2
72ND MEETING ß ACOUSTICAL SOCIETY OF AMERICA 4FO. Brownian Motion in the Cochlear Partition. G. G. HARRIS, Bell Telephone Laboratories, Inc., Murray Hill, New Jersey.--Estimates have been made of the Brownian- pressure fluctuations at the eardrum. These are 15 dB below threshold (MAP) for 3000 cps. It is thus necessaryto look into the ear itself to see whether thermal fluctuations limit the sensitivity of the ear. It is assumedthat the first stage of amplification of acousticalenergy takes place at the hair cell, and that it is the relative shear displacement between the top of the hair cell and the stereocilia that is the relevant parameter. Two estimates of Brownian fluctuation are made. The first assumes a rigid connectionbetween the stereocilia and the tectorial membrane, and thus the source of noise is the fluctuation in amplitude of the basilar-membranedisplace- ment. The second estimate assumes a connection rigid enough to provide the greatest flow of acoustical energy to a hair cell, i.e., proper impedance match, but which is loose enough so that each hair cell has independentthermal displacements. The impedance used in making these estimates were based on the in vivo measured impedances at the eardrum and the anatomy of the middle ear. The first estimate gives a signal to noise ratio of 15 dB at threshold for 3000 cps. The second estimate gives a signal to noise ratio of --37 dB. This cal- culation shows that it is necessary to assume that the stereocilia are attached to the tectorial membrane and that the hair cell bodies are embedded in the organ of Corti with a high degree of rigidity. There is no direct experimental evidence for the assumption of rigid attachment, although there is some indirect evidence from ultrastructural studies. [12 min.] 4F10. Subharmonic Squelch Effect. PETER J. DALLOS, ,4uditory Research Laboratory & Biomedical Engineering Research Center, Northwestern University, Evanston, I1- linois.--It was shown [J. Acoust. Soc. Am. 39, 1253(A) (1966)] that, between the approximate frequency limits of 750 and 3000 Hz, odd fractional subharmonics can be elicited in chinchilla ears. This type of subharmonic vibration was shown to originate in the inner ear owing to mechanical non- linearities. Undertones were recordable both in cochlear microphonic and in the sound field radiated by the tympanic membrane. If a secondary tone coexists with the fundamental (which generates the subharmonic), then it can interfere with the subharmonic, and under appropriate conditions it can completely disrupt this nonlinear vibration. This disrup- tion of a subharmonic oscillation by the presence of a sec- ondary tone is termed subharmonic squelch effect. It is shown that the squelch effect is stronglydependent upon the fre- quency separation between the subharmonic and the sec- ondarytone,as well as uponthe intensity of the latter. Com- parison is made betweenthe squelch effect and the inter- ference phenomenon [E. Wever,C. Bray, andM. Lawrence, J. Acoust. Soc. Am. 12, 268-280 (1940)], and the two are shown to be of completely different nature.[Work supported by the NationalInstitutes of Health, U.S. Department of Health, Education, andWelfare.] [12 min.] 4Fll. Some Relations of the V Potential to Loudness and to Masking. H^u.OWE•J. D^vis, C. BOWERS, ^•n S. K. Hmsa, Central Institute for the Deal St. Louis, Missouri.-- Tone pips and tone bursts (rise time 20 msec) of different frequency and bursts of white noise that were judged equally loudall evoked slow V (vertex) potentials of approximately equal amplitude. For somesubjects, however,the noisewas clearlymore effective than tone burst or pip and in general a fairly strong 4000-Hz burst (or 4800-Hz pip) was less effective than an equally loud one at 1000 (or 1200) or 250 (or 300) Hz. The rate of increase of V potential with the SPL (sound-pressure level) of tone burstswas very slow. The exponent of the powerlaw was 0.15 at 250 Hz, 0.11 at 1000 Hz, and only 0.08 at 4000 Hz. Individual differences across subjectsseem more important than the interval be- tweenstimuli (1 versus 3.2 sec). In the presence of appro- priate bands of masking noise, the input-output curves rise abruptly to approach the unmasked amplitude at a level about30 dB above the corresponding masked threshold. The effect resembles recruitment of loudness. [Work supported by the National Institute of Neurological Diseases and Blind- ness, National Institutes of Health, U.S. Department of Health, Education, andWelfare.] [12 min.] THURSDAY, 3 NOVEMBER1966 PACIFIC BALLROOM AT 1:30 P.•. Session4G. Symposium on Boundary-Layer Noise PRITCHARDH. WHITE, Chairman Invited Papers 4G1. Possible Mechanism for the Surface-Pressure Fluctuations beneath Turbulent BoundaryLayers. MARTn,r V. LOWSON, Research Staff, I/Vyle Laboratories, Huntsville, Ala- bama 35806.--It is suggested that the fluctuating surface pressurefield beneath a turbulent boundary layer is substantially due to the laminarsublayer eruption observed by Einstein and Li and other workers. The eddy-convection-velocity characteristics observed in flow visualization studies of the eruption process are shown to agree reasonably well with those of the fluctuating pressure field. The mechanism suggests newtheoretical models for the study of surface-pressure- fluctuation phenomena. [This work is an extension of that reported in NASA TN D3156; supported by the National Aeronautics and Space Administration.] [30 min.] 4G2. Interpretation of Wall-Pressure Measurements undera TurbulentBoundary Layer. K. L. CaANDIRA•VrA•X (nonmember), Bolt Beranekand Newman Inc., Cambridge, Massa- chusetts 02138.•Some aspects of the correlations and spectra of fluctuating wall pressure beneath a turbulent boundary layer are considered. The correlation function for the pressure field is represented by a hierarchy of mathematical models, each successive model morecomplex than the preceding one. These models are based on some mathematical inequalities that are applicable to a "convecting and decaying" stochastic process. Some of these models are used for estimating attenuation of the pressure spectrum measured by a transducer of finite size. The results 1264 Volume 40 Number 5 1966 Downloaded 09 Sep 2013 to 128.197.27.9. Redistribution subject to ASA license or copyright; see http://asadl.org/terms

