Room acoustics describes how sound behaves in an enclosed space. The way that sound behaves in a room can be broken up into roughly four different frequency zones: The first zone is below the frequency that has a wavelength of twice the longest length of the room. In this zone, sound behaves very much like changes in static air pressure. Above that zone, until the frequency is approximately 11,250(RT60/V)1/2 (when Volume is measured in cubic feet and 2000(RT60/V)1/2 when Volume is measured in cubic metres), wavelengths are comparable to the dimensions of the room, and so room resonances dominate. This transition frequency is popularly known as the Schroder frequency, or the cross-over frequency and it differentiates the low frequencies which creates standing waves within small rooms from the mid and high frequencies. The third region which extends approximately 2 octaves is a transition to the fourth zone. In the fourth zone, sounds behave like rays of light bouncing around the room.