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Apr 1982

Volume 71, Issue S1, pp. S1-S113

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back to top Session C. Speech Communication I: Speech Processing: Analysis and Synthesis
Contributed Papers
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Noise reduction in speech using adaptive filtering I: Signal processing algorithms (A)

R. W. Christiansen, D. M. Chabries, and D. Lynn

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S7-S8 (1982); (2 pages)

Online Publication Date: 12 Aug 2005

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Adaptive filtering is employed in configurations to filter narrow‐band speech corrupted by noise. These configurations utilize either an independent sample of the input noise or rely on correlation properties of the speech to accomplish cancellation. Necessary constraints on the algorithms to retain and/or improve intelligibility for normal and hearing‐impaired populations are presented. Previous work with the LMS algorithm is compared to the performance resulting from new methods which address the problem created by widespread eigenvalues in the speech plus noise correlation matrix. It is shown that failure to deal with this problem in the single reference application results in a muffling resonant distortion of the speech spectrum. Spectral averages of the original and processed versions of the speech segments are compared to determine improvement. Auditory tests are presented in the companion paper.
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Noise reduction in speech using adaptive filtering II: Speech intelligibility improvement with normal and hearing‐impaired subjects (A)

R. H. Brey and M. S. Robinette

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S8-S8 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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An adaptive LMS filter is employed to filter speech in signal‐to‐noise ratios varying from −8 to +12 dB. The filter configuration used is that previously proposed by Widrow, Boll, and Pulsipher and commonly called noise cancellation. Intelligibility measures were obtained using speech selected from (CID) W‐22 word lists. Fifteen lists of 50 words each were selected. Ten normal‐hearing and six hearing‐impaired individuals were subjects. Sound‐field and free‐field measurements, as well as processed and unprocessed data, were obtained and analyzed. Intelligibility was near zero for all SNR's less than 0 dB prior to processing. Following processing scores increased 30% to 40% for both normal and hearing‐impaired groups. For SNR's of +4 to +12 dB, intelligibility scores prior to filtering ranged from 10% to 50%; after filtering the scores improved to within 10% of the noise‐free sound‐field measurement. The specific test configuration, SNR improvement estimates, as well as additional results and comparisons, will be presented.
back to top Session D. Musical Acoustics I: Guitars
Invited Papers
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The guitar frequency response (A)

Graham Caldersmith

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S8-S8 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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The guitar frequency response is found to be a good predictor of two important parameters of guitar tone: initial level of the partials and initial level decay of the partials. A high initial level normally gives a rapid initial level decay, but under some conditions the higher level may more than compensate for the initial level decay and thus give a tone a longer duration. The condition of the strings was found to be much less significant than the frequency response of the guitar body.
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Simple model for low‐frequency guitar function (A)

Ove Christensen and Bo B. Vistisen

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S8-S8 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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The frequency response of sound pressure and top plate mobility was measured around the two first resonances of the guitar. These resonances are shown to result from a coupling between the fundamental top plate resonance and the Helmholtz resonance of the cavity. The Helmholtz resonance is found as an antiresonance in the mobility response but it is not seen directly in the sound‐pressure response. A simple two‐oscillator model is presented for guitar function at low frequencies. All parameters of this model are determined from experimental measurements of the frequencies of the two first resonances and of the Helmholtz cavity resonance. The model reproduces quantitatively the experimental frequency response of sound pressure and of top plate mobility. The influence of the back plate of the guitar is investigated from a similar three‐oscillator model. [Work supported by SHF.]
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Fundamentals of guitar tone (A)

Erik V. Jansson

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S8-S9 (1982); (2 pages)