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Page 1: Interpretation of Wall‐Pressure Measurements under a Turbulent Boundary Layer

72ND MEETING ß ACOUSTICAL SOCIETY OF AMERICA

4FO. Brownian Motion in the Cochlear Partition. G. G. HARRIS, Bell Telephone Laboratories, Inc., Murray Hill, New Jersey.--Estimates have been made of the Brownian- pressure fluctuations at the eardrum. These are 15 dB below threshold (MAP) for 3000 cps. It is thus necessary to look into the ear itself to see whether thermal fluctuations limit

the sensitivity of the ear. It is assumed that the first stage of amplification of acoustical energy takes place at the hair cell, and that it is the relative shear displacement between the top of the hair cell and the stereocilia that is the relevant parameter. Two estimates of Brownian fluctuation are made. The first assumes a rigid connection between the stereocilia and the tectorial membrane, and thus the source of noise is the fluctuation in amplitude of the basilar-membrane displace- ment. The second estimate assumes a connection rigid enough to provide the greatest flow of acoustical energy to a hair cell, i.e., proper impedance match, but which is loose enough so that each hair cell has independent thermal displacements. The impedance used in making these estimates were based on the in vivo measured impedances at the eardrum and the anatomy of the middle ear. The first estimate gives a signal to noise ratio of 15 dB at threshold for 3000 cps. The second estimate gives a signal to noise ratio of --37 dB. This cal- culation shows that it is necessary to assume that the stereocilia are attached to the tectorial membrane and that

the hair cell bodies are embedded in the organ of Corti with a high degree of rigidity. There is no direct experimental evidence for the assumption of rigid attachment, although there is some indirect evidence from ultrastructural studies.

[12 min.]

4F10. Subharmonic Squelch Effect. PETER J. DALLOS, ,4uditory Research Laboratory & Biomedical Engineering Research Center, Northwestern University, Evanston, I1- linois.--It was shown [J. Acoust. Soc. Am. 39, 1253(A) (1966)] that, between the approximate frequency limits of 750 and 3000 Hz, odd fractional subharmonics can be elicited in chinchilla ears. This type of subharmonic vibration was shown to originate in the inner ear owing to mechanical non- linearities. Undertones were recordable both in cochlear

microphonic and in the sound field radiated by the tympanic membrane. If a secondary tone coexists with the fundamental (which generates the subharmonic), then it can interfere with the subharmonic, and under appropriate conditions it can completely disrupt this nonlinear vibration. This disrup- tion of a subharmonic oscillation by the presence of a sec- ondary tone is termed subharmonic squelch effect. It is shown that the squelch effect is strongly dependent upon the fre- quency separation between the subharmonic and the sec- ondary tone, as well as upon the intensity of the latter. Com- parison is made between the squelch effect and the inter- ference phenomenon [E. Wever, C. Bray, and M. Lawrence, J. Acoust. Soc. Am. 12, 268-280 (1940)], and the two are shown to be of completely different nature. [Work supported by the National Institutes of Health, U.S. Department of Health, Education, and Welfare.] [12 min.]

