Volume 67, Issue S1, April 1980
Index of content:
- PROGRAM OF THE 99TH MEETING OF THE ACOUSTICAL SOCIETY OF AMERICA
- Workshop on Using MEDLINE
67(1980); http://dx.doi.org/10.1121/1.2018094View Description Hide Description
Participants will be introduced to MEDLINE, the major data base of the medical literature analysis and retrieval system of the National Library of Medicine (NLM). This computerized data base contains citations of articles drawn from 3000 journals worldwide. It can be searched online at computer terminals either independently or with the assistance of technical information specialists located at over 800 online centers in the United States. MEDLINE citations include author, title, source, and abstract which can be retrieved on a topic.
The Communicative Disorders Program, NINCDS, in cooperation with NLM, improved MEDLINE coverage of communicative disorders by increasing the number of journals and the specificity of indexing terms in these fields. A users' manual and thesaurus have been written to guide communicative disorders specialists in conducting searches on over 2000 topics.
Participants will learn how to use MEDLINE to meet their own literature retrieval needs. The workshop will demonstrate the design of accurate search statements, use of indexing terms, and procedures to find concepts not included in MEDLINE vocabulary. Basic protocols and conventions of the system will be demonstrated.
Experience with MEDLINE is not necessary for successful participation. The users' manual and thesaurus will be distributed to participants.
- Session A. Engineering Acoustics I, Underwater Acoustics I, and Physical Acoustics I: Radiation, Diffraction, and Scattering
- Contributed Papers
67(1980); http://dx.doi.org/10.1121/1.2018095View Description Hide Description
The sound radiation from convex and concave rigid diaphragms represented by portions of a spherical surface was investigated for the purpose of modeling a dome loudspeaker. Pressure responses, radiation impedances, directivity patterns, and some other characteristics were obtained and compared with those of a circular flat diaphragm of the same radius. While the on‐axis pressure response of the rigid flat piston remains constant, the response of the convex dome decreases for about ka ≥ 1 as the height of the dome increases. The concave dome has a wide smooth peak around ka = 1.5 and many peaks and dips for larger values of ka. The radiation resistance of the convex dome increases by the rate of 12 dB/oct or less up to about ka = 2.0 and remains almost constant in higher frequency regions. On the other hand, the radiation resistance of the concave dome has its largest peak at the frequency corresponding to the peak of sound pressure response and shows many peaks and dips in higher frequency region. The radiation reactance of the convex dome is smooth and always positive, while the concave dome also exhibits negative reactance due to the resonance in its cavity. The directivity patterns of either the convex or concave dome do not have minima with zero sound pressure and their radiation patterns are generally wider than those of the circular plate.
67(1980); http://dx.doi.org/10.1121/1.2018096View Description Hide Description
An infinite fluid‐loaded surface, idealized here as a membrane, is supported by two ribs of infinite impedance, situated at equal distances on either side of a third central rib which is driven by a prescribed line force. Formal exact solutions are readily obtained for the structural response and the radiated sound field, and these are simplified and interpreted on the basis that a fluid‐loading parameter at the coincidence condition is small. The arguments rely heavily on the asymptotic structure of the Green's function for the infinite fluid‐loaded membrane, as described at the 98th Meeting [D. G. Crighton, J. Acoust. Soc. Am. Suppl. 1, 66, S1 (1979)]. Parameter ranges in which resonances and antiresonances are to be found are determined, and explicit expressions are given for the amplitude of resonant and antiresonant responses in regard to the sound field, the drive‐point response, and the surface wave fields transmitted across the ribs. Arbitrary configurations of multiple ribs can be dealt with in a similar manner.
67(1980); http://dx.doi.org/10.1121/1.2018148View Description Hide Description
A thin, uniform, rectangular, elastic plate is freely supported in air, and driven so that it vibrates flexurally at a resonance frequency. The modal function, which describes the vibrational displacement over the surface of the plate, is approximated as the product of two cosine functions. The kinetic energy is proportional to the square of this quantity. From this modal function the probability density function (pdf) P(x), for random sampling over the plate surface of the kinetic energy, is calculated to be , where K is a complete elliptic integral. P(x) has a simple pole at the origin, and declines monotonically as x increases. Using this expression and experimental values sampled randomly over the plate surface, estimates can be made of the mean square vibrational level of the plate. Experimental results are given for a steel plate measuring , resonating at a single frequency. These results are composed with the theoretical cumulative function derived from the pdf given above.
