Acoustical Society of America
Gold Medal Award - 1982

Isadore Rudnick

In 1948 it was saed, "Nobody knows quite as well how much you deserve this honor as do those of us who have been intimately associated with you in the laboratory, who have seen the adventurous sparkle of your eye whenever new ideas were being explored, and who know from direct observation with what professional competence, keenness of mind, and skill of hand you tackle new and difficult tasks." The occasion was the presentation of the Biennial Award of the Acoustical Society of America to Isadore Rudnick, a promising young man who seemed to have an uncanny knack for doing research in acoustics. Now 34 years later, the sparkle, the keenness of mind, and skill of hand have not diminished; in fact these remarkable qualities have been enhanced by a stream of successful research experiences.

In these 34 years the honors have come tumbling in: three times Fulbright or Guggenheim Fellow, UCLA Faculty Research Lecturer for 1975–76, winner of the Acoustical Society's first Silver Medal in Physical Acoustics in 1975 and, this past August, recipient of the prestigious international honor, the Fritz London Memorial Award, presented in the past to such distinguished scientists as Nobel Laureates Lev D. Landau, John Bardeen, and Brian Josephson.

Born in New York City on 8 May 1917, Izzy moved with his family to Los Angeles at an early age. Rudnick received his Ph.D. from UCLA during World War II. After wartime research at Duke, he moved to Pennsylvania State College (now University) in 1945, where he studied sound fluctuations in heated air and sound absorption in sand and soil. His important paper on reciprocity calibration of microphones was written at that time, and acousticians interested in laboratory standards use the technique to this day. Rudnick became interested in the effects of high-intensity sound and, working with the Penn State group, conducted the first applied studies of effects of macrosonics on animals and the laundering of clothes. The pioneering quality and depth of penetration shown in each of these studies was already impressive.

The early progress was accelerated when Izzy was attracted back to UCLA in 1948 to join V. O. Knudsen, R. W. Leonard, Leo Delsasso, and the visiting Carl Eckart, in what was rapidly becoming a world center of physical acoustics. There, working with his first Ph.D. student, he attacked the difficult problem of propagation next to a boundary. In closely coordinated theoretical and experimental forays, they corrected a mistake in the Sommerfeld solution of the electromagnetic problem, and went on to make the definitive acoustical measurements that have guided several studies by later investigators. This symbiotic development is one of the hallmarks of Izzy's research: theory and experiment must advance side-by-side; either approach in isolation is sterile.

Rudnick's research in high-intensity effects continued until the late 1950s. His studies, again both theoretical and experimental, of the attenuation of sawtooth waves are classic. Equally impressive and enduring is the work he did on cavitation, saturation of finite-amplitude waves, the nonlinearity coefficient for liquids, sources of acoustic streaming, and the interpretation of the once-mysterious bulk viscosity of liquids. Much of this work has been rediscovered, and some unknowingly repeated, in the recent surge of interest in nonlinear underwater acoustics.

Those were the days when a casual visitor to Izzy's office would find him browsing through Lord Rayleigh's two-volume Theory of Sound, finding pearls of enduring value in that marvelous work with its notoriously poor index. In retrospect, those early fundamental studies of all aspects of general physical acoustics and fluid dynamics were the groundwork that was to give Rudnick the tools and the ideas that allowed him to become a giant among cryogenic physicists.

Izzy's low-temperature work, which started in the 1960s, presents an elegant example of the power of acoustical techniques in advancing the frontiers of physics. The discovery of "fourth sound," a propagating acoustic wave of superfluid in a tightly packed porous medium, was perhaps the most prominent early cryogenic research in which Rudnick was involved. The identification followed an unsuccessful search by other experimentalists after its prediction many years before. Although the flow persistence of He II had been known, detailed definitive measurements were not made until Rudnick employed the Doppler shift in fourth sound, measured the decay time, and found that even saturated currents would decay by only 12% in a time equal to the age of the universe! Those experiments also established the correspondence between the behavior of helium supercurrents and highly irreversible type-II superconductors.

"Third sound," a surface wave on the superfluid adsorbed to a substrate, has also been studied as a phenomenon, and exploited as a tool, by Izzy. His observations made on films as thin as 3.6A have been of great importance in showing that superfluids of atomic dimensions can still be described in terms of macroscopic hydrodynamic concepts. His measurements which used third sound waves to determine the onset of superfluidity in thin films provided the first experimental test of the universality predicted in the Kosterlitz-Thouless renormalization theory of two-dimensional phase transitions.

Ordinary or "first sound" has not been neglected. Indeed, its use as a tool in low-temperature physics has been pursued by Rudnick in typically extraordinary ways. Besides studying cavitation in liquid helium, he has measured the speed and attenuation of sound at frequencies up to 1010 Hz and at temperatures within 10-6 degrees of the lambda point of liquid helium. His measurements have yielded important information about the critical point and critical phenomena.

