Volume 29, Issue 1, January 1957
Index of content:
- PROGRAM OF THE FIFTY‐SECOND MEETING OF THE ACOUSTICAL SOCIETY OF AMERICA
- Session A. Noise Induced Vibration in Aircraft Structures
29(1957); http://dx.doi.org/10.1121/1.1918447View Description Hide Description
The stress spectrum and equivalent fatigue stress of a structure subjected to a random loading are determined on the approximation of a single degree of freedom and the hypothesis of cumulative damage. The results are applied to the development of a similarity expression for the probable value of the equivalent fatigue stress for a panel subjected to jet noise.
29(1957); http://dx.doi.org/10.1121/1.1918449View Description Hide Description
The problem of aircraft structural fatigue under acoustic forcing has led to a need for economical high‐speed destructive testing techniques. One such method, employing a high‐intensity siren, is described here. A unit presently in use and one now under construction are discussed. A method for using data from this discrete tone test in fatigue design for structure subjected to random sound is outlined.
29(1957); http://dx.doi.org/10.1121/1.1918451View Description Hide Description
This paper considers the response of thin elastic plates to sinusoidal acoustic excitation. A theoretical method for obtaining the dynamic deflection and stress in the nonlinear region is given. The theory is compared with test results and shows rather good agreement with these tests. The application of the theory to the design of panels to withstand acoustic fatigue is discussed in the latter part of the paper. The basic assumption involved in the theory is that the panel behaves like a nonlinear spring with a restoring force of , where F is the restoring force, C 1, and C 2 are constants which depend upon the physical characteristics of the panel, and w is the deflection of the center of the panel. The constants C 1, and C 2 are found from the static large deflection curves obtained either theoretically (S. Way, Proceedings Fifth International Congress of Applied Mechanics, Cambridge, Massachusetts, 1938) or experimentally. The amplification factors for the resonant response of the plate are obtained by combining these static large deflection curves with the nonlinear vibrationanalysis as given in “Nonlinear Vibrations,” by J. J. Stoker, Interscience Publishers, Inc., 1950. Experiments were conducted on aluminum panels, some containing Z stiffeners. The panels were excited by a horn and strains were picked up by use of SR‐4 strain gages. Square panels up to length of side/thickness = 120 exhibited nonlinear behavior under sound pressure of 150 db. The magnification factors calculated by use of linear theory were approximately five times those actually experienced, while those calculated by use of the nonlinear theory presented in this paper were of the same order of magnitude as those obtained in the experiments.
29(1957); http://dx.doi.org/10.1121/1.1918453View Description Hide Description
This paper will deal with the response of simple aircraft type skin panel specimens which were forced to vibrate by random and discrete noise in the laboratory. These two techniques of testing are first discussed and are illustrated by means of a short moving picture. The moving picture also shows the nature of initial fatigue damage as well as fatigue crack growth resulting from noise. Finally, some comparisons of stress levels and fatigue life are presented for a panel configuration exposed to both random and discrete noise.
- Session B. General Acoustics
29(1957); http://dx.doi.org/10.1121/1.1918455View Description Hide Description
Until the mid 1930's, the effect of sound on man was the subject of more speculation than investigation. Occasional frightening demonstrations, such as the brainstorm scene from Elmer Rice's Adding Machine reported to this society in 1939, only helped to perpetuate myths about the auditory lethal ray, the sound which would drive people mad, the inaudible sound which paralyzes. In 1940 the National Defense Research Committee addressed itself to the problem of discovering effects of sound on man which might aid the war effort. In 1941 a pilot project was established to investigate and to devise means and procedures for applying useful phenomena if any should be discovered. The activity of this project was sometimes successful, sometimes not, but always spectacular. This paper treats of early applications, some not previously disclosed, instructive though unsuccessful experiments, and subsequent related developments.
29(1957); http://dx.doi.org/10.1121/1.1918457View Description Hide Description
A new technology utilizing electrochemical phenomena has been developed, from which it is possible to design acoustic indicating and control devices requiring extremely low‐power consumption, in the order of 100 to 1000 times smaller than that of transistors. As an example, a mercury cell, one‐volt, battery‐powered, electrochemicaldetector for acoustic energy operates on 10 microwatts in the quiescent state, delivering 300 to 500 microwatts when fully excited. Besides the low power consumption, other advantages of these devices are inherent stability, long life, ease of manufacture, and simplicity of operation. Their principal disadvantage is the limited frequency range over which they are presently operable, 0 to 200 cps. The component parts of these new systems vary widely in the type of function performed. There are, for example, elements for detecting and metering both dc and acfluid flows, or dc and ac electrical signals; units for taking logarithms, derivatives, and products of electrical and/or fluid flow inputs; units for integrating pressures, flows, or electrical signals over short intervals of time; etc.
