Sonosub - An Inexpensive Powered Subwoofer (page 1)

Introduction

This article outlines the construction of the Sonosub, a low cost subwoofer, complete with power amplifier.

This projects started out as a subwoofer for my home theater (HT). After I started working on it, I decided against having yet another box in the room so I decided to give it to my brother for his HT system. Along the way I learned a few things about working with tubular enclosures.

While the results were ultimately satisfactory, several mistakes were made during construction that could have made the subwoofer better than it is. This article describes both what I did as well as what I probably should have done. Learning often means making mistakes and I don't mind making them if it means I will avoid them the next time around. Hopefully, readers will benefit from my mistakes as well by extracting those aspects of the design they find useful, and ignoring the questionable parts.

For the curious, here's what everything looked like at the end.

[Front view; 11,047 bytes] [Bottom view; 12,243 bytes] [Amplifier; 17,042 bytes]

Design Goals and Criteria

During part of 1998, Madisound sold a driver named the INF-10 at a mere $34 USD. These are 10 inch drivers manufactured by Infinity Systems. During this time, I became interested in the construction of cylindrical speaker enclosures. Subwoofers made from Sonotube, a commercial cardboard tube made for pouring concrete footings, are fairly popular with the DIY speaker crowd due to their low cost, simplicity, and high performance. I decided that the INF-10 driver would be perfect for a low cost tubular subwoofer.

For more information on Sonotube subs, check out the Sonotube FAQ by Gordon McGill.

As with all my projects, I start with a list of goals which then guides my design.

1. Low cost. When I started this project, I didn't have a real need for another subwoofer as I already have a pair of subs utilizing the NHT 1259. But since my subs are wired to my main speakers (left/right), I figured this sub could be connected to the subwoofer output of my Pro-Logic decoder. Since this was targeted at home theater use and not for high fidelity, I was willing to sacrifice some quality for cost.
2. Easy to build. Nothing overly fancy. A tube is about as simple as it gets, though it turned out to be more difficult to work with than I had anticipated. This may have been partly due to the 1/2 inch tube I used instead of the typical 1/4 inch tube. The need to provide an appealing finish also affected the construction steps, making assembly more difficult.
3. Use existing parts where possible. If you like making stuff, you know what I mean when I say that I always seem to have "stuff lying around" waiting to be used. This goal is also consistent with #1 above.
4. Low end extension to at least 30 Hz. This is, after all, a subwoofer, so let's make sure it sounds like one.
5. Aesthetically pleasing. Looks count. To me, completing a project means that it's not only useable, but also presentable. This is important for SAF :)
6. Amplifier and crossover electronics. Can't forget this. In keeping with goal #1, the original intent was to use a pair of National Semiconductor LM-12 high power op-amps in bridged mode. I already had a pair... somewhere. All I had to do was build a multiple-input, variable gain, variable frequency, low-pass crossover and preamp to drive the LM-12s. This idea was eventually scrubbed in favor of a Parapix amplifer from Apex Jr. The Parapix solution was not only cheaper but required much less work on my part.
7. Downward firing. A downwards firing subwoofer has some advantages and disadvantages over a forward firing sub. In this case, firing down made sense due to the use of the tube. It also meant not having to worry about grills or prodding fingers from kids (hopefully). Not all drivers should be used in a downwards firing position, as this will cause cone sag over time. How the INF-10 will fare over time, I don't know.
8. Decent SPL capabilities. It needs to have enough output to enjoy movies.

Technical Details and Design

Here are the specs for the INF-10 as published by Madisound. I did not measure the actual driver used.

The design looks like this :

For more information on the INF-10 driver, see Steve Houlihan's and Brian Steele's web sites.

29-September-2000

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