Speaker Cab Resonance

[Happy Face]

Hi. I have an old German 2x12" ported speaker cab about the size of the smaller Bassman cabs. It had guitar speakers in it and I went to sell it but no one offered anything reasonable. Then I saw a pair of neo 12" speakers offered on TB and decided to give 'em a whirl. Thiele Parameters be damned!

You know what? It sounds pretty good. Punchy. And it seems to handle loud volumes without a problem.

BUT - when I play a C C# or D on the A string (close to the nut) or on the E string I get a sound like you get when you have a microphonic tube. It's loudest on the C. Appears with any amp.

It's not like the rattles I've encountered when internal screws need to be tightened nor like a resonance buzz I suffered with on another cab. It's a little reminiscent of a slightly blown speaker, but I don't recall that those are so frequency-specific.

What do you guys think?  Loose something in the cab, a resonance, a bad speaker or something  else entirely?

It's not a deal-killer but I'd love to get rid of it.

Thanks.

   
 

[patman]

I'm no expert...the experts will weigh in...sounds like a speaker is on the way out, but not dead yet.

[amptech]

I´ll step carefully here, acoustics is a dangerous force... especially on internet foras

Just a thought, but if you have some slight voice coil rubbing it might be audible only around the speakers resonance
frequency? At least your low C will hit off somewhere below 70 Hz. Not that it solves anything, but the speakers data sheet  might give more info on what kind of cabinet size (and design, ported or sealed) the driver was meant for.

I installed neo celestions in a similar size cab (musicman 112RH) and it sounds great, but that is a reflex horn design and might be different. I found that trial & error, common sense and patience was my best tool for matching speakers and cabs

[Happy Face]

A little more detective work. The cab is like a smaller Bassman cab (pre-CBS size). I had it up on end so the speakers were vertically stacked. Today I put it on the ground so that the speakers are horizontal.

The buzz was much-reduced. In fact, I thought it was simply picking up some fret buzz on the same note! But when I played a different bass the buzz became louder and when I tipped back up on end, there she was again.

So not sure if speaker or cab. A friend suggested unsoldering the speakers and trying them one at a time. I guess that's the next step, but maybe I'll just live with it for a while because the cab sounds really nice otherwise. I'm surprised how well it handled my Bass Terror turned up pretty loud.

But if anyone else has any ideas, please do speak up. My ignorance of simple things is astonishing!   

[Granny Gremlin]

Since it was a guitar cab, the port tuning may be a little on the high side for bass and the noise could just be you reaching that resonant freq.

All the other things (VC rub etc) are also possible.

Take the speaker out of the cab and play through it - if you have another cab you can put it in then that's ideal, but you can still hear VC rub or cone fatigue without an enclosure.  Do you get the same issue?  If not it is likely the port tuning (replace with a wider and/or longer port; or add a second port.... the internal volume may be small too, you can try stuffing the cab to make it appear larger to the speaker).

The buzz getting louder depending on cab orientation can be caused by many things, including room acoustics/geometry (how the lobes of sound from both drivers interact in the room; additively and/or subtractively) or because the VC rubs more when oriented one way vs the other.  To test that, turn the drivers 90 degrees in the cab and repeat the orientation test.

Note, I suspect that this is port/cab tuning because the resonant freq of a guitar 12" is 65-80 Hz usually, so a 70Hz C is right about where I would expect the port tuning to be. It may not sound like typical resonant peak because it is not the speaker's resonant frequency, but the cab's; higher then the speaker's and more energetic (because you're playing a bass not a guitar).  The speaker cone may also be buckling a bit (cone breakup; can sound kinda like distortion or fartiness, depending).  If the cone breakup is this bad then it could be time for a recone (all bass drivers will fatigue and need to be reconed depite lack of VC issues eventually; you will often be able to see creases or stress marks on the cone, especially if smooth vs ribbed.... super rigid synthetics and metal cones possibly excepted, but assuming standard paper or poly cones, especially paper).

[Happy Face]

Thanks Granny. A very thorough list of items to go through!

I fired up the old Hans Ohm website to try and get info on the cab. He had it listed as an "instrumentalbox" rather than a bassbox or gitarrenbox. Sadly, he had no spec sheet on the ET20, but the others listed as instrumentalbox were listed as being for organ or guitar.

Guess I should eliminate whether a speaker is at fault first, then try some stuffing.

You can see the cab here:
http://www.ebay.com/itm/Echolette-ET-20-speaker-cabinet-/331467757289?pt=LH_DefaultDomain_0&hash=item4d2d0496e9

[patman]

Have you tried pushing on the cone gently and listening for the voice coil?  This is usually where you can tell.  If it's on the way out, you will hear it.

[the mojo hobo]

Because the cab wasn't designed for bass it maybe would benefit from some internal bracing.

[Happy Face]

Thanks everyone.

Must put aside some time and try everything suggested.

Luckily, it's not so bad, and it get drowned out at volume.