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Old 30th January 2013, 01:42 AM   #1141
IG81 is offline IG81  Canada
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Driver is FF85WK. The pipe tuning is based on the factory number for Fs, 115Hz - I ought to measure it for real in a serious build. Maybe I'll make a wood version come spring. Cross-sections for the three segments are 42cm^2, 62cm^2 and 87cm^2 respectively. The start and end of the line are pretty much that of the F-81 and I just went with a 1:1.4142:2 ratio for the central segment.

At 12ft, the F-81's 5/4 mode would fall close to driver Fs, but probably be too low in acoustic impedance to make much of a difference. Who knows, maybe even my 3/4-mode loading does not do much! I wonder what was the rationale behind the F-81 design.

IG

Last edited by IG81; 30th January 2013 at 01:46 AM.
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Old 30th January 2013, 12:53 PM   #1142
xrk971 is online now xrk971  United States
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IG,
I was just noticing that the overall aspect ratio of your F-81 box is very thin - could be wall mounted if you just put the driver on the big panel at the top corner. You would have to slightly modify the plan to allow clearance for the driver. Do you think double layer of foam core was necessary or could a lot of little braces have been used similar to how I made the FIB? If you have a freq response measurement of the pipe output that would be cool to see.
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Old 30th January 2013, 01:10 PM   #1143
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Nice! I'm digging the stepped approach for the horn expansion.
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Old 30th January 2013, 01:28 PM   #1144
IG81 is offline IG81  Canada
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Quote:
Originally Posted by xrk971 View Post
IG,
I was just noticing that the overall aspect ratio of your F-81 box is very thin - could be wall mounted if you just put the driver on the big panel at the top corner. You would have to slightly modify the plan to allow clearance for the driver. Do you think double layer of foam core was necessary or could a lot of little braces have been used similar to how I made the FIB? If you have a freq response measurement of the pipe output that would be cool to see.
I prefer doubling-up the foam-core than using a ton of small braces, it's a faster process for me and I find it makes for a more rigid structure. The internal folds already brace it pretty well in this case too.

Blue trace = Driver output at 2"
Green trace = Vent output at ~2-3", with the cab's back to the wall, so effectively ~1/8 space from the mic's point of view

Click the image to open in full size.

Far-field response:

Click the image to open in full size.

Response at 60":

Click the image to open in full size.

We can see the 109Hz dip in the driver near-field measurement - which is where the actual -wave mode lands, showing the pipe is tuned to 36.3Hz and probably does offer some acoustic impedance to the driver at Fs. We can see on the near-terminus response as well as far-field the peaking above 200Hz from the upper pipe modes. This should be suppressed with suffing.

Quote:
Originally Posted by BP1Fanatic View Post
Nice! I'm digging the stepped approach for the horn expansion.
I'd really hesitate to call that a horn though, it's really more an expanding TL.

IG
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Old 30th January 2013, 02:37 PM   #1145
xrk971 is online now xrk971  United States
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IG,
Thanks for posting the measurements - very nice! So you currently have no stuffing in the line at all? I might have to try doubling up the panels on one of my next projects.
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Old 30th January 2013, 03:31 PM   #1146
IG81 is offline IG81  Canada
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Quote:
Originally Posted by xrk971 View Post
IG,
Thanks for posting the measurements - very nice! So you currently have no stuffing in the line at all? I might have to try doubling up the panels on one of my next projects.
Yep no stuffing at all. Sustained guitar notes in the 200-300Hz area are sort of annoying because of this.

How thick is your foam-core? I get the cheap stuff at the dollar store: 3/16" thick, 20"x30" sheets, 1.25$ a piece.

IG
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Old 30th January 2013, 03:52 PM   #1147
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Dollarama special. Aren't we all using the same stuff?
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Old 30th January 2013, 03:56 PM   #1148
xrk971 is online now xrk971  United States
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Same stuff I use. $1 ea in the states. Readiboard (made in USA).

Bulk Readi-Board Foam Boards at DollarTree.com
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Old 31st January 2013, 07:32 AM   #1149
ravener is offline ravener  Singapore
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Greetings all!

Have read through all the way from the start of the thread till the last page, and I must say I'm definitely intrigued on how simple it is to build an enclosure out of foam board, especially when space is a big issue as I stay in an apartment on the sunny island of Singapore.

I'm more fascinated by the Fibonacci curve rather than the Cornu, reminds me of something I saw on a magazine a couple of years back, probably start a small build using old drivers from the satelite speakers of a Cambridge Soundworks (Amazon.com: Cambridge SoundWorks PCWorks Multimedia Speaker System, Black: Electronics , mine are white though).

Looks like you guys have another one joining the foam core group.
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Old 31st January 2013, 09:45 AM   #1150
xrk971 is online now xrk971  United States
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Ravener,
We have looked at the Fibonacci spiral and it would be cool. It has a rather rapid expansion ratio (1:1:2:3:5:8:13:21:35) so harder to get a long path in a reasonable size. I believe Planet10 posted a Fibonacci spiral plan. Welcome on board!
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