Like light waves, sound waves can be reflected and focused. A flat surface, like that of a cliff or wall, reflects sound better than an irregular surface, which tends to break up sound waves. Echoes are useful in many ways. In a fog, a ship's captain can often tell whether he is near a hilly shoreline by listening for echoes of the ship's whistle. Underwater sonar equipment uses echoes of a supersonic signal similarly to detect submarines.
Depth finders use echoes from the ocean bottom to measure the depth of the water.
A band shell focuses the sound of the band in just the same way an automobile headlight reflector focuses light. The headlight reflector and many band shells are in the shape of a parabolic curve. This curve has the property of reflecting spherical waves in such a way that they form a beam. The band shell concentrates the sound on the audience, preventing it from being dissipated in all directions. If someone whispers at one spot in such a chamber, he can be clearly heard at another spot yards away.