LCR MTM 3-Channel Speaker

I didn't need to build the LCR-MTM, but I had the materials and in the end, it seemed like a good idea. LCR refers to Left, Center and Right - the 3 front speakers in a typical home theater setup. MTM is the mid-tweeter-mid driver topology. This speaker contains 3 speakers (the left, center and right), each in a horizontal MTM alignment. It replaces three Cambridge Soundworks speakers that used to sit on the floor by my TV. This speaker reduces the clutter, lifts the sound off the ground and raises the TV's height.

Truth be told, I could have built a shelf above my existing entertainment stand and placed the 3 existing speakers horizontally. They would have fit in the space just as well. But then I wouldn't have had the fun of building my own speaker and using up some materials I had already purchased some time ago.

The "M" is 4 inch shielded mid-woofer from the former MCM Electronics (part number 55-1840). I bought 6 of these a while ago when I saw them used in Avatar Audio's "Pop" speaker. But they sat for years in a box, waiting to be used. The "T" is an Allison Acoustics tulip-shaped tweeter. This is the newer version of the classic tweeter by Roy Allison. I bought four off eBay years ago as well. One had a bad voice coil leaving me with just the right number for this speaker.

Tweeter specs:

• 1 inch convex diaphragm, ferrofluid damped
• 4 ohms
• 87 dB at 1 meter, 1 watt

Mid-Woofer specs:

• cone - carbon fiber
• 40W, max 60W
• Impedance 8 ohms>
• Sensitivity 85 +/- 2dB/W/M 2.83V
• Qts 0.522
• Qes 0.753
• Qms 2.233
• Vas 2.1 L
• Sd 0.053 m^2
• Re 6.7 ohms
• Xmax 2mm
• Zo 27.2 ohms
• Cms 0.00053 n/M
• Mms 6.45 g
• Fs 88.0 Hz

The tweeter crossover is a 2nd order electrical high pass (4 uF cap, 0.25 mH inductor). The woofer crossover is a 2nd order electrical low pass (13.3 uF cap, 0.28 mH inductor) with the woofers wired in parallel. Each crossover module is glued to one of the large wall panels in each section.

The entire speaker measures 45 inches wide, 6 inches high and 11 inches deep (external measurements). It's made from 1/2 inch MDF. The internal space is 14.25 inches wide, 5 inches high and 19 inches deep or 712.5 cubic inches or 0.41 cubic feet, for the left and right side. The center section is just a tad larger at 14.5 inches wide, 5 inches high and 19 inches deep or 725 cubic inches or 0.42 cubic feet. The box tuning is 62Hz with a calculated F3=54 Hz and F10=45Hz. Four inch long, 2 inch diameter PVC ports are rear mounted alongside the terminal cup. The outside is painted with a satin black acrylic paint. The far left and right exterior panels are veneered with 2 layers of rosewood.

All drivers are surface mounted. The tweeter has a protective metal dome which protrudes about 3/4 inches from the baffle.

The grill is made from 2 layers of 1/2 inch plywood. Black acoustic fabric covers the front and sides. Four ball and socket connectors hold the grill onto the front baffle at the corners.

I'm really pleased with the results. The speaker is rock solid (it weighs a ton!) and sounds great. It's more efficient than the previous speakers so I had to re-equalize the subwoofer on the HT receiver. It blends in nicely with the rest of the equipment and we no longer have three little speakers sitting on the floor.

02-July-2023

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