Single sheet TH challenge

We used 3 more sheets to make extensions for the 2x2 stacks, but for 4-8 db gain (curves for 1 box, 2 boxes and 2+extensions are attached), an extra 3/4 of a sheet per pox makes for a good sound to material ratio :)

They also unclip and pack flat...

It allows a pair of ss15s to hit a 45Hz corner, bumping up 40-45hz response by about 7-8db over a pair of subs with no extensions.

We came to the shape after a lot of experimentation with bits of ply and measurement mics, plus deciding on 600mm maximum extension from the front.

I can post sketches/plans if anyone else is keen, or just experiment yourself!

FYI, forum member Art Welter did something very similar with his keystone sub, and got equally impressive gains :

Richard
Richard,

Nice work!
Your extender adds more path length and is larger relative to mouth size than the ones I use, adding more gain and also lowering the Fb. Mine were designed to assist the low mid in my system, the extra 3 dB they provided down to Fb was a bit of a surprise.

The great thing about the added 4 dB (and lower frequency extension) an extender provides is it is not subject to power compression.

The SS15 has nearly 7 dB of power compression at 700 watt input compared to one watt, the extenders allow reaching a higher output level with a fraction of the power.

Did you happen to try the extender expanding from the full frontal area of the SS15, rather than just the mouth area as pictured?

Art
 
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Jbell_SS15_%25252B_Extension_idea_03.jpg
 
I'm not anywhere close to my shop, (it's graduation weekend for family member) but from memory.

The last ss15 layout I posted is 107" total path length. 3.5" to start at S1, 4.5" upper left, 6.5" upper right, 9.25" bottom right, 16" opening. Should be able to reverse engineer the hornresp inputs from there.

H and E are about 1" apart
 
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Hi r23d,

Post #1518: "...I can post sketches/plans..."

Just the dimensions (size of the opening and depth of the extension) would be nice.

And, what was your measurement setup?

Regards,

The inside dimensions of the opening are 1110mm high by 1670mm wide, and the front edge is parallel with the box 600mm out.

The graph I posted should only be taken as a relative comparison, because we did not use an SPL meter to calibrate outputs to 1w/1m or anything so useful. For the graph posted, I used a behringer measurement mic (ecm8000 I think) about 2m away on the ground in my reasonably large backyard. I expect the results are a bit lumpier than we would get in a nice open area.

FYI we're using Room Eq Wizard v5 software to do sweeps - free download from Home Theater Forum - Home Theater Systems - HomeTheaterShack

To work out the best shape for extended response, I made some big guesses using hornresp, and then we expermented by sitting one sub on it's side with a board on top, placed side panels in at various angles and sat on them :)

Not hugely scientific I know, but we tried quite a few shapes quickly this way, and this setup gave us pretty good results.

Other tidbits :
- We wanted to keep the mouth open for simplicity and visual appeal, so I'm using aluminium channel to stiffen the edge.
- The top and bottom board do vibrate at bit much for my liking, so a brace is coming.
- The boards are butted up using adjustable over centre clamps with sponge rubber between to stop massive vibrations.
- Starting the flare at the horn exit gave us smoother results than using the full box width to start our extension from.
- From memory, the 170hz drop was less nasty with our test boards on a single box.
- We're using secondhand RCF lp200 drivers
- I didn't expect such a gain at the bottom from my guesses in hornresp, so will be testing in a park soon to see if this is a measurement anomoly from my backyard fence or something.

cheers all,

Richard
 
Richard,

Did you happen to try the extender expanding from the full frontal area of the SS15, rather than just the mouth area as pictured?

Art

Yes Art, we did try using the full frontal area to start the extension from, and against my wishful thinking, the results weren't quite so good. Not as smooth, and not as much output, though I'm sorry I don't have any figures right now. I'll see if our audio guru Phil still has Holm impulse plots on his laptop.

I was really hoping it would make little difference, because building the extension would have been much easier, but the odd asymmetric shape has kind of grown on me.

We have almost finished the other pair, so soon will have plots of a matched set each side...

Richard
 
Horn extensions:

Each time you extend a 1/4WL horn or tapped horn by: 0.6 x diameter the mouth, the new mouth area of the extension needs to be doubled. For the SS15 that is easy since the total front of the SS15 is very close to twice the area of its mouth:

The mouth area on the SS15 = 2168cm^2
The total front of an original Jbell SS15 is 4267 cm^2

The mouth is 2168 cm^2. If you calculate that to a circle you get a diameter of 53 cm.
53 x 0.6 = 31.8 cm

Therefore the extension for the SS15 can be made max. +/- 30 cm long (if the extender follows the outlines of the SS15 like my example in post#1525)

Smuggling:

In practice you sometimes can extend the front a little longer, depending on the driver's response. In my previous example of post#1525 we smuggled a little by using bracing to prevent internal resonances within the extension. The horizontal braces, covering the last 10cm of the extension, were used to prevent internal standing waves (A similar approach was used in the Stage Accompany XL-bins for instance). The vertical brace was mainly used to enforce the structure, to support the braces and had cut out sections (not visible in the example).