67(1980); http://dx.doi.org/10.1121/1.2018149View Description Hide Description
Convective component of flownoise (e.g., a turbulent boundary layer) is characterized by length scales of the order of U/f, where U is the vehicle speed and f is the frequency. Nonresonant response of underwater structures to the convective component of flownoise is often of interest. An exact theory of transmission of sound across multiple‐layered structures, involving longitudinal and shear waves, is modified so as to apply at convective wavenumbers. Closed form results are presented for a flat plate.
Control ranges and the optimal solutions of the minimal reflectance problem; homogeneous materials, normal incidence, and narrow bandwidths67(1980); http://dx.doi.org/10.1121/1.2018150View Description Hide Description
We study the optimal solutions of the minimal sound reflection problem for planar inhomogeneous sound absorbing layers at normal incidence and narrow bandwidths as a function of their controls. Homogeneous materials are considered. The controls are the real and imaginary parts of the inverse complex longitudinal modulus, and the density, ρ. The optimal solutions correspond to multilayered systems, with piecewise constant, extremal control values on the sublayers. The dependence of layer structures, i.e., numbers of sublayers, sublayer thicknesses, and parameter distributions, on the controls can be studied as a function of a certain polyhedron in control space. The shape of the polyhedron, which is determined by the boundary values of the controls, will be correlated with desirable characteristics of the resulting layered structures.
67(1980); http://dx.doi.org/10.1121/1.2018151View Description Hide Description
The transmission of flexural waves through various discontinuities in the walls of cylindrical shells is theoretically investigated. The discontinuities consist of changes in wall thickness or wall material. The solution employs matching boundary conditions with all propagating and evanescent waves provided by the Flügge solution of the cylindrical shell problem. The curves obtained predict high transmission losses at the cut‐on frequencies of various axial and torsional waves and at high frequencies approach the flat plate solution. It is also shown that, due to the strong generation of nonflexural waves, the evaluation of transmission losses at a discontinuity by measurement of radial vibrational velocity alone can give a highly misleading result. The attenuation produced by a change in wall material was found to depend more strongly upon the ratios of phase speeds of the materials than their relative stiffnesses, and falls markedly at the ring frequency of the incident pipe due to improved coupling of flexural waves in both materials. The inclusion of damping in the analysis was found to increase the transmission losses. [Work supported by MOD].
67(1980); http://dx.doi.org/10.1121/1.2018152View Description Hide Description
Previous work has considered the effect of free body resonances on the scattering of an incident plane wave by a cylindrical target in water and has related the major features of the theoretical form function resonances to the experimentally obtained farfield reflection patterns. Excitation of these free body resonances is here observed for a glass cylinder in water utilizing a birefringence imaging technique. A fused quartz cylinder is insonified by a long plane propagating pulse and images of the internal stress pattern at resonance frequencies obtained under polarized light. The experimental data are in agreement with a normal modetheoretical computation of the resonance locations and predicted stress distributions for the particular modes.
67(1980); http://dx.doi.org/10.1121/1.2018153View Description Hide Description
A numerical solution to the generalized Burgers' radial wave equation has been developed which allows one to calculate stepwise the harmonic content of a finite amplitude wave in the frequency domain for the case of plane, cylindrical, or spherical geometries. The finite amplitude wave may have any initial harmonic content and the attenuation coefficient of each harmonic is independently adjustable. Remaining in the frequency domain allows much larger steps than conventional programs which alternate between the time and frequency domain. The algorithm is used to verify the farfield behavior of spherical waves as predicted by D. A. Webster [J. Acoust. Soc. Am. 64, S33(A) (1978)] and to investigate the effect of a large second harmonic attenuation coefficient on the generation of a shock wave.
