Volume 20, Issue 2, March 1948
Index of content:
20(1948); http://dx.doi.org/10.1121/1.1906364View Description Hide Description
The equations of propagation of a compressional wave through a moving medium are developed in vector notation. Application is made to the special case of a linear vortex of constant strength m in which the motion is irrotational. Differential equations are set up in general form for the wave fronts and rays. A simple approximation is derived for the distortion produced in an originally plane wave front by transmission through the vortex. The results are in general agreement with observed fluctuation phenomena in acoustic propagation in air.
20(1948); http://dx.doi.org/10.1121/1.1906365View Description Hide Description
The problem of analyzing the spectrum of acoustic signals is discussed for various types of filters. The problem is straightforward for narrow filters but requires, in general, a self‐consistent assumption for broad filters. The necessary assumption is made, and two examples are given to show the nature of the problem.
20(1948); http://dx.doi.org/10.1121/1.1906366View Description Hide Description
When an acoustic load is coupled to an electromagnetic transducer through a length of air column, there is a critical length at which an increment in acoustic load produces a maximum increment in electrical impedance. The method described makes use of this critical coupling to obtain an optimum precision in determining an acoustical impedance. It is noteworthy that the value of the characteristic impedance of the air column is not assumed but is directly measured.
20(1948); http://dx.doi.org/10.1121/1.1906352View Description Hide Description
The phase shift analysis is applied to the absorption and scattering of plane waves by spherical and circular cylinder regions. The boundary condition on the surface of these regions is expressed in terms of an impedance. A method of calculating the required phase shifts and corresponding scattering and absorption cross sections is presented together with the necessary tables. It is found that when the reactive part of the impedance is capacitative, resonance phenomena can occur. Approximate cross sections for low and high frequencies are given. It is shown that at low frequencies, acoustic treatment of scatteringsurfaces will, in general, increase the scattering and, therefore, their ability to increase the diffusion of sound in a room.
20(1948); http://dx.doi.org/10.1121/1.1906353View Description Hide Description
The behavior of the living elastic bodies in the ear must be studied for a full understanding of the ear as sound conductor. Two elementary structural elements possessing restoring forces are the collagen fiber and the elastic fiber. These two tissue elements form membranes, ligaments, tendons, and articulations in the ear.
The physical behavior of these two fiber systems is described. Three areas in the ear are distinguished according to their construction material. Methods of measuring the physical characteristics of the ear structures are described. The linear displacement of the center of the ear drum as a function of static pressure changes in the outer ear canal is given. Static movements of the stapes footplate due to reflectoric muscle contractions were used to calculate the elongation of the stapes ringband in vivo.
20(1948); http://dx.doi.org/10.1121/1.1906354View Description Hide Description
A cavity pressure method for measuring the gain of hearing aids has been developed. It has the advantage of requiring no dead room, allowing a compact set‐up. The method has been investigated as to its validity and the degree to which diffraction effects are avoided. Comparisons with free field data are made. The applicability of results obtained by this method to the specification of gain characteristics for hearing aids is discussed.
20(1948); http://dx.doi.org/10.1121/1.1906355View Description Hide Description
Some marine animals produce sounds which, under certain conditions, completely dominate the ambient noise in the sea. The snapping shrimp (not to be confused with the edible shrimp) are the most widely distributed of these animals; they are frequently less than 3 cm in length and produces the sound by snapping of the claw. Shrimp noise is likely to be found around the world in tropical and subtropical waters less than 55 meters deep wherever rock, coral, or other material on the bottom provides interstices in which shrimp thrive. This paper describing the acoustic output of the snapping shrimp (Crangon and Synalpheus) is based upon measurements made off the Southwestern and Southeastern coasts of the United States, the Hawaiian Islands, and several islands in the Southwest Pacific. Over a shrimp bed, the noise spectrum is found to be roughly independent of frequency from 2 to 24 kc (the upper limit of measurement), whereas the ambient noise normally present in the deep sea decreases with frequency. There is a broad peak in the shrimp spectrum somewhere between 2 and 15 kc. Over a shrimp bed the noise at 20 kc is about 25 db above the ambient noise which accompanies a sea state of 2 (waves less than a meter high, not including swell). For low sea states, shrimp noise is appreciable a mile or more from the boundary of the bed. The maximum diurnal variation of shrimp noise is from 3 to 6 db, the noise being greater at night. Single snaps from isolated specimens have been subjected to a Fourier integral analysis which indicates a spectrum comparable to that measured over shrimp beds where the underwater sound consists of a multitude of snaps. A typical peak sound pressure at the distance of a meter from a single shrimp is of the order of 200 dynes/cm2.
