A few years ago, Royal Device, a self-proclaimed esoteric audio/video company built: "The Biggest SUBWOOFER of the WORLD, for the most esoteric Audio and AUDIO-VIDEO Room of the World. The Highest efficiency ever obtained with NO distortion using NO feedback tube low power amplifiers and NO crossover on the speakers"
The subwoofer has dual 9.5 meter horns driven by eight 18" drivers each. Yes, that's right, eight 18" drivers in each horn for a total of 16 woofers. It has a claimed frequency response of 10Hz at full power - below the level of human hearing. With an efficiency of more than 110 dB at 1W/1meter the system operates at 6400 Wrms total.
The subwoofer horns are built underneath a solid concrete floor, one meter deep.
The acoustic level at listening position(about 6 meters from frontal speakers) are 100 dB with 1 Watt.