Volume 20, Issue 4, July 1948
Index of content:
20(1948); http://dx.doi.org/10.1121/1.1906386View Description Hide Description
A stationary right circular cone was exposed to an essentially plane progressive wave, and the sound pressurep at various points on the surface was determined relative to the free‐field pressurep 0 in the undisturbed incident wave. Measurements were made in the frequency range of , where k is the wave number of the incident wave and a the radius of the base of the cone which had a generating angle of 30°. In particular, the diffraction effect |p/p 0| was determined at the center of the base for various angles of incidence θ, measured with respect to the axis of the cone. The results are generally similar to those obtained for other obstacles with the same circular face, e.g., a disk. As might be expected, the greatest differences are exhibited in the region θ>90°. A “bright” spot appears at the center of the base when the vertex points directly towards the source (θ = 180°). For incidence normal to the base, pressure measurements have been made at several other points on the surface of the cone. The data, represented as a function of frequency and position by a surface, given an approximate picture of the pressure distribution over the whole surface of the cone.
20(1948); http://dx.doi.org/10.1121/1.1906387View Description Hide Description
It is pointed out that a certain class of diffraction problems can be formulated as Wiener‐Hopf integral equations [R. E. A. C. Paley and N. Wiener, “The Fourier transform in the complex domain,” Am. Math. Soc. Colloq. Pub. 19 (1934), Ch. IV] and solved exactly, an observation due to Schwinger [J. F. Carlson and A. E. Heins, Q. App. Math. 4, 313 (1947)]. Following the analysis of the equivalent electromagnetic problem by Carlson and Heins [Q. App. Math. 5, 82 (1947)], the diffraction of a plane wave of sound by an infinite set of parallel equally spaced semi‐infinite plates is calculated. The conditions for plane wave propagation within the plates, for a single reflected wave, and expressions for the reflection and transmission coefficients are given. Except for the limitations by the requirements that only a plane wave be propagated and that there be only a single reflected wave, the reflection and transmission coefficients are found to be independent of wave‐length or the spacing between the plates. Other problems which may be solved by the same techniques are cited.
20(1948); http://dx.doi.org/10.1121/1.1906388View Description Hide Description
It is well known that to obtain very small side lobe radiation the amplitude distribution across a radiating surface should be such that the maximum amplitude is at the center and the minimum at the edges. The Gaussian distribution gives no side lobes, but requires an infinitely large radiator. This paper gives the analysis of the clamped‐edge disk, vibrating in its first normal mode. It shows that the dynamic curve approximates the Gaussian form and that the sound pressure distribution has very small side lobes, the amplitude of the first side lobe being 33 decibels below that of the axial lobe.
For a disk of a given diameter the width eft the axial lobe may be decreased by raising the frequency of the first normal mode, which requires a corresponding increase in the disk thickness. At very high frequencies the disk thickness may become prohibitive. In such cases a thin disk can be forced to vibrate in the shape of the first normal mode by a proper radial distribution of the driving force.
20(1948); http://dx.doi.org/10.1121/1.1906389View Description Hide Description
20(1948); http://dx.doi.org/10.1121/1.1906390View Description Hide Description
This paper gives the derivations of the “directivity index” formulas for several types of sound radiators. The “directivity index” is defined as “the ratio of the total acoustic power output of the radiator to the acoustic power output of a point source producing the same pressure at the same point on the axis.” The utility of the directivity index concept is that it permits power calculations to be made for all radiators in the same manner as for point sources. Directivity index formulas, together with graphs covering practical cases, are given for the following types of radiators:
1. General plane piston in infinite baffle,
2. Circular plane pistion in infinite baffle,
3. Rectangular plane piston in infinite baffle,
4. Sectoral horn,
5. Multicellular horn,
6. Piston set in sphere.
20(1948); http://dx.doi.org/10.1121/1.1906391View Description Hide Description
The vibrational boundary conditions of a free, thin, rectangular, plane‐parallel plate can be satisfied by the sum of an infinite series of plane wave components. When the ratios of the thickness to the length and width of the plate are small, this series converges rapidly enough to give a fair approximation with a reasonable number of terms.
Frequency spectra as well as standing‐wave configurations are calculated approximately for AT and BT cut quartz plates. Some observed data are given for comparison.
20(1948); http://dx.doi.org/10.1121/1.1906392View Description Hide Description
A mathematical expression for the pressure distribution in a circular mercury delay line is derived. It is found that many waves having slightly different phase velocities can exist, and under certain circumstances may interfere with each other to produce distortion effects.
An analysis is made of the voltage developed by the piezoelectric pick‐up crystal, and of the distortion that might result when the carrier is pulse modulated.
20(1948); http://dx.doi.org/10.1121/1.1906393View Description Hide Description
Some measurements have been made of the attenuation of ultrasonic waves in mercury as a function of tube diameter and the inner surface of the tube. A pulse technique was employed and attenuation was measured at 10.6 Mc/sec. by comparing successive echoes of the initial pulse. The results with smooth glass tubes are in qualitative (although not quantitative) agreement with the predictions of the Helmholtz theory for tubes large compared with the wave‐length.
