With deadlines coming closer I finaly have made some progress with Sleipnir. The Whardedale SFB that got me going has several features I do want to change. Insted of having one baffle as a slate I intend to use a three baffles: one for the four bassdrivers with a lot of bracing, standing straight up. One baffle for the midrange driver, angled slightly upwards and with no bracing but with bitumen lamination like the Spendor BC-1. And lastly a third panel angled 45 degrees upwards for the tweeeter

This is a prototype with only two bass drivers I have tried out how to mount them and also cutting out holes with a router, this time with some scrap material below and also to round of the cutout with a router bit.  I am not sure how to place the vertical side panel, well inside the front panel edge as the SFB or covering the edges of the baffles. The finish will be very vintage looking...

All alnico drivers now, I plan to use 16mm MDF and rely on laminations and extensive bracings rather than having 2x25mm MDF,

Even this small baffle lowers the Fr of the drivers.

I added a 4.7ohm serial resistor and a 3.2 ohm paralell  resisor and remeasured with both phases tried out.

The orange one is the the response with the Seas H86 tweeter in the right phase. Reversing (blue) is far inferior and the brown line with no tweeter.  The response as measured on axis from the tweeter at about 75 cm distance shows a decent respsonse. Flat to 10 kHz and a falloff above that without any nasty peaks. Padded down by 10 dB the tweeters are well protected from abuse.

I also tried adding a zobel filter to the woofer to see if it changed the filter resoponse but it is more or less the same so I will not add one. Time to wrap up this one by finding some screws and assemble the speakers, and move on to the next project.

First tweeter trial
I padded down the Seas tweeter by 5-6 dB and tried both phases and none is that good

The black trace is without tweeter. I am surprised at the lift in the top end of the midrange dome. The red trace is with the tweeter in phase with the midrange and the combined output is way to high. The blue were they work out is better in the overall level but the dip at 5 kHz is bad. The measurements are bit rough so I will try to increase the attenuation to 10 dB and see how that pans out. If that does not work out I will try some other tricks with phase tracking around the crossover frequency.

The H86 is not the best of tweeters but  I have four of them and they fit perfect in the cutout for the original tweeter. The Philips tweeters looks like obvious replacement as the rebate does not match with the diameter, so I try to tweek the Seas before falling back on the Phillips, and make some more stringent measurements.

The bass
The drivers are mounted with the boxes filled up with bonded acetate fiber, I do not like to work with fiberglass. The system now have a Q of close to 45 Hz and a Q of  about 1.1. A very good candidate for capacitor coupled closes box. Simply inverting the phase gave good integration with the midrange. Frequency response is on axis of midrange driver at a distance of 75 cm so the room has effect on the response below 400 Hz or so. A nearfield measurment of the  bassdriver show response down to 50 Hz as expected the low pass is a bit earlier than expected. I will make a Zobel filter and see how that affect the interaction between the crossover and the driver. The drivers intrinsic response is flat to 3000 Hz.

The black line is with the driver in phase and the other two is the two loudspeakers. Very good matching.

The Tweeter
Both tweeters are blown, I have some vintage H86 Seas tweeters that are a drop in replacement for the drivers as far as cutouts are concerned. Frequency wise it is far from a drop in replacement. The output is about 10 dB to high. The proper replacement is a seas 19mm metal dome but I will try my hand at the vintage drivers first. If they are padded down 8-10 dB they ought to be very well protected from further misshaps. To bad about the tweeters.

 Progress report
Today I have coated the gray cones of the Seas 21TV-EW woofers with three layers of Visaton LTS-50 speaker compound. Not that the cones need damping, the do not, but the allmost black semiglossy color fits better than the matte gray cone color in original. The drivers had been glued to the old boxes with some white glue. Scraping of that stuff has also removed some of the black color of the chassies so that has been painted. I also need to find some more black screws.

Tomorrow I plan to play around with the Zobel and phase to get a decent match with the midrange driver. Then I will also try to measure the individual drivers. I do not dare to desolder the crossover from the drivers after ruining one ScanSpeak tweeter. Lastly I will add more fiber filling to the cabinets until I reach the lowest resonance frequency and lowest Q. And then I am done with the first pair of those speakers.

Then I might return to my open baffle project Sleipnir...

Kirksaeter Monitor 121
I was asked to try resurect a some loudspeakers. Made in West Germany dates them pretty well.

The familiar foam rot had set in but the midrange and tweeter appeard to be in good condition.

The woofers were open circuit and a dissection suggested that the voice coil had been wobbling around (scrape marks on the bobbin) and causing a break in the coil.

The woofer is a Seas but the model is hard to make out, the sticker printing is really faded

The boxes are well made sturdy and filled with some fiber glass at the back and top. With a high Q driver some improvement can be had by adding more fiber glass or other damping material.

The crossover seems to be quite resonable, probably a mix of 6 and 12 dB filters.

The impedance curve is taxing. Here I had a Seas 8 Ohm woofer replacing the orignal woofer.  The impedance is below 3 Ohm from 150 to 400 Hz the range where music has it's highest power demands. At 6 kHz the impedance dips to 1 Ohm! That is really low but the energy content in music is much lower there so the demands should not be that bad.

With some more damping material added I hope to lower the Qt below 1.0 and the Fr down in the 45 Hz range. The integration between the woofer and midrange might be tricky but I will try to keep the original crossover and use just a Zobel filter and phase switches to get the work done.

