Sound absorption material for acoustical correction
Sound absorbing Fibreglass insulation has three major acoustic applications, namely:
- To decrease the noise level in typing, computer and plant rooms and in factory halls, etc. In a room sound is reflected again and again from the walls, floor and ceiling. Every time a reflection passes the ear, the same sound is heard again thus making it sound louder. Fibreglass on one or more of the reflecting surfaces quietly reduces the strength of the reflections and hence the loudness of the sound.
- To adjust the reverberation time in recording studios, multi-purpose halls, lecture rooms, etc. In a well-furnished lounge, sound dies out much sooner than in an empty room. The former has a shorter reverberation time than the latter because it contains more sound deadening materials. Studios for speech and music should have a reverberation time of about half a second and this is obtained most economically by the use of Fibreglass insulation. For every 3 – 4 m³ of studio volume, use 1m² of 100mm thick Fibreglass, having a density of 24 kg/m3. Fit in patches distributed over the walls and ceiling. Church, school and recreation halls must compromise between speech and music. A reverberation time of about 1 second for an occupied hall is acceptable. It is often suitable to treat the rear wall only with 2 x 50mm layers of fibreglass Aerolite compressed to 75mm, covered with Hessian, and finished off with wooden battens spaced to provide 30% open surface area.
- To increase the sound insulation of lightweight dry-wall partitioning. The Fibreglass does not act as an additional insulator but quickly absorbs the sound in the inner spaces. National benchmark tests have shown conclusively that it is the thickness of the insulation, and NOT the density, which is important in dry-wall partitioning. Fibreglass is a highly efficient sound absorber and equally good heat insulator. Aerolite on the ceiling not only reduces sound penetration of the roof-ceiling combination but also keeps out summer heat and keeps in winter warmth.
Sound striking a hard surface is bounced off of it. When it strikes a layer of Fibreglass, it enters the material and encounters friction, which changes the sound energy into heat energy. A sound absorption co-efficient of say, 0.95 implies that 95% of the energy is absorbed. At the higher frequencies the co-efficient can be greater than 1, the material has a ‘sucking’ effect so that more sound reaches it than would fall on the same area if it were hard.
The high notes are absorbed more than the low ones. This typically absorbs sound (fibrous) and need not necessarily be a disadvantage because the ear is less sensitive to the low frequencies while the high frequency components of noise are annoying. Test results prove that low frequency absorption increases appreciably with increased thickness. To absorb all frequencies well, use a thick layer of Fibreglass. If high frequency absorption is required, a more economical thinner layer could be sufficient.
Rolls and Bats
Pink Aerolite in rolls is very well known as a ceiling insulator for heat. Because of its ready availability and excellent sound absorption, it is also widely used to absorb sound. The denser versions of Fibreglass are supplied in batt form. For critical applications, such as broadcasting and recording studios, batts of different densities are often used. A higher density can of course be had by compressing Aerolite. The basic fibres are the same, the higher density types being more compressed.
Fibreglass sound absorbing materials can be covered with sound transparent materials to suit the surroundings. Dyed or natural Hessian, or ‘shade netting’ prevents erosion of the fibres and would be minimum cover. In air-conditioning plant rooms, wire mesh could be added for extra protection and faced with Sonic Liner. ‘Peg board’ is also often used.