Colossal Sub-woofer

(Above)  A 15″ bass driver is bolted to the throat of each of the eight parallel hyperbolic horns.  The mouths of the eight horns form a tight array opening into an upper corner of the listening room (other side of wall).  The accoustic images of the horns (as reflected by the adjacent wall and ceiling) make the eight-horns function as though there were 32 of them, the equivalent of a single large horn with a mouth diameter of 106″.  The driver array is powered by 400 watts and is designed to cover the 2.5 octaves between 15Hz and 90 Hz.  Frequency response is very flat and distortion is extremely low over this range.  Efficiency is high, so the audio power output is unlimited for any practical purpose.

The left and right channels from a Harman-Kardon Citation Twenty-One preamp feed into a high-quality digital signal processor.  Digital filters separate the bass for each channel from the mid and high frequencies using a 71 Hz crossover frequency and 9 db per octave slopes.  The mid and high frequencies for each channel feed 200 watt Harman-Kardon Citation twenty-two high-voltage / high-current power amplifiers which drive two full-range JBL tower speakers for the left and right channels.  The bass below 71 Hz for each channel feeds two more Harman-Kardon 200 watt amplifiers; one drives the inner four 15″ bass speakers of the sub-woofer and the other drives the outer four 15″ bass drivers.  (There is no directionality for the long wavelengths of the low bass, so no stereo imaging is lost by employing only one sub-woofer.)  The digital signal processor also delays the upper frequencies for the left and right channels by 7 milliseconds to match the time it takes the bass to propagate through the horns and into the room; this maintains the correct phase matching through the crossover range.

A lot of classical music does not have much acoustic energy below 45 or 50 Hz, so commercially available high quality “full-range” speaker systems can handle it reasonably well.  However, organ music, synthesized music and the low audio “special effects” channel for Blu-ray movies are dramatically better with the rock solid bottom end provided by the colossal sub-woofer!  The goal is not to blow out windows (although that very well may be possible), it is just to have very high fidelity audio over the entire audio spectrum.

The massively solid sub-woofer was constructed primarily from 3/4″ MDF, approximately 1,300 screws, a lot of wood glue and some polyester resin.  It can be moved with some difficulty as there are two sections which bolt together with seven half-inch bolts.  It weighs about 850 pounds assembled and with the bass drivers.  Anyone interested in design theory and details should read the following document, and the pictures at the bottom show key steps in fabrication and assembly.

“Hyperbolic Horn Physics and Design”