Edition
Acoustics and Sound Insulation Eckard Mommertz, Müller-BBM, 2009. 112 pages, with numerous diagrams and photos. Format 21 × 29.7 cm. ISBN 978-3-7643-9953-5 Paperback: € 42.95 / £ 35.– / US$ 60.– + postage/packing + VAT, if applicable ∂ Practice series
Ideal interior designs for optimal acoustics Sound installation and acoustics are perhaps not the primary factors that normally influence a building’s design. Nevertheless, if you have ever failed to understand the lecturer in a seminar room, found the noise level in a large office unbearable or lost sleep due to a neighbour’s snoring, it becomes clear how significantly room acoustics contribute to your everyday well being. Every room possesses
Room acoustics More accurate prognosis methods and room acoustics measurements
an acoustic dimension that varies depending on the requirements of its function. The handbook conveys practical and experienced knowledge of acoustical engineering to all expert planners, architects as well as to interested building contractors. In doing so, the manual raises your awareness of how specific acoustic considerations can contribute to the success of a project.
Room acoustics Sound absorption of architectural surfaces
Detailed, typologically specific measures for acoustic and noise-control engineering
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choice of input parameters, i.e. modelling of the surfaces, absorption and scatter properties. If the modelling contradicts the laws of geometric acoustics, e.g. the surfaces are resolved too finely, unrealistic results are the inevitable outcome. Used with care and backed up by experience, room acoustics simulations are a valuable aid in modern acoustic design. With a little more effort it is possible to use the results of the simulation to hear what rooms sound like in advance. The term auralisation was coined for this. In this case the receiving characteristic of the human ear is emulated in the simulation. It is possible to create a realistic aural impression of a room by processing speech or music recordings (recorded without any reverberation) and played back via headphones or loudspeakers. This is a very good way of assessing speech or loudspeaker systems, or also individual instruments. However, emulating the sound of a whole orchestra is still a dream owing to the complexity and interaction of the sounds. Special features of small rooms Describing the sound propagation with the help of geometrical and statistical methods is inadequate for small rooms with a volume less than approx. 100 m3 and low frequencies less than about 160 Hz. The wavelengths are then in the order of magnitude of the room dimensions and the sound pressure level depends on how good the respective wavelength “fits in the room”. Room resonances occur when one dimension of the room coincides with half the wavelength or a multiple of it. Undamped room resonances make themselves felt during speech or music as unpleasant booming. This is particularly noticeable when there is a whole-number ratio between length, width and height because then the same resonant frequencies are superimposed. 18
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Favourable proportions for rectangular rooms require room dimensions that are as dissimilar as possible, e.g. ratios of 1:0.83:0.47 or 1:0.79:0.62. Placing one or more room surfaces at an angle, dividing up large areas of the wall surfaces or using absorbent materials are suitable methods for suppressing disturbing room resonances. Such aspects are particularly important for the recording and listening rooms of studios, but also in classrooms for music. These considerations also play a role in small, possibly glazed, offices. The physical effects can be predicted with more accuracy by using finite element or boundary element methods, for example, where the sound is modelled according to its wave nature. For large rooms, such methods are only suitable for low frequencies at best owing to the computing time required.
Measurements of room acoustics are generally carried out with the help of special measuring loudspeakers. Microphones or possibly a dummy head (p. 11) are used as receivers. The number of measuring positions (transmitters and receivers) varies with the size of the room and lies between about six positions in rooms of classroom size up to more than 100 in concert halls and opera houses. The duration of the measurements varies correspondingly, from 30 minutes to whole days (or nights). The measurements are mostly carried out in unoccupied rooms and the results converted to the occupied condition. Measurements in venues are occasionally carried out in the occupied condition, e. g. directly prior to a performance, with fewer measuring points (duration about 5 – 10 min).
Room acoustics measurements An objective room acoustics quality assessment, or a report on the current situation, is necessary to check both design and construction, in advance of planned refurbishment work, or in the event of complaints. Whereas in the past a blank cartridge fired from a gun or bursting balloons were the methods often used, these days synthetic measurement signals that can cover the entire range of audible frequencies are used. In this way, room impulse responses can be determined quickly and accurately, allowing the reflections structure and objective room acoustics criteria to be evaluated. It is also possible to track down acoustic defects with the help of intelligent measuring techniques. If only the frequency-related reverberation time is required, the decay process of noise signals after switching off can be evaluated.
Measurements on models During the design phase it is possible to measure the room acoustics using models, normally built to a scale of 1:10 or 1:20. The model includes all the surfaces in the room shaped and positioned according to the drawings. Owing to the smaller dimensions, the lengths of the soundwaves are also scaled down, i. e. the measurements are carried out at higher frequencies. The problems are that absorption increases with the frequency and this affects the measurements for sound propagation in air, and it is not easy to transfer the sound absorption properties of materials to the scale of the model. The audience, or seating, is usually represented by soundabsorbent profiled plastic foam (Fig. 2b). Compared to computer simulations, measurements on models have the advantage that the sound propagation remains faithful to the wave nature of sound, i.e. focusing and scattering of the
sound is emulated properly in physical terms. Owing to the different pros and cons of measurements on models and computer simulations, both methods are sometimes used together for particularly demanding room acoustics tasks (concert halls, opera houses).
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Sound absorption of architectural surfaces Many of the surfaces preferred by architects these days, e.g. glass, fair-face concrete, plaster, reflect sound. It is therefore frequently necessary to incorporate sound-absorbent materials into the interior design. Industrially manufactured products, custom-designed surfaces and multi-layer constructions are all potential options. All kinds of sound-absorbent systems are available, from mineral-fibre insulating boards, laid, for example, in suspended ceiling systems, to special products such as microperforated foils. The materials differ not only in terms of appearance and price, but also in their acoustic efficiency, i.e. the sound absorption coefficient.
Measuring sound absorption in a reverberation room Sound-absorbent linings and materials are identified by the sound absorption coefficient for omnidirectional sound incidence. Typical values for many architectural surfaces can be found in the relevant publications. For acoustic products, the sound absorption coefficients are listed in their technical specifications. Most of the data is based on measurements carried out in a testing laboratory to DIN EN ISO 354. According to this international standard, the construction to be tested – an area of 10 –12 m2 – is set up properly in a so-called reverberation room (Fig. 1a). After measuring the frequency-related reverberation times with and without the construction, the Sabine reverberation equation can be used to determine the sound absorption coefficient _S, which is specified in one-third octave bands from 100 to 5000 Hz.
Building acoustics Room acoustics Design and forecasting procedures Noise protection in urban design Samples of completed projects
Examples of wall linings that scatter the sound: a Bruneck Grammar School, Southern Tyrol b Refurbishment of Teatro Reggio, Turin Sala Santa Cecilia, Parco della Musica, Rome, 2002, Renzo Piano Building Workshop a Computer model for acoustic purposes: the room acoustics computer simulations help to provide more accurate forecasts of the acoustic propagation quality that can be expected. b Model built for acoustic tests (scale 1:20), shown here opened; the audience is resented by soft profiled acoustic foam Soundwaves are reflected in various directions from structured surfaces; schematic drawing of a lining with a sawtooth structure. a Low frequencies ignore the structure if the wavelength is large compared to the dimensions of the structure; the sound energy is reflected geometrically with respect to the dotted red line. b Medium frequencies are scattered more or less evenly in different directions. c High frequencies are reflected geometrically from the individual surfaces (dotted blue line) because here the dimensions of the sawtoothstructure are large compared to the wave2b lengths.
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