Room acoustics

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, wavelengths are comparable to the dimensions of the room, and so room resonances dominate.
*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

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 (described at Standing wave) that generate nodes and high pressure zones. [ “Acoustics”, Leo Beranek, chapter 10, McGraw Hill Books, 1954] The mode spacing is a very important matter specially in small and medium size rooms like recording studios, home theaters, broadcasting studios and concert halls.Some methods were created from 1940 - 1981 to obtain the best mode spacing to avoid sound coloration, but none of them succeed, because the mode spacing is not only a geometric problem [ “The master handbook of Acoustics”, Alton Everest, TAB Books] An incorrect mode spacing in a room determinates a bad frequency response from a loudspeaker or a music player [ “Room Acoustics” Heinrich Kuttruff, Applied Science Publishers ]

In order to solve this problem, Oscar Bonello, professor at the University of Buenos Aires, created in 1981 the Modal Density concept that solves the problem introducing concepts from Psychoacoustics ["A NEW CRITERION FOR THE DISTRIBUTION OF NORMAL ROOM MODES" - Journal of the Audio Engineering Society (USA) Vol. 29, Nro. 9 - September/1981. Oscar Bonello] This new Bonello’s Criterion as it was named, analyzes the first 48 room modes and plot the numer of modes in each one-third of octave. The curve must increase monotonically (each one-third of octave must have more modes than the preceding one). This Criterion is now the standard method of designing the room dimensions ["Handbook for Sound Engineers"Glen Ballou, Howards Sams Editors, page 56.]

Reverberation of the room

After determining the best dimensions of the room, using Modes Density criteria, the next step is finding the correct reverberation time. A good explanation of the theory can be found at Reverberation. The reverberation time depend on the use of the room. Times about 1,5 to 2 seconds are needed for Opera Theaters and Concert Halls. For Broadcasting & Recording studios and Conference rooms, values under one second are frequently used. The recommended Rev Time is always functioning of the volume of the room. Several authors give their recommendations [ “Acoustics”, Leo Beranek, chapter 13, McGraw Hill Books, 1954] A good approximation for Broadcasting Studios and Conference Rooms is: TR [1khz] = [0,4 log (V+62)] – 0,38 TR in seconds and V=volume of the room in m3 [ “Clases de Acústica”, Oscar Bonello, Edited CEI, Facultad de Ingeniería UBA ] The ideal RT must have the same value at all frequencies from 30 to 12.000 Hz Or at least, is acceptable to have a linear rising from 100% at 500 Hz to 150 % down to 62 Hz

In order to get the calculated RT in a room, several acoustics materials can be uses as described in several books [ “Acoustic Design and Noise Control”, Michael Rettinger, Chemical Publishing, NY, 1977] , [ “Acoustical designing in Architecture” ,Knudsen & Harris, John Wiley and Sons, NY ] A valuable simplification of the task was proposed by Oscar Bonello in 1979 [ A new computed aided method for the complete design of broadcasting and recording studios”, Oscar Bonello, 1979 IEEE International conference on Acoustics, Speech & Signal Procession, Washington ] It consist of using standard acoustic panels of 1 m2 hanged from the walls of the room. These panels uses a combination of three Helmholtz resonators and wooden resonant panel. This system gives a large acoustic absorption at low frequencies (under 500 Hz) and reduces at high frequencies to compensate the typical absorption of people, lateral surfaces, ceilings, etc

References

ee also

*Acoustics
*Acoustic board
*Architectural acoustics
*Anechoic room
*Digital room correction
*Reverberation

Compare

*Noise control
*Sound proofing