Online Publication Date: 12 Aug 2005

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The guitar is a complex physical system containing many resonances. Fundamental tonal properties such as initial levels and initial level decays are set by properties of the guitar. The decays of partials are approximately proportional to the inverse of the initial level. Furthermore, the initial partial levels, and thus the inverse level decays, are approximately proportional to the frequency response of the guitar body (recorded as radiated sound for a constant driving force). At resonance frequencies, however, the peak level of the frequency response overestimates the initial tone level and underestimates the decay. In spite of a more rapid decay, a high initial level may give a tone of longer duration, i.e., longer duration above a masked threshold. The guitar gives a decaying tone spectrum with the higher partials decaying more rapidly than the lower ones. Initially the decaying vibrations of the eigenmodes play a noticeable role. High sound pressure levels, including high levels at 500–800 Hz and at 1500–4000 Hz, seem to be favorable. Frequency components above 5000 Hz are perceptible only at the attack of tones. [Work partly in cooperation with G. W. Caldersmith.]
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Guitars: Steady state and transient response (A)

I. M. Firth

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S9-S9 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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Measurements of input admittance confirm that the guitar can be considered as a bass‐reflex enclosure with flexible sides. The effect of this is to increase the sound output without distortion below the first resonance of the top plate. The proposed equivalent circuit will be reviewed. New measurements on plucked notes using a digital analyzer show that the onset of sound contains two distinct parts. The main steady state resonances are shock excited at the beginning of the pluck, grow, and decay within the first few milliseconds of the onset. Simultaneously the harmonics of the string grow and remain as the sustaining vibration of the tone after the steady state resonances have died away. It appears that every played note of the guitar contains at its beginning sound representative of the steady state response which contains information about the type of instrument being sounded and its quality, and this is followed by the sustaining harmonics of the string. Experiments with the violin, cello, and harp show this new effect to occur also in these instruments.
Contributed Papers
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Studying a guitar's radiation properties with nearfield holography (A)

William Y. Strong, Jr., T. B. Beyer, D. J. Bowen, E. G. Williams, and J. D. Maynard

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S9-S9 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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Recent studies of an intact string‐excited guitar have shown that the motion of the air in the rose is quite significant over the first two octaves of the instrument's range. Our nearfield holography technique has revealed that the rose is a dominant source of the radiated energy. We have found that modeling the guitar as a rigid cavity with flexible top plate and a sound hole is not accurate. The motion of the back plate in the guitar studied was quite significant, with amplitudes across it 2–3 times greater than at the top for the lower frequencies, and therefore should not be ignored in subsequent studies. Additionally, accepted plate modes seem to occur at more than one frequency in the string‐excited guitar. The recurrence of these modes may tend to smooth out the frequency response curve, and a better understanding of this effect may lead to a more well balanced instrument.
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Inharmonicity of wound guitar strings (A)

Adrian J. M. Houtsma

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S9-S9 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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Wound guitar strings are known to “go dead” after several hours of playing. Increased inharmonicity of string partials is thought to be the primary contributing factor, making exact tuning of strings impossible. Increased inharmonicity with age is mostly due to changes in mass distribution rather than changes in stiffness. String aging can be artificially induced by repeated stretching and releasing of a new string. Measurements of the frequencies of the first ten partials in standard brass‐wound steel guitar strings show that inharmonicity is significantly increased by repeated stretching. The inharmonic effect of stretching can be greatly reduced, however, if the strings are stress relieved by heat after winding.
back to top Session E. Underwater Acoustics I: Shallow Water Propagation
Invited Papers
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Shallow‐water acoustic research at the Naval Research Laboratory (A)

F. Ingenito and S. N. Wolf

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S10-S10 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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This paper reviews shallow‐water acoustic research performed by the Naval Research Laboratory during the past ten years. The earlier work concentrated on acoustic propagation modeling and measurements. Major achievements were the development of techniques to measure the excitation, depth distribution, and attenuation of individual normal modes and successful comparison of the measurements with theoretical predictions. This work culminated in a tested propagation loss model capable of representing the complex conditions characteristic of the shallow‐water environment. More recently, a model of wind‐generated noise based on the transmission loss model has been developed. This model predicts relative level and spatial coherence of the noise field. The experimental program has been expanded to include measurements of signal coherence, ambient noise level and coherence, and array gain. Examples of these measurements and comparisons with predictions will be presented.
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Broadband sound propagation in shallow water (A)

