Index of content:
Volume 24, Issue 2, March 1952
24(1952); http://dx.doi.org/10.1121/1.1906863View Description Hide Description
24(1952); http://dx.doi.org/10.1121/1.1906864View Description Hide Description
The total absorption for theater chairs with back and seats upholstered in mohair has been measured from 250 to 9000 cps in the Defense Research Laboratory reverberation chamber. Values are given for the chairs with (1) seats up, (2) seats down, and (3) occupied by adults.
24(1952); http://dx.doi.org/10.1121/1.1906865View Description Hide Description
The acoustic treatment of an excessively reverberant civic coliseum is described. Before treatment the structure, originally an aircraft hangar, had a sharply peaked reverberation time of eight seconds in the region between 600 and 800 cycles per second. Thirty‐two thousand square feet of two‐inch‐thick Fiberglas panels were suspended in horizontal V's six feet below the parabolic metal ceiling. After treatment, the reverberation time is essentially uniform from 100 cps to 800 cps, with an average value of approximately 1.2 seconds. The method of construction and the mounting of the panels is described. Sound absorption coefficients for these panels are given, and a comparison made between the value obtained from the coliseum installation and the value obtained for 72 square feet of identical panel hung in the Defense Research Laboratory polycylindrical reverberation chamber.
24(1952); http://dx.doi.org/10.1121/1.1906866View Description Hide Description
The transformer action of the middle ear as measured by Bekésy is shown to be the principal cause for the low acuity of hearing for low frequencies. Because of the very low mechanical impedance across the basilar membrane at low frequencies, large acoustical pressures in front of the ear drum produce appreciable acoustical pressures across the basilar membrane. For example, at 100 cps this pressure is 30 times and at 6000 cps it is 1/10 that created across the basilar membrane.
24(1952); http://dx.doi.org/10.1121/1.1906867View Description Hide Description
After an ear is exposed to an intense sound, the absolute threshold for many sounds is raised, usually temporarily. The manner in which the absolute threshold recovers to its normal value after such stimulation is the subject of these experiments. It is found that recovery from such auditory fatigue is not a simple monotonic process. Rather these experiments show that under certain conditions the threshold first recovers to an approximately normal value about 1 minute after the cessation of the exposure but then rises again to a higher value that reaches a maximum at about 2 minutes after the exposure. This diphasic recovery curve, with its characteristic “bounce,” is found when the exposure involves sound pressure levels between 100 and 120 db and durations of the order of several minutes.
Recovery curves are shown for the auditory thresholds of clicks, bands of noise, and pure tones. Fatiguing stimuli include bands of noise and pure tones.
Two subsidiary observations are also reported. First, in many recovery curves there is evidence for a second “bounce” or rise in the threshold after stimulation. Second, after stimulation by moderate intensities the initial recovery (about 1 minute after stimulation) may demonstrate facilitation, i.e., a temporary reduction in the absolute threshold below the normal value. Qualitative observations concerning the pitch and timbre of the test stimuli and of tinnitus following stimulation are related to some of the data.
24(1952); http://dx.doi.org/10.1121/1.1906868View Description Hide Description
Opening and closing the mouth increases the sound pressure produced by bone conduction in the closed auditory canal by as much as six to ten decibels in the frequency range between 40 cps and 700 cps. This difference is explained by vibrations of the lower jaw relative to the skull. The resonance curve of this motion was measured and used to calculate the influence of the lower jaw motion on the sound level in the closed auditory canal. The results show that the measured frequency response of the difference in sound pressure open mouth vs closed mouth, may be explained entirely by vibrations of the lower jaw.
24(1952); http://dx.doi.org/10.1121/1.1906869View Description Hide Description
When a pure tone is mixed with a noise of uniform spectrum, its threshold is raised. At levels above threshold, its loudness and pitch are changed by the presence of the noise. Introducing abrupt changes in the slope of the noise spectrum by filtering out (rejecting) one octave changes these effects in the vicinity of this gap.
