Room acoustics is a subfield of acoustics dealing with the behaviour of sound in enclosed or partially-enclosed spaces. The architectural details of a room influences the behaviour of sound waves within it, with the effects varying by frequency. Acoustic reflection, diffraction, and diffusion can combine to create audible phenomena such as room modes and standing waves at specific frequencies and locations, echos, and unique reverberation patterns. Frequency zones 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 wavelengths are comparable to the dimensions of the room, room resonances dominate. This transition frequency is popularly known as the Schroeder frequency, or the cross-over frequency and it differentiates the low frequencies which create 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. Natural modes For frequencies under the Schroeder frequency, certain wavelengths of sound will build up as resonances within the boundaries of the room, and the resonating frequencies can be determined using the room's dimensions. Similar to the calculation of standing waves inside a pipe with two closed ends, the modal frequencies and the sound pressure of those modes at a particular position of a rectilinear room can be defined as where are mode numbers corresponding to the x-,y-, and z-axis of the room, is the speed of sound in , are the dimensions of the room in meters. is the amplitude of the sound wave, and are coordinates of a point contained inside the room. Modes can occur in all three dimensions of a room. Axial modes are one-dimensional, and build up between one set of parallel walls. Tangential modes are two-dimensional, and involve four walls bounding the space perpendicular to each other. Finally, oblique modes concern all walls within the simplified rectilinear room. A modal density analysis method using concepts from psychoacoustics, the "Bonello criterion", analyzes the first 48 room modes and plots the number of modes in each one-third of an octave. The curve increases monotonically (each one-third of an octave must