These metal boxes with soft radial tube fans in them come in different sizes. Starting with a mere 250 m³ all the way up to 7000 m³. The metal boxes are ideal in a dry place where they will not be affected by corrosion. These boxes are a bit cheaper than the other options namely MDF or Birch boxes. Also, the soft boxes are more quiet than the normal metal boxes, this is because the motors in the box are hanging on a rubber suspension. This eliminates the vibrations and causes the whole thing to become more quiet.