The masked threshold for a pure tone varies from the value for unfiltered noise at the edges of the gap to a value approximately 25 db lower at the middle. This indicates that gapped‐noise may be used to mask out sounds outside of the gap without unduly raising the threshold of sounds in the gap.
Presence of white noise.generally raises the pitch of a pure tone whose frequency is between 500 and 4000 cps. Presence of noise with the gap does not raise the pitch of a pure tone located in the upper half of the gap. For a tone located in the lower half of the gap, the pitch is raised more than it would be in the presence of unfiltered noise.
The changes in the judged loudness of pure tones partially masked by a gapped‐noise reaffirm the importance of the tails of the excitation pattern in their effect on loudness.
24(1952); http://dx.doi.org/10.1121/1.1906870View Description Hide Description
Half‐loudness adjustments have been obtained from 18 observers at 10 intensity levels from 10 to 100 db. Adjustments were made with two methods. An analysis of variance of the attenuation scores necessary for half‐loudness showed: (1) Intensity accounted for a quarter of the variance. This variance indicates the extent to which the attenuation necessary for half‐loudness varies with different intensities. (2) Consistent differences between observers accounted for 35 percent of the variance, indicating that some observers consistently require much greater or less attenuation for half‐loudness than others. (3) One‐tenth of the variance was a result of the fact that the shape of the half‐loudness function with respect to intensity was different for different observers. (4) The error variance, caused by repeated adjustments by the same observer at the same intensity and with the same method, was 22 percent of the total. (5) Other variances were practically insignificant. The average half‐loudness function differs markedly from previously reported results. It is suggested that this discrepancy, as well as the large differences between individuals, is largely a result of the fact that observers have difficulty establishing the correct fractional value, and thus are extremely susceptible to indirect suggestion. A means of avoiding this problem of a nonvalid fraction is suggested.
24(1952); http://dx.doi.org/10.1121/1.1906871View Description Hide Description
The most comfortable listening level and the range of comfortable listening levels for pure tones were determined for a group of normal (nonclinical) listeners in the quiet and against various levels of background noise. In general, the most comfortable listening (mcl) level contour has the general shape of the equal‐loudness contour at intermediate loudness levels—lowest intensity at the middle frequencies and highest intensity at the lower frequencies. The range of listening levels considered “comfortable” against a quiet background varies from about 20 db at the lowest frequencies to about 35 db at the middle frequencies. The variability of the mcl is larger at high frequencies than at low frequencies, and it is about the same magnitude as the variability of heterophonic equal‐loudness matches. The effect of noise is primarily to raise the lower limit of the range of comfortable listening levels and only secondarily to raise the upper limit. As a result, the range of comfortable listening levels is decreased in noise. The possible use of the comfortable listening level test as a gross diagnostic tool in the detection of nerve‐type deafness is discussed.
24(1952); http://dx.doi.org/10.1121/1.1906873View Description Hide Description
Albino mice were tested for susceptibility to audiogenic seizures in sound fields of various frequencies and intensities. An attempt to fix the ears of the mice with respect to the sound source by suspending the mice with a large spring clip which grasped the dorsal skin‐fold proved fruitless, for too few seizures were induced at any intensity. The mice were therefore tested in a small wire mesh cage suspended in front of the sound‐transducer, and the positions of their ears during the treatment were recorded. At 10 kc, at least 92 db was needed to induce seizures in mice which had no previous seizures, while only 83 db were needed for animals which had previous seizures. The most effective frequencies for the induction of seizures were between 12 and 25 kc. Below 10 kc, the effectiveness dropped off sharply and was very low at 5 kc. Studies were not made above 25 kc. The peak of the mouse's audiogram is probably near the frequencies which are most effective for induction of seizures and is thus a little lower than that of the rat and about 1.5 octaves above that of man. The ability of mice to hear at frequencies which are ultrasonic for the human ear is probably the reason for the intense reactions of mice to complex sounds which seem of moderate intensity to man.
