[Ultralight]

I have a sub that, when driven even to moderate level, vibrates a bit on the enclosure surfaces. 

Anyone ever try to 'wrap' some of the enclosure surfaces with vibration absorbing material so that the vibration is damped and does not excite any air?  If so, what did you use?  Or is this a fool's errand?   

Thanks,
UL

[richidoo]

The frequency band of a sub is below the resonant freq of common sub box materials like plywood or MDF. So the box is expanding and contracting synchronous to the signal due to internal pressure, but it is not resonating, which is ringing after the signal ends. Flexing in sync but opposite polarity to the signal will slightly reduce the SPL, but not change the tonal color as much as it would if it was resonanting in the midband where our hearing is very sensitive to the tonal changes caused by the time shift of real resonance or ringing.

Since typical wrap-on vibe damping materials like dynamat, bitumen sheet or paint-on goop don't add any stiffness, they won't resist the flexing due to internal pressure. Also, the commercial sound deadening wraps are designed to damp metal (for cars) which resonantes at a much higher frequency and higher Q than wood. Thin Dynamat type wrap will have minimal effect on MDF. I have used GAF roofing material on plywood, and it does help, but it is 4mm thick, comes in a 100pound 30 foot long roll and difficult to acquire by the roll, must be glued on. If it was a midrange cabinet that plays in a freq. band that is within the resonant freq. band of plywood, then such damping materials would help to reduce resonance of the wood and clean up the sound. That kind of time-skewed resonance does affect tonal color.

You could add internal mechanical bracing to reduce the flexing, but the value of doing that depends on the quality of the driver, amp and the bass alignment of the design. If it is a low price sub, then there are many design choices that compromise performance so stiffening the box may not make any noticeable difference if the driver has a small motor and the amp is underpowered, or the alignment is optimized to be "small but loud" for HT and not optimized for transient detail in music. If it's a high performance "music" sub (sealed, big, heavy) then maybe adding bracing would be audible, but it would probably not be needed because the mfg would have designed the box to flex less to compliment the higher performance driver. If it's a reputable commercial brand my guess is that they have braced the box just enough to match the price/performance target, just as they optimize every other design element to meet the design goals.

It's not hard to glue in some cross-bracing sticks to see what happens. 
Rich

[Peter J]

The frequency band of a sub is below the resonant freq of common sub box materials like plywood or MDF. So the box is expanding and contracting synchronous to the signal due to internal pressure, but it is not resonating, which is ringing after the signal ends. Flexing in sync but opposite polarity to the signal will slightly reduce the SPL, but not change the tonal color as much as it would if it was resonanting in the midband where our hearing is very sensitive to the tonal changes caused by the time shift of real resonance or ringing.

Since typical wrap-on vibe damping materials like dynamat, bitumen sheet or paint-on goop don't add any stiffness, they won't resist the flexing due to internal pressure. Also, the commercial sound deadening wraps are designed to damp metal (for cars) which resonantes at a much higher frequency and higher Q than wood. Thin Dynamat type wrap will have minimal effect on MDF. I have used GAF roofing material on plywood, and it does help, but it is 4mm thick, comes in a 100pound 30 foot long roll and difficult to acquire by the roll, must be glued on. If it was a midrange cabinet that plays in a freq. band that is within the resonant freq. band of plywood, then such damping materials would help to reduce resonance of the wood and clean up the sound. That kind of time-skewed resonance does affect tonal color.

You could add internal mechanical bracing to reduce the flexing, but the value of doing that depends on the quality of the driver, amp and the bass alignment of the design. If it is a low price sub, then there are many design choices that compromise performance so stiffening the box may not make any noticeable difference if the driver has a small motor and the amp is underpowered, or the alignment is optimized to be "small but loud" for HT and not optimized for transient detail in music. If it's a high performance "music" sub (sealed, big, heavy) then maybe adding bracing would be audible, but it would probably not be needed because the mfg would have designed the box to flex less to compliment the higher performance driver. If it's a reputable commercial brand my guess is that they have braced the box just enough to match the price/performance target, just as they optimize every other design element to meet the design goals.

It's not hard to glue in some cross-bracing sticks to see what happens. 
Rich

Thanks for a well thought out, coherent explanation, Rich. 

[charmerci]

I have a sub that, when driven even to moderate level, vibrates a bit on the enclosure surfaces. 

Anyone ever try to 'wrap' some of the enclosure surfaces with vibration absorbing material so that the vibration is damped and does not excite any air?  If so, what did you use?  Or is this a fool's errand?   

Thanks,
UL

It'll help but with a sub that's built so cheaply that it does that, it probably is a fool's errand. Can you build another box for it?

[Ultralight]

Huge thanks to all.  And especially you Rich.  Superb education.

Do have a question for all of you. These are low cost subs but functions superbly.   

What if instead of reinforcing it internally, I simply glue 3/4" thick high density MDF boards externally across the three open flat surfaces?  That should make it rigid like a rock....

Thanks again,
UL

[RDavidson]

Well.....you're really just adding mass going that route.
What you need to add is stiffness, which bracing does.

[Duke]

Huge thanks to all.  And especially you Rich.  Superb education.

Do have a question for all of you. These are low cost subs but functions superbly.   

What if instead of reinforcing it internally, I simply glue 3/4" thick high density MDF boards externally across the three open flat surfaces?  That should make it rigid like a rock....

Thanks again,
UL

Agreed, Rich's post is great. 

For stiffening the panels, I'd use void-free (perhaps a true Baltic Birch) 3/4" plywood instead of MDF.  If you really want to go to the extreme, double-thick the plywood in the centers of the panels, as it's the centers that will tend to flex the most (assuming there is no internal bracing). 

[Quiet Earth]

These are low cost subs but functions superbly.

Hi UL,
  Have you tried placing the subs on a sandbox? Or maybe an Auralex Sub-dude? That seems like a more logical upgrade to me.

[richidoo]

Porcelain tile makes a good glue-on-panel type stiffener. Von Schweikert uses something like this with a hard heavy quartz countertop material along with a compliant layer between. Porcelain is heavy, hard to cut, and not so pretty with the exposed edges. Maybe you don't care about the looks and can find a tile that is the right size. A little smaller is OK, doesn't have to go perfectly to the edge. Glue it on with PL urethane construction adhesive. Ceramic tile will crack, but porcelain is very strong. You can use an abrasive cutting wheel on grinder to cut it.

I think you'll find that removing the driver and wedging in a couple plywood sticks from side to side and top to bottom to be a lot easier.   Don't be afraid to remove the driver. You can't hurt anything, just don't over tighten the screws when you reinstall the driver as MDF is easy to strip screw holes. Definitely glue the cross brace sticks in place securely so they don't fall out of place when the MDF box relaxes over time. You don't want them getting jammed in the driver.

[Ultralight]

Thank you all - everyone of the suggestions from cross bracing to subdude makes sense.  I'm evaluating my final two system and if the subwoofer system stays, I'll implement the suggestions.  I know that there are a lot that can be improved in my more traditional system which includes the subs. 

Thanks!  You guys are champs!!

UL