If the listener were to sit closer to one speaker than the other, the crests of the two sound waves would reach the listener at different times. The waves might then arrive out of phase. If they arrived exactly half a cycle out of phase, the crests of one wave would coincide with the low points of the other. The waves would cancel each other, and so no sound would be audible to the listener. This effect is known as cancellation or destructive interference.
Similar interference occurs with light waves.
The effects can be observed in a simple experiment. Coat a glass plate with paint and then scratch two thin lines very close together. Direct a beam of light at the lines and allow the emerging light to fall on a viewing screen. The light appears as a series of colored bands called interference fringes. Each band will have bright and dark fringes.