Subwoofer Project
This sub is based on two Rythmik Audio Servo kits, each one installed in a separate chamber in a single box. The intent was to replace an original Sunfire True Sub, which I purchased in late '96. While the Sunfire is certainly impressive for the output it provides for it's size, I had grown to dislike the quality of its sound. Unfortunately, my room is very limited space-wise. I had learned from the Sunfire that one of the better placement options in my room happened to be mounted near the ceiling, in the corner above the armoire that houses my a/v gear. Unfortunately, that spot made it very prominent, and any typical commercial sub in that location would have been too large, and aesthetically stood out like a sore thumb. The obvious answer was to build my own.
History
When I was originally was shopping for a subwoofer in '96, I had narrowed my selection down to a Velodyne servo based sub, or the Sunfire. Ultimately, I chose the Sunfire for it's significant size advantage. Since location plays such a major role in bass response, I figured the Sunfire would afford me more location possibilities, and therefore have a better chance of fitting in a location that provided good response. Overall, I was pleased with the purchase. It provided a significant amount of output compared to other products that would have been feasible in my room. It was not without its downsides, however. The Sunfire is a mass loaded passive radiator design. The heavy PR causes the sub to shake very strongly at low frequencies. That is a bad enough situation when merely setting a sub on the floor. The problems are compounded when placing it on a shelf supported by walls. It HAD to be fastened to the shelf to guard against the possibility of shaking itself off the shelf, which meant that the shelf would vibrate along with the sub. While the walls in this 100 year old house are very thick and heavy, even they will vibrate when a 50lb mass is attached to them and shaking strongly at 30hz and below. And just my luck, on the opposite side of the wall that was the "optimum" sub location was: a built in china cabinet. I ended up securing it via some spring mounts, which helped significantly.
As time went on though, I became less and less satisfied with its sound. At the lowest frequencies, there was a lot of noise coming from its driver, which became distracting. Also, its response seemed sluggish. It simply couldn't deal with fast transients well. And while it had fairly strong output down to 20hz, which was impressive for any sub at the point in time when I purchased it, let alone one that small, I was starting to feel that wasn't enough. More and more movies are starting to make use of information below 20hz... the range where bass energy is more felt than heard. And the Sunfire, being a passive radiator design, had a response curve that dropped off very rapidly below its tuning point of 18hz. In fact, in my experience, it started dropping off higher than that. It seemed to drop like a rock at 20hz.
Options
I don't remember how, but somewhere along my search, I learned of Rythmik Audio. Their products caught my eye very quickly. To the best of my knowledge, they have the only servo based product available to the DIY market. There is plenty of detail on their site on how their servo works, but in short, a servo based subwoofer monitors the movement of the driver, and attempts to auto-correct, trying to mirror the signal input as closely as possible. This has the effect of reducing distortion, and improving its frequency response. In other words, servo feedback is a tool to make it more accurate. Velodyne, the other manufacturer I was considering when I purchased the Sunfire, is still one of the few manufacturers of servo based subs, and they remain as one the most highly regarded in the industry.
The Rythmik servo sounded impressive on paper, and the size requirements and specs fit what I was looking for. Unfortunately, I could not find much feedback on the Rythmik servo kits. The only professional review I could find was at the Secrets of Home Theater and HiFi, which was positive. It also seemed to be highly regarded in the diyAudio WiKi. I could only find a couple of users on the web actually using the servo kits, but the feedback I saw was positive. The Rythmik amps also seemed to be well regarded, and their drivers are made by TCSounds, which are high quality drivers also used in the popular SVS subs. So I decided to give them a shot.
Their servo kits work in both a sealed and ported configuration. While there are many debates as to which design is "best", generally speaking, a ported enclosure will give higher output within it's tuning range, a sealed enclosure can have an edge in certain areas of accuracy, particularly transient response. Also, a sealed enclosure is easier to build, since there are no ports or passive radiators, and it's a little more forgiving of less than ideal box design. Because the biggest complaints I had of my Sunfire were it's poor transient response, and sudden drop-off of output below it's tuning frequency (a characteristic of typical ported and PR designs), I chose to go sealed. I did not, however, want to give up output to get the accuracy I was after, so I decided to build a single box containing two servo kits. Two subs in close proximity provide 6db more output over one, which would match the boost within it's tuning range I would have had with a ported sub. But the 6db boost using two kits is available over the entire range of the sub, not just the much narrower tuning range of a ported design, which means two sealed kits will have more output overall, compared to a single ported sub.
Continue on to see the sub design