Interaction of acoustical waves and electrons

It might strike you as odd that a title which includes the words electron and acoustical wave could describe a frontier of physics. We have already heard at this meeting that the electron was discovered in the 19th century, and as everyone knows, acoustics is perhaps the oldest branch of physics. But, what I would like to discuss is something that is rather satisfying, not only because it is rather simple physics, but because it shows that old and honorable subjects still have aspects that are new. The electrons that I shall discuss are the same ones that J. J. Thomson discovered, but in this context they are somewhat “strange” particles since they move in the odd world within a metal. The sound waves, too, are not quite what the Greeks discussed; here they are ultrasonic ones.