Generating high output at low bass frequencies is a demanding task, so the driver, enclosure, and amplifier are designed to work together as a package. These subwoofers with a built-in amplifier are called Active or powered subwoofers. The amplifier in these units have built in crossover, phase, and sometimes equalizer capabilities.
Some amplifiers include an adjustable low-pass crossover, which limits the range of the subwoofer to low frequencies. For example, if a listener's main speakers are usable down to 80 Hz, then the subwoofer crossover can be set so the subwoofer only works below 80 Hz. The crossover section may also include a high-pass "infrasonic" filter that prevents the subwoofer driver from reproducing frequencies below its capabilities. The subwoofer phase control allows a listener to adjust the phase (time) alignment of the subwoofer system relative to the main speakers. Doing so eliminates or minimizes the acoustic cancellation between the subwoofer system and main speakers, the result of them not being in phase within the crossover region as heard at the listener's ear. Phase settings may be a two-position switch (0° or 180°) or a continuous adjustment between these two values.
Some subwoofers are built with their own internal amplifier. Designers of this type can add a degree of corrective equalization to ensure optimum performance. Some also include a user-adjustable equalizer that allows boost or cut output at certain frequencies, and these vary from a simple "boost" switch, to elegant parametric equalizers for detailed speaker and room correction. Some even include a calibrated microphone to measure the subwoofer's in-room response, so the automatic equalizer can correct the response to some pre-determined performance.
Passive subwoofers have a subwoofer driver and enclosure, but do not include an amplifier. They sometime include passive crossovers inside, with the filter frequency determined by the factory. These are used with third-party power amplifiers, taking their inputs from active crossovers earlier in the signal chain. While few home-theater-in-a-box systems use passive subwoofers, this format is still popular in the professional sound industry.