20(1948); http://dx.doi.org/10.1121/1.1906394View Description Hide Description
The velocity and attenuation of bulk waves (longitudinal waves of dilatation) in solid samples of high polymers are measured by an acoustic pulse technique in the frequency range 10 to 30 mc. An ultrasonic pulse of about 2 microseconds' duration is transmitted by a crystal into the liquid contained in a tank about 5 cm long, and returns as an echo to the crystal transducer. The velocity of bulk waves in the polymeric material is found from the change in echo arrival time which occurs when a flat sample is introduced into the sound path, and the attenuation is found from the reduction of echo intensity. Measurements for several rubbers and plastics are reported over various portions of the temperature range −60°C to 100°C. The attenuation generally shows at least one maximum with respect to temperature in this range. Maximum attenuation values for various rubbers at 10 mc range from 200 to 400 db/cm.
20(1948); http://dx.doi.org/10.1121/1.1906395View Description Hide Description
A new procedure is presented for the measurement of the normal specific acoustic impedance of extremely porous screen as a function of frequency. The method, restricted to this case, is based on a special application of the Transmission‐Characteristic Method of impedance measurement and can be applied to screens having impedances even less than 1/100 the characteristic impedance of air. Relatively simple experimental equipment and computation is required. Measured values of the impedance for two samples of extremely porous screen, determined by this procedure, are given.
20(1948); http://dx.doi.org/10.1121/1.1906396View Description Hide Description
A method of driving a single shear plate crystal through a mechanical transformer is described. Particular attention is paid to the application of this drive to a phonograph pick‐up. The advantages of this method of drive over previous methods are discussed.
20(1948); http://dx.doi.org/10.1121/1.1906397View Description Hide Description
This paper describes a newly developed acoustic instrument that is now commercially available for making absolute sound pressuremeasurements over the frequency range 50 cycles to 250 kc. The microphone standard is only , and is therefore completely free of diffraction errors throughout the entire audible spectrum to beyond 20 kc. In order to maintain constant phase shift between the generated voltage and the actuating pressure, the vibrating system is stiffness‐controlled to beyond 250 kc, thus insuring accurate reproduction of pressurewave transients having extremely steep wave fronts. The tiny size of the microphone and its very high acoustic impedance will permit free field measurements throughout the entire audible range without diffraction errors, as well as making easily possible many special acoustic measurements extending well into the ultrasonic region, heretofore very difficult to make. A preamplifier and power supply unit makes the equipment readily portable.
20(1948); http://dx.doi.org/10.1121/1.1906398View Description Hide Description
From data obtained in an extended program for the study of underwater sound propagation, eight examples at 200 c.p.s. have been selected to illustrate long‐range propagation involving reflection from the ocean bottom. Essentially continuous sound measurements were made while the sending vessel moved from 100 meters to 60 kilometers or more away from the receiver. The source was at a depth of 4.3 meters, the receiver usually at 5, 15, or 90 meters. For the examples given the water depths were 3600, 1800, and 95 meters respectively, and with some variety of thermal conditions. There is some evidence of interference extrema. In 3600‐meter water the bottom‐reflected signals were roughly constant to 10 kilometers, partly because in water of this depth the distance to and from the reflecting bottom does not change rapidly with increasing horizontal range. In an extreme case the bottom‐reflected signal was about the same at ranges of 3 and 60 kilometers, and it was about 15 decibels higher than this value at a range of 10 kilometers.
20(1948); http://dx.doi.org/10.1121/1.1906399View Description Hide Description
The apparatus used to measureocean reverberation at 24 kc consists essentially of a sound projector which sends a signal of adjustable duration into the water; a hydrophone which translates the backward scattered sound into electrical voltage; an amplifier which increases this voltage; a cathode‐ray oscillograph which converts voltage fluctuations into spot movements; and finally a camera which records the movements on a moving film. Simple reverberation theory indicates (i) that reverberation level increases 10 db with a tenfold increase of pulse length; (ii) that volume reverberation level decreases 20 db for a tenfold increase in range; and (iii) that surface reverberation level decreases 30 db for a tenfold increase in range. At certain times and under certain conditions, presumably when ocean conditions are those postulated by theory, observed reverberation levels agree with theoretical values. Such agreement, however, is relatively uncommon. Under most conditions, deep scattering layers cause volume reverberation levels to depart markedly from simple theory; also, a combination of refraction, wind, and other factors causes a decrease in surface reverberation level with range which is too rapid to be in agreement with simple theory. When a sound beam is projected horizontally in deep water, both surface and volume reverberation might be expected. Under a rough sea and for ranges less than 500 yards, surface reverberation predominates over volume reverberation. Beyond 1000 yards, even under a rough sea, volume reverberation usually overshadows surface reverberation. Also, for such long ranges, attenuation enters as an important factor and causes the reverberation level to fall off more rapidly than the rate predicted by simple scattering. In shallow water, bottom reverberation (which depends for its intensity on whether the bottom is rock, sand, mud and sand, or mud) is the dominant part of the observed reverberation.