20(1948); http://dx.doi.org/10.1121/1.1906356View Description Hide Description
In Pearl Harbor, where there often were eight hundred ships of all kinds, the underwater noise level was high. No place was found where noise measurements could be made satisfactorily, and therefore it was decided that the best arrangement would be to insulate Auxiliary Repair Docks, and measure the noise of submarines while they were in the docks. This was done by the development of a suitable air bubble screen across the open end of the dock. Such an acoustic barrier was comparatively easy to install, did not interfere with submarines entering and leaving, kept oceansurface oil out of the dock, insulated against low as well as high frequency noises as was required, and, after extensive experimentation, the noise of the screen was reduced to a level that did not interfere with the noise measurements. The insulation of the screen upon the noise of a nearby submarine charging batteries is illustrated by a phonograph recording.
20(1948); http://dx.doi.org/10.1121/1.1906357View Description Hide Description
Thresholds for the detection of the presence of a sinusoidal signal (thresholds of audibility) and thresholds for the perception of the tonal character of the signal (thresholds of tonality) were determined by three listeners over a wide range of audible frequencies. At each of the frequencies tested, the threshold of audibility was found to be significantly lower than the threshold of tonality. The difference between the two thresholds—the atonal interval—ranged from about 2.5 db at 500 c.p.s. to about 6.5 db at 4000 c.p.s. Little difference was found between the variabilities of the two thresholds.
Preliminary measurements indicated that acuity of frequency discrimination changes quite abruptly at the threshold of tonality. Below the threshold of tonality, frequency discrimination is very poor; above the threshold of tonality, frequency discrimination is markedly more acute.
20(1948); http://dx.doi.org/10.1121/1.2142326View Description Hide Description
20(1948); http://dx.doi.org/10.1121/1.1906358View Description Hide Description
If a communication engineer, confronted with a sound wave consisting of speech mixed with audible random noise, were requested to build a device to separate the speech from the noise, he would be hard pressed to produce a mechanism as effective as the human auditory system. But if he were given twowaves, one a sample of speechplus a sample of random noise, the other the same speechminus the noise, he would invoke the elementary mathematical (or electronic) processes of addition and subtraction and oblige in short order with noise‐free speech and with speech‐free noise.
This paper examines the performance of the (binaural) human auditory system in handling the two‐wave problem. The effectiveness of the solution is judged in terms of the intelligibility of speech heard against a background of white noise. If monaural intelligibility is taken as a standard of comparison, it is found that the advantage of binaural presentation of the speech and the noise depends upon the interaural phase relations. The auditory system handles best the problems that are easiest for the engineer, though not as effectively as the engineer would handle them. Intelligibility is highest with noise plus speech in one ear, noise minus speech (i.e., the noisewave plus the invertedspeechwave) in the other. Words are understood almost, but not exactly, as well with speech plus noise in one ear, speech minus noise (i.e., the speechwave plus the invertednoisewave) in the other.
These modes of presentation, in which either the speechwaves or the noisewaves in the two ears are 180 degrees out of phase, yield word articulation scores as much as 25 percentage units higher than the more conventional mode of presentation in which both the speechwaves and the noisewaves in the two ears are in phase. Observations with other interaural phase relations and with monaural‐binaural presentation of speech and noise are also described.
The results suggest a means of providing a small but probably significant improvement in reception whenever speech is heard through earphones in the presence of ambient noise. The scheme is simply to reverse the connections of one of the earphones.
