sound bar in a speaker cab design mystery

hector

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1,077
I see some closed back 2x12 speaker cabs with a horizontal "sound bar" running internally from the speaker baffle to the back of the cab. I assume the purpose is to transfer vibration to the back of the cab. While some major vendors do not have the "sound bar or bass bar". When I put my arm against the back of a cab without the bar, I hear a slight increase in volume and low end frequencies. Much like when you touch your guitar headstock against a table or wall, when it's vibrating. So, I imagine the sound bar is a good thing. However, maybe it's only purpose is to support or strengthen the cab. Also, wondering if it's anything like the sound post in a violin, which travels from the top to the back of the body. The positioning of this post is a large factor in the violin's tone. Can someone with experience shed some light on this horizontal bar's purpose and mystery? Is it structural or tone related?
 

trisonic

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13,152
You'll see them in some old Marshall 4x12s. There's a screw which holds them to the back (so you can remove it). My understanding was that they prevent too much excursion of the backs which would otherwise create noise from the form of unwanted vibration.

Best, Pete.
 

hector

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1,077
Yes. I see. So, it's nothing like the sound post in a violin. Where, the post is part of the tone. With speaker cabs, the desire is to vibrate the speaker, and nothing else, correct? I suppose that would explain the speaker gaskets and why tightening the speakers too much is considered a bad thing (transferring vibrations).
 

khromo231

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Yes. I see. So, it's nothing like the sound post in a violin. Where, the post is part of the tone. With speaker cabs, the desire is to vibrate the speaker, and nothing else, correct? I suppose that would explain the speaker gaskets and why tightening the speakers too much is considered a bad thing (transferring vibrations).
With hi-fi and pro sound cabs, the idea is to have the cone vibrate and nothing else (except the air excited by the cone). The main reason you don't want to overtighten speaker mounting screws is you can bend the frame if you compress the gasket unevenly, or if your baffle is not flat.

With guitar cabs, it can be a little different, as the sound of a vibrating pine box in your tweed Twin or Deluxe is sometimes thought to be part of the tone we associate with those amps. This is probably somewhat true when you're talking lower volumes and cleaner tones. The problem for guitar cab designers is predicting how cabinet resonance will behave at different volumes. Some manufacturers claim to do it, I have my doubts.
 

Jef Bardsley

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2,951
That bar is intended to brace both the baffle and the back panel, to prevent those panels from vibrating more than is absolutely necessary, and in doing so absorbing energy and causing other undesirable effects. They are commonly used in well designed pro sound cabinets.

It's almost certainly just a brace, not a "tone bar", "sound post" or "bass post." Guitar cab salesmen sometimes get carried away with assigning magical properties to commonplace hardware. Uncontrolled cabinet resonance is usually not a good thing!
Well said.

One man's tone secret is another man's tone suck.

Most players would be horrified if their rig sounded like Billy Gibbons' more recent excursions into tonal aberrations, yet the Reverend has managed to make quite a few hit records with them.

In general, though, I think khromo is right - resonances tend to be volume dependent and frequency specific. If a resonance increases with volume, it's likely to get out of control, if it doesn't then it will be drowned out. If it does, then it will likely become 'high Q' - that is, it will affect one narrow frequency band. Your cab might sound good when you play in A, but not so good in C.

The reason people like closed cabs is because they limit the sound to that coming from the front of the speaker, so I think as a rule it's safe to say that they should be built as solidly as possible. Especially if you want to get a solid "thunk" from the bass notes. Sound that leaks through the cabinet will decrease the impulse response and cause intermodulation distortion. I.E., it will sound dull and muddy. ;)


(BTW, I have built enclosures that add musically to the soundfield. However, they have more in common with a grand piano than a steamer trunk. They are very big, very expensive, and quite room dependent. Not the sort of thing a gigging musician wants to deal with.)
 

hector

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1,077
I tested out two of my cabs tonight by sending a recorded loop through them. The one with the post did not vibrate much at all on the back panel. The one without the post had much back vibration, and it's not even a fully closed back. So, I agree that it must be for dampening purposes. While we're on the subject of dampening, what about those who claim advantages to adding insulation foam to the inner walls of their closed cabs? What is the point of that ??
 

khromo231

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Stuffing or at least padding two adjoining sides and the back of a closed or ported cab helps minimize standing waves. The stuffing compresses a little when waves hit it, and converts some of the energy into heat which is harmlessly dissipated. The big concern with closed back cabs, especially those with parallel sides, which is almost all of them, is that mid range sound waves trapped inside the box batter the rear of the cone, which can cause ragged, unpredictable frequency response. It is much more of a problem at higher volumes. It can also improve bass response, duplicating the performance of a slightly larger box.
 

khromo231

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- resonances tend to be volume dependent and frequency specific. If a resonance increases with volume, it's likely to get out of control, if it doesn't then it will be drowned out. If it does, then it will likely become 'high Q' - that is, it will affect one narrow frequency band. Your cab might sound good when you play in A, but not so good in C.

That's a great explanation of why cabinet resonances are so tricky to "design into" guitar cabs. You may be able to tune the resonance of the box itself, but how do you use that musically? Unless you are vamping on the same frequency all night long your cab resonances are hurting more than they are helping.:barf
 

Tone_Terrific

Silver Supporting Member
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34,039
Stuffing or at least padding two adjoining sides and the back of a closed or ported cab helps minimize standing waves. The stuffing compresses a little when waves hit it, and converts some of the energy into heat which is harmlessly dissipated. The big concern with closed back cabs, especially those with parallel sides, which is almost all of them, is that mid range sound waves trapped inside the box batter the rear of the cone, which can cause ragged, unpredictable frequency response. It is much more of a problem at higher volumes. It can also improve bass response, duplicating the performance of a slightly larger box.
Overstuffing, maybe any stuffing, in a guitar cab can kill off desirable characteristics, too. Guitar cabs are weird...not to mention the players:eeks
 

Bruce Clement

Platinum Supporting Member
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Back to the brace - if your cab does have one, be sure it is svrewed in nice and tight. I've heard a few cabs that at certain frequencies the vibrating back panel would knock against the end of the brace because it wasn't screwed in tight. You can imagine what that sounds like.
 

hector

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1,077
Thanks for all your comments, guys. Obviously, the cab selection and speakers make an impact on your final tone. It's always subjective when people comment on the tone of a particular head, when we're all using different cabs, speakers, etc.
 






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