20(1948); http://dx.doi.org/10.1121/1.1906400View Description Hide Description
By using absorbing walls surrounding a small body of water, measuring tanks have been produced which will determine the directional properties of underwater sound instruments down to a level of 25 db below the direct beam. These absorbing media are constructed by inserting fine mesh screen or packed copper wadding in a viscous liquid such as castor oil. These obstructions result in an enhanced viscous action which is nearly independent of the frequency above 10 kilocycles. A six‐inch wall can reduce the reflections by 20 db. Tanks using such absorbing media were used for testingtransducers at the manufacturing plant and were used for determining the approximate characteristics of small sized instruments. Absorbing media were also used in the sound transparent dome housing the transducer and in the back of the QJB transducers.
20(1948); http://dx.doi.org/10.1121/1.1906401View 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 Lifschitz. MacNair's equation was derived from the following basic assumptions: , , where .
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: , , where .
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/10.1121/1.1906402View Description Hide Description
A peculiar problem in communication arises when a talker is wearing an oxygen mask or a gas mask, in that both react on the vocal mechanism and distort the voice at its source. Thus, the loss of intelligibility when a gas mask is worn is much greater than would be predicted from the amount of muffling, and, even when the oxygen mask contains a microphone, the talker's enunciation is not, in general, clear. The reaction on the voice is in part mechanical, by constraining the facial muscles and obstructing the flow of the breath, and in part acoustical, by adding a new cavity onto the vocal system so that the talker has less control over the character of the sound. The requirements for good intelligibility during the use of such enclosures have been discussed in some detail in a previous paper (see reference 1).
The present paper concerns itself with an experimental analysis of the distortion produced by some small enclosures, and an evaluation of present theories of vowel production by comparing their predictions with the experimental results. Such a comparison is probably as severe a test of the theory of vowels as can be devised.
In general predictions from the present state of the theory of vowels give a useful qualitative picture of the nature of distortion by small enclosures, but are not in quantitative agreement with experiment. A criticism is given of the assumptions involved in the present theory, and some suggestions are made toward the future improvement of our knowledge of speech.
20(1948); http://dx.doi.org/10.1121/1.1906403View Description Hide Description
Information on the instantaneous sea surface reflectivity for 30‐kc sound has been obtained. The method required that the submerged source and receiver be rigidly mounted on towers resting on the sea bottom. Transmission of a frequency‐modulated sound pulse results in an interference pattern at the receiver; interference occurs between the direct and reflected sound. Analysis of these interference patterns reveals the following: The average time of persistence of the pattern is five‐hundredths of a second. Under the conditions of the experiment, surface reflectivity is highly frequency dependent. The median value for the reflection coefficient is near to unity. Approximately 10 percent of the time the sea surface focuses the sound, giving a a reflection coefficient greater than unity.
20(1948); http://dx.doi.org/10.1121/1.1906404View Description Hide Description
The distribution of average power in various speechsounds as a function of frequency has been measured with an integrating audio spectrometer. This instrument divides the electrical signal into 14 frequency bands and simultaneously records the integrated square of the amplitude in each band over a measured time interval, usually 30 seconds. From these data and the known over‐all calibration of the spectrometer channels one can compute the spectral distribution of average acoustic power at the location of the input microphone. The operation of the instrument and the procedure of measurement are described. Speech spectra are given which are average results for a trained crew of seven men, speaking into a condensermicrophone in an anechoic chamber.
20(1948); http://dx.doi.org/10.1121/1.1906405View Description Hide Description
The relation between loudness and the repetition rate of short tones was experimentally determined. The effects of intensity, frequency, and tone duration on this relation were also studied. The results can be summarized as follows. (1) A series of repeated short tones can be louder than a steady tone of the same peak intensity. (2) The difference between the loudness of repeated tones and a steady tone of the same intensity is dependent on both frequency and intensity. (3) Two series of repeated tones, of the same frequency and intensity, will not have the same loudness unless their repetition rates and durations are the same. At high intensities, the tones with the faster repetition rate and shorter duration are louder, and conversely at low intensities. (4) The rate at which loudness increases with repetition rate is dependent on the duration. The shorter the duration, the greater the change in loudness with a change in repetition rate; and conversely, the slower the repetition rate, the greater the change in loudness with a change in duration.
All of the relations shown are understandable in terms of the shape of the loudnessfunction, and the complex spectra of repeated short tones. Calculated loudness levels, according to the method of Fletcher and Munson for calculating the loudness of multicomponent tones, agree very well with the observed loudness levels.