An acoustical resonance in a room caused by parallel wall surfaces, also called a room mode or an eigentone. Any set of parallel walls will establish a series of standing waves, the lowest one of which has the wall spacing as a half-wavelength. The sound waves interfere with one another to produce a series of places where the SPL is high and another series of places between then where the SPL is very low,. It is as if the sound wave were stationary between the two surfaces. See also room mode, quarter-wavelength rule.