Free Field. The noise level measured near a piece of equipment will depend on the surroundings in which it is placed and the same machine will sound much louder in a small room with hard walls than in the open air; it will also sound slightly louder if it is standing on a hard floor than if it is suspended high in the air. It is therefore necessary to use a standard environment for quoting noise measurement and this is usually defined as “free field above a reflecting plane”. This corresponds to the noise from a machine standing on hard ground (say, concrete) in still, open air, with no walls or other reflecting surfaces nearby.