Fundamentally, a distinction must be made between airborne noise and impact noise for the sound insulation of building elements. Airborne noise comes from a source that makes the air vibrate directly (e.g. radio, TV). Impact noise of comes from a source that makes part of the building vibrate; the noise is then transmitted to neighbouring rooms and so makes the air vibrate indirectly (e.g. vibration in central heating pipes). Impact noise insulation must be provided for at the design stage. It is very important, from this point of view, to make the necessary arrangements between quiet and noisy premises. The sound insulation that we describe below only takes airborne noise into account and not impact noise. In common usage, “insulation against airborne noise” is often confused with “noise absorption”. These principles are explained below. Sound absorbing products serve to limit the reverberation time and to adjust the acoustic comfort in the room, whereas sound insulation is given to mean the reduction in sound intensity transmitted from one room to another.
Insulation against noise
In practice, the noise between two rooms does not only propagate directly (meaning through the separating wall), but also indirectly (side walls, ceilings, floors, etc.). More specific sound insulation, which takes both the direct and indirect routes into account, is called overall airborne noise insulation; this is a real value that can be measured on site. In contrast, the transmission loss coefficient of a material is a laboratory-measured value based only on the direct transmission of noise through the separating wall. It is expressed in dB. In accordance with Belgian standard NBN S 01–400, the transmission loss coefficient (R) of a building element is expressed in classes, and Hebel is placed in the following classes:
In accordance with standard DIN 4109, a calculated value of the overall sound insulation (R’w,R) can be deduced from the mass per unit area of single rigid walls or ceilings. It is based on construction with closed joints or a noise proof finish. The table below gives some of these calculated values as a function of mass per unit area.
Noise absorption
Sound waves that strike a wall are partly reflected, partly absorbed and partly transmitted.
In accordance with Belgian standard NBN S 01-009, sound absorption is expressed by an absorption coefficient a of between 0 and 1. The value of the coefficient a (sound absorption factor according to Sabine) depends on the frequency of the incident sound and the surface structure of the construction element. The absorption factor (a) of a wall is: α = transmitted sound energy + absorbed incident sound energy α = 1 signifies that all noise is absorbed or transmitted (e.g.: open window) α = 0 signifies that all noise is reflected Reverberation occurs in a room when the incident noise is reflected and absorbed to a lesser extent. The sound absorption of a building element inhibits reverberation in a room. In the event that all the sound energy is completely absorbed, the coefficient value is 1. Due to its cellular surface structure, cellular concrete has a sound absorption capacity 5 to 10 times greater than smooth materials. The graph below shows that Hebel absorbs ± 25% of the noise.
.jpg)
