RPG Bad Panel -- Detailed Plans

[Syncamorea]

The bag should work fine. I know a builder that makes hardwood chairs with bent and twisted boards. He uses a bag for his steam treatment.

Please put up some pictures as you build!

[Adhoc]

Originally Posted by AwwDeOhh

Was going to use .5", but am going with 9/16 (only slightly larger) for a bit more absorption in the equation. i'm retro-fitting some 6" deep panels on the back wall (there's already QRD and some polys inbetween as well as QRD's on the back sides and a prd skyline on the ceiling) to help even out the rooms' decay a bit more (there's aLOT of absorpiton in my small room). I've got the low end pretty much under control (as much as it can be in this room) but the decay falls off more than i'd like in the mid-to-high end. If i don't like how they turn out (for the room), i can always put them on my Gobo for another tracking option.

Yes the acrylic plastic grid won't put up a fight against a drillbit, but it saves me from drawing out the grid, and if i can keep the drill from touching any side of the square, the hole should be darn close to vertical (there is only the very slightest bit of wiggle room w/ 9/16 bit)

With the pattern i'm using there will be 1.5" of "unused" space around the edges, which i figure should be plenty enough for mounting hardware or any trim i might add.

If you read through all RPGs papers on BADs carefully, you will find a
diagram showing panels with 1/2" and 5/8" holes.

Results from the diagram show that 5/8" holes versus 1/2" will give:
Less reflection/diffusion for higher frequencies (usually seen as worse performance)
Less absorbtion for lower frequencies (usually seen as worse performance).

Personally I would not use that plastic grid, 37 squares wide, but mark out all the holes with a good ruler / square directly on one piece of plywood, clamp together a number of plywood sheets and drill them all at the same time.

4 mm or 1/8" plywood is quite bendable even with out steaming, with little risk of cracking if you use 1/2" holes versus a 20 mm (3/4") inch grid. You will need D-shaped sides and also straight pieces of wood between these D-shapes, to nail down the plywood. It will bend up "a bit" in the middle otherwise. (This is from my practical DIY experience on BAD-copying.)

[Syncamorea]

Thanks for refreshing my brain on the 1/2 vs 5/8" data, Adhoc. I'd waded through a bunch of their tech pages and had forgotten that tidbit.

I can't remember if you have mentioned how much of a bend you have in your panels?

Even 3/4" plywood can be bent quite a bit if you cut 3/8" deep reliefs in the back. I've built highly sculptured climbing walls this way that retained strength and stiffness very well. I'd fix one edge and as the bends were being secured to the frame, fill the reliefs from behind with gorilla glue.

I think I've talked myself into using BAD panels in my about-to-start mix room build. I picked up the hardwood flooring last week and am about to rip out the carpet. Yippee!

[Adhoc]

Depths of the curved shapes varies, they where chosen depending on where in the room the panels were placed, 1210 x 70 mm, 1000 x 102 mm, 895 x 185 mm etc. The curve in the pictures below, from my first experiment with the BADs in my previous flat (2007), is 1000x102 mm. Total dim 2000 x 250 mm deep, with insulation inside it works pretty well as a low frequency absorber too.

An easy way to make a smooth curve is to mark the unbent width of the panel as well as the center between the marks on a slim strip of easily bent Masonite (the Masonite should be about 12 inches or so longer than the width of the panel). Hammer 2 nails onto the board to be D-shaped and mark out the center between them. Now, push up the Masonite between the nails, center mark to center mark until you get your wanted depth and mark out the curve with a pencil.

[AwwDeOhh]

Thanks Adhoc.
Ends up i need to use 1/2" holes anyhow...
After drilling 5 rows of holes with the 9/16" bit, i noticed that i'm getting some tear-out between some of the holes. The plywood just doesn't like that tight of spacing. (but the MDF scrap i was using underneath has beautiful holes! )
I've decided to salvage some of the ply anyhow by starting on the other end of the board and going for two 2'x2' panels w/.5" holes. Doing 1/2" holes gives me about 3/16" spacing between holes (with the help of a hole punch/marker to keep the bit from wandering)

If i do as you say and draw out a grid, how big should the squares be? should i just divide it up evenly from end-to-end (to get 33 squares wide)? Should i do another sequence that is 37 squares?

I'll have to get some more material to give the 2x4' panel(s) another go. Luckily i haven't invested too much time before noticing the damage.

[Adhoc]

For 2'x2' panels = square and not rectangular, the grid size should be slighly less than 3/4" x 3/4" if you want a trim around the edges. Check up the spread sheet in post 69 for exact measurent on the different width and height for a square panel. If it is OK with a rectangular panel, you go with equal width an height.

If you buy a forstner drill bit for the holes, you will get less tear out on the backside (compared to a normal drill bit with 118 degree angle). High speed together with low force gives best result.

[AwwDeOhh]

Originally Posted by Adhoc

For 2'x2' panels = square and not rectangular, the grid size should be slighly less than 3/4" x 3/4" if you want a trim around the edges. Check up the spread sheet in post 69 for exact measurent on the different width and height for a square panel. If it is OK with a rectangular panel, you go with equal width an height.

The plastic 'grid' is exactly 5/8" squares (with 1/16" 'lines'). Since i'm already halfway through drilling .5 inch holes, i'm just going to finish these and see how they turn out. Next build i'll grab better quality material and draw a proper grid. I'm just anxious to see what kind of effect these have compared to other diffusors. Was worth a try anyhow...

Originally Posted by Adhoc

If you buy a forstner drill bit for the holes, you will get less tear out on the backside (compared to a normal drill bit with 118 degree angle). High speed together with low force gives best result.

Yeah.. i'm used to working with metal (used to work in a metal fab shop), so most of my equipment is geared towards that. Thanks for the tip.
Too bad i can't fit this thing under my drill-press... that would be perfect!

[OpusOfTrolls]

While a press would be faster, if you have a good drill with bubble level, it would be just as simple.

[AwwDeOhh]

Here's the prediction for my first shorcut attempt


Pics to follow

[senor roboto]

hi!
just curious as I'm sure a bunch of you on here know way more about this than me: would it work to basically "grow," or scale up this design x4 so the grid is 2" squares instead of .5" ? doing that would lower the range of frequencies that the diffuser works at right?

backing would remain the same, a 4" piece of OC705 or similar

[AwwDeOhh]

If you have Excel (windows Office), or even OpenOffice you can use this:

Porous Absorber Calculator V1.59

At the bottom of the Excel sheet, there's a tab for predictions other than porous absorbers; you should find what you're looking for there.

[gullfo]

larger openings tend to affect the HF more than the LF. however the BAD panels are "amplitude grating" which means the numeric periodicity of the panel has a specific relationship to the underlying absorption which defines it operating characteristics... so simply trying to scale it differently than defined by RPG could lead to unexpected results

on making these types of panels - there are a lot more CNC machines around in local shops so you might find it a huge time saver (and reasonable cost) to provide the blanks and a simple CAD file with the layout and let them do the precise cutting of the openings... http://en.wikipedia.org/wiki/Milling_machine

[petertjed]

These Keypacs from auralex Auralex Acoustics - KeyPacs have bigger holes, I can't find any info about them. But they look easy to build.

Best regards,

Peter

[Guillermo]

After reading this post, we recently decided to build some BAD panels for a small 5.1 control room and they work really well, we used 4 mm MDF for the frontal face and 4" airspace filled with rigid black mineral fiber.