A rotary woofer is designed to produce only frequencies lower than 20 Hz; distortion increases as the input frequency exceeds the fan's rotation rate (20 Hz would be about 1200 rpm for the fan), which is itself limited by the need to avoid making higher-frequency noise (due to the sound of the spinning blades). Current models use an AC induction motor spinning at 800 RPM (13 Hz). The woofer is installed and carefully braced so that the blades lie in a circular opening, so that air can be moved between an external chamber, such as the attic of a house, and the main listening space; if the rotary woofer were not installed in such a "baffle" and placed directly in the main space instead, the generated sound pressure from the rear of the unit being 180 degrees out of phase would almost completely cancel that from the front the end, the result of which would not be fit for its intended application.