67(1980); http://dx.doi.org/10.1121/1.2018202View Description Hide Description
Consider the problem of finding the brachistochrone between two points on the boundary of an area of interest having a spatially varying velocity field. The arbitrary velocity field can be synthesized from a set of simpler functions, for example, by a Fourier representation or a power‐series expansion. Now consider the analogous brachistochrone problem, using the same endpoints, for each of the simpler functions. Each of these problems is often solvable analytically. The solution to the original brachistochrone problem can be expressed in terms of the solutions to the set of brachistochrone problems on the simpler surfaces. The theory and an application will be presented. [Work supported by NSF and NOAA.]
Experimental detection of boundary diffraction waves irradiated by an impulsively excited line source of finite length67(1980); http://dx.doi.org/10.1121/1.2018203View Description Hide Description
It has been shown theoretically that the transient acoustic pressure field radiated by a finite line source comprises three distinct components: a spatially discontinuous cylindrical wave (“geometrical wave”) and two directional boundary‐diffraction waves radiated by the ends of the line. This model of the structure of the pressure field, which is that predicted by the Rubinowicz‐Maggi diffraction theory, has also been demonstrated experimentally. The experiments were conducted in a water tank. Bursts of pseudorandom band‐limited white Gaussian noise were used to impulsively excite a line transducer electrically. By crosscorrelating these signals with the acoustic signals that the transducer radiates, and by using a computer to filter the resulting crosscorrelation functions, the waveform of the transducer's spatial impulse response was obtained at a number of observation points in the source transducer's radiated field. The experimentally obtained waveforms agree closely with those predicted by the theoretical calculations.
67(1980); http://dx.doi.org/10.1121/1.2018204View Description Hide Description
Cracks, which constitute an important class of defects in materials, are mathematically idealized by planar voids of zero thickness whose surfaces are traction‐free. A longitudinal or transverse plane wave propagating in a homogeneous isotropic elastic medium will be scattered by such a flaw. The following properties of the scattering cross section will be established. (i) In the Rayleigh limit the longitudinal and transverse scattered amplitudes can be described by 6 parameters which represent (1) the area of the crack, (2) its aspect ratio, (3) the deviation of its shape from symmetry under inversion through its center. (4, 5) two angles specifying the orientation of the crack plane, and (6) the orientation of the crack in its plane. Formulae for the scattered amplitudes in terms of these parameters will be displayed. (ii) For any wavelength whatever and the incident direction normal to the plane of the crack the scattered intensity in either the longitudinal or the transverse mode is symmetric under reflection through the plane of the crack, and the transverse intensity vanishes in the plane of the crack. [Work supported by DOE.]
67(1980); http://dx.doi.org/10.1121/1.2018205View Description Hide Description
The impedance formulation developed by Junger [J. Acoust. Soc. Am. 24, 366–373 (1952)] to describe a thin elastic shell is used for an eigenfunction expansion solution to the plane wavescattering from two elastic cylindrical shells in fluid. The two‐dimensional approach employs the addition theorem as first reported by Twersky [J. Acoust. Soc. Am. 24, 42–46 (1952)]. The effects of the resonances of the shell are shown by comparison with the cases of scattering from two acoustically soft or two acoustically rigid cylinders. The interaction between the cylindrical shells is discussed. Finally, the method of images is used to investigate the scattering of a plane wave from a cylindrical shell near an infinite plane.
67(1980); http://dx.doi.org/10.1121/1.2018206View Description Hide Description
Backscatteringmeasurements using short acoustic pulses were made for finite elastic cylinders immersed in water. These solid aluminum cylinders were terminated with solid hemispherical caps. The overall length of the cylinders is four times the radius. For frequency scaling the factor (or 2ak) was used. Measurements were made perpendicular to the cylinder axis, end on, and at 45°. Results ranging from less than 4.0 to greater than 5.0 in were compared with the theoretical predictions using a T‐matrix formulation made by Varadan, Varadan, and Flax [V. V. Varadan, V. K. Varadan, and L. Flax, J. Acoust. Soc. Am. 60, S81 (1979)].
67(1980); http://dx.doi.org/10.1121/1.2018208View Description Hide Description
From first principles it is possible to calculate schlieren patterns from given transducervelocity profiles. By use of the Fourier projection theorem, the sound field need not be calculated as the integrated optical effect for light passing through the sound field is found directly. By applying the principles of optical spatial filtering, schlieren patterns similar to those observed can be calculated. These patterns depend on sound level, transducer configuration, and type of spatial filtering. The effect of these patterns of a flaw, i.e., a region of no motion on the transducer, will be presented.
