Hz, and THX-certified systems operate below 80 Hz. Subwoofers vary in terms of the range of pitches that they can reproduce, depending on a number of factors such as the size of the cabinet and the construction and design of the enclosure and driver(s). Specifications of frequency response depend wholly for relevance on an accompanying amplitude value—measurements taken with a wider amplitude tolerance will give any loudspeaker a wider frequency response. For example, the JBL 4688 TCB Subwoofer System, a now-discontinued system which was designed for movie theaters, had a frequency response of 23–350 Hz when measured within a 10-decibel boundary (0 dB to −10 dB) and a narrower frequency response of 28–120 Hz when measured within a 6-decibel boundary (±3 dB). Subwoofers also vary in regard to the sound pressure levels achievable and the distortion levels that they produce over their range. Some subwoofers, such as "The Abyss" by MartinLogan for example, can reproduce pitches down to around 18 Hz (which is about the pitch of the lowest rumbling notes on a huge pipe organ with 16 Hz bass pipes) to 120 Hz (±3 dB). Nevertheless, even though the Abyss subwoofer can go down to 18 Hz, its lowest frequency and maximum SPL with a limit of 10% distortion is 35.5 Hz and 79.8 dB at 2 meters. This means that a person choosing a subwoofer needs to consider more than just the lowest pitch that the subwoofer can reproduce. Amplification 'Active subwoofers' include their own dedicated amplifiers within the cabinet. Some also include user-adjustable equalization that allows boosted or reduced output at particular frequencies; these vary from a simple "boost" switch, to fully parametric equalizers meant for detailed speaker and room correction. Some such systems are even supplied with a calibrated microphone to measure the subwoofer's in-room response, so the automatic equalizer can correct the combination of subwoofer, subwoofer location, and room response to minimize the effects of room modes and improve low-frequency performance. 'Passive subwoofers' have a subwoofer driver and enclosure, but they do not include an amplifier. They sometimes incorporate internal passive crossovers, with the filter frequency determined at the factory. These are generally used with third-party power amplifiers, taking their inputs from active crossovers earlier in the signal chain. Inexpensive Home Theatre in a Box packages often come with a passive subwoofer cabinet that is amplified by the multi-channel amplifier. While few high-end home-theater systems