A special bass speaker which is used to reinforce the lower frequencies. The frequency response is limited by means of active or passive filters to prevent it being locatable. In home theatre systems, one or two subwoofers are used and these are usually bass reflex systems. Often, the decoder has an integrated active lowpass filter. According to the THX® specifications, a subwoofer which is a bass reflex system will cover the range of frequencies from 35 Hz to 200 Hz, and this is limited by an active filter in the processor or controller to 80 Hz.