1/10/2002 Revised 11/28/03

Introduction
Components
Crossover
Cabinet
 Results

Introduction

I had been using an extremely cheap center channel that was given to me by a friend. While I certainly appreciated the gesture it was a terrible speaker. I couldn't understand dialog and that's the whole point of a center channel. So I went out and listened to various commercial center channels. Unfortunately, the ones that I liked I couldn't afford. Since I had recently built a subwoofer I decided to build the center channel. The subwoofer turned out okay so what the heck? I ended up spending about $250 for components and about another $100 for the cabinet (not counting the tools I had to buy to build it). And it turned out pretty good... 

Components

 
It took me quite a while to pick out which speakers to design around. The Loud Speaker Design Guide is an invaluable reference for the beginning DIYer. After much browsing I decided on the following drivers: 

•   VIFA P17SJ-00 6-1/2" Shielded Woofer 
•   Focal TC90TdxB - 1" Shielded Tioxide Dome Tweeter 

I picked the Vifa mid/woofer because it was shielded, had a good review from the LDSG and it was affordable. I picked the Focal tweeter because it was shielded and it has a very smooth frequency response. In retrospect I don't think I would use the Focal again because the face plate is not round, making it a pain to recess. Also, I don't think Focal is making it anymore. But, I do like how it sounds. I went with a two way MTM design because it was my first attempt at a crossover and I didn't want to bite off more than I could chew. Here are the speaker frequency responses:

Crossover

I choose a 3rd order crossover crossed over at 2.8 kHz for the woofer and the tweeter. Here is the measured frequency response using ETF   to perform the measurements. I used ETF for two reasons; there is a calibration file for an analog Radio Shack sound pressure meter and its a free demo. Also, it gave results that where repeatable, as opposed to Speaker Workshop which seemed to give a different result every time.The testing results shown here is with the microphone sitting on my desk pointing at the speaker from about a foot away. I'll do a little more formal testing in the future and see if I don't get a little smoother results. 

To design the crossover I used a student  version of Microcap II that I used in school. While it models the frequency response of the filters, I couldn't add the driver characteristics to the network the way a commercially available crossover design program would. I'm fairly happy with the crossover but it does sound a little nasally in the midrange. I think a 2nd order crossover would work better with the Vifas. 

Cabinet

I used Speaker Workshop to measure the T/S parameters of the Vifa Woofer after breaking it in for about two days. I probably drove my cat crazy with the warble tone that I ran while I was away at work. I have reservations about the repeatability of Speaker Workshop but it is the best product for the price right now (free beta version). Its pretty much useless unless you build  a jig to calibrate it. 
 

I used WinISD to design the cabinet using the T/S parameters that Speaker Workshop found. I used a ported design because I wanted to get the maximum bass out of the center channel, reasoning that it should have as full a frequency response as possible since it would reproduce most of the sounds in a movie. WinISD calculated a box volume of 1.2 cubic feet using 2 -2" ports that are 6" long.  The ports are thick cardboard tubes that I got from work. I'll buy adjustable ports for my next cabinet so I can try and tune the box more easily. I used two ports thinking that it would cut down on port noises.WinISD predicted a 3db down frequency of 45hz for the box, which agrees with the measurements taken with ETF. 

Unfortunately, I got a digital camera after I built the cabinet so I don't have any construction pictures but the cabinet is pretty straight forward. Its 3/4" MDF with braces between each speaker in order to make the baffle as rigid as possible. I recessed the tweeters and woofers. I lined the interior with dynamat to reduce resonance. I later lined the interior with egg crate foam mattress because there was a lot of midrange reflection. That helped the sound a lot and from now on everyone of my speakers will have that inside. I tried stuffing the cabinet with polyfill but later took that out as it gave the bass a papery, thumping quality. Checkout the Patman's web page for tons of helpful tips on constructing a cabinet. I pretty much copied his technique for my cabinet. 

The veneer is iron-on oak from Lowe's. You have to be very careful sanding this stuff because its pretty easy to sand through the wood to the glue underneath. The glue doesn't accept stain and will stand out pretty badly.You can see a corner on the back where I sanded too much.The veneer is very easy to apply though. I also used a roll of 3/4" iron-on veneer for the front and back edges.A laminate trimmer for a router will trim the edges flush. 

I used Minwax red oak stain and three coats of semigloss polyurethane to finish it off. The front and back are finished with Rustoleum Stone Creations textured paint. I found a web site that sells veneer for a lot cheaper than Lowe's and will be using it for future projects. They stock a two ply veneer that is much thicker than the stuff from Lowe's and it accepts stain better too. However, it is not iron-on and there apparently is quite a debate over the durability of contact cement over time. 

I'm very pleased with the results. The sound compliments my Martin Logans, which was a concern. I was afraid that the timbre wouldn't match but I haven't noticed a big difference. Dialogue is now understandable and music sounds very good too. Additionally, everyone who has seen it has been pretty impressed with how it looks.  In the future I plan to tweak the crossover and fiddle with the port lengths. I'm thinking about trying the Eros crossover and see how that works. Or I might cough up the money for a real crossover design program and try my luck at designing my own.