24(1952); http://dx.doi.org/10.1121/1.1906874View Description Hide Description
The noise fields generated by a standard turbo‐jet aircraft engine have been measured for three different power settings. Measurements were made at points on circles around the engine having radii of 25 and 50 feet. For the 50‐feet distance the directional characteristic presented for the over‐all sound pressure and for the noise in the different octave bands starting at 37.5 cps. From these measurements the total acoustic power radiated from the engine is calculated to be approximately 69 kw at full engine power. The distribution of this power over the different frequency bands and space angles is shown. The highest total energy per cycle and the highest sound levels are found at frequencies near 100 cps for the higher power settings of the engine. Above that frequency range the total energy per cycle drops approximately as the reciprocal of the square of the frequency. The data should help us understand qualitatively the jet engine as a sound source and are therefore discussed in that respect. On the other hand, the data have practical significance with respect to the design of test facilities for adequate protection of personnel. They are equally important with respect to problems of noise control on an airport.
24(1952); http://dx.doi.org/10.1121/1.1906875View Description Hide Description
Relationships between a listener's identification of a spoken vowel and its properties as revealed from acoustic measurement of its sound wave have been a subject of study by many investigators. Both the utterance and the identification of a vowel depend upon the language and dialectal backgrounds and the vocal and auditory characteristics of the individuals concerned. The purpose of this paper is to discuss some of the control methods that have been used in the evaluation of these effects in a vowel study program at Bell Telephone Laboratories. The plan of the study, calibration of recording and measuring equipment, and methods for checking the performance of both speakers and listeners are described. The methods are illustrated from results of tests involving some 76 speakers and 70 listeners.
24(1952); http://dx.doi.org/10.1121/1.1906876View Description Hide Description
An example is shown of the steps in the design of the structure for noise control of a jet engine test cell from the determination of the design goal to the final acoustic treatment of the cell. An arbitrary design goal was set from a previous survey of city noise. With the expected octave band levels at a distance of 3000 feet, for 10 jet engines with afterburners the quieting necessary is 50 and 55 db, respectively, in the 150–300 and the 300–600 cps bands, these two bands being the most difficult to control. An analysis of the required volume and dependent cost of the structure is given as the function of the amount of quieting needed, the quantity of heated gases exhausted, the temperature of the gases, and the velocity of flow permitted. Two unconventional but economical systems of noise control, (1) a series of acoustically treated plenums and (2) a series of acoustical lined 180‐degree turns, are recommended. Design equations and confirming data from model studies are presented.
24(1952); http://dx.doi.org/10.1121/1.1906877View Description Hide Description
A convenient and theoretically satisfactory method for measuring the sound attenuation of flying helmets is described in which a miniature microphone is employed to measure the sound intensity inside the helmet capsule. A theory of sound transmission through helmet capsules is proposed the validity of which is established by a direct comparison of theoretical with experimental results.
It is thus possible to design helmets which are considerably better than standard R. A. F. types. Comparison tests on one such helmet are described.
Rotational Dispersion in the Velocity, Attenuation, and Reflection of Ultrasonic Waves in Hydrogen and Deuterium24(1952); http://dx.doi.org/10.1121/1.1906878View Description Hide Description
Data are presented on the dispersion of the velocity of sound and on the effect of impurities upon the rotational relaxation times determined therefrom. The attenuation due to the relaxation is shown to agree with the theory, with some evidence of multiple relaxation. Anomalous dispersions of the classical attenuation and of the Herzfeld reflection coefficient are predicted. The latter is confirmed experimentally and also yields a determination of the accommodation coefficient.
