resonating 'c' note in 2x12 cab. Any ideas?

[mfreqmaster]

I bought a used 2061cx cab last week. I discovered after taking it home that 'c' notes, (mainly the lower c's on the guitar, not the upper octave ones), resonated vary loudly. This happens more on clean tones with the neck pickup rather than a bridge sound with distortion. There are some amp settings I can avoid this but would love to get to the bottom of this. Is this a torn speaker cone or something else inside the cab making this?

[gbono]

Try adding some damping material to the walls of the cabinet.

[Jazz P Bass]

The best approach would be to have a function generator, into the amp/cab.
In that way you could sweep the frequencies & find exactly which ones are the problem.
Then it comes down to: is it the cabinet or the speaker? Or a combination.
The cabinet may resonate at C.
The speaker may resonate at C.
Sounds like a tough nut without proper test equipment.

[Enzo]

Yes^^^

Isolate the problem, it is a speaker itself, or it is the cab.

Try pressing on the various cab surfaces. For example a bunch of 4x12 cabs have a post in the center of the rear panel to the center of the front baffle. The post is mounted to the baffle, and pushes against the center of the big rear panel. The end of that post can buzz against the rear. You pull off the rear and add a shim of rubber or something to the end of the post. That exstra dimension takes up any slack, plus the rubber acts as a cushion. That is just one possibility. Your Marshall has such a post, try pushing against the back.

Loose rear panel screws, loose corner hardware. Grille buzzing. Got casters? Maybe one of them. Turn the cab upside down or on its side, any difference?

You may find a handle is loose, or a speaker mounting bolt. You can find a failed joint in the cabinetry. Etc etc. You press on just everywhere looking for places that wioll stop the vibration. That at least tells you WHERE the noise is coming from, or helps.

[Chuck H]

By "resonate" does mfreqmaster mean that the cab is vibrating loudly or that the cabinet seems to pronounce these notes loudly?!? If the cabinet seems to pronounce certain notes loudly it's probably a matter of the cab and speaker having similar resonant peaks. There is a simple solution for that. Detune the cabinet with a port. There are even on line calculators if you do a Google search. You can effectively even out the bass and lower mid response of a cabinet to get rid of "wolfe notes" and improve, as well as smooth out, the bottom end. If this is your problem it's worth looking into.

[mfreqmaster]

OK, I took off the back of the cab. The cab no longer makes the sound. Sigh of relief that my G12 H30s aren't bad. I like the sound of them. Next step will be to probe around the inside looking for loose mechanical connections as Enzo suggests. Thanks.

[gbono]

You may have an air leak that is causing the problem. If you can play some program that replicates the noise and the cab is sealed again you might be able to listen for the leak with some tubing placed next to your ear.

[Chuck H]

I repeat:

By "resonate" does mfreqmaster mean that the cab is vibrating loudly or that the cabinet seems to pronounce these notes loudly?!?

[mfreqmaster]

It pronounces the notes loudly. I wanted to try to pin down any vibrations before I went to cutting a hole in my baffle, which I assume is what porting is. I will do it though if it is a last resort.

[woodyc]

Lots of cabs do that, at some note in the lowest octave on a guitar. Note that 2x12=2ft and thats a quarter wavelength for a low C on a guitar. Its worse when there is an overlap with a multiple of the speaker resonance. Changing one of your speakers to another type (with a different resonance) will help. Acoustic fill is supposed to help, but doesn't, at least not that I've noticed. Obviously an open back works. I wouldn't do a port because frequencies below the port resonance become dangerous for the speakers.

[Chuck H]

Love the idea of changing one of the speakers. I also love my G12H30's. But a very popular speaker combination in 4x12 cabs is a pair of G12H30's and a pair of Vintage 30's. I have a 2X12 cab that had a pair of G12H30's in it. Last week I replaced one with a Vintage 30 but I haven't tried it out yet. I'll go do it right now and report back.

[Chuck H]

I'm going to enthusiastically endorse a G12H30 with a Vintage 30.

I can't say if the V30 will differentiate enough to get rid of your resonance. And, of course, your cabinet is going to have a different interior volume than mine. But I'm VERY familiar with the original tone of my cab with two G12H30's. I like the G12H30/ Vintage 30 better. Deeper, clearer bottom, a little more strike on the notes and better mid EQ for both clean and dirty tones. If you like a soft attack jazz/blues kind of tone you may not dig this. But if your a rocker your going to wonder what took you so long to do it!

[mfreqmaster]

I loosely placed the back panel of the cab on, turned the amp, & it sounded fine. I then screwed in the 2x2 post that connects the baffle to the back panel & the problem surfaced again. I cut out a piece from an old car floormat & attached it to the post, placed the back panel on & things were working fine again. I thought I had the problem licked & started to tighten the rest of the screws. I tightened a few & tested, then the problem surfaced again. I took all of the screws out & tested, hand tightening at first, then just barely using the screwdriver to get the screw to not stick out from the panel. I tested the guitar after every new screw. Finally I had the screws back in the cab, not super tight, but in there. I tested again. A 90% improvement. I really had to work to get the note to resonate & even then it was not as powerful as before (what made it resonate this time was the neck P-90 p/u on a Les Paul, the bridge didn't cause any problems this time). The cab is usable now, before it was not. I think I will play it a few days to see if I'm satisfied.
I have another 2x12 cab with Vintage 30s in it. Unfortunately they are 8 ohm, my G12H30s are 16 ohm so I can't substitute.

[Chuck H]

I have another 2x12 cab with Vintage 30s in it. Unfortunately they are 8 ohm, my G12H30s are 16 ohm so I can't substitute.

You could always shell out the $$$ for a 16 ohm V30. That's the easiest way I know of to get one. But on another note...

It seems like the center post may be causing trouble? Something about the rear baffle and/or front baffle and/or their interaction due to the post seems to be causing the problem. Solutions could be:

Remove the post (not sure it's needed in a 2x12 anyway)

and/or

Add mass to the front and/or rear baffle. You can cut plywood with a jigsaw to fit areas around the speakers and then screw and glue it to the baffles. The rear shouldn't be too tough. Make sure the screw length is enough to grab tight but not punch through. Pre drill oversized holes in the pieces your adding so that the screwing acts as a clamp for the glue.

[Steve Conner]

One thing I've done is fasten the post at both ends. Figure out where it presses against the rear panel (could require some woodwork-fu) drill a hole in the panel that lines up with the centre of the post, and run a big woodscrew into the end of the post.

A while back, I got an EVM12L reissue and made a ported cabinet for it. The ports give a worthwhile bass boost: they don't so much make it louder as deeper. Results will vary depending on the speaker though. The EVM12L was designed for a ported box.