Greg Smith Book Reviews
Introduction to the Book Reviews
September 1997
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There are layers of understanding to any subject. The compuspeak that flows around the Internet has a wide variety of terms that reflect how well someone knows something, ranging from "newbie" to "guru". I've always found that one of the best ways to really learn something deeply is to construct one of the objects you wish to understand. This is certainly true when you're trying to figure the wacky world of audio equipment. What you can pick up while sweating over a DIY project goes well beyond anything you can learn just by using assembled products. Building speakers is an excellent way to get started constructing your own audio equipment. There's a lot to learn before starting a project like that, and the best all-around introduction to the topic I know of is David Weems' Designing, Building and Testing Your Own Speaker System (with projects) (what a mouthful). It walks a fine line, giving tons of handy information with a moderate amount of technical detail while not being overwhelming even for newcomers to audio.
The first chapter of this book contains more useful information about what to look for in a speaker than a year worth of your typical audio magazine. You find out about transient response, dispersion, damping, impedance, efficiency, you name it. The diagrams presented every few pages help illustrate the concepts put forth by the text. This chapter might go a bit too fast for those really new to speaker terminology, but it's certainly quite understandable if you pay attention. At the same time, the material goes by at a good clip for those who already know something about these topics. The next chapter goes into the different kinds of speaker enclosures, describing why it is you need an enclosure for drivers in the first place and what the trade-offs between sealed and ported boxes are. The third chapter gets into detail on how to design and build speaker boxes. Material here ranges from the theoretical, like why path length differences justify keeping drives aligned vertically, down to nitty-gritty information on what type of wood and damping material to use.
The next two chapters give you a mathematical basis for determining how large to make the inside of your speaker, for both closed and ported systems. This is a practical guide that's short on theory and long on tables and graphs to make the process as easy as possible. The main weakness of this section is that Weems doesn't really describe how you can look at the parameters describing a bass driver and figure out whether it's better suited for a closed or ported box in the first place. The short answer in this case is that you should divide the driver Fs by its Qts, which should give you a ratio between that's typically between 25 and 200. Values less than 50 are only suited to closed boxes. Between 50 and 80, a closed box is still better but it might be possible to vent it with a port. Values of 80 to 100 are equally likely to work with either type of box tuning. If the ratio is over 100, a ported system is the only real choice if you want good bass extension. A lot more information on practical information like this that dovetails nicely with Weems' coverage is a short article in the 4/97 issue of Speaker Builder, a Marc Bacon piece titled The Speaker Builder's Travel Guide. For information about getting back issues, look at their home page or call (603) 924-9464/fax (603) 924-9467. While you're there, they've also got a killer sample package offer that gets you issues of their magazines along with a free copy of J. Gordon Holt's Audio Glossary--if you don't have this yet, this is a primo way to get a copy along with the best DIY audio magazines available. Anyway, back to Mr. Weems.
The next chapter covers crossover networks. This is the area that is most frequency neglected by introductory level speaker texts. Weems starts with the simple filters every speaker book describes, but there's practical information on things like impedance compensation, contouring, and notch filters that other speaker building texts either ignore or made too complicated for a beginner to understand.
New to this edition of the book is the "Fun with a Computer" chapter. This gives you simple BASIC programs for computing box dimensions and designing simple enclosures. Most of the chapter discusses the LDP program Weems has included in the book (greatly expanded from the little program listed in the 3rd edition). At a whopping 20 printed pages of difficult to follow BASIC code, this is certainly not cutting edge speaker software. It's not bad, though, if you have the patience to type it in or are willing to drop the $15-$30 for a version on diskette (money that, for most people, would be better put toward one of the far more featured commercial speaker design programs). The typical high-end consumer will likely be somewhat shocked by the book's eighth chapter, "How to Choose and Use Your Speakers". Here you find heresy promoting infrasonic filtering, lamp cord as speaker cable, and using a fuse to protect your speakers. While I don't agree with everything Weems says here, I've been spoiled by too much time at the top part of the market. For those putting together seriously inexpensive systems, his advice is pretty sound (so to speak).
"Testing your Speakers" continues to offer pragmatic advice on what to do with the speakers you use. Some advice on listening for problems with speakers is offered, and (gasp!) the suggestion that they can be corrected with an equalizer is presented. The rest of the chapter goes into the technical details of how you use a frequency generator and some test equipment to actually measure the parameters of the drivers you encounter.
If you're not feeling up for learning enough to design your own speakers, the next chapter presents four projects you can build without as much planning. Sadly, the earlier edition included ten such projects included in the context of the material they highlighted, but these four are a decent sampling. Anyone with a few dollars to spend and some time could build the first project, a little box with a 4" full-range driver (check out that whizzer cone!) you can buy at Radio Shack. This is more complicated than most single driver configurations because it includes a filter to get rid of the nastiest peak in the driver response, which means you get try a touch of crossover work as part of the project. No more heavy shopping at the Shack after that, because the later projects will require getting some catalogs from companies like MCM, Parts Express, or Meniscus in order to buy the better drivers they carry. The fourth project uses a dome tweeter and 6.5" woofer from Vifa in a ported box. If built properly, I'd expect this one to be fairly competitive with similar speakers that cost around $200/pair, with construction costs closer to half that. This is an excellent way to save some money in exchange for your time.
Four appendices cover useful formulas for speaker building, how to wind your own inductor coils, source code to the LDP program, and a partial listing of mail order sources for good speaker parts (buy any issue of Speaker Builder to find many more companies than are listed here). An extensive glossary gives short explanations of many of the terms tossed around by the rest of the book.
Designing, Building, and Testing Your Own Speaker System is a very down to earth introduction to the subject. While the texts by authors like Vance Dickason and Martin Colloms are better suited to those looking for heavy technical details, Weems gives a much easier to follow, gentle introduction to the most important things to know about speaker building. Anyone who would like to know more about how the speakers they listen to work, whether it's simply curiosity or the desire to build them, would be well served by buying a copy of this book.
.....GS (gsmith@westnet.com)