Experimental Investigations on the Origin of the Anomalous Absorption of Ultrasonic Waves in Liquids24(1952); http://dx.doi.org/10.1121/1.1906879View Description Hide Description
There are two classes of liquids which show anomalous absorption of ultrasonicwaves, class A1 (nonassociated liquids) and A2 (associated liquids). In mixtures of two liquids of class A1, both density and sound velocity vary almost linearly with concentration, in spite of the anomalous variation of sound absorption. The smaller the ultrasonicabsorption of a liquid of class A1, the greater is the decrease of absorption per unit concentration in a dilute benzene solution of the liquid. This does not hold in liquids of class A2, such as alcohols. The inverse correlation between the absorption of ultrasonicwaves and infrared radiations was found in the case of liquids of class A1. It was concluded from the above facts that the absorption of ultrasonicwaves in nonassociated liquids should be a result of the so‐called “molecular absorption,” in the same way as in gases. In the case of associated liquids some part of the absorption is probably ascribed to the molecular absorption, but the remaining part may be caused by another relaxation mechanism, a slow change in local order of molecules. The contribution of the molecular absorption to that, in highly viscousliquids. is also estimated.
Transmission of Ultrasonic Waves through a Thin Solid Plate at the Critical Angle for the Dilatational Wave24(1952); http://dx.doi.org/10.1121/1.1906880View Description Hide Description
The transmission, through an isotropic solid plate immersed in a liquid, of ultrasonic waves incident at the critical angle for total reflection of the dilatational wave, is examined experimentally and theoretically. It has previously been held that total reflection does not occur at this angle when the thickness of the solid is less than or comparable with the wavelength of the dilatational wave in it. It is now shown that, owing to interference between the dilatational and rotational waves, total reflection does occur at or very near this angle for a considerable range of thickness in this region. It is therefore possible to use the total reflection method for the determination of the dilatational velocity in a solid even when only thin specimens of the solid are available.
24(1952); http://dx.doi.org/10.1121/1.1906881View Description Hide Description
Four groups of new methods of indirect ultrasonoscopy and ultrasonography are developed: those using temperature‐sensitive chromotropic compounds, those using leucobases of dyes, those using thermo‐stimulable phosphors, and those using special temperature‐sensitive phosphors. Various techniques are described and some results shown in the illustrations. Finally, a new method of direct ultrasonography is proposed. This method makes use of the possibility to detonate certain labile chemical compounds by ultrasonics.
24(1952); http://dx.doi.org/10.1121/1.1906882View Description Hide Description
Divergent compressional waves in solids differ from similar waves in fluids, even though the particle displacement is parallel to the direction of propagation in both media. The wave propagating from a radially oscillating spherical cavity in an infinite solid medium sees an acoustic radiation impedance which is a function of the Poisson's ratio of the medium as well as of the usual parameters. The radiation resistance has the same form as in a fluid medium, but the reactance is a negative or stiffness reactance, except at high frequencies in media of low rigidity. When an impulsive pressure is generated in the cavity, as by an explosion, the form of the radiated pulse is a damped oscillating wave train which does not closely reproduce the original pressure pulse.
24(1952); http://dx.doi.org/10.1121/1.1906883View Description Hide Description
In the following paper are included: the graphical synthesis which originally led to the belief that the tangential motion of a phonograph playback needle would cause distortion of the output signal; the mathematical analysis of the forces which would tend to cause such motion; and descriptions of equipment, tests, and conclusions reached.
These conclusions are, in brief, as follows: In a common type of crystal pick‐up, equipped with the usual steel needle, tangential motion of the stylus tip does occur as postulated, ranging in amplitude up to 0.0012″, the amplitude of the motion being a function of the recorded frequency, the amplitude of recording, the radius of the groove in which the signal is recorded, recording speed, record and needle wear, and needle pressure. In addition to the tangential motion caused by the recorded signal, a vertical resonance occurring in the needle‐pick‐up arm system causes low frequency tangential motions of the needle tip (24 cps) with amplitudes of up to 0.0015″. Attempts to detect the distortion of the output signal caused by tangential motion were unsuccessful principally because of the presence of other distortions which masked the tangential motion distortion, and which could not be removed from the equipment used. However, since the existence of tangential motions and their amplitudes were established, it became possible to compute the cross‐modulation products caused by such motion.