This kind of fork-shaped tube was used to study the motions of different parts of a vibrating Chladni plate. If the ends of the tube were held over two zones that were vibrating in the same direction, a sound would be heard, but if the vibrations were in opposite directions they would cancel each other out (interfere) and the sound would be greatly reduced.

Source: Steven Turner; Curator, Physical Sciences, National Museum of American History, Smithsonian Institution. 

Picture: Steven Turner