"The speaker plays a signal—a note—that resonates in the pipe, and creates a standing wave," Waterhouse says. "That means that in some parts of the pipe, the air pressure is low, and in other parts it is high." He is backing down the length of one of the pipes, making chopping motions with his hand. "These U-tubes are full of mercury." He points to one of several U-shaped glass tubes that are plumbed into the bottom of the long pipe.