Proper recording and playback of sound requires exact standardization of camera and projector speed. Before sound, 16 frames per second (fps) was the supposed norm, but practice varied widely. Cameras were often undercranked or overcranked in order to improve exposures or for dramatic effect. Projectors were commonly run too fast in order to shorten running time and squeeze in extra shows. Variable frame rate, however, made sound unlistenable, and a new, strict standard of 24 fps was soon established. Sound also forced the abandonment of the noisy arc lights used for filming in studio interiors. The switch to quiet incandescent illumination in turn required a switch to more expensive film stock. The sensitivity of the new panchromatic film delivered superior image tonal quality and gave directors the freedom to shoot scenes at lower light levels than was previously practical.