Sound doesn’t always spread uniformly. You may have experienced this if you shout while you walk through a tunnel. You hear echoes and a person at another part of the tunnel will hear you much more clearly than he or she would in an open area. Whispering chambers are often found in science museums. They are rooms with walls in an elliptical shape. If you stand at one focus of the ellipse and another person stands at the other focus you can hear each other speak even if you whisper while other people in the chamber are talking loudly. Sound can also be transported through the ocean with less spreading using an acoustic waveguide, similar to the way light can travel in an optical fiber (see the chapter on Light). The ocean water can separate into layers at different temperatures. The speed of sound depends on water temperature, and if there is a layer of cold water where sound speed is lower under a layer of warmer water where the speed is higher, then some of the sound moving through the cold layer will be reflected at the boundary back into the cold layer, keeping it from spreading. The name of this channel is SOund Frequency And Ranging (SOFAR) Channel. It is 100 to 200 meters below the surface in the cold waters off Alaska but 750 to 1,000 meters deep in the warmer Hawaiian waters.