I recently acquired a pair of Altec 834A (Monterey) speakers and I'm quite pleased with the sound. I did hear a problem in the 4 to 5k range and considered using EQ to tame what sounded like typical horn coloration. The Monterey uses the Altec 3000B compression driver with a sectoral horn. Earlier, more collectible versions of the horn were made from zink and were multicell. The later "B" version is made from fiberglass and is not multicell. All versions use a very light, delicate microphone diaphragm. After digging around I found a test report from a 1960 Audio Magazine. The reviewers were using a new measurement technique; tone burst response. They detected serious ringing at 4.7 and 5.K (see first photo) Output at 5K was actually higher after the burst than before!
My question is, where is this ringing coming from? I know about tar filled larger horns from Altec, so is it the horn structure itself ringing, the driver itself, or the shape of the horn? I would love to tame this problem, because otherwise they are a great sounding speaker.
My question is, where is this ringing coming from? I know about tar filled larger horns from Altec, so is it the horn structure itself ringing, the driver itself, or the shape of the horn? I would love to tame this problem, because otherwise they are a great sounding speaker.
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Good question! Those tweeters are certainly valuable these days, but with the ringing, one wonders why. I don't have any, so can't help (only Altec 1" and 1.4" drivers). But there is probably someone here who does. If not, you can ask over on the Altec forum.
If you mean the Altec user's board, it's impossible to register there these days, so I can't post or even search there. If you mean the Lansing heritage site, I'll give it a shot, but things move glacially over there unless you're discussing current high end JBLs. Female vocals are where the ringing is most audible. The extension on this driver is amazing. The curve I posted doesn't show it because of the rolloff of the measuring microphone, but I understand it has usable output to 27K, which must be rare for such an old design.
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I can take your message over to the Altec site via a link if you wish. Please add some details about the registering.
FWIW, stock 511s ring too, even bolted to a baffle board. It is, as you say, very annoying. My cure is to fill the bells, and essentially pot the outside of the horn in a thick layer of polyurethane construction adhesive. Done properly, the horn is then very neutral sounding. A similar cure would probably solve the 3000B problem.
Something like that could easily be removed from the diaphragm again, if it turned out to be not the reason:
Foam Self-Adhesive Anti-Vibration Insulator Pads 15mm (36)
Foam Self-Adhesive Anti-Vibration Insulator Pads 15mm (36)
Any email used during registration comes up as "banned." You are then sent to the help desk, but you can't contact the administrator unless you are registered. Classic Catch 22.
The diaphragms are exceedingly delicate, I'd be a little wary of trying this one.
What do you use to fill the bells? Details or photos would be great.
Polyurethane construction adhesive:
Henkel 828471 PL Premium Polyurethane Construction Adhesive, 10.2-Ounce - Amazon.com
Home Depot has it, too.
It takes about 3 x 10 ounce tubes to fill one bell, so 12 tubes total to fill both bells. Figure another 8 - 10 tubes to properly coat the horn body. It's a bit of a pain, and takes a week or more to do right, because the method requires applying a coat of no more than 1/2 inch thick, letting that cure for days, then putting on more. Yes, it weighs a lot when you're done.
It helps to first paint the 511 with this stuff:
http://www.amazon.com/QuietCoat/dp/B004ZFZARY/ref=cm_cr-mr-title
It helps a lot all by itself. The polyurethane not only further cures the ringing, but stiffens the too-flexible walls of the horn.
There may be other materials that work as well or faster (perhaps foamed polyurethane), but whatever it is needs to have good damping properties. People have used thick layers of undercoating, and that probably works fine, although I was a bit leery of the smell and having the the petrochemicals outgassing in my house.
I'll see about photos, but all you'll see is a 511 swaddled in lumpy tan-colored plastic 🙂
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I have 3000B's not 511's, so I'm sure I won't need 12 tubes. At what frequency do 511's ring? If it's just the horn itself ringing wouldn't Mortite (window calking rope) applied to the outside do the trick?
I've never tried Mortite, but I've seen people claim it didn't work very well for them. I think one said his 511 sounded like a bell covered with Mortite 😱 Not sure the Mortite would help the wall flexibility, either.
As for the ringing frequencies, Zilch made some revealing measurements:
AudioKarma.org Home Audio Stereo Discussion Forums - View Single Post - Z19 Crossover
Note the weird tails on the CSD.
The thin fiberglass walls of the 3000B horn do seem to be pretty flexy. I would like whatever I do to be reversible if at all possible. That construction adhesive (AKA Liquid Nails, right?) is a real pain to remove. My take is that you seem pretty convinced that the ringing is caused by the horn structure ringing, and not the diaphragm itself.
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Definitely the horn ringing, at least in the case of a 511. Originally you could tap the bell and hear it ring. Zilch used a DE250 driver for his test, which has a very flat, resonance-free response.
Cal used candle wax to damp his 511s, I believe. Even Dynamat should work - IF the horn is the problem. We don't yet know that it is. Wrap it up tight in Ace bandage for a quick fix. That should kill any horn ring.
Sorry you can't get unto the Altec board. The Hostboard it runs on is very shaky and quirky. 🙁
Sorry you can't get unto the Altec board. The Hostboard it runs on is very shaky and quirky. 🙁
It's actually duct seal putty on the backside of the lips. The candle wax is the romantic side of me coming out. You know, fill the bell with wax, add some wicks, turn the lights down low, pour a glass of rose...
...and crank the **** out of your favourite Jimmy album.
Pure romance.
...and crank the **** out of your favourite Jimmy album.
Pure romance.
I have left a link over there asking for help.
Hey, if Jimmy would marry me, I'd switch sides in a New York minute. 😀
Hey, if Jimmy would marry me, I'd switch sides in a New York minute. 😀
The stuff I used on a NHT1259 sealed sub ~15 yrs ago dissolved the surround and glue joints, though I imagine EPA has forced it to be relatively benign by now. Still they'd have to cure a bit before bringing them indoors.
If doing something like that today, I'd head on over to the local spray truck liner folks and see if they'd do a few coats 'on the cheap' when clearing/cleaning out the spray system.
GM
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