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Ask any group of audio enthusiasts about whether or not a center channel speaker is necessary, and you're sure to get lots of opinions both for and against the idea. Those for the idea will say a center channel speaker is essential for the proper localization of mono (center) sounds, such as dialogue. Those against it will say that it screws up imaging, and it's also difficult to get a perfect match across the three front speakers, so sounds that are panned from one side to the other will tend to sound different as they cross the center.

I decided to try to build one. Hey, this was going to be my first full-range project, and seemed like a good way to practice my limited speaker-design talents without having to build a matching pair of boxes...!

Getting Started
Conventional wisdom has it that a center channel speaker in a DPL system generally requires the following characteristics:

1. Timbre-matched with the Left/Right speakers: - necessary if the characteristics of an aural event panning from left to right are to remain the same when it passes through the center.
2. Same or greater power-handling than the Left/Right speakers: - During a typical movie, a good portion of the audio will be routed to the center channel.
3. Broad horizontal dispersion: - necessary if you're catering for listening positions that are located off-center of the TV.
4. Shielded: - a basic requirement if you want to locate any speaker next to a TV.

These are nice goals in theory, but let's take a realistic look at this center channel idea...

Timbre-Matching
It is difficult to build a center-channel speaker that is timbre-matched to a pair of commercially-built speakers, especially if you have no knowledge of the drivers or crossovers making up the commercial system. The "sound" of a speaker is also likely to be very dependent on its mounting location, so unless the left, right and center speakers are mounted on identical TVs at identical distances from nearby objects, they will all sound different anyway.

Power-Handling
It is difficult to get the same power-handling with a center-channel speaker as you do with the other front speakers, mostly because of its intended mounting location - on top of the TV - which tends to limit its size somewhat. However, most of the DPL receivers allow you to configure the center channel to play in "narrow" mode, diverting all the frequencies below 90 Hz or so to the Left/Right speakers. This allows you to use a center channel speaker that's smaller than the Left/Right speakers and still get satisfactory results.

Horizontal Dispersion
The "broad horizontal dispersion" requirement does make sense, but it seems to be one that many commercial speaker companies seem to be ignoring, based on the large amount of MTM (Midrange-Tweeter-Midrange) type of speakers being sold for use as center channels. Either they know something we DIYers don't, or they're sacrificing dispersion for the better power handling provided by twoo bass drivers.

Selecting a driver
I wanted to build a center channel speaker that had the same (or very similar) dimensions as my main speakers, the Missions. This means that the woofer element of the speaker could be no larger than 5.25 inches in diameter. Secondly, I wanted to meet the requirement for broad horizontal dispersion. I was therefore against the idea of using an MTM design. The only problem was that a trditional MT design would have to stand upright on top of the TV, and that looked like a recipe for disaster to me, with little Kathryn now starting to take her first steps.

So, I decided to try out one of the coincidental drivers that are presently available from the companies that cater to DIY speaker builders. Coincidental drivers are part of the larger class of coaxial drivers, where the tweeters are mounted on the same axis as the bass driver. However, in the case of a coincidental driver, the tweeter is mounted at the apex of the cone, usually on top of the pole of the woofer's magnet structure. The result is supposed to be closer to a true point source than conventional speaker designs. Horizontal dispersion is also supposed to be improved when compared to conventional speaker designs, and this was the main reason why I chose to use this type of driver. A good example of commercial speaker systems using coincidental drivers are the KEF Reference series of speakers.

I decided to use the Synchron SYN 519A coincidental driver. This driver has a rated diameter of 5.25 inches, and its published specifications indicated that I could use it in a ported box slightly larger than my Mission 751s, so it matched at least two of my requirements. However, the 519A is not shielded, and I did not know if it would make a good match for my main speakers, but there did not seem to be many choices in the 5.25 inch shielded coincidental driver arena.

The Synchron SYN-519A has the following T/S parameters, as published by Madisound, and measured by me.:

Madisound recommends a 6 litre box tuned to 61 Hz for this driver. I decided to use a 12 litre box tuned to 50 Hz which seems to be a better match for the measured specs. I'll tell you why below...

Shown below are a few graphs showing the predicted bass response of this driver in the two enclosures:

As can be seen from the graph, if I'd chosen to go with Madisound's recommendation, the result would be a bass response that peaks over 2dB at 100 Hz. I've heard that this may actually produce good results with small speakers (giving them more apparent bass), but I was more concerned about the effect it would have on vocals. Also, the recommended enclosure would produce a low frequency cutoff of 70 Hz with my driver, and I want it to extend a bit lower to provide a better match for the 90 Hz high-pass filter that will be introduced by my DPL receiver.