29(1957); http://dx.doi.org/10.1121/1.1918459View Description Hide Description
Experimental work is now being done on the transmission of sound through air containing solid particles. A pulse technique is used in which a sine burst containing about twenty complete waves is sent through the particle‐filled air. Examination of the transmitted pulse with and without particles yields a measure of the absorption of the sound by the particles as well as the dispersion. The absorption can be determined to an accuracy of about ±0.01 db/ft, and the velocity changes to an accuracy of about ±0.2 ft/sec. The size and number of particles per cubic centimeter are determined by catching the particles that fall out of a known volume and examining them by means of photomicrographs. Results of these measurements will be presented and a comparison will be made between the theoretical absorption as calculated by Epstein and Carhart and the experimental absorption. The experimental arrangement will also be described. The work is being supported by the Sandia Corporation.
29(1957); http://dx.doi.org/10.1121/1.1918461View Description Hide Description
Measurements have been made of spectra of sound radiated by submerged cavitating jets of water. Circular nozzles of diameter , , and were employed. Sound pressures were measured in half‐octave bands over the frequency range 125 cps to 11 000 cps. Ambient pressure was varied from atmos to 2 atmos, and the dissoved‐air content of the water was controlled. The sound spectra can be correlated when presented wholly in terms of dimensionless parameters. The spectral distribution of the radiated sound power conforms with the theory of the sound from single bubble cavitation, presented previously [H. M. Fitzpatrick, J. Acoust. Soc. Am. 27, 202(A) (1955), paper D4].
29(1957); http://dx.doi.org/10.1121/1.1918464View Description Hide Description
It is known that a gas bubble pulsating in a sound field in a liquid containing dissolved gas can grow by a process called “rectified diffusion.” If the liquid is undersaturated with the gas at the existing static pressure, growth of the bubble will occur only if the sound pressure exceeds a threshold value. The threshold sound pressure for growth by rectified diffusion has been measured for individual air bubbles trapped at the center of a water‐filled sphere resonating at a sound frequency of about 25 kc/sec. The data cover a range of bubble sizes, static pressures, and air contents of the water. The measured thresholds are several times larger than Blake's theoretical values (Acoust. Research Lab. Harvard Univ. Tech. Memo. 12, September, 1949) or values calculated from several refinements of the theory.
29(1957); http://dx.doi.org/10.1121/1.1918466View Description Hide Description
An X‐cut quartz crystal is optically wrung on each end of a stainless‐steel tube of which the length as a function of temperature is known. Electrical pulses are applied to one crystal at such a rate (PRF) that the pulses, received on the other crystal and viewed on an oscilloscope, coincide. The coincidence is among the first received pulse corresponding to a particular electrical pulse, the first echo corresponding to the electrical pulse next preceding, and so on. The speed of sound is twice the path length times the PRF, which is set by a stable sine‐wave oscillator having fine control and then measured with a counter. The result is meaningful for nondispersive liquids only. The results, which are easily reproduced to within 1 part in 35 000, will be presented for water over the temperature range 0 to 100°C.
29(1957); http://dx.doi.org/10.1121/1.1918468View Description Hide Description
The theoretical determination by perturbation methods of the eigenvalues of spheroidal free surfaces by S. A. Moszkowski, in connection with his recent developments in nuclear shell theory [Phys. Rev. 99, 803 (1955)], stimulated interest in solving the problem experimentally. Accordingly, spheroidal bodies of water with free surfaces were constructed, and 20 to 30 consecutive acoustic eigenmodes were identified and measured in each body. The experimental eigenvalues agreed with theory to ±0.2%. These results suggest that the method can be used to solve the Helmholtz equation with the boundary condition ψ = 0 on the surface of other smooth but irregular figures of revolution. (This work was supported by the Office of Naval Research.)
- Session C. Architectural Acoustics
29(1957); http://dx.doi.org/10.1121/1.1918470View Description Hide Description
The authors worked in close cooperation with the architects and Music Department throughout the entire design, construction, and furnishing of the building. Measurements of the sound insulation and of reverberation characteristics were made in Schoenberg Hall (525 seats), Rehearsal Rooms, and Practice Rooms during and after construction of the building. An optical study of reflections in a two‐dimensional model (devised by R. W. Leonard and described by him at the June, 1954, meeting of the Acoustical Society) was used to determine the shape of the ceiling for Schoenberg Hall that would provide a uniform distribution of sound throughout the seating area. This objective was well realized; for a sound source (a band of “white noise” centered at 500 cps) on the front of the stage, the sound level drops, on the average, only about 1.5 db from the front to the rear seats. Optimum reverberation and diffusion are obtained from strips of absorption on the side walls. The reverberation curves and the sound insulation data for typical rooms will be reported.
29(1957); http://dx.doi.org/10.1121/1.1918472View Description Hide Description
Several measurements which have proven useful in the engineering aspects of architectural acoustics are described in this paper. Data obtained in measurements of various sized auditoriums and large coliseums are presented showing the engineering use of reverberation time measurements, articulation tests using practiced or naive listeners, pulse measurements for isolation of echoes, and noise measurements. Some preliminary results and possible use of definition measurements in schoolrooms and some auditoriums also will be discussed.