4Fll. Some Relations of the V Potential to Loudness and to Masking. H^u.OWE•J. D^vis, C. BOWERS, ^•n S. K. Hmsa, Central Institute for the Deal St. Louis, Missouri.-- Tone pips and tone bursts (rise time 20 msec) of different frequency and bursts of white noise that were judged equally loud all evoked slow V (vertex) potentials of approximately equal amplitude. For some subjects, however, the noise was clearly more effective than tone burst or pip and in general a fairly strong 4000-Hz burst (or 4800-Hz pip) was less effective than an equally loud one at 1000 (or 1200) or 250 (or 300) Hz. The rate of increase of V potential with the SPL (sound-pressure level) of tone bursts was very slow. The exponent of the power law was 0.15 at 250 Hz, 0.11 at 1000 Hz, and only 0.08 at 4000 Hz. Individual differences across subjects seem more important than the interval be- tween stimuli (1 versus 3.2 sec). In the presence of appro- priate bands of masking noise, the input-output curves rise abruptly to approach the unmasked amplitude at a level about 30 dB above the corresponding masked threshold. The effect resembles recruitment of loudness. [Work supported by the National Institute of Neurological Diseases and Blind- ness, National Institutes of Health, U.S. Department of Health, Education, and Welfare.] [12 min.]

THURSDAY, 3 NOVEMBER 1966 PACIFIC BALLROOM AT 1:30 P.•.

Session 4G. Symposium on Boundary-Layer Noise

PRITCHARD H. WHITE, Chairman

Invited Papers 4G1. Possible Mechanism for the Surface-Pressure Fluctuations beneath Turbulent Boundary Layers. MARTn,r V. LOWSON, Research Staff, I/Vyle Laboratories, Huntsville, Ala- bama 35806.--It is suggested that the fluctuating surface pressure field beneath a turbulent boundary layer is substantially due to the laminar sublayer eruption observed by Einstein and Li and other workers. The eddy-convection-velocity characteristics observed in flow visualization studies of the eruption process are shown to agree reasonably well with those of the fluctuating pressure field. The mechanism suggests new theoretical models for the study of surface-pressure- fluctuation phenomena. [This work is an extension of that reported in NASA TN D3156; supported by the National Aeronautics and Space Administration.] [30 min.]

4G2. Interpretation of Wall-Pressure Measurements under a Turbulent Boundary Layer. K. L. CaANDIRA•VrA•X (nonmember), Bolt Beranek and Newman Inc., Cambridge, Massa- chusetts 02138.•Some aspects of the correlations and spectra of fluctuating wall pressure beneath a turbulent boundary layer are considered. The correlation function for the pressure field is represented by a hierarchy of mathematical models, each successive model more complex than the preceding one. These models are based on some mathematical inequalities that are applicable to a "convecting and decaying" stochastic process. Some of these models are used for estimating attenuation of the pressure spectrum measured by a transducer of finite size. The results

1264 Volume 40 Number 5 1966

Downloaded 09 Sep 2013 to 128.197.27.9. Redistribution subject to ASA license or copyright; see http://asadl.org/terms

Page 2: Interpretation of Wall‐Pressure Measurements under a Turbulent Boundary Layer

72ND MEETING ß ACOUSTICAl. SOCIETY OF AMERICA

indicate that, at high frequencies, temporal rather than spatial fluctuations of the pressure field contribute to the "fixed-microphone" spectrum as measured by a transducer of finite size. Some comments are included concerning the influence of transducer size on measured narrow-band crosscorrelations. [30 min.]

