These drivers are often used for public address systems. The principle is that the relatively large cone forces the air through a comparatively small opening. This can only work if the air moves at a greater speed than that of the cone's oscillating movement. The efficiency of the driver is thus increased. They may be seen as acoustic impedance transformers. However, even these speakers have their physical limits. The limits are set by the point at which the speed of the air flow becomes so great that turbulence and friction are generated. This leads to compression distortion which become noticeable as the "distortion factor".