T. Akal and F. B. Jensen

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S10-S10 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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A review of ten years of research in shallow water acoustics at SACLANTCEN is presented. Both experimental and theoretical results are given, with general features of shallow water propagation being emphasized. The experimental basis for this study is an extensive set of shallow water propagation data collected in different coastal areas of the Mediterranean Sea and the eastern North Atlantic. These data, in turn, have been subjected to a comprehensive modeling effort using wave‐theory models (normal mode and parabolic equation) in order to identify the fundamental mechanisms governing sound propagation in shallow water. As a result of this model/data comparison we have obtained a better understanding of the complicated frequency‐dependent changes in propagation conditions with changing environment, i.e., sound‐speed profile, water depth, bottom type, etc. Propagation levels in shallow water are generally found to be highly dependent on bottom type. In particular the coupling of sound into shear waves in the bottom is found to be an important low‐frequency loss mechanism. It is also shown that the optimum frequency of propagation in shallow water is strongly dependent on water depth but less dependent on bottom type. This phenomenon will be dealt with in some detail.
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Geoacoustic models for propagation modeling in shallow water (A)

D. M. F. Chapman and Dale D. Ellis

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S10-S10 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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Acoustic propagation in shallow water is viewed as a guided‐wave phenomenon, with the sea surface and seabed forming the boundaries. At subkilohertz frequencies, the acoustic properties of the seabed to a depth of several wavelengths can have a strong effect on propagation. The computer modeling of propagation requires estimates of such parameters as sound speed, density, attenuation, and layer thicknesses, which collectively are called the geoacoustic model of the seabed. Direct measurement of these quantities is difficult, and methods must be devised to infer these values from other experiments, often employing acoustic techniques. At DREA, we have adopted the approach of independently determining as many geoacoustic parameters as possible, and adjusting less precisely known parameters within reasonable limits to effect an agreement between theory and experiment. To this end, we have used sub‐bottom reflection profiles to determine sediment types and layer thicknesses, large and small scale seismic refraction experiments to estimate sound speeds, and processing of sub‐bottom vertical reflection data to estimate volume attenuation. Techniques used by other researchers will be reviewed. Examples of geoacoustic models and comparisons with experiment will be presented for shallow water sites on the Scotian Shelf and the southwestern approaches to the English Channel.
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Coupling of acoustic normal modes in shallow water (A)

Suzanne T. McDaniel

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S11-S11 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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In shallow water, coupling between acoustic normal modes may occur due to interaction with irregular boundaries. Two treatments of mode coupling are presented and discussed: a stochastic theory applicable to scattering due to ocean surface and bottom roughness, and a deterministic theory applicable to large‐scale range dependent features. The advantages and limitations of each of these methods is discussed and results are compared with those obtained from alternative methods of treating the same phenomena. It is concluded that both the stochastic and deterministic mode‐coupling models provide a powerful tool for increasing our understanding of the effect of boundary irregularities on shallow water acoustic propagation. [Work supported by NAVSEASYSCOM, NSEA‐63R‐1.]
Contributed Papers
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Shallow‐water propagation: The role of the branch‐line integral in the Pedersen‐Gordon model (A)

Marshall Hall

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S11-S11 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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Solutions to the wave equation for sound‐speed profiles which are terminated by an isospeed half‐space can be expressed as a sum of discrete modes plus a branch‐line integral (BLI). The BLI is often insignificant, but can be of great significance in duct propagation at frequencies near the cutoff freqency of the duct. In one example [D. C. Stickler, J. Acoust. Soc. Am. 57, 856–‐861 (1975)], the BLI dominated the mode contribution for ranges out to 30 km. The Pedersen‐Gordon normal‐mode model, on the other hand, terminates the profile with a “furry” half‐space in which the sound speed approaches zero as z−1/2 as the depth (z) approaches infinity. The Pedersen‐Gordon model is applied to Stickler's shallow‐water duct, and the limit of the results for the sound field (based on only a mode sum), as the sound‐speed gradient in the half‐space (evaluated at its interface with the duct) approaches zero, is determined. This limit is found to be equal to Stickler's result for the sum of both the BLI and the mode contributions. The theoretical reasons for this equality are discussed.
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Comparison between perturbative and exact treatment of bottom attenuation for shallow‐water, low‐frequency conditions (A)

