For roughly the last three years, I have been planning, designing, studying, and pondering how best to implement a high performance subwoofer in my music reproduction system.  That the system would be designed and built by myself was a foregone conclusion.  I just like to build things!  Now the only question was which type of alignment to use.  Every since I started learning about sound reproduction, I have been fascinated by high efficiency systems.  It seems like such an elegant method of achieving the desired goal.  This led me to first try high frequency horns.  Liking what I heard and reading various impressive descriptions of the sound of bass horns on the internet, I decided to build a subwoofer horn.  Now the trick was to design something that would still leave me some space to live in.  After much design work, I finally settled on an alignment that would give smooth response down to a decently low frequency while still not being too terribly large.  The result can be seen at right.

In designing this horn, I was primarly constrained by my budget. This is what led me to base the design around my pair of Peerless 850146 drivers (10" drivers). These drivers are not normally what would be considered horn drivers. They have an efficiency bandwidth product (EBP) of only 56.7 while conventional wisdom would say that a horn driver break 200 on this figure. Lucky for me this 'rule' is typically misapplied - the missing factor is the frequency range the horn is being designed to cover. These drivers have a poly cone and a moderate excursion (9mm). This actually used to be considered a large excursion, believe it or not. The poly cone is probably not a very good feature for a bass horn driver, but it will have to do for now. For a bass horn (and any other subwoofer, for that matter), the more linear volume displacment the drivers have, the better. Plugging the drivers' Thiel-Small parameters into my design spreadsheet (based on Leach's horn model detailed in AES preprint #1405 and elsewhere), I saw that I could achieve fairly high efficiency (~108dB/watt predicted) with these drivers by designing for a very narrow bandwidth - 26.7hz to 100hz. This prediction was supported by simulation using David McBean's Horn Response program.

My other design constraint was that the subwoofer had to be small enough to get up my stairs, around the corners, and fit in my closet. My original plan was to slide the sub into the closet that's in the corner of my room so I would have true corner loading while avoiding the situation of having the speaker pointed into the corner. To meet this goal, I was constrained in both width, height, and depth. At the same time I needed enough area on the side of the box for the horn's mouth. Resolving all this led me to design the box to break apart into two sections. This would allow it to be taller to get the needed mouth area, and the pieces would fit around corners individually when stood on end.

Something else that had been hovering in the back of my mind was that I would have to move this beast after only a year's time; I'll be graduating and moving somewhere else. This is what prompted me to think of using foam for a substantial part of the box's construction. My thought was that this would save weight, and thus my back. This construction method would also have the advantage of letting me make smooth curves for the horn flair using a hotwire cutter. Whether or not smooth curves in a bass horn provide any audible benefit is debatable, but I did not want any frequencies much above 100hz, so there was nothing wrong with this approach (reflectors should be better for reducing high frequency attenuation through the bends). In addition, having a sharper acoustical cutoff above 100hz should help reduce harmonic distortion by naturally attenuating the upper harmonic frequencies.