The materials from which sound absorbing products are made, such as ceilings, islands and acoustic panels, among others, usually have a porous structure, consisting of numerous air-filled, small, interconnected pores with deep penetration, or a fibrous structure. This structure allows sound waves to easily penetrate deep into these materials.
Part of the sound energy of a sound wave hitting a material is converted into heat energy through friction, air viscosity inside the pores and fiber vibration. In this way the sound is partially absorbed by the material on which the sound wave falls.
The basic parameter characterizing the sound absorbing properties of building materials is the reverberation sound absorption coefficient a, which takes values between 0 and 1.
A value of 1 means that the sound was absorbed 100%, while 0 means that the sound was completely reflected from the surface. Current architectural trends in commercial buildings promote the use of many reflective surfaces such as concrete, glass, stoneware, etc. The sound absorption coefficient of these materials is close to zero. Their excessive accumulation in an interior causes long reverberation times, poor speech intelligibility and promotes reverberation noise. These factors have a negative impact on the acoustic climate of commercial interiors. Therefore, it is also important to use materials or systems with sound-absorbing properties.
However, it must be stressed that the key to achieving a good acoustic climate is to combine both sound-absorbing and sound-reflecting materials and use them on the right surfaces in the right proportions. Manufacturers of acoustic suspended ceilings declare sound absorption properties with a single sound absorption figure aw, which is obtained in accordance with PN-EN ISO 11654:1999 “Acoustics.
Sound-absorbing products used in construction.
However, the sound absorption coefficient a w alone is not sufficient to check whether the normative requirements for acceptable reverberation time values and minimum sound absorption values will be fulfilled. For this reason it is very important to choose products as finishing materials for which the manufacturers make available the values of the sound absorption coefficient in octave frequency bands so that the reverberation time values in octave bands can be calculated and the requirements of the standard can be verified.The sound absorption properties for spatial sound absorbing products such as ceiling islands and vertical elements, so-called baffles, are determined by the equivalent absorption area Aeq, expressed in m² per single element. It determines the size of a 100 % absorbing surface that absorbs exactly the same amount of sound as the tested element (island or baffle).
For the selection of design solutions according to the requirements of PN-B-02151-4, for most types of spaces the calculation of the reverberation time must be carried out using the available sound absorption coefficients for the given types of materials.
There are, however, spaces for which it is sufficient to know the acoustic absorption coefficient of the room. In this case, information from manufacturers on the acoustic absorption that a material introduces into the room is very helpful. This solution is used when the element is suspended from the ceiling at a considerable distance and does not obstruct the surface of the ceiling above it. Knowing the acoustic absorption introduced by a given sound absorbing element solves the problem of what sound absorption coefficient to take for the calculation of the reverberation time for the surface of the suspended island facing the ceiling.
Verification of the requirements presented in the standard and performing the required calculations require knowledge of interior acoustics and sound protection. To verify whether a room meets the requirements of the standard, it is best to seek the help of a specialist in the field of acoustics who will make the necessary calculations and propose suitable materials and solutions or verify the proposals of the interior designer or contractor. It is best to install sound-absorbing suspended ceilings according to the manufacturer’s recommendations. This is due to the high sensitivity to changes in the sound absorption coefficient in the low and medium frequency range depending on the installation method used. The manufacturer, while declaring the sound absorption coefficients in octave frequency bands, should also provide the conditions and installation method for which the coefficients were measured. For example, increasing the distance of a suspended ceiling from the soffit to some extent usually leads to an increase in absorption in the low frequency range.