The
internal parts of the BM consist of reinforced concrete monolith, the walls of
which are 30cm thick, with internal dimensions of 8.6m x 7.1m x 6.6m. The
monolith is situated on springs. The weight of the monolith equipped with
absorbent wedges is about 300 tons. The walls of the room resonate at the
approximate frequency of 5.5 Hz. The wedges are 1m long and the dimensions of
their base are 24 x 24 cm. The absorbent wedges are cut to size using a water
jet and have a volume density of 100 kg/m3 and they are covered with a very
thin knitted nylon fabric (silon). The room is equipped with four 180 wedges
on all four walls including the ceiling and the floor. The total weight is
about 15 tons. So the net volume of the room between the edges of the
absorbent wedges is about 160 m3. Besides the wire stepping network, stepping
grids are installed from the entry to the centre of the chamber, the area
which is 3 m x 2 m in the middle of the free-field room. The grids are
removable and they allow for measuring of even relatively heavy equipment with
a weight over 500 kg. The dimensions of the BM entry is w-1620 mm x h-1950 mm.
The detectors and other electric devices within the BM can be interconnected by cables with measuring equipment situated in the adjacent laboratory. The free-field room is used primarily by Petrof and for acoustic measurements for companies that need to measure acoustic parameters. The free-field room does not have any problems with vibrations thanks to its construction design – the lower limit frequency is 80Hz.