The significance of the results for the theory of masking is discussed.
20(1948); http://dx.doi.org/10.1121/1.1906359View Description Hide Description
Short bursts of random noise were used to study the perceptual growth and decay of the noise experience. Noise, unlike sinusoidal tones, does not change materially in spectrum as the duration of the sound is varied.
A short burst of noise must be more intense in order to be equal in effectiveness to a longer noise. Thus the threshold of hearing is lowered by increasing the duration of the noise up to durations at least as long as 1 sec. The loudness of an intense noise, however, depends upon its duration up to durations of only 65 msec. Judgments of the slowest rate of decay of a noise which sounded indistinguishable from an abrupt termination also led to the concept of a critical time of approximately 65 msec. for the sensation to decay to threshold from any steady magnitude of stimulation.
From these data it is inferred that the auditory system acts as if the growth and decay periods of the noise perception depend upon differences in latencies among the various neural paths in transmitting the cochlear activity to the higher centers in the brain. According to this hypothesis, the activity in the slowest pathways arrives at the higher center 65 msec. after the activity of the fastest pathways. By means of this hypothesis it was possible to predict the judgments of time of termination of noises in the two ears when the intensity of the noise differed between the ears.
20(1948); http://dx.doi.org/10.1121/1.1906360View Description Hide Description
Noise interrupted at a steady rate has essentially the same spectrum over the range of frequencies transduced by the earphone as does continuous noise. The frequency corresponding to the rate of interruption is not intensified in the spectrum. Consequently, the ability of listeners to respond differentially to the rate of interruption cannot be explained on the basis of a simple resonance theory of hearing. The point at which an interrupted noise becomes indistinguishable from a continuous noise depends upon the rate of interruption, the sound‐time fraction, and the intensity of the noise. For a sound‐time fraction of 0.5, the presence of interruptions can be detected at rates well above 1000 per second. Differential sensitivity to changes in the rate of interruption (with a sound‐time fraction of 0.5) is poor above 250 interruptions per second. Also at these high rates the listener loses his ability to match the frequency of a pure tone to the rate of interruption. Presumably the ability to perceive interruptions in a random noise depends upon the synchronous firing of the fibers in the auditory nerve. This hypothesis is supported by the correspondence between auditory sensitivity to changes in the rate of interruption of a noise and the tactual sensitivity to changes in the frequency of a vibrating pressure applied to the skin.
20(1948); http://dx.doi.org/10.1121/1.1906361View Description Hide Description
Very large amplitudes of the velocity and acceleration produced by a sound wave in water may be evoked by exposing thin laminae of the liquid to an airborne sound wave. In the case of water and air at atmospheric pressure and at a frequency of 10,000 c.p.s., the power advantage is shown to be a factor of about 3600 when the thickness of the lamina is small compared with 14 microns. On the other hand, the use of liquid laminae is ineffective when the effects desired depend on the pressure amplitude of the sound wave, as distinct from the velocity or acceleration. Sketches of an experimental arrangement are shown.
20(1948); http://dx.doi.org/10.1121/1.1906362View Description Hide Description
The absorption and velocity of sound in binary mixtures of water with alcohols, glycols, and glycol ethers have been measured at frequencies in the range of 5–25 megacycles, using the optical diffraction method originally developed by Debye, Sears, Lucas, and Biquard. However, the method of absorptionmeasurement has been improved by utilization of a high gain, secondary multiplier‐type photo‐tube viewing the first‐order diffraction spectrum, the change in intensity of this spectrum being an index of the sound absorption in the liquid. This method has proved more rapid and precise than other diffraction techniques described in the literature.
Ultrasonic absorption peaks at intermediate concentrations have been found in mixtures containing acetone, ethanol, propanol, isopropanol, and tertiary butanol and in the monoethyl and monobutyl glycol ethers. In both series, the magnitude of the peaks increases as one progresses from the ethyl to the butyl derivative, and there is a simultaneous shift of the peak toward higher water concentration. No absorption peaks were found in mixtures containing methanol, glycol, monomethyl glycol ether, or dioxane. Velocity peaks at intermediate concentrations have been found in all of the mixtures listed. Mixtures of tertiary butanol and methanol show neither absorption nor velocity peaks.