Building a 1/4WL horn or tapped horn from rectangular sections:

If you want to build a 1/4wl horn or tapped horn and use it to maximal its full wavelength you can build it from 6 rectangular sections. If you want to use it up till 1.5 its full wavelength you can build it from 9 rectangular sections…and so on.

The total pathlength of an original JBell SS15 is 227.54cm.
In case of r23d, he extends it with 60cm

227.54 + 60 = 287.54 cm
The new full wavelength is therefore 119.5Hz

If you want to use it up till 119.5Hz you can build it from 6 equal rectangular sections. 287.54 cm : 6 = 47.92cm

Therefore in r23d’s case, the extension of 60cm is way too long to be made from one rectangular section.

(Edit: See Jbell! It's 8:46 am here and way too late so I hope I didn't make to many mistakes in my calculations :faint:)
 
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Hi jbell,

Post #1526: "...H and E are about 1" apart"

That sound a little tight. Could you (or anybody who has a 3015LF on hand) take a look at the attached drawing of a 3015LF, and make whatever corrections to the dimensions? I don't have a 3015LF, and thus have to go from pictures, and by Eminence's data sheet specs.

Regards,
 

Attachments

  • 3015LF_preliminary_notes.pdf
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Yes Art, we did try using the full frontal area to start the extension from, and against my wishful thinking, the results weren't quite so good. Not as smooth, and not as much output, though I'm sorry I don't have any figures right now.
Richard
Richard,

Thanks for the reply.

Proximity to walls and houses etc. still has a large impact on outdoor LF testing.
Were both the extender measurements and the dual cabinet only measurements taken in the same exact place and orientation?

Was the mic the same distance in front of the extender as ist was from the front of the cabinet?

Art
 
Richard,

Thanks for the reply.

Proximity to walls and houses etc. still has a large impact on outdoor LF testing.
Were both the extender measurements and the dual cabinet only measurements taken in the same exact place and orientation?

Was the mic the same distance in front of the extender as ist was from the front of the cabinet?

Art

Good point Art - the measurements were all taken without moving subs or the measurement mic, so this means the speakers were closer to the mic by about 600mm with the extensions on. That would be 2m away with two subs, 1.4m away with the extender added.

If this means the speakers were closer by a factor of 2/1.4=1.43, then should they be 1.43*1.43=about 2 times louder, or about 3db anyway?

This puts my gains in a more conservative light.

Once I have all four working, I'll be heading out to a more open area for more testing at bigger distances and let you know ho I go.

cheers,

Richard
 
Good point Art - the measurements were all taken without moving subs or the measurement mic, so this means the speakers were closer to the mic by about 600mm with the extensions on. That would be 2m away with two subs, 1.4m away with the extender added.

If this means the speakers were closer by a factor of 2/1.4=1.43, then should they be 1.43*1.43=about 2 times louder, or about 3db anyway?

This puts my gains in a more conservative light.

Richard
Richard,

Once the measurement distance is within about the diameter of the radiating surface of an enclosure, the inverse distance law (6.02 dB loss per doubling distance) will not apply exactly, the so called “horn bubble” makes for different levels in the very near feild.
If I understand your mic placement, 1.3 dB at minimum would need to be deducted from the closer extender measurement .

As large as your horn extender is, 2 meters from the front of the extender, and the same distance from the front of the cabinets would be the minimum distance to get a decent comparison. If you can find a large enough area with any wall or building at least two wavelenghts of the lowest frequency of concern (around 56 feet for 40 Hz) tests at a greater distance than 2 meters will give more accurate comparative results.

A 3 dB difference is the equivalent of doubling power, assuming no power compression. Sounds like your extender probably does about that, still a good deal.

To sound twice as loud at 1000 Hz requires a 10 dB difference, at 20 Hz a 5 dB difference, at 50 Hz roughly a 6 dB difference.

Art
 
Hi

Phil here...R23d`s partner in crime making the SS15`s with barn doors.

I have had a dig around my laptop and I think I have found the measurements for barn doors off the horn and off the end of the box.

Barn%2520doors%2520end%2520%2526%2520horn.jpg


So green is off the end of the box and red is off the horn. This was done in a large warehouse with a concrete floor. Measurement mic on floor. I still don`t believe Holm at the low end, it just doesn`t seem to relate to real life? Would like to try this all in Smaart when I get a) the time and b) my work measurement setup !!

Top respect to Mr R23d ( neighbours and family) as he did a few days continuous measurements in his garden of these SS15`s and the Unity horns we`re building!

Phil B
 
Hi

Phil here...R23d`s partner in crime making the SS15`s with barn doors.

I have had a dig around my laptop and I think I have found the measurements for barn doors off the horn and off the end of the box.

Barn%2520doors%2520end%2520%2526%2520horn.jpg


So green is off the end of the box and red is off the horn. This was done in a large warehouse with a concrete floor. Measurement mic on floor. I still don`t believe Holm at the low end, it just doesn`t seem to relate to real life? Would like to try this all in Smaart when I get a) the time and b) my work measurement setup !!

Top respect to Mr R23d ( neighbours and family) as he did a few days continuous measurements in his garden of these SS15`s and the Unity horns we`re building!

Phil B

Something really looks a bit off there. Did you do an impedance response test on your SS15 to see if it was a good match for the HornResp model?