sentences:  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.This suggests that for sound to be perceived at 7–8 Hz, the 7–8 Hz SPL would have to be 12.3 dB higher than the 92.0 dB SPL required at 15.5 Hz, which means that the 7–8 Hz SPL would require at least 104 dB of SPL to be perceived.For 3 Hz to be perceived, the SPL would need to add another 12.3 dB, or reach 116 dB of SPL, to be perceived.