73RD MEETING ß ACOUSTICAL SOCIETY OF AMERICA

Fum,•¾, 21 AvruL 1967 GEORGIAN ROOM, 2:00 V.•.

Session N/EF. Sound Generation, Transmission, and Reception

CYRIL M. HAuuxs, Chairman

Contributed Papers (15 minutes)

N/EF1. Acoustic Admittance of a Subsonic Wall Bound- ary Layer. C. L. Mom,•.¾ (nonmember), Bolt Beranek and Newman Inc., Cambridge, Massachusetts 02138.--At fre- quencies such that the steady (dc) boundary layer over a solid surface is acoustically thin, the effect of the boundary layer on incident sound waves can be incorporated into the surface acoustic admittance. The dc layer can interact with the sound field in two ways. (1) Mean-flow gradients modify the acoustic (ac) boundary layer. If the ac layer (viscous and thermal) lies within the dc sublayermwhere mean-flow properties vary almost linearly with distance from the wallreit is possible to estimate the modified ac layer ad- mittance. (2) Outside the ac layer, sound waves are re- fracted by mean-velocity and sound-speed gradients. This further modifies the over-all surface admittance, but the effect is small in the frequency range over which the admit- tance concept applies (boundary layer thickness<<wave- length).

N/EF2. Need for a New Acoustic Source. WILLIAM H. CRA•*T, Lockheed Missiles and Space Company, Huntsville, •llabama.--Arguments are presented that develop the need for an acoustic-source arrangement which simulates convec- tive-pressure fluctuations better than present methods. The type of source needed is described and a method for achieving such a source is proposed.

N/EF3. Tone Generation by Subsonic Flow Through Sharp-Edged Circular Nozzles. HANN0 H. Hm.L•.R (non- member), Bolt Beranek and Newman Inc., Cambridge, Massachusetts 02138.--Intense discrete-frequency tones are generated when gases flow at high subsonic flow speeds (0.3<M<0.8) through circular orifices with sharp edges on both ends and with diameter-to-length ratios of approximately 1 to 2. These tones are shown to result from a feedback mechanism; their generation does not require the presence of adjacent resonators. Schlieren-optical photographs are pre- sented that indicate a periodic vortex shedding. Because the feedback wave is superposed on the flow, the frequency of the tone, which is identical to the vortex-shedding frequency, is not directly proportional to the flow velocity; a frequency law relating Strouhal and Mach number, is presented. [This work was done while the author was at the Institut ffir Technische Akustik der Technischen Universit//t Berlin, and was sponsored by the Deutsche Forschungsgemeinschaft.]

N/EF4. Turbulence Measurements of the Mixing Flow of a Nozzle with a Single V Notch. R. C. Poa'r•.R, Research Staff, IVyle Laboratories, Huntsville, •tlabama 35806.--Hot- wire-anemometer measurements are reported for the cold subsonic turbulent mixing region formed by a circular nozzle with a 45 ø notch. Such a nozzle simulates a single corruga- tion, typical of those used for jet-engine silencers. The meas- urements showed that the notch flow induced a large sec- ondary flow, causing a rapidly widening mixing region. The intensity of the turbulence in this initial mixing region was slightly reduced from that measured in the corresponding mixing region from the opposite circular side of the nozzle.

The scale of the turbulence was found to be similar in both mixing regions. These results were used to estimate the sound produced by various parts of the flow. The fact that the turbulence scale and intensity do not increase in the wider mixing region formed downstream of the notch results in less noise generated. Possible techniques for optimizing corrugated nozzle silencers are then considered.

N/EFS. Transmission Phenomena in Slotted Acoustic Tubes. Jos•.va WooI) RoG•.Rs, Electrical Engineering De- partment, Bucknell University, Lewisburg, Pennsylvania 17837.•This report describes some of the transmission phe- nomena in slotted acoustic tubes. Only round cylindrical tubes with uniform slots are considered. A graph with a family of curves illustrates the behavior of the attenuation and phase constants as they vary with frequency and slot width. The slotted tube is a high-pass filter. When all of the variables (including slot width, wavelength, phase, and at- tenuation constants) are properly normalized with respect to the radius of the tube, two curves are found to be sufficient to describe the complete variation of the two constants. These phenomena are explained in terms of the analogous electrical-transmission line loaded with shunt resistance and inductance.

N/EF6. Frequency Spaces for Cylindrical and Spherical Enclosures. RONALD KLIGMAN (nonmember) AND RICHARD WA½•.RaOUS•., Physics Department, The ,4roerican Univer- sity, •'ashington, D. C.--The normal-mode frequencies for a rectangular enclosure may be depicted as points on a rectangular lattice in a three-dimensional Cartesian-frequency space. This results from the uniform spacing of the zeros of the sine and cosine functions that comprise the solution to the wave equation in rectangular coordinates. In the case of cylindrical or spherical enclosures, the wave equation in the corresponding coordinates yields solutions that contain func- tions whose zeros are not uniformly spaced. Such are the Bessel and spherical Bessel functions. Thus, the points repre- senting the normal-mode frequencies of such enclosures do not lie on a regular lattice. However, it is shown that be- cause the spacing of the higher-order zeros of these func- tions approaches uniformity, the frequencies of the cor- responding higher modes can be represented by a rectangular lattice with good accuracy. This is a convenience in estimating the numbers of modes lying within a given frequency in- terval. [Work supported by the U.S. Navy Office of Naval Research.]

N/EF7. Reception and Self-Noise oœ Simple Lines and Perforated Pipes in Motion. EWALD EICHLER, Raytheon Company, Bedford, Massachusetts 0173.0.•Extended acoustic receivers that sample a pressure field by means of a simple propagating wave and their realization in the form of a microphone-terminated perforated pipe are studied. Motion through the medium subjects a receiver to apparent changes of frequency, to loss of surface reactance, and to flow noise. The receiver gain generally decreases with increasing veloc- ity. In the perforated pipe, the phase speed of sound gen- erally exceeds the speed of sound in the free medium; because

The Journal of the Acoustical Society of America 1615

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