Head keeps rolling off my cab?

[hover]

Originally Posted by joelb79

Changing the mass of the speaker cabinet (which is functioning as a complete system given the speaker is acting as a spring) will be the only way to change this. Setting an SVT on top does not count as changing the cabinet mass. Thicker wood throughout would. The whole point is that the mass change has to happen in the system and not what is put on top of the cabinet.

Exactly why I was curious how thick the actual baffleboard is. If it too is made from 1/2", without a full brace scheme across it, it's doomed from the start as the baffle is too "live".

Bracing the cab properly alongside an adequately thick, properly braced baffle will go miles in killing the mechanical resonance of the cabinet...but goes completely against the "light-weight" credo...and, well...you get what we have here.

I still feel it's a product of improper / inadequate bracing.

[toothguy]

Try using rare earth magnets.

[joelb79]

Hard to make that judgement call without seeing the inside of the cabinet. What page was of this thread did the OP open up the cabinet to show the inadequate bracing? Why is everybody now saying that Don @ LDS doesn't know how to make a speaker cabinet?

To the OP: If there wasn't a head on top of the speaker cabinet, would the cabinet itself would walk around the floor at that note?

[joelb79]

http://www.youtube.com/watch?v=qmFt0OVTA70

perhaps this video is a better demonstration of what I am saying is happening, with the blue part being the amplifier head. The whole speaker cabinet itself is the spring. Once the resonant frequency is met, the fun begins. C# anybody? The cabinets resonant frequency is 138.6Hz.

[B-string]

Bracing can be done for not only mechanical rigidity but the change the self resonate frequency to a more desirable point. The hope is to change that point above or below the audio band. You can not eliminate (MECHANICAL) self resonance, you just shift it to a point where it is no longer a concern.

[JoeWPgh]

I think this is a fair assessment. My first thought was 1/2" ply is problematic. Chances are a 3/4" top would probably do it. As it's a finished cab, there's not many 'pretty' solutions. I would look to see if it's possible to laminate a piece of 1/4 or 1/2 to the underside of the cab top.

Originally Posted by joelb79

I do not think that extra bracing in the cabinet will fix this issue. You are dealing with mechanical resonance. When the cabinet is built and constructed; it will have it's own resonant frequency, much like a wine glass has a resonant frequency which when played loud enough will break the wine glass. A cabinet can have its own resonant frequency which when played will cause the entire cabinet to move around.

http://en.wikipedia.org/wiki/Mechanical_resonance

The bracing would make the enclosure more rigid but would do nothing much to change the mechanical resonance of the entire system. I agree this is not exactly a perfectly functioning cabinet since it has this tendency to mechanically resonate at frequencies that the bass guitar reproduces; but bracing will not fix it is my theory. Changing the mass of the speaker cabinet (which is functioning as a complete system given the speaker is acting as a spring) will be the only way to change this. Setting an SVT on top does not count as changing the cabinet mass. Thicker wood throughout would. The whole point is that the mass change has to happen in the system and not what is put on top of the cabinet.

Wine glass breaking with sound:

http://www.youtube.com/watch?v=17tqXgvCN0E

I'm just not certain that bracing will change this behavior unless you can show the cabinet is itself vibrating.

http://www.youtube.com/watch?v=3mclp9QmCGs

Mechanical Resonance caused the failure of the Tacoma Narrows Bridge.

[joelb79]

Originally Posted by JoeWPgh

I think this is a fair assessment. My first thought was 1/2" ply is problematic. Chances are a 3/4" top would probably do it. As it's a finished cab, there's not many 'pretty' solutions. I would look to see if it's possible to laminate a piece of 1/4 or 1/2 to the underside of the cab top.

The change of that much mass would push the resonant frequency much lower (1/2" vs 3/4") eliminating the problem. Bracing wouldn't change the mass much at all (according to Mechanical resonance theory).

[B-string]

I believe you are IHO applying the theory incorrectly here. If the top and bottom panel resonate at 138 Hz and you tie those panels at the center (brace) you divide that resonant by four equal quadrants (now 34.5 Hz). If your intent is to maintain the resonant band. These problems are dealt with all the time. Noise, vibration, harshness points are shifted with small changes to overall mass.

Edit: That IS the whole point of bracing in the first place. Did Don miss something, maybe, it is possible the instrument and eq the person uses presents a problem his testing did not find. There is no shame to be had from product improvement. PERIOD.

[JoeWPgh]

I don't disagree B-String. The problem is that a 1/2 ply box demands very exacting bracing, that may very well hold up better on paper than practice. To what I've read, the question is how to fix what's there, (presumably on the cheap) I think a lamination to the top (depending on what bracing may contradict the solution) will add mass to the the offending panel, as well as damp it's inherent resonance.

