At ordinary sound pressure levels (SPL), most humans can hear down to about 20 Hz.[1] Woofers are generally used to cover the lowest octaves of a loudspeaker's frequency range. In two-way loudspeaker systems, the drivers handling the lower frequencies are also obliged to cover a substantial part of the midrange, often as high as 2000 to 5000 Hz; such drivers are commonly termed mid woofers. Since the 1990s, a type of woofer (termed subwoofer), which is designed for very low frequencies only, has come to be commonly used in home theater systems and PA systems to augment the bass response; they usually handle the very lowest two or three octaves (i.e., from as low as 20 to 80 or 120 Hz).