To properly understand the phenomena of room acoustics, different models must be applied to envisage the propagation of sound at high and low frequencies. The limiting frequency which identifies the transition from one model to the other depends on the volume of the room. In a car it is at about 400 Hz, in an average living room it is at about 180 Hz, and in a concert hall it is at about 30 Hz. There is no clear dividing line between the low frequency range and the high frequency range.
At frequencies far above the limiting frequency the dimensions of the room are considerably larger than the acoustic wavelength. Therefore the geometric acoustics model applies, i.e. sound radiates from a source and is then reflected by the walls of the room. The amount of energy a sound ray looses with each reflection depends on the nature of the walls. With the help of this model a number of room acoustical parameters can be derived. The best known parameter is the reverberation time. It is a measure for the reverberance of a room and is closely related to the sound level generated by a sound source within the room. The longer the reverberation time, the higher the sound level.
At frequencies distinctly below the limiting frequency the simple model of geometric acoustics does no longer apply. The phenomena occurring at low frequencies can only be illustrated by the acoustic wave theory. The sound field is characterized by room eigenmodes. These are three-dimensional standing waves within the room developing at certain frequencies. Each eigenmode is associated with a different sound pressure distribution. Moving around in a room where an eigenmode has been excited, you will register strongly fluctuating sound intensities.