DeWayne White and Stanley A. Chin‐Bing

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S11-S11 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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A comparison between treating bottom attenuation perturbatively and exactly in a normal‐mode expansion has been made for a representative shallow‐water, low‐frequency problem. The exact treatment introduces bottom attenuation through complex sound speed and thus complex depth functions, whereas the perturbative approach uses real valued depth functions and introduces mode attenuation only in the range function. Cases examined indicate transmission loss versus range calculations resulting from the two approaches disagree significantly near cutoff. Comparison with FFP calculations indicates the exact (complex depth function) solution is correct. When more than one mode is present, the other modes dominate the mode nearest cutoff; and although this mode is in error, it does not seriously affect the results. For those cases where the perturbative and exact solutions significantly differ, the error in the perturbative approach is mostly due to an incorrect normalizing factor; mode attenuation differences also account for part of the error. Results are discussed and a correction to the mode normalization factor derived from the perturbative solution is suggested.
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Investigation of forward sound field amplitude and phase fluctuations by boundary perturbations in a normal‐mode duct (A)

Michael R. Layton

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S11-S11 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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A physical model of a shallow‐water duct has been constructed with a 5‐cm (4λ) isovelocity water layer overlying a 10‐cm homogeneous sand bottom. Using a projector operating cw at 100 kHz, a propagating normal‐mode pressure distribution consisting primarily of the lowest‐order mode was established in the duct. A pair of small hydrophones were then positioned in the water layer at identical range and depth, with a horizontal separation between them on the order of 1.5 cm (one wavelength). Prior to investigating the effect of a small perturbation either on the water surface or at the water‐sand boundary, the difference between the received signals from each hydrophone was nulled using electronic means. The differential signal behavior was then examined for a variety of experimental conditions, which included surface roughness, bottom roughness, receiver depth, receiver range (from projector), and receiver separation. [Work supported by ONR and DARPA.]
back to top Session F.Physical Acoustics I and Engineering Acoustics I: Radiation
Invited Paper
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Photoacoustic generations and applications (A)

Andrew C. Tam

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S12-S12 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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A wide variety of acoustic pulses (different magnitudes and temporal/frequency profiles) can be produced in a sample by optical absorption of various light beams, i.e., photoacoustic (PA) pulse generation. Various generation mechanisms are possible: thermal expansion, gas evolution, chemical changes, dielectric breakdown, electrostriction, etc. PA pulse generation has found several important applications, including ultrasensitive spectroscopic detections of linear and nonlinear optical absorptions, noncontact ultrasonic measurements in hostile environments, PA imaging techniques, PA operation of mechanical devices, and so on. This paper presents some basic understanding of a PA generation process, and examples of important recent applications.
Contributed Papers
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Nearfield of an arbitrary piston in an infinite rigid baffle (A)

Alan H. Lubell

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S12-S12 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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Numerical calculations of the nearfield of several types of under‐water radiators made in 1965 for NAA Columbus led to this author's rediscovery of the edge integral for pistons in an infinite rigid baffle. Interest in this very efficient numerical method applicable to any shape of plane piston was rekindled in conjunction with an ongoing investigation of the impedance of a plane symmetric axisymmetric stepped piston radiator (LL model 98 underwater loudspeaker). A program was devised for the TRS‐80 Mod III computer and Used to duplicate a recently published absolute pressure field map for the circular piston [New et al., J. Acoust. Soc. Am. 70, 1518–1526 (1981)]. The agreement between the maps will be discussed as well as the cost of computation to a specified accuracy. Real and imaginary components of pressure on the piston face will be given for several values of ka. The value of the edge integral method for understanding certain aspects of diffraction theory more fundamentally than is possible with the Helmholtz integral theorem will be touched on.
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Three‐dimensional, farfield measurements of a Gaussian profile ultrasonic transducer (A)

