The Sleipnir

The history

The inspiration is the Wharfedale SFB. A damped open baffle roughly quadratic in shape large surface area for the woofer one largish midrange and tweeter facing upwards. Using what I have on the shelfs meant some modifications.

Design philosophy

My design is a homage to the SFB using as much as possible of what I had at home. I have tried getting the same well-damped baffle as the original but with a technique that fits me and also results in a lighter baffle. I found the original baffle complicated to build the big sloping slate to deep and room dominating. An upright woofer baffle reduced the depth but the baffle then looked really boring. A curved baffle would be fun but that is above my wood working skills. So I made it in the three sections angled gradually upwards.  Originally I intended  to paint the front white Hammerite to get a retro look and also to get a sharp contrast to the black silicone strips. Then I and the wife saw a AudioPro speaker in glorious orange or fiery yellow color. I built some prototypes of slice of weatherbeaten 12 mm ply that I had. Cutting it in half resulted in two 120x80cm  boards that were braced with some pine planks. Mounting all the eight 10" drivers as well as the midranges and a single cap crossover resulted in some interesting observations. The bass was far deeper than I had expected it also had a slam I had been missing since the Klipsch horn days. Using  Rammstein I tested loudness ability and it was plenty. 

Hardly surprising the baffle vibrated badly very badly in fact. 

The drivers

 All drivers are vintage Seas Al nico drivers with paper cones including the tweeters. The 12" and 10" bass driver is replaced with four 10" drivers  increasing the bass speaker area from 800 something cm2 to 1300 cm2.  The drivers also travels quitly. Some drivers I have tested have spiders that buzz at large cone movements. With a long voice coil overhang and pliable rubber surrounds a Qt of about 0.7 and Fr of 27 Hz  the stage is set for generating deep bass. With eight drivers power handing should not be an issue. Avoiding signals below the baffle range has to be taken into consideration. The baffle loads the driver lowering the bass resonance to  xx Hz.  I suggest either having a good bass filter or using a serial cap especially if using a record player as LPs sometimes generate subsonic noise. The Midrange is handled by a much smaller 6.5" than the original 10" but then it only handles midrange eliminating large cone movement and greatly reducing power requirement. The tweeter is a set of three 3" set at different horizontal angles. I have some additional cone tweeters to add if I need some more sparkle to sprinkle at the ceiling or back wall. 

The baffle

A sand-filled baffle is to complex and to heavy for my taste. A open baffle has more promblems with vibrations than a box. A brute force solution of double 25 mm MDF would be very heavy and perhaps inferior to my solution  (no ofence meant Troedsen ;) )

I have read numerous claims that open baffles intrinsically are less prone to vibrations and resonances than boxes. I should argue that the opposite is true. Within its working range the baffle is exposed to as high pressure differences as any box.  And an open baffle has both sides radiating outward. Secondly Newtons second law apply equally to baffles as to as it does to boxes.  

 So numerous approaches were used to deal with the the problem. The square were cut up in three subbaffles that were connected by silicone joints. The back braces that are at the very side on the SFB were moved in by 15 cm on each side. Sliding in the back legs leaves the edges unbraced but what can I do!  The large woofer baffle is extensively cross-braced some bitumen is also added. The main idea is to push the resonances above the working range. What look like 32 short corner braces are meant to be shock absorbers. The short end is glued with a stiff epoxy glue to the MDF board holding the bass driver. The long end is glued with lossy acoustic grade silicone glue to the stiff cross-braces. (The rest of the cartridge was used in the midrange baffles.) Here the four woofers are very close to the cross-braces and bracing of the back legs. The midrange use another principle. No braces are used but extensive lamination is used. As the other baffles it is made out og 16mmm MDF silicone glue was used to add a 12 mm soft bitumen loaded soft board on top of that a 4mm bitumen layer and finaly 10 mm of felt for a total thickness of 32/42 mm. I have some foam-bitumen-foam damping mat at hand but in this neovintage setting it was just wrong to use.  The aim was both to get the resonances below the critical midrange and also minimize the acoustical transparency of the baffle. While the woofer section is vertical the midrange slopes back about 15 degrees. Finaly the tweeter baffle is angled 45 degrees upwards it is laminated with  some felt. So in total the baffle approximates a bent shape. All drivers are rearmounted the woofer openings were not chamfered for strenght reasons. The mid and tweeter cuts were chamfered 45 degrees. I used HiFiKit sealing strip that is flexible and also collapses to form a seal with very little bounce compared to weather sealing strips. 

The crossover

The SFB use single electrolytic cap in series with the tweeter.  Sleipnir use a more elaborate crossover. Initial tests used a analog active crossover to get an estimimate on were to have the crossoverpoints and the relative sensitivity of the drivers. Suggested slopes are on the 12-18 dB/octave range. 6 db is to shallow to give useful protection in many cases and 24 is complicated to get right. The tweeter and midrange has a resistor across the terminals with a value of twise the driver this will give substantial reduction in the peak at the fundamental reaonance improving the crossover action in this range. 

A more compex LCR filter can be used to equilize the peaks of the midrange and tweeter drivers.  In my experience the filter values are not critical as even very approximative corrections are better than no correction at all. 

Construction details

1. The seam between the woofer baffle and the floor is sealed with a rubber strip to maximise bass extension. Otherwise this leak would shorten the acoustical length around the baffle. 

2. The crossover boards situated behind the tweeter and midrange baffles are used to break up reflections between the two paralel back legs and also to break up the left to right symetry of the backside of the baffles.  

3. Speakon connector were used cables and crossovers are of decent quality but nothing fancy. More along the line of sound engineering than waste on audiophile grade. 

4. From a mechanical point of view I would have prefered attaching the drivers with bolts but having the fronts dotted with close to one hundred bolt heads was out of the question. The simplest solution is to use ordinary wood screws. A more elaborate way would be cxx glued into the baffle. 

Now on to the actual building!

I routed the cutouts for all 8 drivers in a rather cold garage today.

This is how front mounting the drivers would look, it looks cluttered!

With rear mounted drivers it look cleaner.

Tomorrow I plan to go for the midrange driver, I will just have to get a longer center bolt for my router. The one I have just clears a 16mm board and I also need some better light as I will not only route the holes but also chamfer the edges at 45 degrees. I better have some trial runs in some scrap before the real ones...

The driver cutouts went well, with a longer center bolt. The chamfering not so good. Next up are rectangular cuts for the tweeters.

This is the full with midrange baffle. The 6.5" driver will get additonal baffle loading  from the bass driver baffle below and the tweeter baffle above in the final design.

Here is a closer look at the chamfering.

Here are the routed tweeter baffles. I had the advantage of sunlight with the garage door open, on the other hand the temperature was -12 C.
The routing in straight lines went OK. It was tricky to time the stops and I also discovered that trying to fix a minor residue a the bottom after routing by using the router on free hand was a bad idea...

The seas cone drivers goes down to 1000 Hz at least the two side cones are angled outwards. I will do the chamfering an other day both fingers and toes were cold in the end.

I end with a mockup of how the three panels will come together.