G-K 212MBE cab lining and tweeter falling off

[LOLeaux]

I bought a pair of these. Following some comments from BFM, and believe me, I'm not contradicting anything he and other long-time members have said, I lined one with 1-2" of mattress topper PU foam. The 9V resonance test didn't really convince me, and after lining one of the two, I have to admit I'm not that enthusiastic about lining. Clearly, lined is better, but IMHO, not by much.

Here's an uncalibrated, near-field, only relative frequency response plot of unlined and lined.



While applying the lining through the port and feeling the tweeter configuration with my fingers, the tweeter actually fell off from the horn that's attached to the front panel. I was gentle! Easy (tape) fix, but not confidence-inspiring. I've e-mailed G-K and will let you know.

 

[tallboybass]

Very interesting! Looks like the lining helped that crazy thing that happens around 600 hz though.

Surprised no one has commented so far.

 

[B-string]

Interesting! Some smoothing of response.

 

[will33]

Meh, they hit the dirt at 120, nice bass cab. Those things also suffer from a total lack of bracing. Try running a 3/4" dowel or other 1x1 stick or something front to back, side to side and top to bottom. Killing some panel resonance would be another improvement and would help things.

 

[billfitzmaurice]

If the two charts were identically measured the loss of sensitivity below 300Hz indicates over-damping, with material that's either too thick and/or too dense. But even over-damped there should be no difference at 100Hz, so I suspect that they weren't identically measured. Moving the lined plot up so that the 100Hz response of both charts coincides would give a more valid comparison. It also may reveal higher output below 80Hz from the damped cab, which,along with the taming of the various peaks and valleys above 300 Hz, is exactly what damping is supposed to do.

 

[LOLeaux]

Very interesting! Looks like the lining helped that crazy thing that happens around 600 hz though.

Surprised no one has commented so far.

The round-trip distance from front to back wall is a wavelength at about 600 Hz. I wonder if lining just the back wall would have been enough.

Meh, they hit the dirt at 120, nice bass cab.

Perhaps by "nice" you meant something else. If so, ok, opinions always welcome.

I suspect that the low-frequency rolloff isn't entirely the cab. I used a Shure Beta 58A vocal mic that is spec'd to roll off about 5 dB from 200 to 50 Hz in vocal use and perhaps with a large-diameter source the proximity effect would be even worse. I intended only to show the effect of lining this particular cab as best I could and didn't/don't claim to have accurately measured the cab response in any particular environment.

Those things also suffer from a total lack of bracing.

The cab is in fact braced, with a 5/8" to 3/4" (metric?) plywood brace about 4" wide from front to back and centered plus smaller ones front-to-back midway on the long sides.

If the two charts were identically measured the loss of sensitivity below 300Hz indicates over-damping, with material that's either too thick and/or too dense. But even over-damped there should be no difference at 100Hz, so I suspect that they weren't identically measured. Moving the lined plot up so that the 100Hz response of both charts coincides would give a more valid comparison. It also may reveal higher output below 80Hz from the damped cab, which,along with the taming of the various peaks and valleys above 300 Hz, is exactly what damping is supposed to do.

Insightful and interesting comments, as always, Bill. Although the cabs were measured side by side at almost the same time, they are two different serial numbers (although measurements taken before lining the one cab are similar especially near 600 Hz), the mic position re the drivers was only eyeballed, their location re two walls was a couple of feet different, etc. I repeated some measurements to suggest that the repeatability was less than the mid-bass ripple, but I wouldn't bet that it approached +/- 1 dB. One of those things that if I'd waited to do right, wouldn't have happened at all. Time to get back to playing the bass!

 

[will33]

Well, the vocal mic will throw things off a bit but if you got the 2 tests done identically, the charts should still be of some use in comparison to each other. As stated, the 600hz thing would be a function of cancellations due to the dimensions of the cab, which lining does smooth out. Should sound a bit less "boxy" in general. Following Bill's measurement description, it may indeed have better lower response than it looks on the chart.

 

[will33]

The lowend rolloff looked surprising to me as it should have ample box size to do better than that with most any 12 that's spec'd anywhere near what you'd use for bass guitar.

If the tweet is piezo or some all plastic component you can glue it back together. If it's an hf driver threaded or screwed onto a plastic horn, it's likely the horn that's cracked somewhere.

 

[billfitzmaurice]

I used a Shure Beta 58A vocal mic

That at least partially explains the lack of low end; placement of the cab in the room matters as well.

The round-trip distance from front to back wall is a wavelength at about 600 Hz. I wonder if lining just the back wall would have been enough.

Every panel to panel distance introduces a different resonance that needs to be damped, the baffle to rear distance is just the most obvious.

 

[garagebassman]

I bought a pair of these. Following some comments from BFM, and believe me, I'm not contradicting anything he and other long-time members have said, I lined one with 1-2" of mattress topper PU foam. The 9V resonance test didn't really convince me, and after lining one of the two, I have to admit I'm not that enthusiastic about lining. Clearly, lined is better, but IMHO, not by much.

Here's an uncalibrated, near-field, only relative frequency response plot of unlined and lined.



While applying the lining through the port and feeling the tweeter configuration with my fingers, the tweeter actually fell off from the horn that's attached to the front panel. I was gentle! Easy (tape) fix, but not confidence-inspiring. I've e-mailed G-K and will let you know.

