Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Cabinets-Speakers => Topic started by: tubenit on April 27, 2009, 06:36:08 am
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Anybody tried this and can report on how it works?
I'm gonna give away a trade secret here: if you want to make a 12" speaker less directional, cut out a circular piece of 1/2" or 3/4" open-cell polyurethane foam (I use Unifoam S82N) the same size as the speaker cutout in the baffle of your cabinet, with a 3" diameter hole in the center. Attach it to the grille, centered in front of the speaker. I use a very light coat of spray contact adhesive, applied to the foam only, to get it to stay in place. If you later decide to remove the foam, it can be easily pulled off, leaving no residue on the grille.
The 3/4" foam doughnut will make the response of a 12" speaker very consistent over a 70-80 degree angle, at the expense of some overall loss of brightness. The response of the speaker at 40 degrees off axis is almost identical to what it was without the foam [Edit: which is to say that it rolls off at about 12 dB/octave above 2kHz], but the on-axis peaked high frequency behavior is completely gone.
The 1/2" foam doesn't do quite as good a job of evening out the response differences - there's still a little peakiness on axis - but the overall sound is a little brighter. I have two otherwise identical 1x12 combo tube amps, one with 3/4" foam, the other with the 1/2", and my present preference is for the 1/2". YMMV, but it's worth at least what you paid for it.
Reading the reports of some others, it looks like this perhaps works more effectively in perserving the tone than a "beam blocker" does?
With respect, Tubenit
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Interesting...
http://www.hugeracksinc.com/forum/viewtopic.php?t=54594&postdays=0&postorder=asc&start=0
http://www.univalve.net/vbulletin/showthread.php?t=3401
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This is what you want: http://www.billfitzmaurice.com/XFCabs.html
What they are trying to do is reduce the aparrent diameter of the driver which may have a small effect at certain frequencies. The other thing that they are trying to do is attenuate the highs from the cone area which will lower overall efficiency somewhat. different foam densities will have a variable effect. What would work better than this is a small line array of six to nine 6" full-range drivers in a straight line vertically oriented. It would be just as loud as a 4x12 and have excellent dispersion. However, guitar players want to see a big wall of speakers behind them- hell with what it sounds like, as long as it looks cool. This was the reasoning behind Marshall's (non)design, and also why Pete Townshend is deef as a post. It's what happens when you design with your eyes instead of your ears. Get a closed back Champ with no larger than an 8" driver if you want a balanced even sound. Oh, you gotta have that 12" even though it is about as useful as a V8 in a VW. bug.
Since no self respecting guitar player can get their 'face' on properly with a skinny six foot stack of sixes, the XF was implimented. It's actually a tweak on an old Sun design that looks and works better than the original. the drivers are cross-fired and angled up so that your ears can hear the highs as well as your boots could... It's heresy, but that's how the laws of physics and acoustics operate. Case in point- Steve Tyler and Mick Jagger would never get laid for their looks without their talents now, would they? 8)
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> give away a trade secret here
Hasn't been top-secret since 1947.
JBL 1217-1290 "potato masher"
This comes from WWII radar research. A large flat radiator is omni at low frequency but beamy at high frequency. In radar, a goal was to make it more beamy, but the same techniques can make it less beamy.
With light you use curved glass, a lens, thick in the middle, to get a spotlight beam. If you were stuck with a spotlight and needed a floodlight, you could use a negative lens (thin in the middle).
Glass does nothing to radar, and blocks sound. However you can get a lumpy approximation of what glass does with a semi-solid matrix. Perforated metal. Open-cell foam.
Directivity is not entirely bad. Without it, a 12-inch cone would droop badly above 1KHz. Indeed, "at the expense of some overall loss of brightness".
A hole in a slab is a very rough approximation to a lens. Look at the potato masher: it has multiple layers with different size center holes. I'm not sure a 12" paper cone is uniform enough to warrant such fussiness. (The JBL is a slow-taper horn so the sound field is very uniform.)
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I just installed the "Mitchell Doughnuts" in my Twin last night. The other guitarist in my band installed 2 beam blockers in his Peavey Classic 50. We have a gig tonight so we shall see, but last night we could tell that the beam blockers did not get rid of the beamy highs out front, but did seem to throw some highs to the sides a bit better. My amp, with the doughnut, had a much more even tone all around the room 24 by 24. Also, the ice pick beam in front was not as bad as his. Now, given his amp was always worse with this, but it was still bad. Our problem is that at gigs we are right up on our amps and the drummers symbols, so to us the amps sound good, but out in the audience they are too loud. I'll let ya' all now how tonight goes.
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One simple approach may be to get your amps (speaker cabs) tilted back, and maybe up higher off the floor. Amps stands or a couple of chairs might do the trick. That will fire more sound at you and less directly at the audience. You might also try re-positioning your amps or speaker cabs: a) against a far back wall; or b) KOC recommends firing your amps sideways or at an angle across the stage. That way the players can better hear one another, and there's less direct fire at the audience.
Another solution, used by Joe Bonomasso, is a partial plexiglass baffle board in front of the speakers. Somebody manufactures these things. (Ginger Baker used them at the last Cream concerts.)
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I've been using one of those plexi shields for a couple years now. It's a four pieces unit that folds accordion style. Even with my 10W EL34 Champ with a 1x12 (always mic'ed). I'm also right on top of my amp in most places I play. Sometimes I run some guitar thru my monitor, but often times don't need to. I do 50% of the vocals, so I wanna be sure to hear that.