Need advice on grille project


2010-09-07 10:40 am
I had a close call today in which I narrowly avoided having to have one of my beloved EVX-180B drivers reconed. The near miss prompted me to finally design some grilles for the cabinets I have them in.

These cabinets are large. The drawing below doesn't really do them justice - the area the grille needs to cover is 27.5" x 44.5". The speaker shown is an 18" woofer, and the ports are all 4" diameter, flared to 6" at the baffle.

Now, back when I was constructing the cabinets, I had the forethought to purchase some perforated metal for the grilles. In the interest of acoustic transparency, I bought the pattern with the greatest open area I could find - 1/4" hexagons on 9/32" centers, giving ~79% open area. The problem I have now that I have worked with this material a bit, is that it is a bit flimsy - only 22 gauge (0.0299"), and may not be able to span the whole width of the cabinet nicely without a bit of extra support to stiffen it.

Looking at the image below, the box on the right is my first concept for the grille. Looking from the front, there is a waterjet-cut bezel of what I am thinking will probably be about 14 gauge steel (0.0747") which will stiffen the mesh and hold the whole assembly down via screws into threaded inserts in the baffle, followed by the aforementioned mesh, followed by a frame made of 5/8" x 5/8" x 0.065" wall square steel tubing, to act as a standoff and provide stiffness to the whole assembly. In addition to a basic rectangular frame, the square tube would run beneath the other legs in the bezel as well, so that only the area over the woofer would be unsupported. All of the metal parts are to be welded together, tacked / stiched as necessary to eliminate rattles.


Now, regardless of how carefully the mesh is straightened and welded along its perimeter, the chance that it will be tight and stiff enough to resist deflecting across a 24" span over the woofer is small. Hence the second concept on the left, which includes a few extra legs on the bezel (and possibly additional elements underneath these on the back side of the mesh) to help stiffen the mesh over the woofer, at the expense of adding metal right in front of the driver, which may be a bad idea - I'm actually not sure. That's why I'm posting here - I don't know how bad putting material right in front of a driver is. This cabinet is currently covering three octaves, from 32 to 256 Hz. In the future, I hope to build additional midbass cabinets to drop the demand on these to only two octaves (~32-128 Hz). The additional bezel legs would diminish the benefit of the wide-open mesh, but if the cruft in front of the cone has little effect on bass frequencies, that may not matter. I've never used grilles before so I have no experience with this.