29(1957); http://dx.doi.org/10.1121/1.1918474View Description Hide Description
Establishment of satisfactory room environment for music rooms used for practice purposes has been a problem of considerable difficulty for some time. Presently established criteria seem to be almost unanimously unsatisfactory. Part of this difficulty seems to be the failure of the directors to make themselves clear as to their requirements. Another obstructive factor seems to be ambiguity in the presently accepted room environment optima. Through extensive experience in this field, discussing such rooms with many directors and instructors, and in planning and testing such rooms, new viewpoints will be suggested which may be found helpful in achieving more satisfactory results.
29(1957); http://dx.doi.org/10.1121/1.1918476View Description Hide Description
29(1957); http://dx.doi.org/10.1121/1.1918478View Description Hide Description
Sound absorption coefficients of acoustical materials are commonly measured by the reverberation chamber technique, thereby requiring knowledge of the characteristics of the chamber. Use of acoustic pulses allows determination of reflection coefficients and absorption coefficients without the use of a special chamber, since the pulse reflected from the sample of material may be isolated from other pulses arising from reflections from other surfaces if the pulses be short enough. An essentially plane wave front is obtained by use of a parabolic reflector behind the source of pulses. To compare results from the pulse measurements with data from reverberation chamber measurements, variation of angle of incidence must be carried out in the pulse measurements, and frequency must be varied in the chamber measurements. A preliminary measurement at one‐pulse frequency shows satisfactory agreement between the two methods.
29(1957); http://dx.doi.org/10.1121/1.1918480View Description Hide Description
Extensive measurements of the variation of effective absorption coefficient with sample area have been made by Chrisler [J. Research Natl. Bur. Standards 13 (August, 1934)]. There is some indication that the measured variation follows a simple law based upon a very elementary consideration of diffraction effects. An attempt is made to deduce such a law from a more detailed consideration of diffraction effects at (a) the single edge of a semi‐infinite, absorbent sheet and (b) the two edges of an absorbent strip. An explicit result is obtained in case (a) which leads to a calculated “area effect” of the type observed by Chrisler. The result is, however, of limited validity and cannot be applied to sound striking the absorbent material near grazing incidence. It is shown that case (b) would lead to a result valid for all angles of incidence.
Measuring Techniques for Surface Noise, Absorption Coefficients, and Transmission Coefficients for Materials Exposed to Random Noise Fields in Small Enclosures29(1957); http://dx.doi.org/10.1121/1.1918482View Description Hide Description
In order to reject ambient noise, surface microphones were developed by placing absorbent enclosures around nondirectional microphones. The advantages and errors of this method are discussed. A sound diffusing box is used to calibrate these microphone enclosures. The correction factors are qualitatively analyzed for a cylindrical enclosure. The sound diffusing box was used to measure the transmission loss of some materials. A directional (cardioid) microphone placed inside the sound diffusing box was used to measure directly the reflected sound energy of small samples of materials. A method for converting these measurements to absorption coefficients is described.
29(1957); http://dx.doi.org/10.1121/1.1918484View Description Hide Description
The measurement of the sound power produced by many types of apparatus is often desirable and sometimes necessary when neither an anechoic chamber nor a reverberation room is available. When measurements are made in an enclosure where the absorption of the walls may be arbitrary, some means of determining the effect of the enclosure upon the observed sound pressure level measurements is necessary. One possible means of attack has been suggested [R. J. Wells, “Power apparatus and systems,” Trans. Am. Inst. Elec. Engrs., No. 21, 1170 (December, 1955)]. An alternative method is presented here, having the significant advantage that no measurements need be made until after the apparatus whose sound power is to be measured has been installed in an enclosure having reasonably arbitrary absorptioncharacteristics. The procedure requires the taking of two sets of sound pressuremeasurements—one set with the apparatus operating in the enclosure as normally constructed, and a second set after the sound absorbing properties of the enclosure have been modified by the introduction of acoustic material of appropriate area and absorption coefficient. From these data, the sound power output of the apparatus may be determined.
- Session D. Instruments and Methods
29(1957); http://dx.doi.org/10.1121/1.1918486View Description Hide Description
This method of showing fluid motion utilizes the optical interference patterns observed in a flow‐birefringent fluid when viewed between crossed polarizing plates. The fluid used is an aqueous solution of milling yellow, a commercial dye product of National Aniline Division of Allied Chemical and Dye Corporation. The optical interference patterns are related to the stress distribution and streamline directions in the moving fluid. Motion pictures are made of the patterns. Fluid preparation, fluid properties, and photographic techniques will be discussed. Color motion pictures will be shown for low‐frequency excitation for such elements as orifices and tubes. (This work was supported by the Office of Ordnance Research, Department of the Army, under Contract No. DA‐23‐072‐ORD‐583.)