Qualifying freefield and reverberation rooms for frequencies below 100 Hz

Abstract

Laboratory measurements at frequencies below 100 Hz of sound power, absorption and transmission are hampered by the presence of various resonance phenomena at the room's eigenfrequencies and the lack of suitable acoustic lining material. In its extensive efforts to create non-fibrous, compact and robust alternatives to the conventional absorbers, IBP has developed, among others, a special broadband resonant absorber which shows an amazingly high absorption efficiency down to 20 Hz with a total lining thickness of only 250 mm. This compound baffle absorber (CBA) enables not only the installation of small freefield rooms with a considerably extended frequency range but also, in the form of merely 100 mm thick modules, the equipment of reverberation rooms for the determination of the sound power of sources, the transmission loss of construction elements like e.g. walls, doors and windows as well as the absorption performance of acoustic damping materials at low frequencies. A brief status report on current developments towards improved measuring and testing procedures was given at Euro Noise 1998 in Munich [1]. In this paper preliminary experimental results are presented for low frequency data of sound sources, acoustic materials and construction elements. The growing interest in their performance at frequencies down to 50 Hz calls for a thorough discussion among those engaged in differing approaches and procedures in other test laboratories. Innovations which could soon lead to new room and building acoustic design tools and construction elements will also be the topic of the second alternative fibreless absorber (ALFA)-colloquium to be held at the IBP during 18/19 October 1999.