Standing Wave

One additional form of indirect sound is known as a standing wave. This may occur when the wavelength of a sound is the same distance as some major dimension of a room, such as the distance between two opposite walls. If both surfaces are acoustically reflective, the frequency corresponding to that wavelength will be amplified, by addition of the incoming and outgoing waves, resulting in a strong, stationary wave pattern between the two surfaces. This happens primarily with low frequencies, which have long wavelengths and are not easily absorbed.

(from Audio System Guide for Houses of Worship, Shure Educational Publication)