In the series of alternative, non-fibrous sound absorber (ALFA) components a suspended ceiling system has been launched on the market which allows an optimum acoustic design of office and conference rooms. The smooth sheet metal cassettes possess an almost invisible perforation of less than 1 % of their surface due to tini holes with a diameter of only 0,5 mm. With these OWAtecta® perfora-cassettes neither a mineral fiber nor a fleece cover or another acoustical lining is necessary to absorb the sound waves effectively. They are as easily installed as the common suspended ceiling systems, i. e. by using suspensions and guides.
Figure 1 shows the sound absorption coefficient as measured according to DIN EN 20 354 for several distances D from the raw to the suspended ceiling. For a distance between raw and suspended ceiling of 200 and 600 mm, as often used in practice, the maximum efficiency of this novel acoustic ceiling lies between 125 and 500 Hz. This frequency range is of high importance whereever a sound absorbing ceiling is absolutely recommended in room with sparse fashionable furnishing the surfaces of wihich are non-absorbent in general. At frequencies between 500 and 2000 Hz, where the perfora®-ceiling absorption is low, the necessary sound absorption is ensured by the carpets, curtains and the persons themselves in a room. This leads to a relatively balanced reverberation time over the frequency range and a decreased sound level in a room.
Figure 2 shows the suspended ceiling as installed in a class room. By this ceiling the acoustical conditions were so decisively improved that the reverberation time recommended for common class rooms has been reached over the entire frequency range.
This measurement has contributed to the audibility in the classroom and helps to prevent hearing impairment of teachers and students.
Kleine Löcher, große Wirkung. Trockenbau-Akustik 14 (1997), H. 8, S. 34-37.