use passive subwoofers, this format is still popular in the professional sound industry. Equalization Equalization can be used to adjust the in-room response of a subwoofer system. Designers of active subwoofers sometimes include a degree of corrective equalization to compensate for known performance issues (e.g., a steeper than desired low end roll-off rate). In addition, many amplifiers include an adjustable low-pass filter, which prevents undesired higher frequencies from reaching the subwoofer driver. For example, if a listener's main speakers are usable down to 80 Hz, then the subwoofer filter can be set so the subwoofer only works below 80 Hz. Typical filters involve some overlap in frequency ranges; a steep 4th-order 24 dB/octave low-pass filter is generally desired for subwoofers in order to minimize the overlap region. The filter section may also include a high-pass "infrasonic" or "subsonic" filter, which prevents the subwoofer driver from attempting to reproduce frequencies below its safe capabilities. Setting an infrasonic filter is important on bass reflex subwoofer cabinets, as the bass reflex design tends to create the risk of cone overexcursion at pitches below those of the port tuning, which can cause distortion and damage the subwoofer driver. For example, in a ported subwoofer enclosure tuned to 30 Hz, one may wish to filter out pitches below the tuning frequency; that is, frequencies below 30 Hz. Some systems use parametric equalization in an attempt to correct for room frequency response irregularities. Equalization is often unable to achieve flat frequency response at all listening locations, in part because of the resonance (i.e., standing wave) patterns at low frequencies in nearly all rooms. Careful positioning of the subwoofer within the room can also help flatten the frequency response. Multiple subwoofers can manage a flatter general response since they can often be arranged to excite room modes more evenly than a single subwoofer, allowing equalization to be more effective. Phase control Changing the relative phase of the subwoofer with respect to the woofers in other speakers may or may not help to minimize unwanted destructive acoustic interference in the frequency region covered by both the subwoofer and the main speakers. It may not help at all frequencies, and may create further problems with frequency response, but even so is generally provided as an adjustment for subwoofer amplifiers. Phase control circuits may be a simple polarity reversal switch or a more complex continuously variable circuit. Continuously variable phase