Richard O. Claus and Paul S. Zerwekh

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S12-S12 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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An ultrasonic transducer has been constructed using a concentric array of annular metallic electrodes on a thin circular piezoelectric substrate. The transducer is driven through an impedance matching and weighting network to produce an electric field within the substrate that is an optimized piecewise linear approximation to a two‐dimensional Gaussian function [P.S. Zerwekh and R. O. Claus, IEEE 1981 Ultrason. Symp., Chicago, IL]. In this paper, the calculated farfield Gaussian amplitude distribution is presented and compared to experimental data measured using a microprocessor‐controlled automatic scanning system at a distance of 500 wavelengths in a water tank. This data is also compared with similar analytical and experimental results obtained for a circular piston transducer having only one active surface electrode. Applications of the transducer in acousto‐optical modulation systems and in the ultrasonic inspection of metallic surfaces is discussed. [Work partially supported by NSF and NASA.]
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Sound radiation from an axisymmetric radiator in an infinite baffle (A)

Hideo Suzuki and Jiri Tichy

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S12-S12 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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A method that can be applied to the radiation problem of a convex or a concave radiator in an infinite baffle is described. This method utilizes the least‐square‐error method, and allows the radiator to take almost any kind of axisymmetric shape even though the accuracy of the result depends on the shape. The frequency responses of the on‐axis sound pressure are calculated for three cases of the radiator shape, a convex dome, a concave dome, and a combination of a truncated concave cone and a convex dome at the center. The accuracy is confirmed by comparing the results obtained by the present method with those obtained by previous methods [Hideo Suzuki and Jiri Tichy, J. Acoust. Soc. Am. 69, 41–49 (1981)]. Three error indexes are suggested for an estimation of the accuracy of the results.
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Vibration and acoustic response of a damped circular plate in an infinite baffle (A)

Michael Latcha and Adnan Akay

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S12-S12 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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Transient sound radiation from an impactively excited circular plate clamped at the outer edge in an infinite baffle is obtained by numerical integration of Rayleigh's integral. Calculated sound pressure wave‐forms are compared with the experimental results to demonstrate the sound radiation mechanism due to “rapid deformation” of the plate. These results show that on the axis of symmetry perpendicular to the plate, the sound pressure wave reproduces the plate velocity, as opposed to the classic case of a transiently excited rigid piston in an infinite baffle which reproduces the piston acceleration.
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The mass law for a finite panel (A)

Patrick Leehey

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S13-S13 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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It is a classical result that the sound power per area transmitted by a limp infinite panel decreases by 6 dB per frequency or panel mass doubling. Several years ago, Sledjeski and I demonstrated that a plane wave transmitted normally through a baffled rectangular membrane yielded a transmitted sound pressure level which asymptoted at high frequency to a constant at any fixed on‐axis position. It is shown that this results from the summed effect of all those panel modes that are driven sufficiently above their resonance freqencies that their responses are masslike and the modes themselves are acoustically fast. The panel then acts as a rigid piston with directivity which increases by 6 dB per frequency doubling. The last statement is verified experimentally. [Work sponsored by ONR Sensor Technology Program.]
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Asymptotic results for the motion of finite fluid‐loaded structures (A)

