Crossover Specs    Driver Comparison

Design Overview

This speaker is initially based on the A652 design done for Audax by Vance Dickason.  The MTM (or. D'Apollito) design allows the drivers to operate with improved efficiency and lower distortion in the lower frequencies.  This particular speaker is no bass machine but it makes a terrific surround or satellite running reasonably flat down to about 50 Hz.  The original design called for a 1.2 cubic ft. enclosure but due to the ever present SAF (*) I scaled these down to a more manageable .83 cubic ft. internal volume (it also made for more evenly measured boards).  I used a smaller 1.5" diameter flared port on these final speakers compared to the 3" diameter port used on the prototype mostly for aesthetic reasons, but also due to the diminished port length.  I don't need to max these speakers out very frequently so I have not noticed any unnatural port noises due to the smaller diameter. 

I've recently purchased a digital sound meter but have not performed exhaustive response measurements on these drivers yet.  When I do I will be sure to update these pages.  

Driver Selection

This speaker is based on the A652 with the not-so-minor change of replacing the $190 dollars worth of Audax drivers per speaker with about $50 worth of Dayton Loudspeaker drivers from Parts Express.  Since the final theater system is comprised of 5 speakers this amounted to a total savings of around $700.  While I'm very pleased with the final product, in retrospect, I might have done better to have selected the Audax 6.5" Aerogels as they more closely matched the efficiency and response curve of the original Audax carbon fiber drivers the original design called for at a more reasonable $25 per driver.  This might prove an interesting upgrade in the future (see the driver comparison for specifics).

As for the tweeter I chose to go with the Dayton 1" titanium due to it's good specs and, due to budgetary concerns, slightly lower price (bad reason, I know).  These drivers I again have not been terribly disappointed in their performance.

I will hedge a little and say that I'd like to reappraise these selections after really doing a fair comparison with other top notch drivers from manufacturers like Focal, Audax and Morel.  If you have some experience designing speakers like this I'd love to get your opinion on favorite drivers for this sort of application.

Crossover

The crossover is a 4th order Linkwitz-Riley type at 2.8 kHz which is designed to produce system efficiency around 90dB with the original drivers.  This is probably down about 2-3 db due to the difference in driver selection.  The crossover uses a modified 3rd order design for the low-pass section of the circuit which provides multiple break points at about 200Hz and 2.8kHz.  This setup should produce an anechoically flat response up to the 2.8kHz frequency followed by a 3rd to 4th order acoustic slope at the crossover frequency to provide a flat summation of the two driver planar output.  The high pass section also requires a 3rd order topography to produce a 4th order acoustic slope with the circuit resistance adjusted to produce as shallow a transfer function "knee" as possible.  When both sections are acoustically summed together the final response of the speaker is 60Hz - 20kHz +/- 2.2dB with very flat off-axis response.  See Crossover Specs for a detailed list of parts and schematics.

Enclosure Construction

The internal volume used to calculate the response curve was .83 cubic ft while the internal volume used to calculate the construction was adjusted up 10% to .925 cubic ft. to account for bracing and filler.  I chose to use .75" MDF for it's density and superior milling characteristics.  This proportion also provided for easy to measure boards as you may note below.

Since much of the front baffle would be cut away and because I wanted to flush mount the drivers I cut an extra board for the front and glued two together to make a 1.5" front board.  From this I was also able to use a router to put a 1/2" round over on all front edges to minimize edge diffraction and improve the off axis response.  I used the same round over bit to flare the port opening to improve port efficiency by minimizing turbulence. 

I typically use screws for construction but for this time I tried something a little different.  I would first apply a generous amount of yellow woodworking glue to the interior half of each edge.  Once clamped the glue would ooze only slightly on the exterior but would make quite a nice glue seal on the interior of the butt joint.  After clamping I would use a air gun to apply 2 1/2" finish nails all along the joint.  This long of a nail firmly secures the joint during drying so I could remove the clamps and continue with assembly and provide solid reinforcement once the glue was dry.  Not only was this much faster than screws it was also much more precise and less trouble to hide in final finishing.  This was especially true on the front baffle. 

After the boxes were assembled I continued with the finishing.  For the front speakers which are housed behind fabric centered doors in the cabinetry I applied two coats of black spray paint to provide a good color base.  On top of that I applied two coats of black textured fleck stone paint that leaves a granite like texture and coloring. Finally I applied two coats of clear polyurethane to seal and protect the finish.  This process was very effective at hiding blemishes and making a great looking final finish.  The back speakers were a little easier.  Since they would be out in the open and visible the SAF indicated that it would be optimal that they blend into the surroundings as much as possible.  The walls of the theater were painted with a tan suede texture paint.  Initially I tried to have a high gloss paint made to match but we immediately realized that the texture difference made the colors appear to clash.  Hence, the back speakers are now also a tan suede texture and (I have to concede) look great in the space.

See the The Theater for pictures of the finished theater space.

Design Details

* SAF = Spousal Approval Factor