum interference consisted of from 4 to 256 sinusoids, superposed in a linear adder. Three different spacings of the components in frequency, and several different distributions of power among the components, were studied. Tests with continuous‐spectrum random noise were made for comparison.
The over‐all interference power required to reduce intelligibility to a given level decreases as the number of components in the interference increase. The drop is about 10 db in the decade from 4 to 40 components. Beyond 40, there is much less change. Even 256 components, however, mask measurably less effectively than random noise of equivalent power in the band 200–6100 cps. For a given number of components, the line‐spectrum interference most detrimental to intelligibility has the same number of components in each frequency band of equal contribution to intelligibility, and its components are uniform in amplitude. The bearing of these findings on the theory of intelligibility and on procedures for predicting intelligibility from physical parameters is discussed.
29(1957); http://dx.doi.org/10.1121/1.1908862View Description Hide Description
The threshold of intelligibility for a word in a wide‐spectrum noise is shown to be a decreasing function of the frequency with which the word occurs in general linguistic usage (word frequency). The drop in threshold is about 4.5 db per logarithmic unit of word frequency. This rate is independent of the length of the word, although the thresholds for words of given frequency of occurrence are lower for long words.
The effect of restricting the listener's alternatives in an intelligibility test to a specified number of words is calculated from this relationship. These calculations come within 1 db of published experimental data. Theoretical functions relating intelligibility threshold to word length are also calculated from the word‐frequency effect, on the assumption that listeners can discriminate the length of a word at levels too low for it to be identified. These functions are in general agreement with the experimental results.
Implications for intelligibility testing procedures are discussed.