D. Innes and D. G. Crighton

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S13-S13 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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The response of a finite plane fluid‐loaded structure under local excitation is governed by a generalized Wiener‐Hopf functional equation whose solution has previously been found asymptotically for large plate size or light fluid loading (the general approach being nonetheless valuable in that it encompasses the full range of edge conditions and the absence or presence of a surrounding baffle). Here we use the smallness of a “fluid loading at coincidence” parameter ϵ to find asymptotic solutions for a strip membrane or plate, unbaffled and with free edges, under line force drive, and covering the whole range of normalized frequency Ω and strip width L. Resonant response is identified, both at moderate frequencies where fluid loading is light, and at low frequencies where it is heavy; the resonant response is calculated, both on the structure and in the fluid. Approximate methods, yielding results in precise agreement with those derived here, are devised for numerous areas of the (Ω, L) plane. [Work supported by ONR Code 421 and DTNSRDC Carderock Code 1900.]
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Waveform shape near Mach cone arrival for the pressure perturbation created by a modulated laser beam moving over a water surface at supersonic speed (A)

Allan D. Pierce

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S13-S13 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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Previous work by Lyamshev, Kolomenskii, Bozhkov, and others on acousto‐optical sound generation is extended to a detailed study of the pressure perturbation following Mach cone arrival. The solution of the Westerfelt‐Larson equation for sound generated by laser beam heating is presented in integral form using the Green's function for a supersonically moving point impulsive source. The laser beam points downward into the water, has a Gaussian beam shape (radius a), and moves rectilinearly with supersonic speed V; heat generation per unit time varies locally with z as e−μz, where μ is the optical absorption coefficient; time dependence of laser beam intensity is as 1 + m cos ωt where modulation index m is less than 1. The formal result requires a three‐fold integration, which is approximated using a local coordinate system with origin at farfield observation point, one axis pointing normal to the Mach cone. Approximate integration proceeds with the assumption that time interval following first arrival is short compared with r/(V2 − c2)1/2, where r is cylindrical distance. Given that a ≪ 1/μ, a ≪ c/ω, and r ≫ 1/μ, the transient pulse initially has a period of the order of 2a/c, and subsequently evolves into a super‐position of two signals of gradually decreasing frequency. The two frequencies are explained in terms of Doppler shift theory and ray travel times. Amplitude of the first peak decreases as 1/r1/2 and undulates with distance parallel to the beam track, the periodicity being 2πV/ω.
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Parametric study of noise and vibration signals (A)

Jean Nicolas and Gilles Lemire

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S13-S13 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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Acoustic and vibration signals have been studied in various ways and conditions. Assuming that the information provided by a vibration accelerometer and a pressure microphone have some correspondence, we have investigated how far one can extend or not this relationship. The results of our experiments have shown that one must clearly separate the amplitude related parameter (power spectrum) from the functions yielding both amplitude and phase contents (coherence, acoustic intensity, etc.). The most significant parameter happens to be the acoustic velocity, obtained in the nearfield and compared to the structural velocity. The acoustic velocity has been obtained by an approximation of the pressure gradient given by a two‐microphone probe. The procedure of calibration of the velocity has been found far more critical than the one for the acoustic intensity. The acoustic velocity found experimentally for a point source has been compared to that predicted by the exact theory; results show very good agreement both for the near and farfield. The revealing aspect of this study points out the fact that acoustic velocity is a complex function (intensity is only part of it). Therefore coherence function might be evaluated between the vibration and the acoustic velocity, opening the field for more knowledge about the influence of the reactive field and the radiation efficiency effects.
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Aerodynamic mechanisms in jet‐edge‐resonator oscillation (A)

Samuel A. Elder

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S13-S13 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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See Also: Erratum

Show Abstract
Undulatory motions of a separated shear layer in a rectangular slot are found to give rise to three forces that are believed responsible for jet‐edge and jet‐edge‐resonator oscillation at low Mach number. The three are (1) a transverse force on the downstream edge which gives rise to the transverse dipole feedback radiation postulated by Powell, (2) a longitudinal force on the downstream wall, which results in longitudinal dipole radiation believed responsible for oscillation in shallow cavities, and (3) a longitudinal force in the body of the fluid which is associated with pipetone drive. By direct integration of the Lighthill tensor over the entire region of fluid motion between the edges, the magnitude of each force is estimated. In every case the results are directly proportional to shear layer width, and cannot therefore be derived from theories using the vortex sheet approximation.
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