Bookshelf speakers with a tweest.

[MJL21193]

I have just finished a small class A amp, the K10A and I thought this world be perfect for my home office. I saw this as an opportunity to build (yet another ) a pair of speakers and use up some of the drivers I have laying around.
I wound up using the Vifa PL18 for the mid-bass and something a bit different for the tweeters. I used Unibox to determine the best box size and tuning, keeping in mind the end purpose. The office is not big and space is limited so I would need to compromise. Ideal vented box size for the PL is ~18 litres (giving an F3 of ~41Hz) but this was just a bit bigger than I wanted.
12 litres is more like it and the low end response still looks very reasonable at an F3 of ~49Hz.

[MJL21193]

With only 10 watts, it's obvious that I'm not going to break any SPL records. I would have liked higher efficiency but that would mean buying more new stuff. My new policy - out with the old before in with the new.
Box size and alignment issues out of the way I look to an idea that's been rolling around in my head for a while: a solid wood baffle with a tweeter wave guide carved in. Yes, solid wood. Yes, carved in.
The tweeter is a 2" paper cone salvaged from an old find. Not high quality by any means but I did test their frequency response and listened to how they sound and they are more than acceptable on both fronts. It starts to roll off at ~14k (just like what's left of my hearing ) but they are very linear, very smooth.

Doing some work around the house last summer, I had an off cut of a 2 x 10, just an ordinary hunk of spruce lumber. It was fairly straight grained and free of major defects and knots, long enough to get two baffles.



Of course this is solid wood and I've done my share of preaching about not using it for speakers. The biggest reason is the initial shrinkage after assembly and there is also seasonal expansion and contraction that can work to break joints. The wood felt dry but I didn't want to take any chances on it cracking after the speaker is put together so I put it in the "kiln". This is my kitchen oven and I "cooked" it at 300 degrees on convection for 30 minutes. I repeated this cooking 3 more times over the next 3 days. In this time the width went from 9.125 inches down to 8.875 - that's 1/4" shrinkage. I left them in the office for a week to acclimate to the moisture level there before doing any work on them.

Layout and cut the recess for the woofer and do a rough gouging out of the waveguide:



Then lots of sanding. Luckily spruce is pretty soft - sanding it is easy and it doesn't gum up the paper, disks and belts like pine does.

[lazyfly]

I'll be interested to hear about your results with regards to potential cabinet colouration. With luck it'll make everything sound like a classical guitar - but the grain isn't tight enough

Spruce is fairly stable and once you get a sealant on it you wont have any problems.

Have fun sanding out the wave guides!!

[planet10]

Can you post a pic of the tweeters?

dave

[MJL21193]

Originally posted by lazyfly
I'll be interested to hear about your results with regards to potential cabinet colouration. With luck it'll make everything sound like a classical guitar - but the grain isn't tight enough
Have fun sanding out the wave guides!!

Hi lazyfly,
The baffle is nearly 1.5 inches thick so I don't think there will be any resonance issues.
Sanding is all done days ago. I mounted the tweeter to do some frequency response tests to see if I was doing more harm than good and further refined the horn with more measurements in between. It looks good now, measurement wise.

Originally posted by planet10
Can you post a pic of the tweeters?

Hi Dave,
The tweeters were from a pair of old Lloyds speakers, talked about
here. Nothing special but they do sound pretty good and measure reasonable. I thought they would be a good fit here.

[planet10]

Really nice job on the waveguides John.

Those little 2" thimble tweeters really puch way above their weightclass... they can be improved by fitting phase plugs... these experimental ones were made with shortened foam ear plugs.



The idea of wave guide loading them isn't new... you'll on occasion find them attached to plastic waveguides in Lloyds-class speakers, althou i suspect in those cases it was more about marketing & bling than sonics.

dave

[lazyfly]

Really nice work MJ!

Solid timber is the bee's knee's

[MJL21193]

Originally posted by planet10

Really nice job on the waveguides John.

Thanks Dave.

Originally posted by planet10

Those little 2" thimble tweeters really puch way above their weightclass... they can be improved by fitting phase plugs... these experimental ones were made with shortened foam ear plugs.

I considered the phase plug and may give it a try. I have 4 of these so 2 to experiment with. What effect will it have on the response?

Originally posted by planet10

The idea of wave guide loading them isn't new... you'll on occasion find them attached to plastic waveguides in Lloyds-class speakers, althou i suspect in those cases it was more about marketing & bling than sonics.

I did it for looks but also to try to improve the efficiency of the tweeter a bit (it has). I have one completed now, minus the crossover. Working on the crossover design with the DCX2496, making real time adjustments to determine the best point and slopes.

Originally posted by lazyfly
Really nice work MJ!

Solid timber is the bee's knee's

Thanks lazyfly,
Even withe the drying that I did, there's no guarantee that the baffle won't split. To fight the seasonal expansion and contraction I rabbited the baffle to receive the sides, top and bottom and used #20 biscuits to reinforce the joints. Lots of yellow carpenters glue and clamped up over night.
The baffle has been sealed inside and out several time to stop moisture absorption.
I considered just a clear finish for the baffle but got inspired. I grabbed my propane torch and seared the wood, rubbed it down with steel wool and repeated the burning/rubbing until I had even colour and a pronounced grain. A rubbed on coat of mahogany stain and a few coats of clear urethane. It still needs to be rubbed down with steel wool again and waxed to give a satin lustre.

[planet10]

Originally posted by MJL21193
I considered the phase plug and may give it a try. I have 4 of these so 2 to experiment with. What effect will it have on the response?

In theory, more top and more coherence due to halving the size of the resonant cavity formed by the "dish" of the cone. Also gets rid of the cavity (oli can) resonance of the empty space in the voice coil.

I got the idea from Disk Sequerra

I did it for looks but also to try to improve the efficiency of the tweeter a bit (it has)

Should be mostly at the low end of its passband... should also lead to less flare in the dispertion where the tweeter takes over.

dave

[MJL21193]

Originally posted by planet10


In theory, more top and more coherence due to halving the size of the resonant cavity formed by the "dish" of the cone. Also gets rid of the cavity (oli can) resonance of the empty space in the voice coil.

I did it, just need to let the silicone that I glued it in with dry. Quickly done with 1/2" dowel and my drill:



If I like the results, I'll do a better job of a pair from maple then paint them black.

Until I can test the tweeter with the phase plug, I can do no more crossover design.
Here's how the "finished" one looks "in situ". It's the one I have been testing and for it's size, it has extraordinary bass. I put the ports (two, 3cm tubes) at the top of the box. Originally I planned on putting them in the front but it would have ruined the appearance IMO. Having them on top discourages me from piling papers on there.