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 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.
The sound wave has reflections at the walls, floor and ceiling of the room. The incident wave then has interference with the reflected one. This action creates standing waves that generate nodes and high-pressure zones. In 1981, in order to solve this problem, Oscar Bonello, professor at the University of Buenos Aires, formulated a modal density concept solution which used concepts from psychoacoustics. Called "Bonello Criteria", the method analyzes the first 48room modes and plots the number of modes in each one-third of an octave. The curve increases monotonically. Other systems to determine correct room ratios have more recently been developed
Reverberation of the room
After determining the best dimensions of the room, using the modal density criteria, the next step is to find the correct reverberation time. The most appropriate reverberation time depends on the use of the room. Times about 1.5 to 2 seconds are needed for opera theaters and concert halls. For broadcasting and recording studios and conference rooms, values under one second are frequently used. The recommended reverberation time is always a function of the volume of the room. Several authors give their recommendations A good approximation for Broadcasting Studios and Conference Rooms is: TR = – 0,38 TR in seconds and V=volume of the room in m3 The ideal RT60 must have the same value at all frequencies from 30 to 12,000 Hz. Or, at least, it is acceptable to have a linear rising from 100% at 500 Hz to 150% down to 62 Hz To get the desired RT60, several acoustics materials can be used as described in several books. A valuable simplification of the task was proposed by Oscar Bonello in 1979. It consists of using standard acoustic panels of 1 m2 hung from the walls of the room. These panels use a combination of three Helmholtz resonators and a wooden resonant panel. This system gives a large acoustic absorption at low frequencies and reduces at high frequencies to compensate for the typical absorption by people, lateral surfaces, ceilings, etc.