4G3. Relation between Turbulent-Flow Structure and Wall-Pressure Fluctuations in a Turbulent Boundary Layer. WILLZA• W. WILLMARTa (nonmember), Department o[ Aero- space Engineering The University o[ Michigan, •tnn •lrbor, Michigan.--Recent measurements show the relationship between the turbulent structure near the wall and the wall-pressure fluctuations. Detailed measurements of pressure-velocity and velocity-velocity correlations suggest a flow structure consisting of hairpin-shaped turbulent eddies originating near the wall that are elongated in the stream direction as they are swept down stream and move away from the wall. The model for the turbulent eddies supplies a qualitative description of the observed convection and decay of wall-pressure fluctuations. The accuracy and limitations of contemporary wall-pressure measurements are discussed, along with some suggestions for future experimental work. [Work supported by U.S. Office of Naval Research.] [30 min.]

4G4. Flow-Noise Measurements in Water. GERALD J. FRANZ, David Taylor Model Basin, I/Vashington, D.C. 20007.--Available measurements of flow-induced noise and closely related turbulent-boundary-layer pressure fluctuations in water are reviewed and nominal-spectral- density and cross-spectral-density data for the fluctuating pressures are presented in dimensionless form. Included are data from pipe-flow facilities, gravity-propelled and towed bodies, and self- propelled vehicles. The effects of size, shape, smoothness, type, and location of hydrophones; curvature, roughness, fairness, thickness, homogeneity, and impedance of domes; Reynolds number of the flow; and various active boundary-layer control schemes are examined. Experi- mental difficulties encountered in water measurements are enumerated and compared to similar difficulties encountered in air. Finally, needed future experiments on large gravity-propelled models and on full-scale submarines and surface ships are described. [30 min.]

4G5. Flow-Noise Research Related to the Sonar Self-Noise Problem. PATRICK LEEI-IEY, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139.--A sonar dome is excited into bending vibration by random pressure fluctuations of turbulent-boundary-layer flow over the dome. A reverberant acoustic field is set up within the dome. The field appears as an important source of self-noise in the sonar transducer at moderate to high ship speeds. Much of the research on the corresponding aerodynamic-noise problem is directly applicable to the study of this underwater-noise problem; in particular, work characterizing the random pressure field for subsonic boundary-layer flows may be carried over directly. Dome response and the consequent internal acoustic radiation, however, are severely influenced by internal and external water loading. In addition, hydrodynamic coincidence effects play a markedly different r61e in the underwater problem than in the aerodynamic problem. Those aspects of flow-noise research pertinent to the sonar self-noise problem are reviewed in this paper, and directions for further research upon the influence of water loading in the prediction of self-noise levels are pointed out. [30 min.]

4G6. Response of Flexible Panel to Turbulence. LucIo MAESTRELLO, The Boeing Company, Renton, l/Vashington.•Using a relatively simple functional representation of space-time correla- tion of the wall pressure fluctuation and by the use of the Lyon-Dyer method, the motion of a simple supported panel can be predicted, and the results agree reasonably well with the author's experimental results. The most striking feature of the excitation mechanism is called coincidence. This is a condition under which the wavenumber and frequency spectrum coincide with the turbulence and panel spectra. The matching and mismatching in wavenumber and frequency scale clearly indicate that the mean-square displacement peaks corresponding to the various panel modes coincide with those of a pressure at a given frequency. The frequencies were chosen so that 2•ri/K•* corresponds to the convection velocity, K•* being the wavenumber at the spectrum peak. If, for certain frequencies and wavenumbers, mismatch between panel spectrum and turbulence spectrum should occur, a reduction in panel-displacement amplitude is observed. Such a mismatch is also reflected in the acoustic power radiated where similar reduction is obtained. [30 min.]

4G7. Turbulent Boundary Layer of Space-Flight Vehicle. RICHARD I-I. LYON AND K. L. C•A•DIRA•A•I (nonmember), Bolt Beranek and Newman Inc., Cambridge, Massachusetts 02138.--The turbulent boundary layer (TBL) is an important aerodynamic environment for major portions of a space-vehicle structure. For most launch vehicles, the period of large dynamic pressure, and hence strong TBL excitation, coincides with supersonic flight. This implies high-speed convection of the pressure-field speeds generally in excess of the bending- wave speed on the structure. New procedures for calculating the response of cylindrical struc- tures to a high-speed TBL have recently been developed at BBN. These procedures require information concerning the TBL pressure spectrum that can, in part, be related to flight data obtained from flush-mounted microphones. The use of these flight data for response estimation is the topic of this paper. [30 min.]

The Journal of the Acoustical Society of America 1265

Downloaded 09 Sep 2013 to 128.197.27.9. Redistribution subject to ASA license or copyright; see http://asadl.org/terms