In connection with the construction of a series of halls in Vienna, the largest having a volume of a million cubic feet, cheap and effective absorbers had to be found. The absorption values were to lie between 70 and 80% from 250 to 1000 cps and to fall off at higher frequencies to compensate for the increasing absorption of the audience and that of the air. More than 600 frequency curves had been measured, and all absorbers finally fulfilled the set conditions. They were tuned to 500 cps to give the required low-frequency absorption and, after tuning, damped until the absorption maximum dropped to about 75%, when the frequency range then was of the required width. Three of the studied absorbers of particular interest are: (1) Aluminum sheets 0.5 mm thick, perforated with narrow slits, and corrugated to increase stiffness, manufactured in rolls 20 m long and 1 m wide. The sheets are backed by basalt wool (5 kg/m2). (2) Bricks 10 cm thick, 13 of the cross section being taken up by channels traversing them, the air mass in the channels being tuned by a highly damped air space. These absorbers are to be used in the training and the ball halls where mechanical rigidity is imperative. (3) Frigolith—this is a porous polystyrene material, extremely light, cheap, not hygroscopic, fireproof, and can be colored. Since its pores are closed, it is a poor sound absorber, but its intrinsic absorption suffices to damp resonant cavities cut in the material. The final absorber unit is made up of a box containing a grill like a honeycomb, each grill space communicating narrow channels with the exterior.