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"Lemniscus" 2-Way Monitor

Project, Text and Images courtesy of Scott Hinson

Project History

This speaker had a couple of interesting requirements. I had to use drivers I had on hand, and it had to be useful by itself and as a surround sound speaker in our living room.  So, what drivers did I have on hand?

Woofers

Tweeters
The Drivers

I chose the Vifa and the Strykes for this project. I think I've got another project lined up for the Peerless woofers, so that left the Vifas.

I've used the Vifa tweeters before.  Some were made before they changed the dome material and suspension, some after.  I've never been that happy with them.  For the price they are hard to beat, but that's because there's not a whole heck of a lot of competition.  The Seas 27TFFC is generally a better tweeter than the Vifa's but both domes have been crushed.  Now, they've popped back out, but I don't generally like to see something go squish and then assume it will work just fine. So that left the Stryke ribbon tweeters. One of these day's I'll spring the $9.00 for new domes.

The RT2 tweeters are sold under the Dayton brand as the 275-085 PT2 Planar Tweeter. You can access it's Spec Sheet and Response Graphs by clicking the links. (Note: Requires Adobe Acrobat 4.0 or higher)

The Lemniscus 2-Way

The Crossover

Simulated Frequency Response of Finished Speaker

You'll notice the hump in the bass response centered at about 120Hz.  I *think* this is an artifact of the measurement process I use. I have the D'Appolito book on measuring loudspeakers, but I haven't read the whole thing yet. I use LAUD for measurements so I should be able to do it correctly.  If it is actually there, it sure doesn't sound like it. (In fact if anything the bass response is too refined!)  Whatever it is I'm doing wrong I think Stereophile is making the same mistake, ever notice EVERY ported speaker they measure has a bump in the bass response. Maybe we aren't scaling the port response in right. I get the same issue with sealed and ported though....hmmm....

You'll notice the crossover point is right at 3000Hz.  Also notice the sensitivity, it's quite low.  This is because I designed the speaker for a listening distance of 2meters not 1 meter.  The tweeter looks like a line source a little at 1meter so, I moved the measurement reference back some.  50 Watts per channel is more than enough to really get this thing moving. I went through 8 different crossovers, before deciding this one sounded the best.  Of course I reserve the right to change it at any time.  That said for ~$80 worth of drivers per side, it's quite a good little speaker.  Here's a list of the different crossovers I tried:

Schematics for Above Response


Tweeter Schematics
Click Picture to Enlarge



Woofer Schematics
Click Picture to Enlarge
For the alternate, higher sensitivity crossover, here's the schematics, and response.  Remember this is a very good sounding crossover, with a lot higher sensitivity.  I didn't choose it for my own because it was ever so slightly shouty on vocals at higher volumes. The ear is an amazing thing.

Alternative High Sensitivity Crossover
Click Picture to Enlarge Response Graph

Alternative Crossover Schematics


Alternative Tweeter Schematics
Click Picture to Enlarge



Alternative Woofer Schematics
Click Picture to Enlarge

Constructing the Lemniscus

The Enclosure

The bass alignment is a standard QB3. For this you need a 17L enclosure tuned to 42Hz. 

The enclosure dimensions are 9"Wx14"Hx12.5"D.  It is made out of 3/4" MDF, with one vertical shelf brace. Since the largest un-braced panel  is 7.5" x 12.5", the enclosure is quite dead.

The diagram of the enclosure construction is shown to the left. The woofer is flush mounted and the tweeter is not.

    1. Enclosure Materials

As you can see there's not too much lumber involved in this little bookshelf. I think it can be built using a half sheet of MDF, but I just used scraps and parts I had lying around.


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    2. Test Fit

Test Fitting the pieces together prior to gluing.


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    3. Enclosure Construction

This is a picture of the inside of the speaker during construction. Note the alignment of the port and the holes cut in the brace. The single shelf brace runs vertically. If anyone wanted, they could add a horizontal brace or two without requiring changes in the crossover. If I ever rebuild them I will do this. The cabinets are very dead, but not as dead as they could be, given that the DIY'er can spend as much time as they want on a cabinet.


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    4. The Baffle

This is the first test fit of the drivers after I cut the holes in the baffle. Measure twice, cut once!


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    5. The Raw Parts / Components

The Vifa P17WJ-08, the RT2, and some acoustastuff. I used about 1/2 bag.


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    6. Acoustastuff Placement

This shows the physical location of the stuffing. All behind the vertical brace. This is done so that it breaks up an internal reflection from the woofer off the back wall and to keep it from falling down on the woofer.


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    7. Aligning the Woofer

This picture shows how I get the speaker holes to be square with the enclosure. I place a square on one side of the enclosure and get two of the mounting holes to line up with it. Using a pencil I then trace the circles, remove the woofer and use a center punch to set the drill holes for the T-nuts.


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    8. Wrapping up

This picture just shows the ability to bi-wire if necessary. The actual reason for putting the two sets of terminals is that the final enclosures were also used as test enclosures. This time I only built one box, in other designs I've built 6 or 8 boxes, rebuilding as necessary until I was happy.


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The Finished Speaker

About the Designer

Scott Hinson has a B.S.E.E. from The University of Texas at Austin. He is currently an RF hardware design engineer during the day. After work he enjoys golf, movies, audio electronics and, of course, building loudspeakers.

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