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Old 2007-04-18, 01:48
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Soulinsane Soulinsane is offline
Pirate Lawd
Join Date: May 2004
Location: Hanger 18
Posts: 6,520
Speaker cab foam mod

This is a super easy mod to be done to just about any speaker cab to tighten low end and low mid range response. It will eliminate a cabs tendency to sound boomy by lowering the noise peak resonance frequencies while at the same time extending the low end frequency a few Hz.

But first, while you have the cab open, it is a good time to inspect your speaker wiring, speaker mounting, cab structure strength, and the cabs seal integrity.

Speaker wire inspection: If you have not read the speaker wire mod I posted earlier in this thread then now would be a good time to do so. Upgrading your wiring is a higher priority than added foam.

Speaker mount inspection: Make sure the speaker(s) bolt all the way through the baffle, even if front mounted, with a T-nuts ( or clamps or normal nuts) and bolt screws. Most cabs use "nut and bolt" mounting but a few cabs use just wood screws. This will not provide the speaker seal(s) with proper tension and the speaker(s) will eventually vibrate badly and fall off the baffle. Replace "screwed in" speaker mounting with "nut and bolt" type speaker mounting.

If you have "nut and bolt" mounting then give each nut screw an even tension twist with a screw driver, but not to much or you will strip the threads. Just make sure nothing is loose and every "nut and bolt" have an even amount of tension. Its critical to insure the speaker seals effectively against the baffle board.

Cab structure inspection: Most cabs use glue to hold the joints together in conjunction with internal bracing 1x1' batons stapled or glue in to better seal the cab. I guess its a cheap and good way to make cabs, but it can be strengthened further. It's not necessary in most cases but if you want to strengthen the cab structure then just add wood screws through the 1x1' baton bracing into the cab shell. I suggest predrilling the screws unless you want the bracing batons to split. Once done you should be able to hide behind your cab during a nuclear attack and feel safe.

Cab seal integrity inspection: Now this is critical, even if you have a ported or open back cab; and most important for closed back cabs as no air should ever escape. A cab needs to be sealed air tight to prevent air from escaping where is shouldn't for many reasons. I would love to explain everything but it took me endless hours of researching detailed material to see why it is important. It would take me pages to explain, so I would rather get right to the simple methods of how to properly seal a cab.

Get some caulking and go over ever bracing baton corner within the cab. Do not use it around the speaker(s)! A speaker has its own seal so no need for added sealing.

Around the handles and removable back board use a weatherstrip tape to seal the cab.

FYI: I took shit a step further and built an air tight isolation box around my jack port. I did this because I could see light around the jack so I knew it wasn't air tight. I used wood, glue, wood screws, and caulking to completely seal the jack box. Now my cab is completely sealed.

Inspection and preparation is now done, time to add foam.

There are many expensive sound damping foams. Inside a cab there is no need for expensive foam unless you have money to burn for bragging rights. I bought a single twin bed egg crate foam mattress at Walmart for $10. It's not pretty but no one will see it inside the cab. It is made of the same stuff and works the same as the expensive audio foam. Don't stuff the cab with blue jeans or popcorn foam... please. Its to dense and not porous enough to brake up standing sound waves.

You don't want foam in contact with the speakers, so don't add foam to the speaker baffle. Parallel solid surfaces benefit the most from having foam attached. A cab doesn't have to be packed with foam to achieve the desired effect. Use 3m glue spray on the foam surface to be attached and then stick it to the inside of the cab. Don't spray the glue inside the cab! Other glue will work too but allow a few days to cure. I use wood glue on my foam and it is still holding strong.

How does it work and what about space reduction inside the cab? The needed foam for this mod should be porous and if compressed into a solid block of foam should only consume several cubic inches. Its reduction on cab airspace is effectively null. Depending on how dense the foam is ( profession audio foam used on studio walls is very dense ) it could take more of less relative space inside the cab. Since most egg crate foam is so porous and air creates most of its volume, a cabs inside airspace with foam remains about the same. The speakers still have far more than ample airspace to work safely and effectively for their natural lives. What is important about foam is that air vibrated by the speaker(s) becomes dampened once contacted by the foam. The foam absorbs the energy/vibrations and doesn't bounce it back like solid wood surfaces. These bounced back vibrations are called standing waves ( like an echo ) and will influence speaker vibration in a unwanted way.

Foam will destroy standing waves, kill boomy tone, and expand lower end frequency response. Bass cabs and closed back guitar cabs see the best results from this mod.

If there are any questions then please post them.

The result.
From a different angle.

Good luck
Authorized Mercury Magnetics tech/dealer
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