Bending a little a Bass Trapp...

barefoot · Post by **barefoot** » Thu Aug 14, 2003 3:34 am

Feni,

I would build the front panel like this:

Mount a rectangular or oval center block in the middle of a rigid table (work bench, door, etc.) Spread glue over the inside surfaces of two or more thin sheets of material from which you want to build the panel. This could be thin plywood, veneer (alternate the grain directions), plastic, aluminum or whatever. Just make sure you use glue that has a long enough working time and bonds very well to the panel material. Also cut the sheets slightly over size because they will not align perfectly. Lay the glued sheets over the center block and table then clamp them down around the perimeter using a rigid frame. You may also want to place a heavy object in the center as well. Allow the glue to cure then remove the panel from the jig. Trim the edges to their final dimensions and that's it.

Thomas

knightfly · Post by **knightfly** » Thu Aug 14, 2003 4:59 am

Wow, Thomas, all you need is an 8" edge-wound voice coil, a 300 pound magnet, and you're SEISMIC -

feni · Post by **feni** » Thu Aug 14, 2003 7:01 am

Wow ! wow ! Wow ! Many many many thanx !!!!!! Barefoot !
it's 6;00 in the morning at my time (HongKong), but i must salute to u before going to sleep, i'd a long day, today !
best regard and thanx again !
Feni

barefoot · Post by **barefoot** » Thu Aug 14, 2003 7:50 am

Oh yeah. It would also be very easy to make something like this out of fiberglass cloth and plastic resin. Tap Plastics http://tapplastics.com or some other such supplier has everything you would need except for the mold to lay it over.

Thomas

Todzilla · Post by **Todzilla** » Thu Aug 14, 2003 10:11 pm

Regarding Everest's use of curved surfaces, I believe his usage is solely as polydiffusors, without bass trapping functionality. I have mimmicked this for my ceiling diffusor, although it seems to have much more value as a diffusor than a bass trap.

Check it out:

http://www.toddejones.com/hugebuilding.html

The most crucial thing of all is that it looks cool.

giles117 · Post by **giles117** » Thu Aug 14, 2003 11:14 pm

What would be waaaayyy cool would be if you had a floorplan of what you are building on your page.

Bryan Giles

feni · Post by **feni** » Fri Aug 15, 2003 1:12 am

Dear barefoot
Will the Panel trap cal. formula remind true with this new design ? or some adjustment needed ?
Thanx !
Feni

barefoot · Post by **barefoot** » Fri Aug 15, 2003 3:17 am

Feni,

If you build a broadband absorber, the calculation isn't critical. Most of the design formulas I've seen use the resonator box depth as the normalized air volume element. A reasonable approximation would be to add 1/2 the depth of the bulge to the box depth in order to account for the extra volume behind the bulge compared to a flat panel.

If you really want to get precise, I would suggest lying the panel down like a bowl then filling it level to the perimeter with water, sand, rice, beans or some other such thing. Measure the volume (Vf) of the fill. Then calculate the exact extra box depth (d) that will account for this volume.

d = Vf/(H*W)

Where H and W are the height and width of the resonator box.

So the modified box depth you would use in your resonance frequency calculation would be:

D' = D + d

Where D is the actual box depth.

Thomas

feni · Post by **feni** » Fri Aug 15, 2003 5:51 am

U are great Barefoot ! !
it's a cool panel trap for me, so, if i make the box heavy enough, will it become a stand alone unit or better follow the same old rule ? " stay near the wall ! "
Thanx a million !

Feni

Eric_Desart · Post by **Eric_Desart** » Fri Aug 15, 2003 10:27 pm

Hello Barefoot,

barefoot wrote:Yes, the entire perimeter of the front panel should be sealed with the flexible suspension.

Actually it might be easier to make the front panel bulge out in both the horizontal and vertical planes, such that the perimeter is flat. Hopefully this illustration makes sense.

Thomas

Nice picture/design.

Stupid question having little experience with that.
Which drawing package did you use for this 3D effect?

Best regards
Eric

barefoot · Post by **barefoot** » Sat Aug 16, 2003 2:49 am

Feni,

Portable is fine, but it's best to stay near the walls - most especially near the corners - since this is where the absorbers will be most effective.

Eric,

I just "hand" drew it in Paint Shop Pro.

Thomas

feni · Post by **feni** » Sat Aug 16, 2003 4:20 am

Thanx barefoot !
it is a chellanging DIY ! must call for help !
Best regard !
Feni

illium · Post by **illium** » Thu Sep 11, 2003 8:35 am

i tend to agree with zikaj about this designs lack of effectiveness as a membrane absorber...

coming from the background of one who used to build wooden acoustic instruments for a living (violins, violas, cellos, guitars, mandolins, etc..) a bent peice of wood (ie, one that is under stress) will not excite as freely as a flat peice of wood, and also will not excite evenly, as the resistance is not even... in other words, certain areas of the board would excite at certain pressure levels, but others would not.

this is why on instruments that have a curved soundboard (like a violin) you carve a solid peice of wood down into a dished shape starting with a peice of wood that is the entire depth of the dish... it is also vital to make sure the thickness of the curve is the same across the entire dish.... instruments made from plywood are much more difficult to excite, and instruments made from BENT plywood are even worse!..

as a diffusor however, bending the plywood would increase it's functionality...

although, now that i've said that, i think i understand your reasoning for suspending the panel in a flexible membrane... this way, any force moves the whole panel, rather than flexing it, as in the traditional panel absorber design. so the shape of that panel becomes irrelevant to it's function in the absorber....

would this reduce or increase the absorbers effectiveness? does a flexing panel or a suspended moving panel have a better result?

hope i'm not making a fool of myself. (:

_illium

barefoot · Post by **barefoot** » Thu Sep 11, 2003 9:11 am

illium,

Yes, this design uses a flexible suspension around the perimeter of the panel. It's more like a loudspeaker cone, rather than a violin soundboard. It should be more efficient than a flexible membrane because almost the entire panel can move with equal amplitude. A flexible membrane has maximum amplitude in the center falling off to zero amplitude at the edges.

Thomas

illium · Post by **illium** » Thu Sep 11, 2003 9:24 am

this design would also give a wider frequency range for the absorber (Q)... since the flexible meduim is actually the cloth around the panel, it is far more flexible than wood, and would react to higher frequencies and lower volumes.

btw: i would suggest titebond alphalytic resin glue (off-white bottle, red cap, not blue!!) for combining the panel layers. also, soaking the panels in water (warm to hot preferablly, though that would be hard with something this size...) will help them to bend more freely... i would soak the panels over night... just leave an outdoor hose/sprinkler running over them all night... that's how we used to make our skateboard ramps. (: wetting the boards will make them have a smoother contour than just bending them dry, and will allow a deeper dish..

_illium