New York Speakers Part 2: Subwoofer Design

My starting point was the Cambridge Soundworks Ensemble IV, which is a passive 5.25" dual voice-coil driver in a 4th order band-pass box, whose longest dimension is around 12". Obviously a driver of that size has limited volume and extension, but it was tested to be "appreciably loud" down to 40Hz, not too shabby. However because my subwoofer was going to be powered, and because one of the chambers needed to be large enough to house two satellite speakers, I allowed myself a slightly larger cabinet and thus could expect greater volume and extension versus the Ensemble IV.

My first choice for woofers was the Tang-Band 6.5" woofer, whose Neodymium motor makes it lighter and have a smaller displacement, both major advantages for my system. It's designed specifically for these low volume multimedia "subwoofer" applications, which in reality are glorified woofers since bass below 40Hz is around zero. My first draft of the enclosure basically copied the Ensemble IV design, scaled up about 30%. It's got dual ports because the required cross sectional area needed to prevent vent noise would have necessitated a port longer than the enclosure itself. To get the response curves of the cabinet designs I use WinISD, a very neat free program.

In this design, the sealed chamber is the one that houses all the electronics. The vents pass through this camber into the adjacent one, something I needed to do because of the vent length requirement. This design supposedly rolled off at 45Hz, with a max SPL a bit under 100dBm. I actually had to resonate the cabinet a bit to get this volume, as this driver is pretty inefficient. The more I looked this design over, the more it nagged me. The chamber volume for the satellites was pretty small, so my satellite choices were either sealed cubes using 3" full-range Tang-band drivers, or a tiny two-way using a Hi-Vi 3" midrange and a Dayton 3/4" tweeter, neither of which had any low extension. I'd have to cross over around 150Hz, which would be high enough to make the sub pretty directional. I also did some playing with box volumes and decided I was well alway from the point of diminishing returns in terms of bass extension:box volume, so I should make the enclosure a tiny bit larger and get much better bass, as well as have room for real satellites.

Next I experimented with the idea of using two 5.25" Tang-Band woofers, in a more vertical cabinet. With parallel woofers, I can theoretically gain 6dB of SPL versus a single woofer, and these drivers are so compact anyway the box is barely any larger.
With the new enclosure I repeated the iterative process of satellite design. I've got to size the two chambers according to the equations for bandpass boxes, so that the upper-end roll-off mates well with the satellites, whose enclosure volumes depend on the volume of the sealed chamber. It's a darn over-constrained system, and on top of that it's non-linear! I decided to go with the same two-way system using the Hi-Vi/Dayton pair, but in larger sealed enclosures. My issue with this design is it's a bit inefficient, from a design point of view. Parallel woofers are used when greater extension and power handling are needed, and no single drivers exist which fit the bill. Given the relatively low-performance needs of my application, it seemed unreasonable to think that I couldn't find the perfect midbass driver.

At this point I remembered Adire Audio drivers. You might wonder why the link is broken. Well, it's not. No more then two months after I bought the driver, the company closed its doors. I'm an idiot. Anyway the Extremis 6" midwoofer is impressive; a long-throw design with 13mm Xmax, a stiff cone and cast basket with Neodymium motor. Adire was well known for their Shiva and Tempest woofers in the DIY subwoofer circles, and they applied their wide linear-range motor technology to a smaller driver. A single one of these has more output and power handling capability than a pair of the 5.25"s, so I went with this driver. Now I kinda wish I hadn't given the design will be irreproducible, but oh well it's unique. I'm sure with modifications a Tang-Band 6.5" can be used. Another advantage of this driver is the 8 ohm impedance, which means it will play nice even when bridge amplified with a chip amp, something I'll deal with later. Here's the final cabinet:

You can see the 3D modeling comes in handy here, as the vented cabinet is pretty crammed full of stuff, and it was good to see that the driver magnet has clearance from the plate amp components, something hard to visualize with pen and paper. Rather than listing the enclosure specifications, I'll just give the SketchUp files used to create it: the raw box with port, the cut list, and the driver model. The vented cabinet has about 30 cubic inches too much volume. I figured I'd just glue in some dampening blocks on the walls to consume this extra. Also, you can see the sealed cabinet, in which the satellites are stored, has a lip on the driver face. This is to protect the driver's rubber surround.