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Pappa's TL
Pappa's TL
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Old 15th August 2013, 08:20 PM   #1
Jim Shearer is offline Jim Shearer  United States
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Project Conception (the teaser )

Last winter I had some fun using foam core board to build a couple of speakers after reading XRK971’s threads--a Cornu spiral horn using Fostex FE108eS and a little FH3 inspired design using Vifa TC9FD. I found the Cornu horn fatally flawed (for my taste) due to a suck-out in the 100-200 Hz region. The other one with the little 3.5” Vifa was a surprising success: while limited in SPL, it had remarkable bass extension and a sonic presentation similar to a Metronome. I love Mets. I’ve built four pairs of them now.

So as I was plotting my summer build, I knew I wanted a driver larger than the ones I have been using lately; maybe an 8” again. Haven’t used an 8” since I made the Half Changs with FE206e for the kids a few years back. Thought about the FE208eS in a BIB. Then financial considerations pointed toward a less costly option. How about the recently released FF225wk?

I consulted with SWMBO. Not another Met. Maybe a bit smaller than a BIB. Definitely a different veneer--she’s had enough of red oak.

Thinking back to the FH3 inspired Vifa build, I felt that it had a sonic presentation similar to a Met. And isn’t it something like a Met folded in half? maybe... Some years ago when I started building speakers as a way to while away the hours in retirement, I didn’t intend to get involved with design. Then along came MJK, who began to mentor me. Since I lack the creativity to come up with a radically new design, all I can do is mess with prior designs. Some of the ideas are new(er), and (many) other designs date back to the early years of audio. So I started with Martin’s MLTL worksheet, then moved on to his TL sections worksheet.

This design is intended to fulfill my goals: a larger sonic image on orchestral music than the 4 to 4.5” drivers I have used; able to produce 90 dB SPL peaks on orchestral music without getting congested; able to provide an authoritative presentation in the 60 to 120 Hz octave. So it’s not the last word in bass extension; not going to make your ears bleed, but plenty loud for me-- loud enough that I couldn’t have a conversation without shouting.

I showed Martin what I was working on. While I was making progress, it was slow going, making the multitude of changes in the TL worksheet to optimize the design. Martin took pity on me. He knew I was after a specific TL configuration, so he made a custom worksheet for me; with this sheet, all I have to do is define the basic cabinet geometry, then the worksheet calculates the sections automatically for both the basic model in the first half of the worksheet AND the more complicated second half of the sheet which incorporates the details of cabinet geometry as well as room boundary effects. I can now run dozens of simulations with only a few minutes of work on my part!

to be continued....
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Old 15th August 2013, 09:11 PM   #2
fringy75 is offline fringy75  New Zealand
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Hi Jim.

I shall be reading this with great interest
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Old 15th August 2013, 09:26 PM   #3
xrk971 is offline xrk971  United States
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Pappa's TL
Me too... thanks for your kind words about the little Vifa powered Fh3 inspired speaker. The custom worksheets sound fantastic. To be fair to the Cornu, mounting position and height on the wall really impact the bass suck out.
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Old 16th August 2013, 05:21 PM   #4
Jim Shearer is offline Jim Shearer  United States
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The design is an expanding TL with a very slight constriction just before the terminus. Looking at the design and how it behaves in simulations (and discussing it with Martin), I am quite sure that this is a TL, not a horn. The ‘mouth’ (which I am taking as the 8” tall x 10” wide constriction) simply isn’t large enough to act as a horn in the bass frequency range. Hence the name: Pappa’s TL.

With that introduction let me say that despite superficial appearances:
This design is NOT a Frugal Horn! It is not endorsed by either Scottmoose or Dave @ P-10. They have resisted calls to release designs for a Frugal-Horn using larger drivers. Having worked through a similar design, I can see why: the cabinet gets big & clunky AND the low freq extension isn’t what most folks would expect-- at least not with the cabinet size & bass alignment I want to use. While you can push the tuning lower in a moderately large cabinet, you end up needing significant EQ to get the bass SPL to match up with the level of the mids & highs--then you have problems running up against Xmax. Of course, if you want a really huge box, then deeper bass becomes a possibility. Just plug the numbers for the FF225wk into the BIB calculator...

Shown below are the TL profile from MathCAD and a diagram of design:

to be continued....
Attached Images
File Type: jpg profile.jpg (39.9 KB, 1610 views)
File Type: jpg Pappas TL sketch.jpg (43.3 KB, 1631 views)
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Old 16th August 2013, 09:45 PM   #5
xrk971 is offline xrk971  United States
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Nice! That is not that big actually... I am working on a design for a 60 in tall 20 in wide box - so this is small in comparison. OK, we are waiting for the punch line. How many dB and how low does it go?
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Old 17th August 2013, 12:44 AM   #6
rjbond3rd is offline rjbond3rd
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Ooooooo, very interesting. And how characteristically generous of Martin. I can't wait to see where you go with this!
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Old 17th August 2013, 02:52 AM   #7
xrk971 is offline xrk971  United States
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I wonder if it will look anything like this... (speaker placed 16 in away from back wall).
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File Type: png PappasTL-Freq-1m.png (26.3 KB, 1524 views)

Last edited by xrk971; 17th August 2013 at 02:54 AM.
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Old 17th August 2013, 03:43 PM   #8
Jim Shearer is offline Jim Shearer  United States
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Forgot to mention in the last post that the enclosure is 10” wide internal, so with 3/4” ply the box is 11.5” wide external.

And now: Here are some of the graphs from MJK’s MathCAD simulation of Pappa’s TL.

You may note that the TL is tuned to 54 Hz, despite the FF225wk having an F(s) of 44 Hz. As mentioned previously, if you tune lower without making the enclosure larger, then the SPL drops at low frequencies. My preference is to keep the box more modest in size and give up some bass extension. Hoffman’s Iron Law in action.

And before anyone asks: don’t bother putting other drivers into this cabinet, the results will most likely be disappointing. The only one I checked which wouldn’t fall on its face is the F200A--it would sound OK, but should really be put in a cabinet tuned to get the most from it rather than settling for merely OK. The FE206en and FE208eS don’t work.

Measurements will be coming, but at the moment things are a bit busy. After Labor Day, Martin will bring his PE Omni-Mic over & have a listen.

Graphs:
-- acoustic impedance
-- SPL (from top of worksheet)
-- electrical impedance
-- woofer displacement
-- in-room SPL
-- impulse response

to be continued...
Attached Images
File Type: jpg acstcimp.jpg (51.2 KB, 1461 views)
File Type: jpg spl-1st.jpg (46.6 KB, 1445 views)
File Type: jpg TL-imp.jpg (37.3 KB, 191 views)
File Type: jpg woofdisp.jpg (38.4 KB, 78 views)
File Type: jpg spl-room.jpg (45.0 KB, 147 views)
File Type: jpg impulse.jpg (36.3 KB, 140 views)
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Old 17th August 2013, 05:28 PM   #9
xrk971 is offline xrk971  United States
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Nice work there Jim. So it sounds like you have actually built it? AkAbak came up with the same tuning freq and I like how you are keeping the SPL high rather than push for low extension - 54 Hz is good for most music. Placement from wall seems to be a critical factor. How far back is the wall in your in room simulation?
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Old 17th August 2013, 09:21 PM   #10
Jim Shearer is offline Jim Shearer  United States
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Hi X,

IIRC, the placement is center of the baffle 3' out from the corner on a 45 deg line. (It's whatever the default in MJK's MathCAD worksheets is.)

More about the in-room response tomorrow.

And yes, I have already built it.

Cheers, Jim
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