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Isadore Rudnick
 Figure 3. Professor Rudnick holding a large Rijke tube to demon- strate thermoacoustic sound production in the mid-1970s.
For very thin adsorbed helium films, the restoring force is the van der Waals attraction between the helium atoms and the substrate. As the films get thicker, their free surface relieves any pressure gradients, so thick-film wave motion in the su- perfluid component is restored by temperature gradients. This wave mode, known as fifth sound, was first observed in Izzy’s lab for helium in a resonator that was partially packed with fine powder so that the open part above the packed powder provided the pressure-released boundary condition.
Zero Sound
Zero sound is a collisionless mode of normal 3He quasiparti- cles characterized by an asymmetric oscillation of the Fermi sphere, as explained by Landau using a Boltzmann equa- tion model. Any of Izzy’s graduate students would instantly acknowledge that such an obscure theoretical description would never sit well with “The Mentor’s” desire for a true physical explanation. His discomfort led to the recognition
18 | Acoustics Today | Winter 2017
that both longitudinal and transverse zero sounds were, in fact, just the modes one would predict for an ideal visco- elastic liquid. In the conclusion of a typically insightful and carefully worded article, Izzy gently chastised the low-tem- perature physics community for failing to cast zero sound into this simple phenomenological model that had been so well-known by earlier physicists, going back to Maxwell in 1867, “from bygone days, when courses in hydrodynam- ics and elasticity were normally in the physics curriculum” (Rudnick, 1980).
The Teacher and the Showman
Izzy was a soft-spoken and thoughtful man. As his teaching assistant for an upper-division physics course on acoustics, one of us (Garrett) would listen to his lectures and scribble notes furiously in an attempt to capture the cascade of insights that he would unleash. Rarely, there would be a pause that would allow a glance around the classroom. Typically, half the undergraduate students looked bored and the other half were asleep. You had to listen carefully to appreciate his wisdom.
On the other hand, Izzy’s lecture demonstrations were “loud and clear.” Luckily for us, many of those demonstrations were filmed and are readily available as part of his Collected Works (Maynard and Garrett, 2011). Izzy loved using dem- onstrations in his teaching. He is shown in Figure 3 hold- ing his large Rijke tube. His public lectures at UCLA, given under the title of “An Evening of Demonstration Experi- ments in Physics,” would attract standing room only crowds. Izzy would use the high-intensity sound field of a siren to levitate objects and make cotton burst into flames as it ab- sorbed the abundant acoustic energy (while also removing all of the chalk dust from the blackboards). Such demonstra- tions would always end the show because the campus police invariably were called to the auditorium to investigate the source of such a piercingly loud sound.
In 1976, Izzy was selected to present the 51st UCLA Faculty Research Lecture, an annual honor given to a distinguished scholar who would then give a public lecture in Schoenberg Hall. Rather than just a talk, he prepared a series of live demonstrations using superfluid helium, not an easy trick in a music building. That lecture was recorded and later con- densed into a 17-minute film, The Unusual Properties of Su- perfluid Helium. It won Best of Category at the 21st Annual San Francisco International Film Festival in 1977, beating out several entries from major industrial sponsors. The film closed with a superfluid fountain spraying liquid helium throughout a clear glass Dewar in response to a saxophone

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