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Old 16th April 2007, 10:48 PM   #1
Hardman is offline Hardman  United States
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Default Hemp FR8C Enclosure Suggestions

I recently received a pair of Hemp Acoustic FR8C drivers and am looking for suggestions on a box alignment. I think the final goal is a BIB, but I am looking for a simpler build first to get a feel for the driver and confirm it’s ability before I commit to a BIB. Maybe something with an aperiodic vent (BR for this driver is still quite large) or dipole.

Does anyone have experience with this or the previous version of this driver that is willing to share their design?

Thanks - Chris
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Old 16th April 2007, 11:53 PM   #2
pjanda1 is offline pjanda1  United States
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I would aim for the BIB. I presume you've seen the MLTL's a few threads back. But in the mean time, why not build the little BR that Dan Wiggins designed? The Aperto:

http://www.hempacoustics.com/kits.php

I first used mine in 40L BR's. You can find alignments that work if you tune it low. The group delay will be low too, but the downside is more excursion a ways above tuning. Dan Wiggins knows his stuff. I'm sure there are benefits to the tall baffle, as he says. However, you could also just copy the internal volume and vent in whatever aspect looks good to you. I'd lean towards a wide baffle. You sure won't be disappointed with the BR, and later you'll find the BIB to be a whole different world.

pj
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Old 17th April 2007, 02:44 AM   #3
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PJ,

Thanks for the reply. I saw the 22 liter design on Hemp’s web site, but thought something larger modeled better.

I settled on a BR of about 45 liters or so with a 3” port. I don’t think port noise will be too much of an issue, so I would really like to put the port on the front of the box (box is about 23.5” tall X 14” wide)? I may reuse this box someday for another application which most likely would be wall mounted. Is there any foreseeable issue with a port on the front of the enclosure? What size port did your 40 liter box use? Suggestions on stuffing/enclosure lining would be helpful too.

I am crossing my fingers, as I have a small SE OTL amp that I hope will be able to drive these speakers when finished.
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Old 17th April 2007, 03:27 PM   #4
pjanda1 is offline pjanda1  United States
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As I think about it, it was a 50L enclosure. The port was 2.5" and it didn't chuff (we're only talking 2mm of excursion here!). It was on the back. I was thinking of doing a more permanent BR, and I did plenty of searching for a consensus on port position. If you can do it, rear ported is safer. However, if you put them on a wall, there is no way it will work. The wall should eliminate the need for BSC, so that is a good thing. A front port has some chance of midrange leakage, even if it doesn't chuff. If you are worried about it, you could do side ports I suppose. If you can use MJK's spreadsheets you could model some different port placements. I wouldn't worry about it. Plenty of folks seem to place ports willy nilly without troubles.

I didn't experiment with damping or stuffing as it was a temporary solution. Don't put much of any "stuffing" in there, or you'll mess with the port action. I damped the walls with thick carpet padding (held on with a combination of Super 77 and PL Premium Construction Adhesive). You know the stuff, a bunch of different foam chunks all glued together: cheap and effective. If you want to get fancy, you could use this stuff:

http://www.rawacoustics.com/item__RA...cal_1,821.html
or this:
http://www.partsexpress.com/webpage....=sonic_barrier

This stuff is mainly to damp the cabinet walls, not to tame standing waves. You might start only placing the material on the back, top and one side, as per GM's recommendation. Too much damping can make it sound dead. I put a double layer behind the driver to help damp the backwave. I used more damping than is ideal, as my cabinet was mainly particle board. Build something decent and you can go easy with it.

Tell me about this SE OTL. It sounds like fun. OTL's strongly prefer flat (and high) impedance curves don't they? Instead of the BIB, perhaps something like the Kenobi would be ideal. You could run the toobs on the Hemps and drive the woofer with something else. Then woofer sensitivity wouldn't matter either.

pj
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Old 17th April 2007, 03:56 PM   #5
Hardman is offline Hardman  United States
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Here is the scoop on the SE OTL http://www.transcendentsound.com/single%20ended.htm . I have had this amp for a few years now. The Transcendent Sound amp is a great “little” amp, but I have yet to find/build a pair of speakers that really works well with this amp. A few years ago I built some 95 db 8 ohm two way line arrays using Tang Band drivers and an AC ribbon, but in the end the speakers still need a little more head room. I guess a horn setup would work nicely, but I have yet to venture down that path.

Anyway, back to the Hemp… did you use any filter with the BR box or BSC? I plan to make the baffle 14” to 15” wide. In the end these speakers will be driven by a different amp and most likely end up in a BIB (these speakers belong to a friend who has solicited me to help him construct the enclosures). Again, a BR is less involved and cost less than constructing a BIB. The BR boxes should hold my friend over until he is able to scrape some additional cash together for the BIB. Besides, I think he is still working on the WAF as the BIBs are quite large.
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Old 17th April 2007, 04:32 PM   #6
pjanda1 is offline pjanda1  United States
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I didn't use any BSC, but it sure wouldn't have hurt. I used 15" wide baffles fairly close to the floor (12", tilted back) and back wall (1-2ft). I initially had them on higher stands (24") without the tilt, but getting them closer to the floor helped the midbass quite a lot. I had them set on cement blocks such that there was a fairly wide surface extending from just behind the baffle to the floor.

pj
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Old 23rd May 2007, 06:41 PM   #7
traw is offline traw  United States
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what values would i need for a bsc? i have the fr8c's in a 50 liter or so box tuned to 45hz. still a touch hot on the top, want to shift the tone a little.
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Old 23rd May 2007, 08:49 PM   #8
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You can build a smaller box than a BIB but not much simplier...

The release plans for Demetri (folded ML-Voigt for Hemp FR8/FE207e/FE167e) were just released.

http://planet10-hifi.com/boxes-fostex.html#demetri

Click the image to open in full size.

dave
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Old 23rd May 2007, 08:59 PM   #9
tinitus is offline tinitus  Europe
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Dave, thats a nice one

Hardman, I reckon you know that your nice "little" OTL should double its output in 15ohm - I guess it will need som impedance smoothing
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Old 24th May 2007, 05:47 AM   #10
Hardman is offline Hardman  United States
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Quote:
Hardman, I reckon you know that your nice "little" OTL should double its output in 15ohm - I guess it will need som impedance smoothing
Yes I am aware of this (amp should be good for about 4 watts at 16 ohm), but have yet to find a speaker project that takes advantage of the wattage gain.

Update – I didn’t ever build the BR boxes for the Hemp driver, but have instead started the BIB. I plan to use a different amp with the BIB too. I hope this thing is worth all the effort... I mean wood. Looks like the Hemp BIB consumes five 4’ X 8’ sheets of wood!
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