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Old 13th September 2013, 04:06 PM   #1
DrBoar is offline DrBoar  Sweden
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Default Building &Testing Decca Corner Horn

With the summer finaly coming to a close, I thought I do some work outside before the rain and cold sets in.

The square cuts from the lumber yard.
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I then followed the buildning instructions in the HFN &RR article as refered to in prevous posts. Adding some guide rails in softwood for on the sides
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I then seald the corners with a latex. Many small corner fillets were added after the latex sealing.
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Finaly the dividing panel was glued, screwed and sealed in place. Some of the other panels were cut to shape and some oddly shaped braces were made.
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Now the whole thing will sit over night and set glue and sealant. I will have to make some 45degree edges things I have not done before. So I think I call it a day and rehydrate myselft with a beer

I think it will give some horrible resonances as is is totaly devoid of any damping material, but it will be fun trying to tame it, I hope!
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Old 13th September 2013, 06:09 PM   #2
IG81 is offline IG81  Canada
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Pretty cool, you always have fun projects going on! I guess you chose OSB as an inexpensive "prototyping" material? What driver will you use in there - I think I remember reading Philips 9710 in some other thread?

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Old 13th September 2013, 10:09 PM   #3
DrBoar is offline DrBoar  Sweden
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9710 for sure, some car triaxial, some Peerless bass reflex driver, perhaps also Fane 8M Studio, KEF B200 to get both drivers that should work and those that should not. Beech Plywood is six times the price of OSB. I think OSB is closer structure wise to Plywood than MDF is. If I ever build a real pair I would, at least now, think that I would do them in MDF 12 or 16mm and then have a nice front cover, be it a mirror or plywood.
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Old 18th September 2013, 07:18 PM   #4
DrBoar is offline DrBoar  Sweden
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More braces than a group of teenagers
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The Philips 9710 and the cutouts for the driver, including the one to clear the magnet
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This is a size comparison with a large TLS. Weigth wise the Decca is a comfortable one arm lift the TLS is not!
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I was worried that the slot in the bottom would be affected by the floor so I measured the impedance on hardwood floor, concrete floor and a thick rug on top of hardwood floor. Not much of a difference. The pipe is tuned to 50 Hz and the harmonics peaks seem to be quite small.
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The right way around for some music
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Hardly an ideal setup but here I go.
Way more sensitive than TLS, the bottom octave is missing when I compare with the Tyrland with a fundamental resonance at 20-25 Hz.

With the pipe totaly empty and with such thin walls I expected a lot of boxy colorations. It sounds really good! what is going on here? Playing loud bass and moving my hand over the various surfaces there is not much vibrations apart from the lower third of the front.

That small IMF folded TLS I built using a modern very good Peerless PP cone driver sounded horrible with empty pipe. This one with a shouty 9710 sounds really nice.
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Old 18th September 2013, 07:45 PM   #5
xrk971 is offline xrk971  United States
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Nice work Dr Boar. Sometimes a big empty box can sound nice. the shoutyness is reduced by back firing driver. The large surface area and high volume serve as built in damping? If you can get me the detailed CSA vs distance data I can model it for you and we can try to see what is going on. Very cool.
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Old 18th September 2013, 10:51 PM   #6
Squeak is offline Squeak  Denmark
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So the high frequencies drBoar? That's the main attraction of these things. Does reflecting sound of the wall work, and is the angle for that good in this design?

Last edited by Squeak; 18th September 2013 at 10:55 PM.
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Old 18th September 2013, 11:44 PM   #7
sreten is offline sreten  United Kingdom
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Hi,

My first speaker many years ago was the real Decca. No internal damping.
Driven by a Tripletone mono small valve amp of probably high impedance.

It rocked, big time, even given my 16 year old sensibilities. The very
thin walls damped and resonated at the same time, pretty unusual.

As ever the driver ended up being blown, from about 5 distorted Watts
and being fairly clueless I never managed to fit a driver that got in any
real way close to the original 8" driver, but FWIW my opinion many
years later is the Visatoin B200 is your best bet for a Decca clone.

rgds, sreten.
later is the
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Old 18th September 2013, 11:45 PM   #8
sreten is offline sreten  United Kingdom
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Hi,

My first speaker many years ago was the real Decca. No internal damping.
Driven by a Tripletone mono small valve amp of probably high impedance.

It rocked, big time, even given my 16 year old sensibilities. The very
thin walls damped and resonated at the same time, pretty unusual.

As ever the driver ended up being blown, from about 5 distorted Watts
and being fairly clueless I never managed to fit a driver that got in any
real way close to the original 8" driver, but FWIW my opinion many
years later is the Visaton B200 is your best bet for a Decca clone.

rgds, sreten.
__________________
There is nothing so practical as a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow
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Old 19th September 2013, 06:52 AM   #9
DrBoar is offline DrBoar  Sweden
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Of 48 corners in the house only one is a proper horn corner.

I took a fast set of measurements with the Decca in the concrete corner (green) moving the Decca out on the floor Increase the impedance peaks as well as moving the tuning by 2 -3 Hz. The impedance peak at 180 Hz is moved up to 210 Hz.

Flipping the box over increass the tuning by 10 Hz from 45 to 55 Hz as judged by the zero phase transistions. The other peak is at 230 Hz or so.

When doing this measurements I listened to some music in the background. I got variations in the impedance curves, to verify the interaction I turned the volume up and sure enough the measurements with the decca in the corner whent haywire!

Click the image to open in full size.

Last edited by DrBoar; 19th September 2013 at 06:52 AM. Reason: forgot the graph
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Old 19th September 2013, 02:19 PM   #10
DrBoar is offline DrBoar  Sweden
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Some preliminary frequency response measurements. Made with a hand held mike perpendicular to the driver and the capusule just in front of the inner double cone. This was the only way I could do the measurement in the corner.

Black curve is with the pipe filled with damping material from the two crossbraces of the front to the open end of the pipe. It was hard getting rid of the lower impedance peak of the pipe, needed lots of stuffing.

Red it with empty pipe on floor There is a peak at 40 Hz messing up the fundamental dip. Now it looks to be at 50 Hz but it could be towards 45 Hz. There is major disturbance at 200 Hz and some minor ones at 120 and 300 Hz.
Click the image to open in full size.

These pipe outputs were measured just inside the middle of the front shelf between the two feets
The black curve is the corner one that seems to do wonders for supression of 5th and 7th harmonics.
The red curve is in the room on hardwood floors brining up the harmonics by 10 B or so.

Brown curve is on thick carpet and that seem to attenuate the 40 Hz output by some few dB and then not doing anything until above 1 kHz that seems to be better than the hard floor.

Click the image to open in full size.


The third harmonic seem to be well taken care of. If I by some gentle damping around the driver and in the first section could tame the higher harmonics.This is one interesting speaker.

Ralph West recomended the Lowther 6A, the Visaton B200 have the same frequency lift as Lowther and 9710 but the Qes is even higher than the 9710. It would be interesting to see how the Decca handle a modern bass driver with heavier cone and longer stroke but that will come at a later date.

Last edited by DrBoar; 19th September 2013 at 02:38 PM. Reason: speling ellorr
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