The observed absorptions are considerably greater than would be expected from classical theory. Consequently, the experimental data appear to require a new, or at least modified, theoretical approach. The problem is discussed briefly in the light of modern theories of molecular association, liquid crystal formation, etc.
20(1948); http://dx.doi.org/10.1121/1.1906363View Description Hide Description
- PROGRAM OF THE THIRTY‐FOURTH MEETING OF THE ACOUSTICAL SOCIETY OF AMERICA
20(1948); http://dx.doi.org/10.1121/1.1916867View Description Hide Description
By the most comfortable listening level for a pure tone is meant the level which the listener chooses as most comfortable for listening when he listens to a pure tone of which he can change the level at will. Ranges and mean values of most comfortable listening levels are given for pure tones in the range 128 to 8192 c.p.s. (at octave intervals) for listeners with normal hearing and listeners previously exposed to gunfire and airplanenoise, with resultant marked hearing loss in some cases. Tentative most comfortable listening level contours are suggested. The use of a most comfortable level contour for specification of a hearing aid amplifier is discussed and an example given of the effectiveness of such an amplifier as compared to one specified by a most comfortable loudness level contour.
20(1948); http://dx.doi.org/10.1121/1.1916906View Description Hide Description
Bone conduction thresholds were taken at frequencies from 1500 to 12,000 c.p.s., using the induction‐type electrodynamic bone conductor and the air and bone conduction test assembly described in earlier papers. Stray air radiation from the bone conductor is an important factor at the frequencies considered; therefore, the bone conductor was enclosed by a sponge rubber jacket to minimize the air radiation, and the ears were covered by the test assembly boxes lined with absorbing material in order to protect them from the residual radiation. The present investigation is an extension of earlier work done at frequencies at which stray air radiation effects are negligible, and the results obtained are combined with those of the earlier study to yield a tentative bone conduction threshold from 80 to 12000 c.p.s.
20(1948); http://dx.doi.org/10.1121/1.1916915View Description Hide Description
Equations showing the relation between the volume, V, of an auditorium and its optimum reverberation time, T 0, were published independently by MacNair and Liftschitz. MacNair's equation was derived from the following basic assumptions: , (1) , (2) where ρ = instantaneous value of the energy density of sound during decay. T 1 = time of decay to minimum audibility. E = rate of supply of energy during the steady state condition.
This paper shows that an equation, relating T 0 and V, can be deduced which more nearly fits all the empirical facts by basing the derivation on the following assumptions: , (1) , (2) where G = a constant, n = a constant which should be less than unity.
The resulting equation, for , is . This is in agreement with the empirical data and with the equation derived by Maxfield and Albersheim from acoustic “liveness” considerations.
The psychologists and physiologists studying the mechanism of hearing should be interested in these results, as they have a bearing on the manner in which the ear integrates the sound being perceived.
20(1948); http://dx.doi.org/10.1121/1.1916916View Description Hide Description
Some experiments have been conducted to determine the effect of intense high frequency airborne sound on mice and a variety of insects. The sound source was a high frequency siren. The frequency used was about 20 kc, and its acoustic level, in the region where the subjects were placed, was between 160 and 165 db (relative to 10−16 watt/cm2). With sufficient exposure—from 10 seconds for flies and mosquitoes to 3 or 4 minutes for roaches and caterpillars—the sound proved lethal in all cases. More detailed work was performed on mice and the roach, Periplaneta americana. In both cases it was definitely established that the heating produced by sound absorption was sufficient to cause death. In addition to the heating there are other effects, notably tissue rupture, as evidenced by the almost complete distruction of the wings on flies and mosquitoes and the rapid deterioration and final disappearance of the external pinna of a mouse who had received a sublethal dose. During observations it has been impossible to completely avoid personal exposure to the sound field, and some of the effects observed under these conditions will be described. These included momentary dizziness, and heating of exposed parts of the hand.