dB1661	Back to Speakers
This project is a home theater surround speaker which I designed to go with my dB616TL mains and dB 616TLC center channel. It's a sealed two-way T-M-M-T configuration with a switchable rear tweeter for selection of dipole or bipole operation. The way I came up with this as the answer to my surround speaker needs is discussed on my web page htdesign. The basis of the rectangular box with outward and front/rear firing drivers came from wave patterns shown in the ripple tank simulations. This layout seem to provide the best dispersion pattern for wall mounted speakers producing a sound field across a listening area. It turns out that the optimum location for the speakers will be above ear height and slightly behind the listerner's sitting position. Dipole operation, obtained by reversing the rear tweeter's polarity, seems to move the sound image location farther to the rear. The decision to operate in dipole or bipole mode may depend greatly on your preference and on the movie sound director's intent, so experimentation is recommended. For some pics of projects built by some other DIYers, scroll down.
The crossover is a modification of the dB616TL crossover adjusted to account for the 4 ohm impedance of two tweeters.
You can use a double pole, double throw (DPDT) switch to change polarity on the rear tweeter. I used the type with no center position -- ON - ON. Connect the front tweeter directly to the crossover normally. Connect the switch to the crossover. Solder a pair of jumpers from one set of switch input tabs to the opposite poles of the other set. The output is taken from the center pair of tabs. Double check you wiring -- it's possible to create a short... IMPORTANT: Wire each speaker as a mirror image. The front tweeter for a speaker to the right of a listener is on the left of the speaker if you're facing it; for a speaker to the left of a listener, it's on the right.
I drilled holes in the terminal cups for the switch and mounted them with Goop, making sure there was a good seal. The orange wires in the pic are the wires that cross the +/- on one set of tabs to -/+ on the other. Good idea to label the wires because when you're assembling, it's easy to lose track. You might consider mounting the terminal cup and switch somewhere else on the speaker that you can reach when the speaker is wall mounted. I didn't do that...
Label the switch positions.
The following are some pics of the construction and more detailed drawings are farther down the page. Construction is simple. Make sure joints are sealed well. I used a combination of finishing nails and clamps.
The ends overlap the top and bottom. The front and rear overlap all. I cut the front/rear baffles and ends 1/8" oversize (add 1/4" to the length and width) all around then trimmed the 1/8" with a straight cutting router bit with bearing.
The box is simple. 3/4" MDF. I made the front and rear baffles and ends 1/8" oversize in each dimension to allow trimming with a router trimming bit.
Parts and Cost for One Speaker Item Quantity Cost/Each Price 6 1/2" Woofer 2 $16.20 $32.40 1 1/8" Silk Dome Tweeter 2 15.50 31.00 3/4" MDF, 4' by 8' Sheet 1/2 20.00 10.00 Polyfil Stuffing, 20 Ounce Bag 1 2.89 2.89 Terminal cup (260-311) 1 3.95 3.95 Crossover Parts       C1 (027-430, Dayton MPP) 1 3.55 3.55 C2 (027-420, Dayton MPP) 1 1.77 1.77 Ce (027-358, Non-Polarized Electrolytic) 1 1.80 1.80 L1 (266-806, 18 ga. AirCore) 1 2.90 2.90 L2 (266-816, 18 ga. Air Core) 1 3.75 3.75 Req (016-6, 10W)* 1 .39 .39 Rs (016-2, 10W) 1 .39 .39 Rp (016-12, 10W) 1 .39 .39  DPDT switch, 3A/125v (060-086)  1  1.20  1.20 Grille Cloth 1/2 Yard 7.50 3.75 Grille Guides, Small 12 1.75 1.75             Total $101.88 * 016-6 is no longer available at Partsexpress. You can substitute two 12 ohm resistors (016-12) paralleled or use the non-inductive version (004-6).
These are variations on the db1661 built by James Beane. He used Parts Express 0.75 cu.ft. MTM enclosures. I really like his solid wood end caps and wrap around grille. James told me he used some threaded rod (allthread) to stand the cloth off to clear the speaker surrounds and tweeter dome.
James blocked out the tweeter hole on the front baffle and moved the tweeters to the sides -- so they face front and rear along the wall. This is a vented alignment. Two Dayton 6 1/2s in 0.75 ft3 which is close to an optimum volume for an F3 of about 50 Hz.
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