Gideon Maidanik

The Mach wave field radiated by supersonic turbulent shear flows

Mach wave field radiated by supersonic turbulent shear layers estimated on basis of velocity gradients and density fluctuations

Radiation Properties of Cylindrical Shells

A theoretical method, similar to that applied recently to flat panels, is developed for estimating the radiation efficiency of a cylindrical shell. The radiation efficiency of the cylinder is significantly greater than that of a flat panel of equal dimensions at and about the ring frequency where the longitudinal wavelength is equal to the circumference of the cylinder. This peak in the radiation efficiency is shown to be associated with the increase in flexural‐wave speed due to curvature. A series of measurements of radiation efficiency of cylinders was conducted. The agreement between theory and experiment is satisfactory.

Propagation and Reflection of Sound in Rarefied Gases. I. Theoretical

An analysis of sound propagation in rarefied gases is presented. The analysis emphasizes molecular surface interactions with solid boundaries, and the feasibility of their study by acoustical means. A semiphenomenological theory is developed for the regime in which molecular‐surface interactions dominate. Dispersion relations are obtained which agree with experiments.

Radiation Efficiency of Panels

Theoretical predictions are compared with measurements of the radiation efficiencies of gypsum board and concrete panels.

High‐Frequency Plate Damping Due to Gas Pumping in Riveted Joints

Previously derived expressions for the damping associated with gas pumping at riveted and other multipoint‐fastened joints are restated and discussed. Quantitative predictions based on the theory, which considers the pressure dependence of gas viscosity on the basis of gas kinetics, are shown to be in reasonable agreement with data obtained with three different gases over a wide range of pressures.

Comments on “Propagation of Forced Sound Waves in Rarefied Gasdynamics” [L. Sirovich and J. K. Thurber, J. Acoust. Soc. Am. 37, 329–339 (1965)]

A recent paper of Sirovich and Thurber [J. Acoust. Soc. Am. 37, 329–339(1965)] presents comparisons of their theory with experiments. The comparison is inappropriate since the theory does not describe the experimental situation. Their theory is appropriate to free waves, propagating a distance that is large as compared with the mean free path between intermolecular collisions. The experimental data at low pressures have been determined by apparatus dimensions that are small as compared with that mean free path.

Determining impact source location in structural networks

An impact, or similar event, generates a transient in a structural network consisting of connected one‐dimensional systems. A sensor, attached at an arbitrary point in the network, will ‘‘hear’’ a complicated time series of pulses. A few of these pulses are recorded and used to generate a time‐reversed series which is injected at the sensor location. If the network is stationary in time, the impulse response function (IRF) can be used to show that the introduced series will constructively interfere at the original impact point and this location can thereby be determined. The procedure is demonstrated computationally using a time‐domain IRF. The relationship between the above and ‘‘reciprocity’’ and ‘‘inverse filtering’’ will be discussed.

Controlling the response of an oscillator using a coupled set of satellite oscillators

The response of a system comprised of multiple dynamic systems is analyzed. The results shown here are for the main or master oscillator in that system. The balance of the dynamic systems is designated as satellite oscillators. Controlling the response of the master oscillator is described in terms of the couplings to the master oscillator, the frequency distribution of the satellite oscillators, the loss factors, and the masses of those oscillators. The frequency distributions and masses of the satellite oscillators are specified via normalizations with respect to the resonance frequency and mass of the master oscillator in order to generalize the approach. It is shown that contrary to reported results by others, there is no requirement to optimize the frequency distribution of the satellite oscillators to maximize the control of the response of the master oscillator. Further it is shown that increasing the loss factor of the satellite oscillators beyond certain values does not bring further benefit in c...

Global and local responses of coupled dynamic systems

Responses of a master oscillator coupled to a set of satellite oscillators are evaluated. A set is designed by specifying a normalized frequency distribution. When the designed distribution is completely degenerate, the influence of the coupling is global. The coupling generates two global peaks, one on each side of an antiresonance that replaces the peak in the response of the uncoupled master oscillator. The loss factor for these global peaks matches that of the uncoupled master oscillator. As the degeneracy is gradually removed, the evaluated response gradually reveals, within the distribution bandwidth, contributions from individual satellite oscillators. The loss factors and the frequency separation of the two global peaks then become dependent upon the coupling. As the degeneracy is further removed, the local presence of the satellite oscillators assumes the dominant feature in the response whereas the presence of the two global peaks subsides. Since the designed distributions relate loss factors di...

Recollections of Professor Manfred Heckl

Professor Manfred Heckl was one of the scientific giants in the area of acoustics. He combined theory and experiment so well that both were made simple and transparent. I met Manfred at BBN in 1960. I realized he was one of those individuals that you could approach even when the question was not properly formulated. He helped and taught you how to formulate the question and directed you to the answer. He was an exceptional mentor and generous colleague. When he left BBN to return to Germany to assist in establishing Muller‐BBN and to be groomed to succeed Cremer’s professorship in Berlin, I lost a major impetus in my early development in acoustics. We tried to maintain close professional contacts, but his personal touch was missed and the efforts were not successful. Periodically, we did have face‐to‐face professional contacts and these were immensely enjoyable and beneficial, but the day‐to‐day influence was gone and I was poorer for it. Early in our relationship at BBN we became friends and we remained ...