The sides of a megaphone hold the sound waves in and allow them to escape in only one direction, thus intensifying the waves. A stethoscope is a megaphone in reverse. It funnels sound waves from a relatively wide area into a small area. Sound waves show other properties that resemble those of light. If so, they cancel out one another, and no sound is heard.
Interference helps in the formation of sound beats.
If two organ pipes, for example, are tuned a few vibrations apart, they produce a throbbing tone when sounded together. If the difference is three vibrations per second, the waves will be out of phase three times in each second and will be in phase an equal number of times. When they are out of phase, there is a moment of comparative silence. When in phase, however, they reinforce each other and increase the intensity of the tone. Each produces tones in a different way.