Last year, I purchased a Technics receiver with Dolby Pro Logic (DPL) capability, to add that extra dimension of surround sound while viewing my favourite movies. Of course, having a Pro Logic receiver to play with solves only part of the problem - you need a DPL-encoded source, and of course the speakers. Well, I have the source material, but in the speaker department, I was short one element - a center channel speaker.
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...!
These are nice goals in theory, but let's take a realistic look at this center channel idea...
Selecting a driver
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!
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:
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.