Shown below is a graph that displays the predicted response of Madisound's alignment (using the published specs) and compares it to the predicted response of my home-grown alignment (using the measured specs):

Note the big difference below 80 Hz, especially the extra 12 dB of output 40 Hz! True, the output of my home-grown alignment is down 9 dB at this point, but its response is flat down to 50 Hz, and it will make a much better match for the high-pass filter in my DPL receiver. Also, the DC resistance of the crossover's inductor will tend to increase the effective Qts, which produce even better results at the frequencies just above the cutoff point of the box. Finally, after working through a couple of designs, it seems that a 12 litre box for this driver is much more aesthetically pleasing (to me) than a 6 litre version. My mind was made up at this point - I'm going with the bigger box!

Construction
I'm not going to bother to go into too much details here, as building a fairly generic rectangular box is pretty straightforward.However, I should mention a few things that I did differently.

The first thing is I used 3/4 round edge molding on all edges of the box - this is supposed to reduce diffraction problems, but I chose to do it more out of aesthetic reasons than anything else.

Secondly, as the wood I used for the box - 1/2 ply (1/2 MDF would've been better, but it's not available locally)- is thinner than the molding, I built up its thickness to match the molding by adding a few layers of fiberglass and resin on the inside surface of all panels except the front panel.

Thirdly, for the front panel, I added the fiberglass and resin to the OUTSIDE surface, and "flush-mounted" the driver this way, by adding extra layers until the fiberglass and resin was flush with the driver's mounting flange. The results looked really good, if I do say so myself! The box ended up with the external dimensions of 18 cm x 27 cm x 34.5 cm.

Finally, I attempted to finish the box in a gloss-black "piano" finish, but this turned out to be too difficult to do - and it really didn't match my TV either. So, I settled for a "faux-vinyl" finish, which I created by painting the box black, then spraying it with clear lacquer from a distance of about 18". Again, the results look pretty good.

Here are a few pictures of the completed box:

Tuning
I tuned the box to the correct frequency by using a 5 cm diameter port at the rear of the enclosure (so that the effect of "organ-pipe" noise generated by the port on the midrange response would be minimized). I'm presently experimenting with different port lengths - right now I'm getting pretty good results with a 14 cm long port. Internally, I used foam and some acoustic fiberglass to damp midrange reflections within the box.There is still a fair amount of midrange "noise" emerging from the port though, so mounting it the back turned out to be a pretty good idea.

Crossover
Because I know very little about crossover design, I decided to go with one that Madisound designed specifically for this driver. This turned out to be a pretty good choice I think. My initial thoughts when I first heard the overall response of the system with Madisound's crossover was that it sounded pretty good - the only minor quibble I had was I thought I heard a slight notch in the response in the upper midrange, and I would've liked to hear the effect of a slightly higher crossover frequency.

Results
The speaker sounds really good, with that nice "lean" bass that I like so much. After doing some initial frequency response measurements, it seems that my ears and these initial response measurements agree quite closely with one another. A close-miked frequency response of the system produced a frequency response graph that was flat up to about 1.8 kHz, then selved down about 6dB from that point. When the effect of the baffle was taken into consideration (by doing another frequency response measurement in the "far" field), the result was a fairly flat frequency response up to about 10 kHz - the measurement limit of my equipment - with the exception of a small notch between 2-5 kHz.

The graph below shows the close-miked response of the driver and the port, including 2nd and 3rd harmonic distortion, taken at a 2.83V drive level. The rising 2nd harmonic distortion below 100 Hz suggests that the compliance of the spider and/or surround is not exactly the same in both directions of cone travel. This distortion should be unnoticeable in "normal" use anyway, as the speaker will be crossed over at 90 Hz.

Update: 16-AUG-2004
The glue holding the surround to the driver's cone apparently failed, turning into a sticky mass - probably because of our nice warm tropical climate! I used WD40 to remove the old glue, and while doing this noticed that the surround was apparently designed to be fastened behind the cone, not in front of it as was the case. So, after I removed the old glue, I stuck the surround back in place, but behind the cone this time.   Seems to look pretty good, what do you think?

Update: 16-AUG-2004
This center channel speaker performs OK at low volumes, but there are signs of driver excursion problems at higher levels, even with the 90 Hz HP filter enabled. I'm now planning to redesign the system, using two 5.25 drivers to handle everything below 300 Hz.  If you've got any ideas on which drivers to use, please let me know!

Brian Steele