[smeet]

Originally Posted by B-string

If the top and bottom panel resonate at 138 Hz and you tie those panels at the center (brace) you divide that resonant by four equal quadrants (now 34.5 Hz).

Off topic, but wouldn't the resonant frequency in this case go higher, to 552Hz? Or am I thinking about this incorrectly?

[B-string]

Originally Posted by JoeWPgh

I don't disagree B-String. The problem is that a 1/2 ply box demands very exacting bracing, that may very well hold up better on paper than practice. To what I've read, the question is how to fix what's there, (presumably on the cheap) I think a lamination to the top (depending on what bracing may contradict the solution) will add mass to the the offending panel, as well as damp it's inherent resonance.

I'm not sure a full panel would be needed. I believe it best to let Don decide what is the best way is to deal with it. I'm sure he wants to know about the concern and wants to learn how best to avoid it ever happening again. Cause real world use and paper are not often on good terms with each other.

[joelb79]

One could use instruments of measurement to measure panel flex to prove what you are saying. I would think this is the way to prove your theory with the offending box loaded as is. as far as I've read from following this thread that nobody has proven that the panels are vibrating causing the issue. I think the speaker cabinet itself is moving causing the top piece which is not resonating to float off.

I would tend to believe your theories if the cabinet was not ported. I might also believe this if somebody posted a picture of the internal structure of the cabinet. This note is far above the tuning frequency as well.

Does the speaker cabinet move on its own across the floor at C#3 without an amp head mounted on top? this would be the demonstration of the speaker cabinet itself reaching resonance.

I also think this thread might be headed for page 51 if we keep up the cheerleaders.

[Downunderwonder]

Panels must flex to create the effect. It's not telekenesis.

[B-string]

Yes the panels must be active for anything to be reactive to it.

[B-string]

Originally Posted by smeet

Off topic, but wouldn't the resonant frequency in this case go higher, to 552Hz? Or am I thinking about this incorrectly?

No you are correct, I had a case of brain flatulence.

[BogeyBass]

Originally Posted by smeet

Off topic, but wouldn't the resonant frequency in this case go higher, to 552Hz? Or am I thinking about this incorrectly?

yes bracing will raise the resonant frequency.

and anything above 1200 hz will have very little panel flex/movement

lower the resonant frequency = more panel movement.
best way to lower the resonant frequency of a panel is to cut a hole in it. hence the 4 holes in the baffle.

better way to lower the resonant frequency is to suspend a weight to it. hence the 4 speakers mounted to it.

how do you then raise the resonant frequency of the baffle to offset these horrible but needed things done to it( speakers cutouts+ suspended weights attached to it )

brace the hell out of it to raise the resonant frequency to 1200hz or higher.

Or the well noted cheerleader method.

which i must say out of all the arguments and disagreements we have had on TB.

I would say we all agree we like cheerleaders

[1n3]

Originally Posted by joelb79

I do not think that extra bracing in the cabinet will fix this issue.

Disagree.

You are dealing with mechanical resonance.

Agree. Mechanical resonance is generally a function of mass, rigidity, and internal damping. Bracing increases rididity, which raises the resonant frequency. All speaker cabinet panels resonate, but usually at high enough frequencies where the damping losses are significant, and amplitudes are lower. Here, the top can resonate at a frequency and amplitude that is problematic (obviously). Bracing could fix this.

[::::BASSIST::::]

I built my fEARful 15/6 with 1/2 inch Italian Poplar, which is a pretty soft, non-stiff wood. In an attempt to minimize vibration I braced the crap out of it, including the baffle. As a result the cab, certainly including the baffle, doesn't vibrate at all.

I got chided a bit for over-bracing around the driver cutout, but don't regret it at all. The tad bit of extra weight and work was worth it, even if just for that extra piece of mind. This is one of the benefits of building your own cab, if you can manage it. You know exactly how its built and you can put in the due diligence of building it right.

[creis2]

The speaker baffle seems to be 3/4" or 1". When I went with the build, Don said he wasn't going to use 1/2" on the baffle. I forgot if he said 3/4" or 1".

The cab doesn't move when I play, even at loud volumes and with the castors on.

[hover]

Originally Posted by joelb79

Why is everybody now saying that Don @ LDS doesn't know how to make a speaker cabinet?

I do not recall a single person saying that or even implying it. I think you're overstating greatly. What I do see is an otherwise unforseeable resonant frequency issue that may or may not be inherent to this one design or even just this one box, and unfortunately the bracing scheme has not met / or anticipated completely this to make the box acoustically "dead". As was said, no shame in it, it's an opportunity to learn, but no one is yelling "burn the witch" here ?!?!? Last thing I'd want to do is slander an up and coming custom shop. but what this is is a learning opportunity I'd think Don would like to pursue.

Here's a thought: You never know...a glue / dado joint around the baffle just may have broken in transit. That would surely cause ANY panel to go nuckin futs. You never know.