Altec 828 Fiberglass Resin Resonance Taming

[alfredo]

I have a set of Altec 828 cabinets that I would like to tame some of the resonances. Has anyone ever tried using layers of fiberglass resin to do this? I'm thinking it may be effective especially where the bent plywood used for the horn flare is located.

I have had a little experience in the car audio world using this stuff and whole enclosures are sometimes made of this stuff.

This of course would be used in addition to the wood bracing discussed in previous posts.

Thanks,

Alfredo

[Audio_by_Goodwill]

Originally posted by alfredo:
[QB]I have a set of Altec 828 cabinets that I would like to tame some of the resonances.

<font size="2" face="Verdana, Helvetica, sans-serif">Can you be a bit more descriptive in what your hear? I've recently started listening to one of my early 828s. It's plywood, but the horn isn't braced.... I'm guessing it was a trial, as I believe later cabinets saw the return of the horn braces. Any way.... my cabinet doesn't seem to flex, although, I'm not turning the volume up much, as it's quite thrown together at this point.

I have been thinking recently about what kind of modifications I may do to my cabinets, structurally. One of the ideas I've been kicking around is re-working the back panel. My cabinets have a full-size removeable back, and others have only a half-size removable back. I'm thinking about having a removable panel just big enough for the woofer. Since the back would no longer be fully removeable, I would have further bracing options.

[Audio_by_Goodwill]

Originally posted by alfredo:
Has anyone ever tried using layers of fiberglass resin to do this? I'm thinking it may be effective especially where the bent plywood used for the horn flare is located.

<font size="2" face="Verdana, Helvetica, sans-serif">I didn't address your main question in my last reply, so I'm responding a second time.

I have no experience with fiberglass resin, so my answer is mostly speculation. I don't think I'd use it for two reasons. First, I'd be afraid that I'd end up with a messy looking final result, that would be next to impossible to undo. Second, I'm not confident that fiberglass would work well on the large flat surfaces..... I think fiberglass is pretty flexible when flat, so it would be more mass addition rather than adding rigidity.... which doesn't help much with bass, if I recall GM's past postings on the subject of cabinet integrity. I could see applying it to the backside of the horn flares only. I think I'd try adding small braces to the flares and maybe fiberglass over the braces, if I wanted to go the fiberglass route.

This gives me another option to consider when I start working on my cabinets, and specifically the backside of the horn... Thanks!!

[alfredo]

The one thing I did notice is when I had my other cabinets (JBL 4560's with JBL 2205B drivers), it did not vibrate when I but my hands on the sides of the cabinet or inside the flare area. The vibrations on these 828 cabinets are quite noticable.

My concern is that I'm thinking of installing JBL 2245s that I have in the bottom of the 828's and would imagine the vibrations will get worse.

I will try the bracing when the time comes and will decide later on the fiberglass.

The idea of having a panel just large enough to get the speaker out sounds like a good idea.

Thanks,

Alfredo

[Old Guy]

Essentially- bracing adds stiffness and mass. Fiberglass resin by itself won't add much of either.

With a good glas content it will add stiffness.

I have seen all sorts of solutions- way back in the 70's I saw a permanent install where the horn flares were filled with concrete- it had the horn in the port so the woofer was on the bottom.

[dread64]

Just my 2 cents worth,

I filled those cavities (behind the bass horn flares) with sand. Which pretty much killed the resonances eminating from them, but it also added just over 100lbs to the weight of each cabinet. So, if you will be moving them around alot, this would NOT be the method for you. However, if portability in not an issue, then few things will deaden these resonances as well as sand.

Dave

[Audio_by_Goodwill]

Originally posted by dread64:
I filled those cavities (behind the bass horn flares) with sand.

<font size="2" face="Verdana, Helvetica, sans-serif">My concern with sand is that it won't add any strength to the box, but at 100 pounds, it may very well end up a burden to the plywood structure....besides being a bear to move. I'd like to first strengthen the cabinet, and the horn itself. After that, I may contemplate filling the area behind the horn with sand, but at my home volume levels, I doubt I'd need it.

[bigjunglejim]

Hi there.
I have Altec/Stanley Screamer's covered with shot fiberglass. Fiberglass and the handles make them "R" (Road finish) vs "utility" They vibrate. I don't think adding resin will do the trick unless you use a lot of it, so it acts like a brace. Even the MDF boxes that are loaded with resin vibrate. Bracing is the key. I wouldn't put the bracing on the inside, I'd use it on the outside only. The curve of the horn acts like it's own brace. The problems associated with vibrations comes from the large flat pieces like the top bottom, sides and back much more than the horn.

Altec added vertical braces on the back of the 828 as an improvement. If it were me, I'd try four or five horizontal braces on the back instead of the two vertical braces and not change the way the box opens. Even with the bracing, if you crank it up, it will vibrate some.

The SS-3210-R subs I built have fiberglass on the outside and three 2x4 braces running up and down each of the sides (I don't remember how many on the back, top and bottom) and they still vibrate some. Another way of doing the bracing would be to zig zag the braces making triangles of the flat panels. Or use a four or five horizontal braces with a diagonal brace between each of the horizontal braces. That would make a bunch of small triangles between the braces. Perhaps that would be even better. Has anybody tried doing as I suggest?

It would be easy to make a back for your cabinet for the soul purpose of playing with different bracing systems. Once you find what works best, apply it to your original backs.
Good luck and keep us posted.

[Old Guy]

Back in the 70's and probably influenced by outside forces, we built some huge bass horns out of Fiberglas. The problem was they resonated. We just couldn't get a decent bass outdoors, the best were Shearer horns. I read a bunch of pages about horn design, enough to scratch the surface, and armed wit a programmable calculator we came up with these things. For outdoor concerts we employed an elegant solution. We built them double wall.

The concept was, put up the driver box, attach the big trumpet to the front, and then fill the double walled horn with water. Once in place they weighed some 800 pounds. For the day, they thundered. We tried to market the idea but it wasn't real practical. Fun however. I miss those days, when audio experimenting could be written off as a business expense. These things were some 9 feet deep to just the driver cones.

[bigjunglejim]

Hi Old Guy
Do you have pictures and/or the plans for your water filled horn? They sound very interesting. You most likly have read one or more of my posts regarding the custom made ServoDrive subs I have. For sound reinforcment, I've been coming up with all kind of ideas on how to use them. Perhaps your ideas may work for my subs.
JJJ