Sound level in a room
K: Direct sound field of an omnidirectional source
D: Diffuse sound field
rh: Reverberation radius
The sound level is a measure for the loudness level of a sound source. In rooms the sound level is characterized by two different sound fields, the direct sound field and the diffuse sound field.
The direct sound field is the sound field that would develop around a sound source if the room did not have any boundaries. With increasing distance from the source, the sound level decreases continually. In case of omnidirectional sources that radiate the sound uniformly in all directions, the level decreases by 6 dB when doubling the distance.
The diffuse sound field is created by the fractions of sound that are reflected by the boundary surfaces of the room. They propagate haphazardly in the room and fill it evenly with sound energy. Consequently, the sound level in a diffuse sound field is the same irrespective of the relevant position.
The sound fields superimpose each other. Near the sound source the sound level is dominated by the direct sound field, and at greater distances the diffuse sound field is dominating. The distance from the sound source at which the level of the direct sound field is equal to the level of the diffuse sound field is called reverberation radius.
DIN 18041 and ÖNORM B 8115-3 define a group of rooms the common denominator of which is the requirement that it may not get too loud inside these rooms during normal use. Further below you will find some examples. Each standard lays down two conditions, and it does not matter which of these is fulfilled.
The first condition according to DIN 18041 is satisfied when furniture, people, and additional room acoustical treatments taken together have an adequate sound absorbing effect. This is determined by converting the total equivalent sound absorption area of the room into an average absorption coefficient α–of the room’s boundary surfaces. In the frequency range from 250 Hz to 2000 Hz this average absorption coefficient should be greater than or equal to 0.35. The second condition according to DIN 18041, however, refers to the reduction of the noise caused by the diffuse sound field that can be achieved by room acoustical treatments. If the noise reduction ΔL in the frequency range from 250 Hz to 2000 Hz is greater than or equal to 3 dB, the room acoustical treatments are considered to be reasonable.
In ÖNORM B 8115-3 two frequency-dependent tolerance limits are defined. The first applies to the average absorption coefficient αm,B of the room’s boundary surfaces. The second applies to the average absorption coefficient αm of the room’s boundary surfaces taking account of the furniture within the room.
Offices, Call Centers:
There exists a wide range of suspended ceilings for offices by means of which noise can be sufficiently reduced. Today, however, more and more often thermoactive ceilings are used. Such ceilings, which are mostly made of concrete, contain integrated water pipes which enable the ceiling to be heated or cooled to a certain temperature. They may not, or only partially be covered by suspended ceilings with thermal insulation. In this case vertically suspended absorbers (so-called “baffles”) or single elements with good thermal conductivity, metallic frames, and a surface of perforated metal sheets must be used, supported by sound absorbers on the walls, in room dividers or furniture.
Surgeries, operating and hospital rooms:
Due to hygienic reasons these rooms usually are equipped with furniture that can be easily cleaned. Such furniture, however, almost absorbs no sound. Here acoustics can be greatly improved by even a small number of additional sound absorbers. Many manufacturers offer special systems complying with the high hygienic requirements.
Canteens, bars, restaurants:
If a restaurant is to present a quiet and pleasant ambience, room acoustics play a decisive role. Sufficient sound absorption can be achieved by using upholstered chairs, an abundance of thick curtains, and long table cloths. Where such accessories are incompatible with a modern ambience marked by steel and glass it is still possible to achieve good acoustics by using jointless acoustic systems that look like a plastered surface.
Ticket offices, bank counters, sales rooms, airports, train stations, museums, exhibition rooms:
All these rooms can offer a friendly atmosphere if they do not resound too much. The necessary sound absorbers can often be unobtrusively integrated into the architectural design.
Hallways, foyers:
The atmosphere of staircases, hallways and foyers usually is also more pleasant if not all of their surfaces are rigid. To reduce the sound level, walls or ceilings should at least be partially provided with absorbent linings.