The subwoofers were conceived as 6^th order Chebyshev alignments. While reading Thiele's original paper, I had an epiphany when I read his statement that "box size can be traded for amplifier power." Hey - amplifier power is cheap, at least compared to SAF :-)

  After examining several of the Class I 6th order alignments, I obtained some Peerless 12" woofers with a suitable Qt to work with these alignments. The woofers chosen have the following Thiele/Small parameters (as measured by LEAP):

  The final design ended up being in between Thiele's alignments 16 and 17. The alignment parameters are as follows:

Translating this into speaker parameters for the above drivers:

The magic here lies in the high Cas/Cab ratio - a box about two thirds the size of a standard Butterworth alignment (100 liters vs 146), with a much lower f3 to boot (18 Hz vs 25 Hz). 18 hz out of a 3.5 cubic foot box, and it doesn't even say B*se! The penalties are reduced efficiency near cutoff (in accordance with "Hoffman's Iron Law"), requiring the amplifier boost, and higher group delay. The benefits (besides SAF) are reduced cone excursion compared to sealed or 4th order vented alignments, both below and above cutoff to about 1.6 x f₃, leading to lower distortion, higher output capability near cutoff, and reduced chance of damage from extremely low frequency signals.

  Cabinets were constructed from 1" MDF. Multiple braces are placed on all panels. Since the size of the cabinets precludes solid wood construction across any panels, the top and bottom panels are veneered with oak, with the corners rabbeted and inlaid with solid oak, providing the appearance of solid oak endcaps to compliment the satellites. The front and rear corners were rounded over using the same template as the satellites. Veneer is a plastic-backed African Mahogany, rolled right around the corners for a solid-block-of-wood effect. The back panel is not veneered completely, since I ran short of material to complete both panels. The interior is lined with 1" thick fiberglass, wedged into place, to reduce internal standing waves. There are currently some 12" RS metal grilles over each driver to protect them from two year olds. The completed cabinets weigh about 80 pounds each.

REFOAMING

  The foam on the surrounds rotted out after about 9 years, necessitating some work. A search was commenced, aided by the Bass List membership and the LDSG; however, no suitable replacement drivers were available. The trouble with the 6th order alignment is that it is very sensitive to Thiele/Small parameters, and nothing was available that would work in the same box. So the drivers were sent to a firm in Arizona for re-foaming and Leap parameter measurement. The parameters are still within < 3% of the manufacturer's specifications across the board!

REDESIGN PROGRESS

  The subwoofers are undergoing a smaller-scale redesign than the satellites. The primary change will be replacement of the small (2" dia.) port with 3" flared PVC ports obtained from Madisound. When I built these speakers, someone informed me that the "golden" port length was 6", regardless of tuning, and to pick a diameter that resulted in this length. For the very low tuning of this small box, this results in a very small vent. The current ports were carefully chosen to have a clean interior surface, and both the internal and external edges are radiused. Despite these precautions, there is objectionable port noise around resonance at high levels.

  Obviously, the boxes will be re-tuned to match the alignment, and results will be posted here (including more measurements). According to my calculations :-) there is enough room in the boxes to fit the longer port length without elbows.

  So, how does all this stuff actually work? Travel on to the next page for system measurements!