Acoustics in Sports halls

Problem statement Sports teachers (and other users) often suffer from bad acoustics in sports halls. A survey revealed that most complaints are about tiredness, throat problems and hearing problems. Bad acoustics in sports halls are mainly caused by an uneven distribution of the sound absorbing material in a hall. Often, the roof absorbs most of the sound where the floor and walls up to 3 meter reflect most of the sound. This lower part of the walls needs to be strong, flat and hard to rebound balls. In this way, vertically reflected sound is being absorbed faster than horizontally reflected sound. This difference causes sagging Schroeder curves and therefore long reverberation times. Dutch standards for acoustics are mainly based on reverberation times, so sagging curves are unwanted. Besides the problem of the long reverberation times, another problem is found. Measurements show that sports halls constructed with stone-like materials absorb sound according to the expectations of the material properties. Sports halls constructed with perforated steel sound absorbing panels give different results. These panels seem to behave differently on sound absorption than expected. Especially low frequencies seem to be absorbed extremely well by the panels. The research question therefore is: ‘Why do perforated steel sound absorbing panels seem to behave differently in absorption coefficient in sports halls than expected from laboratory test results?’ Because this question is part of the larger subject ‘acoustics in sports halls’, it is necessary to investigate the acoustical behaviour of perforated steel panels for a broad demarcation of the subject. Approach In order to find an answer to the research question, two hypotheses are tested by measurements. The measurements are done in a laboratory and in a scale model. The results are analysed. The conclusions give insight in the sound absorbing behaviour of perforated steel sound absorbing panels. Besides that, the conclusions give guidelines to the design phase of this graduation project. Hypotheses: 1. A perforated steel panel behaves differently in practice than in a laboratory situation on absorption coefficient because the shape of the panels causes sound absorption of parallel striking sound based on a phase shift principle. 2. A perforated steel panel behaves differently in practice than in a laboratory situation on absorption coefficient because the backing construction has influence on the result. Laboratory measurements Absorption coefficient measurements are done in the reverberation room of the acoustical laboratory. Different samples of the roof structure are tested. By comparing the results, we get insight in the influence of different parts of the roof structure. Besides that, hypothesis one can be tested: the influence of the backing construction will become clear. Scale model measurements The scale model makes it possible to test the influence of different shaped roof structures in a small model. By using the same, reflecting material for all walls, we get insight in the influence of changes in roof shapes. Some of the roof structures are (partly) covered with a sound absorbing material to compare the results of those to the other variations. This research should give an answer to hypothesis two. Results The scale model measurement results show that profiled structures cause a considerable decrease in local sound pressure compared to a hard, flat surface. This decrease is largest for low frequencies. The surface with sound absorbing material on top causes the biggest decrease, which is caused by the sound absorption of the facing material. The ‘sound absorption by shape’ is caused by the phenomena of diffusion and interference. It is not visible in reverberation times (T20), just in histograms and Schroeder curves. The laboratory measurements show that the influence of the thermal insulation layer; a part of the backing construction, is large on (low frequent) sound absorption. The rock wool panels give a high peak in the sound absorption graph for 100 Hz. This very good sound absorption is probably caused by the panel being its own panel-resonator and porous absorber in one. The influence of the ‘cannelurevulling’ is small, like the influence of the vapour barrier and different perforation degrees. Conclusions Perforated steel sound absorbing panels absorb more low frequent sound than expected. This good sound absorption is probably achieved by the profiled shape of the steel panels in combination with the backing thermal insulation. The profiled shape causes the effect of sound absorption caused by the effects of diffusion and interference. The backing construction absorbs a large part of the low frequent sound because the hard, stiff and heavy rock wool panels are probably a porous material and panel resonator in one. The research does not give strong guidelines to improve the roof structure. The gained knowledge is used in the design of a wall panel. The walls up to three meters are still the weakest link when talking about acoustics in sports halls. The designed panel should improve this situation.Green Building InnovationBuilding TechnologyArchitectur