18Watt.com

Hi, I built my own cab (2x12, closed back, WGS 30W and 60W mis-matched spkrs in it now), and find that it's boomy. I used the 2x12 plans floating around on here and modified them some...I made the cab deeper, and slanted the baffle inwards so it is about 6" from the back at the top, and maybe 12" at the bottom. The better to hear what's going on on stage.

It really is noticeable using a LP, neck pup (of course), at low C and below. I have tricked the amp as much as I really can (bass=0, treb =10, and I have played with bass cap values). I'd really just like to know if drilling a couple of holes behind the speakers with a 3 or 4" hole saw might 'relieve' some of the bass? Or does anyone have any other 'tricks' to tuning a cab. Otherwise, everything is sounding AWESOME, and I'd rather not dig back into the head, since it is very close to 'spec'. Maybe removing the back and cutting some of it out might help?
It's this home-brew cab; just looking for pointers!
Thanks!

When you increase the volume of a cab, you generally increase the bass, You could try putting a layer of acoustic wadding across the back panel of the cab. Removing part of the back is another way, although it may not be as tight as with a closed back.

Thanks, ZP.
Yes, I'm noticing this more when I crank 'er up than at a lower volume. I'm about to take this rig out to play, so had it up to stage volumes yesterday.
So playing with the void space in there may affect the bass response? Neat. I will give that a try before running the back thru the table saw!

I'm going to reinforce what ZP said. I build a speaker cab for an Ampeg B15n that had 1/2 of the inner surfaces covered w/ thin acoustic batting. There were a few boomy notes. I covered all the internal surfaces and now it sounds great.

Try insulating the interior before you cut anything.

Sounds like a plan.
Thanks, gentlemen. I'll post back results after I get some time to try that out!

I would suggest first just covering the rear panel with some wadding and see how it goes. The wadding helps get rid of resonances which cause the boominess. But too much may make the cab sound dead.

Is this special-order stuff, Z_P, or do you suppose any sort of insulative blanket would suffice? Thermal insulation is easily had; unfortunately, where I am (Maine...) acoustic stuff is special-order....

The material I used is a felt type covering which I used on the back pieces of the two cabs I have built. It is sold as a cab covering which I rememeber we used in the sixties to cover the wooden cased amps whioch took a beating the back of cars and vans.
It is perfect as it is thin but absorbent. It was on a site selling tolex and grill cloth etc. I do not have the name to hand but will post it if I find it.

Dd

Thanks, D.D. I am pretty sure I know what you mean....almost like old-style carpet padding. I can get that down the fabric place.

Yes it is just like that.

Dd

Carpet padding comes in a few thicknesses. And I'd ask if they have the felt (fiber) variety. It won't take much. When I was building bass cabs (back in the days of JBL's with 12 lb. magnets) I found that installing it with a hump or two worked better than fastening it flat against the walls.

KennyO

CurtissRobin wrote:I found that installing it with a hump or two worked better than fastening it flat against the walls.

That's a great idea. Never heard of it before, thanks!

Another option might be to decouple the amp from the floor.

My closed 2x12 was a bit (but not too much) boomy on certain notes. I found that by getting it off direct contact with the floor, that mostly went away. I put it on a wheeled wooded furniture dolly. Looks fine, I can roll it, and it doesn't boom. I imagine that one of those speaker isolation pads might work as well.

Just another idea to try.

andrekp wrote:Another option might be to decouple the amp from the floor.

My closed 2x12 was a bit (but not too much) boomy on certain notes. I found that by getting it off direct contact with the floor, that mostly went away. I put it on a wheeled wooded furniture dolly. Looks fine, I can roll it, and it doesn't boom. I imagine that one of those speaker isolation pads might work as well.

Just another idea to try.

That's not a bad idea. I use an amp stand sometimes, that might do the trick also.

Good suggestions guys, thanks! I'll try a pad. It has a weird resonance when I play 1st C...pretty good otherwise. I still have to stuff it with insulation or something, tho! {lazy}

GibsonGM wrote:Good suggestions guys, thanks! I'll try a pad. It has a weird resonance when I play 1st C...pretty good otherwise. I still have to stuff it with insulation or something, tho! {lazy}

Hey GGM,

I'm not exactly an expert, but have made a few custom cabs, so here's my 2 yen ...

1) I'm not clear about all the dimensions of your cab, but the slant probably reduced the interior volume of the cab (this is critical in a closed, unported cab). I know you said you made it deeper, but 12" bottom to 6" top is a pretty radical slant, so interior volume is probably down by 20% (just a guess). A while back I explored "optimum" enclosure size for a 12" 100dB speaker, and the ideal cab is the size of a closet! The thing with adding foam or felt "wadding" is to create an artificial volume by soaking up some of those interior waves. It also breaks up some of those specific "pitch assaults" o the backside of the speaker cone. Just throwing a small wool blanket on the floor of the cab might work wonders! (wool and felt seem to be better than cotton ... so I've heard)

2) The slant complicates how the waves move around in the interior. How exactly that works is apparently a very deep science ... at the same time, most folks say it still requires a lot of experimentation. In other words, even the scientists admit that the calculation to real-time achievement of great sound is somewhat hit-and-miss.

3) Porting is REALLY complicated, while open back is a lot more forgiving. But if you need that closed back "projection" and tightness, you have to work at it. As has been said, some speakers just don't work with open back, and some don't work closed. (very generally-speaking, low-efficiency speakers do well open, and more powerful and efficient speakers like closed environments)

I get you about lazy (welcome to the club!), so my suggestion is to just throw any towel inside and see what happens. If it seems to work, you have motivation to try other stuff. And as has been mentioned, irregular bunching (or waves or wadding) will definitely work better than just a flat piece of carpet felt. Don't stuff the cab too much, or it will muffle the tone.

Best of luck!

Going back more than a few years I built a handfull of speaker cabinets. The acoustic science of the time, that I could find, said that the purpose of insulation inside of a speaker cabinet was to reduce or eliminate standing waves that would tend to occur between parallel sides of a cabinet. The standing waves would alter the frequency response and creat undesirable peaks and valleys in the response and exagerate resonant frequencies.

This would be the boominess you're having trouble with.

Also that you only had to cover one of each parrallel side with insulation to be effective so the bottom, back and one side would be plenty. Of course overkill with insulation on all sides wouldn't hurt either.

We used the pink fiberglass stuff they used for attics and it seemed to work well.

This was all very much before the innertubes so we had to find everything in libraries. When you started looking, you could find all sorts of stuff in those places.

Is current science much different?

Hope this helps.

Thanks everyone - I have been following the replies on this old thread with interest!
I STILL haven't opened her up to cure the issue, since I haven't used the cab in a band setting in quite a while. Sounds great at lower volumes, so it's not bothering me at home.
I do have a bunch of insualtion "tiles" - they're not batts, but loose squares, almost an acoustic wadding sort of material. I intend to open it up soon and do just what you said, Skynyrd...I want a dead cab anyway, so stuffing seems to make a bunch of sense.

Too bad, it only booms on your first "C"...has a resonance there. I'm actually surprised it's not worse, being so custom sized!

im no expert on sound or speaker cabs but i use to work in a car stereo shop where we built custom cabs for 12"+ speakers one thing I learned is different wavelengths reflect refract and defract differently when compared to others in a set volume of area if it were me before I went and spent more money I think i would try taking your baffle and move the top closer to the front of the box thereby shortening the travel distance of the soundwaves which might take away some of the boomyiness and brighten the higher tones.

just my 2 cents as I said im no proffesional