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Acoustic InformationInformation about the sound properties of the GSP Range of TilesOne of the questions we are asked most regularly is "What is the acoustic performance of GSP tiles?". We do not place a big emphasis on acoustic values since the ceiling is only one of many sources of noise-reduction in any given space. A room may contain carpets, soft furnishings and drapes for example, all of which will play a much bigger roll in sound absorption than a ceiling and yet who asks about the acoustic performance of a carpet or a sofa! To assess the acoustics of any material it is first necessary to look at the two quite different basic ways of measuring performance: Sound Transmission and Sound Absorption. Sound TransmissionOtherwise known as sound insulation or sound attenuation. Measured as sound transmission class (STC). This is the measurement of sound insulation between one room and another. The units used are dB (decibels) but this is not always quoted. STC is an American test, equivalents are B.S.5821:Part 1:1984 and ISO 717/1.For ceilings, this is important when a moveable partition butts up to a continuous ceiling allowing the sound to follow a path over the partition passing through the ceiling on both sides. STC can be given as a single figure or range e.g. 35dB - 39dB, the higher the value the better. The STC refers not only to the performance of the ceiling tile, but also the grid, suspension and any seals being used at the partition head (in very critical areas such as multi-screen cinemas, the grid must be isolated from the walls and slab via special rubber inserts in the suspension). Because there are so many variables related to STC, every ceiling is different, but full scale laboratory tests can give a good indication of the performance of a particular ceiling system before erection begins. To improve the STC, the material thickness should be increased, e.g. 12mm thick plasterboard will give better results than 9mm etc. Sound AbsorptionMeasured as sound absorption coefficients (SAC). This is a measurement of the airborne sound within a room which is absorbed by the material, e.g. the ceiling. This is given in the form of a table of absorption coefficients usually measured over frequencies from 125-4000Hz (Hertz). The unit for sound absorption is the Sabine but is rarely used. SAC is an American test, equivalents being BS2750:Part 3:1980 and ISO 140/III.Another term you may come across is NRC, this is simply a single figure calculated from the average of the SAC results over the whole range of frequencies and is often used instead of SAC. Indicative tests can be carried out in the laboratory at quite reasonable cost, but again the sound absorption of the tiles are only a contribution to the room as a whole since other factors like carpets, soft furnishings etc will have a dramatic effect on the total room performance. Full tests involve erecting a complete ceiling system (about 12m2) in a reverberation chamber and are relatively expensive. To improve the SAC in ceiling tiles, it is necessary to perforate the face of the tiles and place an absorptive pad (Rockwool or fibreglass) on the back of the tile. Generally, the greater number and bigger the holes (classed as the "open area"), the higher the value. GSP TilesThe sound transmission performance of standard GSP tiles is good simply due to the nature of the material (plaster) which has a relatively high density when compared with other ceiling materials. A typical STC value for a standard T206 tile with no backing would be around 38dB depending on the other ceiling components (partition head seals etc).If a higher performance is required the thickness of the tile can easily be increased by changing the back mould to accommodate this. For sound absorption, since plaster is not naturally a good material for absorbing sound waves, the characteristics of a standard plaster tile are relatively poor. With GSP tiles however, due to the thin section of plaster used, the tiles do perform well as resonance panels and in practice are quite acceptable for all but the most demanding environments. The following table shows a comparison of sound absorption coefficients.
The points to note are:
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