A low frequency technique for inverting of backscattering data for shape and sizing of acoustic and elastic scatterers67(1980); http://dx.doi.org/10.1121/1.2018209View Description Hide Description
A new inversion scheme for backscattered data in acoustic and elastic scaltering is being investigated. There are indications that the ramp response in the time domain yields considerable information about the size and gross shape of the scatterer, albeit fine details may be lost. The theoretical computations show that the Raleigh and lower resonance regions of the scattering dictate the ramp response and that the scattering at higher frequencies is secondary. Computational and experimental results will be presented to support the ramp response concept.
- Session B. Noise I: Jet and Aerodynamic Noise
67(1980); http://dx.doi.org/10.1121/1.2018248View Description Hide Description
A semi‐empirical model for predicting the noise generated by jets exhausting from circular nozzles is presented and compared with small‐scale static and simulated‐flight data. The present method is an updated version of that part of the original NASA Aircraft Noise Prediction Program (1974) relating to circular jet noise. The earlier method has been shown to agree reasonably well with experimental static and flight data for jet velocities up to ∼ 520 m/s. The poorer agreement at higher jet velocities appeared to be due primarily to the manner in which supersonic convection effects were formulated. The purely empirical supersonic convection formulation is replaced in the present method by one based on theoretical considerations. Other improvements of an empirical nature have been included based on model‐jet/free‐jet simulated‐flight tests. The effects of nozzle size, jet velocity, jet temperature, and flight are included.
67(1980); http://dx.doi.org/10.1121/1.2018249View Description Hide Description
Previous studies have shown that increasing the annulus width of a conventional coaxial nozzle with constant bypass velocity will lower the noise level. In the present model‐scale study, the annulus was shaped by an eccentric mounting of the annular nozzle with respect to the conical core nozzle. Acoustic measurements were made in the flyover plane below the widest portion of the annulus and at 90° and 180° from this point. The model‐scale spectra are scaled up to engine size (1.07‐m diameter) and the perceived noise levels for the eccentric and concentric coaxial nozzles are compared over a range of operating conditions. The implications of the acoustic benefits derived from the eccentric nozzle to practical applications are discussed.
67(1980); http://dx.doi.org/10.1121/1.2018250View Description Hide Description
Previous studies have shown that an inverted‐velocity‐profile coaxial nozzle for use with supersonic cruise aircraft produces less jet noise than an equivalent conical nozzle. Furthermore, decreasing the annulus height (increasing radius ratio with constant flow) results in noise reduction benefits. In the present model‐scale study, the annulus was shaped by an eccentric mounting of the annular nozzle with respect to the conical core nozzle. Acoustic measurements were made in the flyover plane below the narrowest portion of the annulus and at 90° and 180° from this point. The model‐scale spectra are scaled up to engine size (1.07‐m diameter) and the perceived noise levels for the eccentric and concentric inverted‐velocity‐profile coaxial nozzles are compared ovcr a range of operating conditions. The implications of the acoustic benefits derived from the eccentric nozzle to practical applications are discussed.
Acoustic Tests of Three Single Stream Referee Nozzles at the NASA‐Lewis Research Center Outdoor Jet Acoustic Facility67(1980); http://dx.doi.org/10.1121/1.2018251View Description Hide Description
The characteristics of the jet noise produced by three single stream referee nozzles have been investigated statically at the NASA—Lewis Research Center outdoor jet acoustic facility. These nozzles were a conical convergent, a 10‐cm‐diam conical convergent, and an 8‐lobe daisy nozzle with equivalent diameter flow area. The same nozzles have been previously tested in other facilities such as the Royal Aircraft Establishment 24 ft acoustic wind tunnel, and the NASA Ames Research Center 40 × 80 wind tunnel. The test conducted covered pressure ratios from 1.4 to 2.5 at total temperatures of 810°K and ambient. Data were obtained using four different acoustic arena arrangements. Data taken with the different arrangements are compared. The results obtained are also compared with data from other facilities and with a prediction procedure developed elsewhere [i.e., Stone and Montegani (1979)].