Summary

Each speaker has dual 8-inch woofers operating below 400Hz and an all-dome middle and top end. They are 361/2" tall, 19" deep, and only 71/2" wide.


Design

To get these in the TV room, they had to meet certain criteria:

  1. They had to fit in an 8" wide space.
  2. They had to use my existing drivers (8", 2", and 1").
  3. They had to be attractive.

I have a few 8" woofers, but the only way to fit an 8" woofer in an 8" space is to turn it sideways. That pretty much defines the system. The woofers are surplus subwoofer drivers with very good specs on the low end but not very good on the high end. They don't work well in a 2-way system - in fact they came from a 4-way system. The drivers want 17.35 liters each, and the enclosure gives them that, yielding a 64Hz f3 with a Q of 0.707. That may sound a bit high, but a MiniDSP performing a Linkwitz transform provides them with response down to their final f3 of 30Hz, which is near their fs of 25.2Hz.

The midrange and tweeter were chosen because they both exhibit low distortion. The Dayton 2" aluminum dome midrange covers the range from 400Hz to 5000Hz, and the Tang Band 1" ceramic dome tweeter goes up from there.

The "preamp" is a pair of MiniDSP 2x4 modules programmed as a 4-way crossover. Two of the channels are 400 Hz lowpass filters with the Linkwitz transform applied. One channel is a 400Hz - 5kHz bandpass for the midrange, and the last channel is a 5kHz highpass for the tweeter. The crossovers are 48dB/octave Linkwitz-Riley. The volume and input selection are done with a Teensy LC Arduino clone running a custom program to read an IR remote control. The Teensy was chosen because it has a real DAC, which is used to act like a pot on the two MiniDSPs. One of the things it does is ramp the volume up after unmuting, or when changing inputs. The brightness of the input, power, and mute LEDs varies with room light, getting dimmer as the room is darkened. The IR remote is the same one used by the DirecTV receiver, so it follows the volume and mute of the TV.


This drawing is of the MDF core of the enclosures with the front panel and one side off. The center drawing is a section showing the rear, top, bottom, and the center divider. The cutout for one woofer may be seen on the side panel.

The MDF parts are:

  • 4 ea. 18" x 35" panels for the sides
  • 4 ea. 71/2" x 19" panels doubled up for the bases
  • Everything else

"Everything else" is 4" wide by however long. With all of the parts being the same width, it doesn't matter if it was cut perfectly to size. The 4" parts are all cut with the same setup on the table saw. They have to fit. If you need to change the internal volume of the enclosures, this is the easiest dimension to manipulate. Here is the cutlist for the speaker enclosures. The 4" parts are cut 477/8" long because I can't fit 8' in my car and the cutlist software won't allow me to specify "just cut them in half".

The core is wide enough at 51/2" that the drivers could just be mounted directly on it, but that would be ugly.

Enclosure Assembly

Assembly is typical of a speaker box, so I'll just hit the high spots. The bases were glued up first, the bottom panel glued and screwed to that, and then assembly as usual from there.

I used biscuits. They make it as simple as putting a part in place and clamping it, because once in place it won't move.

The sides, top, bottom and rear panel were all assembled as a unit, leaving the front open. The holes for the woofers were made after the core box was assembled, as were the holes for mounting the grilles and maple panels. The enclosures are mirror images, so they will be symetrical in the listening room.

The side panels just clip on with grille fasteners, something I would never do again. Screws and rubber cushions maybe. They could be used to damp the enclosure resonances. Not that there are any large resonances. Small internal braces break the panels into smaller unsupported surfaces, raising the resonances above the 400Hz woofer cutoff.

Painting the Texture

This stone paint is different. It takes two coats, but the second coat can be applied anytime. After painting, the tape is carefully removed. The cure time is 24 hours, but it stays slightly sticky for days. It doesn't smell nearly as bad as regular paint, and appears to be lumps of paint suspended in a solvent like mineral spirits, which does not dissolve it but evaporates leaving the lumps on the surface. Now they have gallon cans that you can use with a roller. That would have been much easier to use.

Baffle Assembly

The baffle has two parts - the 4" wide MDF front panel and the hardwood baffle. The front panel is screwed to the baffle from the MDF side. Position the front panel so it is centered on the width, but 13/16" or so from the top of the baffle. This will allow for the thickness of the top panel and the thickness of the paint. I used 1" screws, pre-drilling the holes, making sure not to put a screw where the midrange or tweeter would be.


They sound amazing. They don't sound compressed or held back in any way. Imaging is good. Frequency response really is down to below 20Hz, thanks to the Linkwitz transform the MiniDSPs are providing. As mentioned above, the room is giving me low frequency fits, but I'll get that solved. It sounds just like the curve looks - a couple of holes and peaks in the upper bass area.


I have most of the parts to build another pair. What would I do differently?

  • No two-tone panels. There were too many cuts and glue-ups involved in the 6 panels (56 total cuts).
  • Woofer grilles narrower and starting farther back so part of the enclosure is visible. The baffle would then be only 5-1/2" wide, a lot like this Brodmann.
  • Baffle only 1/4" to 3/8" thick. It doesn't do anything but look good, and thin is a lot easier to cut holes in.
  • Darker wood. Shedua or maybe Sapele dyed a "red mahogany". Or maybe burl veneer.
  • Three feet made from 1" birch dowel rod that poke out the bottom 1/8" or so.
  • Full passive speakers - no amplifiers

Modified: 2016-09-10 19:04:05 GMT