I read about the Wharfedale SFB long time ago but thought it was an outdated design. Then I read Troels Gravesens new interpretation of the design and got intrigued. I have heard rely large open baffle speakers sounding good (Audiotronic Megatrend with each speaker having 18 KEFB200 8" drivers and a baffle area close to 2 m2) I have a scruffy packing crate plywood sheet 1200x1600x12mm. I have eight Seas alnico 10" woofers with a Qt of 0.7 and Fr of 34 Hz. And several other Seas vintage drivers. To that I bought 12 meters of wood with the cross section 120x21mm to the cost of about 15 dollar.

I cut the board in half and cut holes for the drivers, then I added a 120 mm deep frame around the perimeter of the boards as well as some extra braces with leftovers.

I screwed the driver in place with 104 screws!
Added a crossover consisting of electrolytic capacitors from West-Germany.
So 4 Seas 25TV-EW per side then 1 Seas 21TV-EW capacitor coupled and finally  3 of the Seas 9TV-LG that is mounted on a metal bracket angling the outer tweeter about 15 degrees to the side.

Mounting the drivers on the baffle lowers the resonance from 34 to 28 Hz. Two impedance peaks at 55 and 70 Hz turns up as well

A Omnimic measurement just in front to one of the dust caps show the dips in response at 55 and 70 Hz otherwise good response from 30 to 300 Hz.

Listening test
Room 6x4 m with the speakers on the standing about 1 m in front of the shorter wall.
Equipment: Philips CD473 NADC320 BEE, the cheapest cables and loudspeaker cables with visible oxidation.
Reference equipment: Maranz CD5001, Cambridge C500 SAnsui BA-F1 Stridbeck TLS (think IMF80) Decent cables.

Music  NADs first test CD, BIS CD-232 Kroumata Percussion Ensemble, Laleh and Rammstein.
I have heard the KODO drummers from the NAD cd and the Kroumata group several times live as well as heard it using my large TLS speakers. I had little expectations for these flimsy baffles.

To my surprise it sound really good, the baffles connect to the room in a way that I last had at home with DIY Klipsch corner horns. The piano on the NAD CD is in the room more so than how it sound with the large TLS. The percussion on Kroumata and Kodo is better with the baffle closer to what I remember of live sound. More than just bass there is a bass that gives space and volume to the music.

Lalehs singing and acoustic music is portrayed well. Finally Rammstein at close to concert level to get that physical experience. Loud and clean and only about 2mm cone motion. Probably not much bass below 50 Hz or so. The baffle vibrated a lot as did all the braces.

Conclusions:
I have to build proper baffles, a modern interpretation of the SFB. I will not have a sand filled baffle as the original Wharfedale nor have 2x25mm plywood as Troedsen. But some lamination of MDF and bitumen as suggested by Hobby HiFI.  Drivers from Norway, eight of them per set adding the kick, the name of the speakers has to be Sleipnir.

I had one final nagging question, does a final 90 degree bend reduce the 5th harmonic at 200 Hz? So at long last I cut out a hole of 190 cm2 in the back board and sealed the bottom of the box.

Note the opening in the narrow back below the driver.  Below is the impedance curves with the two different openings.

 Frequency response at the pipe end does not change. The measurements are months apart and the old one (blue) with the speaker at a wall and the new with the speaker in the middle of the room (green). That 200 Hz peak sure is persistent!

Then I have a new pair of Voigt pipes, designed by late Bosse Hansson of Opus 3 Records. They are designed for Lowther, with 19mm chipboard as the building material and 2x 19mm in front an back they are very different from the flimsy Deccas.
Tuning of the pipe is similar.

In both cases I used a Peerless 8" PP driver that works well in the pipes

Damping inside the pipes are similar with felt covering the walls of the first half of the pipes.

Listening to Adele 21 and some other records the pipes sound quite similar. Playing loud bass and the decca vibrates a lot not only the large front baffle but also the narrow top plate. The other pipe does not vibrate at all or very little.

So that 200 Hz peak is not tamed with a new location for the pipe opening. Nor does the flimsy constructions cause the sound to radically different compared to a massive pipe of similar dimensions. The proper reference would be to build a second DCH but in much thicker material as baffle loading and other factors comes into play here.

I tried adding braces of OSB or chipboard, screwed and glued see pictures. I measured the pipe output and it was no different than the unbraced.

The original HFN&RR article claimed that the removal of the brace for the front panel was a improvement. That with a 9mm plywood baffle. Here I ended up with 11mm OSB-wood glue-11mmOSB-silicone glue-12mm asfaboard.

So to conclude
1. In its original state it works far better than I expected
2. Covering the first half of the pipe in felt helps a lot in the time domain by reducing internal reflections around the cone. It probably also give some panel damping as well.
3. The the bracing shows that the original articles claim that the damping is provided by the thin-walled panel vibrations is not true. Building the cabinets in 12mm MDF or even thicker and more wellbraced cabinet should work well. The well behaved original design is more due to geometry and many non paralel  walls than the walls functioning as absorbers.
4. Moving the pipe opening to the back of the cabinet does not change neither bass output nor harmonics. Having the opening facing a stone slab or a thick carpet makes no difference.
5. It sounds good, it looks cool and the backward facing position works well with many fullrange drivers that sound forward and shrill when facing forward.