This is a great read for me...thank You for posting.

First I agree that the tweet's a cheapy...my cab was delivered and I found the little screen cover at the bottom of the cab (fell off during shipping) , and removed the grille to extract and save it. I know i could have it fixed (bought new from rmc audio)...but I was too "jazzed" to play it and gig it (with MB500).

but, more importantly, and after reading a lot of good info and intelligent posts from people here (including in this post 'BFM')
I did go ahead and line it after about 8 mos of giggin it. I was always very happy with it except for at one room where a hollow stage was not helping, seemed to resolve more or less with a small carpet under cab (these cabs are kinda "too light" to put wheels on like nearly every other cab I've owned)

I did always "appreciate" the gritty harshness of the mids , and lost a touch of that after the lining (1-2" poly fill/pad), but possibly smoothed things out overall, and maybe evened out the bottom a tad, but I have been considering getting a second, and was purposefully planning to not lineit until i could try to compare the two.

Thanks to this thread, I can "see" what I "may" be hearing, or at least am perceiving to hear ?

In any case, I also agree that this cab may provide an(other) instance of SOME improvement with lining in it, certainly didn't hurt the sound, I also don't really feel that mine improved to the point where I would consider the lining "necessary" ....but as always, other ears may differ !

in any case . thanks for posting it....but now I'll need to find another "excuse" to buy a second 212MBE ! doh !

 

[rpsands]

With this particular cab I'm pretty sure you won't see as much improvement from lining as you would from adding a couple cross-piece dowels. Based on what I've heard about the interior being mainly unbraced.

 

[LOLeaux]

With this particular cab I'm pretty sure you won't see as much improvement from lining as you would from adding a couple cross-piece dowels. Based on what I've heard about the interior being mainly unbraced.

Despite several speculative posts and plentiful misinformation about these cabs to the contrary, my new cabs are in fact braced with a 5/8" to 3/4" (metric?) plywood brace about 4" wide from front to back and centered plus smaller ones front-to-back midway on the long sides.

I would probably buy a pair of 8 ohm cabs rather than 4 ohm or choose a smaller amp if I had a do-over. My ShuttleMAX 12.0 600W/ch exactly matches the thermal cab rating, but as I have learned from others here on TB, at low frequencies the excursion-limited power handling is much less. How much less I don't know for sure, but with a 5-string, I am definitely monitoring the LF boost and keeping an ear on it at larger gigs.

 

[WingKL]

Ooo, is this one of those stealth engineering updates after all the heat on TB?

 

[garagebassman]

Ooo, is this one of those stealth engineering updates after all the heat on TB?

fwiw...mine also has similar bracing described.....and i've had it for nearly a year....it was the first 8 ohm one I could get when most dealers were listing/selling only 4 ohm version

 

[ulynch]

fwiw...mine also has similar bracing described.....and i've had it for nearly a year....it was the first 8 ohm one I could get when most dealers were listing/selling only 4 ohm version

Ditto here. I've had my 4-ohm for over a year also, and it's similarly braced.

Oh, and thanks to LOLeaux and BFM. It's a fascinating test to start with, made more interesting by smart folks providing insight.

- D

 

[rpsands]

If there is bracing I have doubts that it's sufficient, especially on the top and bottom. I rapped on the panels of a few MB series cabs (1x15 and 2x10) and they felt extremely boomy to me.

* It's entirely possible that the 2x12 model, which came out a lot later, came with bracing where the 15/210 models did not.

 

[billfitzmaurice]

If there is bracing I have doubts that it's sufficient, especially on the top and bottom. I rapped on the panels of a few MB series cabs (1x15 and 2x10) and they felt extremely boomy to me.

As unscientific as it might sound ye olde knuckle test does work. If the sound heard is a solid high pitched whack chances are the cab is adequately braced. If it's a hollow sound akin to a drum it isn't.

 

[Psilosounds]

I feel like a certain amount of cab resonance is what makes these sound as lively and large as they do. Call me old school, but I would rather have a cab that adds a bit of its own life, even if slightly quirky (as long as you learn what it is doing and where), than a cab with total focus and ruler flat response that sounds boxy and lifeless.

 

[will33]

I feel like a certain amount of cab resonance is what makes these sound as lively and large as they do. Call me old school, but I would rather have a cab that adds a bit of its own life, even if slightly quirky (as long as you learn what it is doing and where), than a cab with total focus and ruler flat response that sounds boxy and lifeless.

That sometimes can sound good in guitar cabs. In a bass cab, any energy wasted vibrating the box is energy not making sound. The box resonance also cancels the corresponding frequency the driver is producing. You can have stiff panels and still not have "total focus/ruler flat response" should you so choose. Take 2 identical designs, make one box stiff and the other flimsy, the stiff one will play louder even though the drivers are the same.

 

[billfitzmaurice]

That sometimes can sound good in guitar cabs. In a bass cab, any energy wasted vibrating the box is energy not making sound. The box resonance also cancels the corresponding frequency the driver is producing. You can have stiff panels and still not have "total focus/ruler flat response" should you so choose. Take 2 identical designs, make one box stiff and the other flimsy, the stiff one will play louder even though the drivers are the same.

+1. Flexing panels on a bass cab can't add anything to either tone or output, they can only take away.