The introduction of modern technology and computerised machinery into industry has reduced the occurenceoccurrence of high noise exposure situations, but also introduced other types of occupational noise of more moderate occupational noise levels. In many cases, the change to moderate noise levels has been achieved by building insulated control rooms from which industrial processes are supervised. The noise in such control rooms is often dominated by noise in the frequency range of 20 to 200 Hz (low frequency noise), caused by ventilation and air conditioning systems as well as by the lower attenuation of the low frequencies by the walls, floors, and ceilings. Other occupational environments, such as office areas, house a number of noise sources that generate low frequency noise at moderate levels.