Sound absorption. Design for sound
The acoustic design of a room depends on the activity to be performed in it. From an experimental point of view, we can appreciate that there are differences between talking in a cathedral, for instance, and in a cinema. The “echo” feeling that our voice experiences in the cathedral is in clear contrast with the fast disappearance of our voice in the cinema. The explanation to this phenomenon is the large difference in reverberation time of both places. Thus, the reverberation time is a good starting point to identify the acoustic characteristics that a room must have. The American physicist and founder of architectural acoustics, Sabine (1868-1919), defined the reverberation time by means of a simple formula, where the latter depends on two variables:
- Dimensions of the room: Volume (V) and surface area (Sk)
- Materials absorption coefficient (?k)
Therefore, the absorption coefficient to be used is the main difficulty when calculating the reverberation time. From the first measurements by Sabine, tables of absorption coefficients of different materials have been published. However, these values are just for guidance and must be used very carefully since they come from diverse sources. Many reports on commercial sound absorbing panels, such as plates or screens with mineral wool and glass fibre as core materials, have been developed in the noise control field (industry, transport, infrastructures, etc.), and therefore their results are not valid regarding room acoustics, where they are applied inside large spaces which are very different in architectural terms, such as auditoriums, commercial or public premises, houses, etc. In addition, most of the superficial areas in these buildings are covered in plaster, wooden panels, curtains, carpets or other finishes that modify the acoustic characteristics of materials, and most of the absorption coefficients of which have not been measured. Who is the sound absorption coefficient? Absorption is a phenomenon affecting sound propagation. When a sound wave reaches a wall, a part of its energy is reflected, another part is absorbed by the wall, transforming itself into heat energy, and a third part is transmitted towards the other side of the wall. This way, we can define the sound absorption ability of a material, identified by its absorption coefficient, as the relationship between the energy absorbed by the material and the energy reflected by it. Its value is expressed in a scale from 0 (fully reflective material) to 1 (fully absorbent material) for each sound frequency. Firstly, attention should be drawn to the fact that the aim of room acoustics does not always involve sound absorption. For instance, the panels located over an orchestra shall be as reflective as possible (very low absorption coefficient) so that the orchestra can be listened to from the stalls at sufficient level. On the contrary, the side walls and the back of the auditorium will have to be covered in absorbent materials to prevent the presence of seasonal sound waves (annoying echoes) in the audience areas. Secondly, it is important to bear in mind that a room does not respond the same way to a high-pitched noise than to a low-pitched noise, and this is due to the fact that absorption differs for each frequency. In practice, the truth is that sound absorbing materials that are usually installed in ceilings and walls have higher absorption coefficients for medium frequencies, and especially for high frequencies, than for low frequencies. Therefore, and due to the fact that goodness of absorption of a room is aimed at high frequencies, it will be necessary to pay special attention to the more disadvantaged cases, born by the low frequencies. There is at this point an additional problem, and that is that the noise of installations (air conditioning, electricity, electronic equipment, etc.) focuses on these frequencies. Why for my project? Both in new building works and in the alteration of built spaces, the main advantage posed by knowing the absorption coefficient of materials, is that is enables to assess the result of the installation for different coatings, and to select this way the material that will meet the conditions (in terms of reverberation time, sometimes incorrectly named “echo”) required by the end customer. In order to determine the appropriate reverberation time of any type of premises, or improve the existing one, it is necessary to know the absorption of internal coating materials to be installed. The Standard UNE EN ISO 354 “Measurement of sound absorption in a reverberation room” determines the absorption coefficient of the materials used in treatments on walls and ceilings, as well as on objects, furniture and textiles, among other. We shall not forget that we are at the design phase of the project: the virtual model would lack in reliability if we were missing this parameter. The situation will become more critical if the materials have already been installed at the construction phase. The necessary measurements and the replacement of the existing coatings would imply unforeseen technical and economic efforts and the resulting delay in the project time frame. For the development of acoustic and architectural projects, starting from the test results, there are currently powerful software applications for the acoustic conditioning of rooms that help integrate data and simulate rooms in detail (coating materials, doors and windows, furniture, textiles in the room, number of people, etc.). These software applications already have the results of the tests conducted by the manufacturers of construction and conditioning materials, characterised by their absorption coefficient, as measured in the lab.