Contains research objectives and reports on three research projects.U. S. Navy (Office of Naval Research) under Contract Nonr- 1841(42)National Science Foundation (Grant G-9330

Gentle, K. W.

Ingard, K. Uno

Maling, G. C., Jr.

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XII. PHYSICAL ACOUSTICSProf. K. U. Ingard L. E. Johnson G. C. Maling, Jr.Dr. L. W. Dean III T. J. Kinzer III J. W. Ryan IIIK. W. Gentle H. L. Willke, Jr.RESEARCH OBJECTIVESIn general terms, our research objectives concern the physical problems involvedin the generation, propagation, and absorption of sound and vibrations in matter.Specifically, our program of study includes:1. Acoustics of moving media.2. Acoustical studies of molecular structure of liquids.3. Generation of very high frequency sound waves ("hypersonics").4. Nonlinear acoustics.K. U. IngardA. THE INFLUENCE OF TURBULENCE ON SOUND PROPAGATIONOVER A PLANE BOUNDARY*A theoretical analysis of the influence of turbulence on sound propagation, which waspresented in a previous report, l has been extended and experimental studies have beenmade. Assuming a Gaussian distribution function for the fluctuations in phase betweenthe direct and the reflected sound, we calculated the fluctuation in the sound pressureas a function of distance from the sound source. Experiments have been performed ona model in the R. L. E. anechoic chamber. A paper describing the results of this workhas been submitted for publication to the Journal of the Acoustical Society of America.U. Ingard, G. C. Maling, Jr.References1. U. Ingard, The influence of atmospheric turbulence on the sound field over aplane boundary, Quarterly Progress Report No. 55, Research Laboratory of Elec-tronics, M.I.T., October 15, 1959, p. 144.B. HEAT-MAINTAINED OSCILLATIONS*A nicrome ribbon heater has been constructed for making measurements on heat-maintained oscillations in a Rijke tube. The Q of the tube has been varied by changingthe position of a fine-mesh screen in the tube, and the onset of oscillations has beenmeasured as a function of heater power, screen position, and air-flow rate through thetube. Typical results are shown in Fig. XII-1. The dependence of the onset of oscil-lation on flow rate through the tube has also been measured. At constant heater power*This work was supported in part by the U. S. Navy (Office of Naval Research) underContract Nonr- 1841(42).(XII. PHYSICAL ACOUSTICS)-51290-10 o IIOCx 930AIR VEL-15wz -200UJ -25= E5 10 15 NODISTANCE FROM OPEN END OF TUBE (INCHES) SCREENFig. XII-1. Relative sound-pressure level and Q as aof a screen in the Rijke tube.function of the position(1290 watts) the tube will oscillate when the air-flow rate is between approximately100-250 ft per min.These experimental results can be compared with the analysis performed by Merk 1who used a heat-transfer function for ribbons obtained by Carrier. When the appro-priate constants are used, it is found that the analysis predicts that the oscillationsshould occur over a much greater range of air-flow rate than is observed experimen-tally. The analysis is also not sensitive enough to the Q of the tube to be able to pre-dict the dependence of the onset of oscillations on the screen position.This work is being continued, with emphasis on gaining a better understanding ofthe scattering of sound by a heat source.G. C. Maling, Jr.ReferencesI. H. J. Merk, Appl. Sci. Research A6, 402 (1957).2. G. F. Carrier, Quart. Appl. Math. 12, 383 (1955).C. INSTABILITY OF LIQUID CONDUCTORS*Investigations on the stability of liquid conductors, which were presented in a pre-vious report, are being conducted. The program involves subjecting a homogeneous*This work was supported in part by the National Science Foundation (Grant G-9330).5045 Q40(XII. PHYSICAL ACOUSTICS)trough of conductor to high current densities and observing the conditions under whichpinch instability develops. The liquid conductor used is an alloy of sodium and potas-sium which becomes liquid at room temperature. This material was chosen for itshigh electric conductivity, which permits high current densities without overheating,and low density. The low density is a singular advantage, for it permits the coveringof the alloy with a hydrocarbon liquid of almost the same density, thereby almost can-celling hydrostatic pressure as a restoring force to surface deformations. The resultis that static instabilities in a low-density, high-current medium can be readily stimu-lated. In the experimental array, care is being taken to maintain symmetry and keepthe conductor in a homogeneous magnetic field.The techniques of preparing and handling the alloy have been developed. It is storedbest under dried, outgassed kerosene and transferred with a syringe. Other techniqueseither expose it to air, which corrodes it rapidly, or disperse it into droplets that failto coalesce readily.A chamber has been built to keep the material under test in an inert N2 atmosphere.If the alloy should pinch and vaporize, it will not ignite the metal and hydrocarbons.Preliminary trials revealed slight pinches at a current of 300 amps into a cross2section of approximately 3 cm , but these were at the ends where the cross sectionwas reduced because the alloy did not wet copper electrodes. A pilot test revealedthat sodium metal electrodes gave improved results, since the alloy readily wetssodium, and experiments are continuing with this configuration.A bank of six storage batteries supplies up to 1000 amps through a stainless-steelrod current-limiting resistor immersed in water.K. W. Gentle, U. IngardReferences1. D. S. Wiley and U. Ingard, Bifurcation instability of a liquid conductor in anaxial magnetic field, Quarterly Progress Report No. 63, Research Laboratory ofElectronics, M.I.T., October 15, 1961, p. 133; U. Ingard and D. S. Wiley, Searchfor a stable pinch in a horizontal mercury column, Quarterly Progress Report No. 63,p. 135.

Physical Acoustics

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Physical Acoustics

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