Until the publication of a recent article by Rudnick, "zero sound" had been understood by using a Boltzmann equation model proposed by Lev Landau. That model described zero sound as a collisionless mode of normal 3He quasiparticles characterized by an asymmetric oscillation of the Fermi sphere. As any of his graduate students would quickly acknowledge, such an obscure theoretical description would never sit well with "The Mentor's" desire for a true physical understanding. His discomfort lead to the recognition that both longitudinal and transverse zero sound were in fact just the modes one would predict for an ideal viscoelastic liquid. In the conclusions of that typically insightful and carefully worded article, Izzy gently chastised the low-temperature physics community for failing to cast zero sound into this simple phenomenological model which had been so well known by earlier physicists from "bygone days, when courses in hydrodynamics and elasticity were normally in the physics curriculum."

"Second sound" is a propagating temperature-entropy wave in a superfluid and it too could not escape Izzy's exploitation to further our knowledge of quantum fluids. He recognized that the simultaneous measurement of first, sound, and fourth sound over the P-T plane and a knowledge of the density of helium at one point would yield all the information necessary to fully characterize the thermodynamics of superfluid helium. Those measurements, made in his laboratory, resulted in an order-of-magnitude improvement in the precision and self-consistency of the thermodynamic tables for this unique fluid. He also introduced a clever method of transduction for second sound which he then employed to measure the superfluid healing length and to test critical point scaling near the lambda transition. This transduction technique is now very popular in quantum fluid experiments.

Just as an ordinary pressure wave in bulk liquid helium becomes fourth sound in porous media, Izzy predicted that second sound would also undergo a transition to a new propagating sound mode in confined geometries. His predictions were confirmed by the simultaneous discovery at UCLA and Penn State of the latest in the list of superfluid collective modes, "fifth sound." This new mode is currently being exploited in his laboratories to yield fresh insights into the flow and thermodynamic properties of superfluid helium.

Professor Rudnick's insistence on measuring what he discovers, the unity of his theoretical and experimental approaches, have demonstrated to the entire community of low-temperature physicists that acoustics has a major role to play in the forefront of physics research.

More recently, by the modification and modernization of a 150 year old experiment originally performed by Michael Faraday, Izzy has been catapulted into a leading role in the rapidly expanding field of nonlinear transition to chaos through subharmonic bifurcations. So fluid dynamicists, too, will now begin to recognize the power of acoustical techniques when in the hands of an ingenious experimentalist–theorist.

In 1975 the UCLA Academic Senate selected Rudnick as the fifth-first annual Faculty Research Lecturer. The transition has featured such eminent figures as Arnold Schoenberg, Jakob Bjerknes, Hans Reichenbach, and Carl Eckart. Izzy, realizing that it was technically impossible to conduct experiments on liquid helium from the stage of the large auditorium transported the audience into the fairyland of superfluid helium by filming the experiments in his laboratory and showing the motion picture as part of the lecture. The film was subsequently entered in an International Film Festival in San Francisco where it took first prize in its category.

In the early years, Izzy's teaching of young Ph.D. candidates involved hours spent in private dialogues with Izzy thinking out loud, quietly challenging the neophyte to extend, contradict or support his oral jousts. Whenever possible a simple qualitative experiment was developed to dramatize his new understanding of a phenomenon. Many of these experiments became testaments to his philosophy that "today's research becomes tomorrow's class laboratory." By 1980 the series of experiments demonstrating the effects of nonlinearity had grown to an impressive array, and the UCLA Committee on Public Lectures announced "An Evening of Demonstration Experiments in Physics." A half hour before "performance" time the lecture room was packed; later the event was repeated, with supplementary experiments thought up in the intervening weeks, again to a "full house." Science history buffs are reminded of the famous and immensely popular Royal Institution Christmas Lectures of the 19th century; the era of Tyndall and Faraday had returned, if only briefly, to the "hip" audience of Los Angeles. It was a performance that again demanded repetition, and a Plenary session of our Society's 100th meeting was regaled by a cavalcade of exotic experiments in nonlinear acoustics demonstrating acoustic levitation, acoustic streaming, shock waves, sound riding on sound, "the acoustic motor," and "the acoustic log-starter."

Izzy would be the first person to object to the omission of his students' names from this short history. The doctoral students who have blossomed under Rudnick's benevolent guidance are Robert W. Lawhead, William Galloway, Herman Medwin, David Lieberman, Daniel Filson, Norman Meyer, Edward Lax, John Brewster, Joseph Zemanek, Moises Levy, Kenneth Shapiro, Richard Stern, Martin Barmatz, Reynold Kagiwada, Giusseppe Natale, James Imai, Michael Kriss, James Fraser, Stephen Scott, Richard Williams, Taylor Wang, Daniel Commins, Haruo Kojima, Kenneth Telschow, John Scholtz, Joseph Heiserman, Steven Garrett, and Roset Khosropour.

The gold medal is sometimes awarded for services to acoustics. Rudnick's list of service to our Society could hardly be more extensive: Member of the Executive Council 1958–61, Chairman of the Technical Committee on Physical Acoustics 1961–63, Vice-President 1962–63 and President of the Acoustical Society 1969–70; Member of the Governing Board of the American Institute of Physics 1965–68; U.S. Delegate to the International Commission on Acoustics 1978–84.

It is a rare person who is being honored by this Gold Medal: a brilliant acoustician who has made ingenious and fundamental contributions to the mainstreams of physics; a dedicated teacher who has effectively communicated his wisdom and his principles to two generations of students; a fine and gentle colleague who has served the acoustics community with love and distinguished leadership. Herman Medwin
Fred Fisher
Richard Stern
Steven Garrett