bracing problem with speaker cabs

[bob123]

hi all

i have just finished building a set of pa speakers. each has 2x12" drivers a horn tweater and a 6" bass reflex port.
they sound amazing but when i put some loud music on with a lot of bass. the back of the case vibrates only in the middle.
i think this is due to the size of the speakers and the fact that i only used 12mm mdf to build them the dimentions are
50cm x 100cm x 30cm

is there any way that i could thurther brace these speakers to prevent the back from vibrating with the bass?

some trick that some of you veterains out there have learned.


thanx for any posts.

[richie00boy]

Do a search on 'bracing', this was discussed very recently. Adding ribs seems like a good choice.

[johninCR]

With only 12mm mdf, you need some serious bracing. It's not just the back that is flexing. Hopefully one of the panels is removeable. If so use at least 4 shelf braces, 1 parallel with the 1m dimension and 3 front to back at 25cm 50cm and 75cm. Cut big holes in each so air can flow freely and internal volume changes minimally.

If your only access is through the driver cutouts, you'll need a number of ribs, triangles to connect ribs along intersecting dimensions, and front to back and side to side cross braces (also attached to the ribs). It might not look pretty, but no one will see the inside anyway. Do one first so you can see that as long as you don't change the volume too much, there's no such thing as overkill for bracing a 12mm thick cab. The sonic difference will be quite significant.

[bob123]

thanks the problem is that the speakers are designed to be light as well as big hence the 12mm what is the best way to brace whilest keeping the weight down.
i am able to open the back as they are screwed down with 14 screws.

oh and i have put padding in yet as i have not got round to this. what difference will this make to the sound as it is near perfect at the moment. also will this partialy stop the vibrating of the back.

[johninCR]

Some shelf bracing will add very little weight and will increase the structural integrity immensely. They do need to be glued to the back panel also, so afterward your only access will be through the driver cutouts. Damping with provide little benefit to a vented cab compared to good bracing.

[bob123]

thanks for the help.
i think i will use the shelf bracing method, but is there a way i can do this whilst still being able to get the back off as the screws for the drivers are on the inside and i will not be able to open them.

[johninCR]

Just glue the braces to other 3 sides and screw the back plate into the braces as well.

[HiSPL]

This is probably a day late and a dollar short, but the best way to keep the weight down without losing stiffness is by using cabinet grade birch plywood. Thats why the pro's use it.....

[THOR]

if you install tension rods inside the cabs, they will get very rigid. The rods are installed in the center of the panel and should be cut too short. How much too short will depend on the size of the panel. The rods are secured by large screws from the outside which bows the panel inward. You want to bow it as much as is practical, Thus i cant tell you how much too short to make the rods but the panels should touch them (and be glued)after the preload "dish" is applied to the panel. The force required to bow the panel is the amount of "preload" placed on the diaphragm like panel. The wooden rods can be made of 1 or 1 1/2" closet rods . After thier installation, in order for the panel to vibrate at all,the force of the vibrations must exceed the "preload" force , something a loud speaker can not do! The rods should be glued to the inside of the panel as well, but not of course the back panel. RODS SHOULD CONNECT ALL SIX SIDES.This is the lightest and strongest you can get.

[BillFitzmaurice]

oh and i have put padding in yet as i have not got round to this. what difference will this make to the sound as it is near perfect at the moment. also will this partialy stop the vibrating of the back.

With wideband PA woofers fully lining the cabinet with 25mm of polyester batting is an absolute must; otherwise your midrange response will have more peaks and valleys than Switzerland.

12mm plywood is quite adequate, preferably birch, but only when properly braced, as pointed out by the other posters. Full shelf bracing extending from the baffle to the back spaced 15cm apart will cure the problem. Cut large holes in the shelves to lighten them and allow free passage of air inside the box. Line all shelves with batting; you don't have to cut holes in that .

Your drivers should have been mounted from the cabinet front so that the bracing would not interfere with their removal; this would be a good time to go back and correct that. The drivers should be mounted with bolts and T nuts; screws aren't sufficient.