Before start making diffuser...

[Rairun]

Hi again
Before starting to make diffuser i just wanted to make sure everything is ok with my plan. Is there anything you want to add or warn me about?

[Jens Eklund]

Why not try:


Advanced N41+10,0 panel: Shifted down 10 depth units
Design frequency 600 hz
Number of wells: 41
Deepest well in mm: 245
Fin width in mm: 3
Width of wells in mm: 25


or


Advanced N41+10,0 panel: Shifted down 10 depth units
Design frequency 525 hz
Number of wells: 41
Deepest well in mm: 280
Fin width in mm: 3
Width of wells in mm: 25


/Jens

[collo]

I concur with Jens' suggestion of using the advanced option to get a lower design frequency for a given depth.

Wth 25mm well widths and a depth of 280mm, you won't see much of the pattern. If this is important, you might want to go to a lesser number of wider wells.

What finish are you going to use? I'm currently building three N13's 150mm deep with 35mm wide wells, and am using a polyurethane finish. When sanding between coats, I can just fit my hand between the fins but the floor plates on the deepest wells are out of reach.

I'm working with about the same number of fins and can attest that there is a bit of work involved. It will go easier if you use a backboard.

Also have a think about how to hold the fins upright as you glue.

[Rairun]

Originally Posted by Jens Eklund

Why not try:

Advanced N41+10,0 panel: Shifted down 10 depth units
Design frequency 525 hz
Number of wells: 41
Deepest well in mm: 280
Fin width in mm: 3
Width of wells in mm: 25

/Jens

Thanks I'll do that.

Originally Posted by collo

What finish are you going to use? I'm currently building three N13's 150mm deep with 35mm wide wells, and am using a polyurethane finish. When sanding between coats, I can just fit my hand between the fins but the floor plates on the deepest wells are out of reach.

Polyurethane finish seems fine, i can use it.

Originally Posted by collo

Also have a think about how to hold the fins upright as you glue.

Yeah, it may be difficult to achieve. I'll think of something.

Btw, what is that extra thickness added bottom after changing it to N41+10,0? Does backboard thickness has an importance here?


And one thing i can't be sure is what to use for cavities. Fiberglass or styrofoam? And why?

[collo]

The grey area at the bottom of the advanced panel shows unused depth units. This allows the panel to be shallower than the standard panel for the same design frequency.
Alternatively, you can use the saved space to have a lower design frequency.

The only thing the backboard affects is the overall physical depth of the panel (= build depth + backboard thickness)

Since a proper QRD is made of rigid material with airtight seams, it doesn't matter what, if anything, you put in the cavities.
I used polystyrene, but that was to keep things aligned during gluing, not for any acoustic reason.

We will be needing photos...

[Rairun]

I see. Btw...

Originally Posted by collo

What finish are you going to use? I'm currently building three N13's 150mm deep with 35mm wide wells, and am using a polyurethane finish. When sanding between coats, I can just fit my hand between the fins but the floor plates on the deepest wells are out of reach.

Why not just sand and finish all stuff before starting to glue them? Thus i won't have to reach the deepest wells.

[tonio]

Highly recommended to paint/stain, poly the fins & inner frame prior to build, as doing so after the build is a PITA.

[collo]

Sealing the timber with the finish before building would make things a lot easier. You can also finish the whole sheet before cutting and then do the edges after cutting.

You would not be able to use normal wood glue though, because it no longer can get into the grain of the timber. Whatever glue you opted for would need to bond non-porus surfaces.

- You could use silicone, but that is more of a sealant than a glue for small thicknesses.
- Epoxy would be messy and expensive.
- Aquadhere Durabond (Titebond II) foams up and has bubbles covered by a skin - not sure how well it seals for thin application.
- Probably best to use a specialist polyurethane glue like the Sikaflex products or equivalent.

Whatever you go for, let us all know how well it performs

[Rairun]

Collo, sicaflex products are sold here. Can you specify the exact model name?

And i wonder why do sections need to be airtight?

[Jens Eklund]

Originally Posted by Rairun

And i wonder why do sections need to be airtight?

If not, there’s a risk of absorption taking place (which we don’t want, at least not in the effective scattering range).


Sincerely Jens Eklund

[collo]

I'm only guessing, but looking down the list of Sika products in the previous link, 'Sikabond' might be the one.
Have a talk to a salesperson (if you can find one)

[Rairun]

My final plan:




I purchased 4mm plywood sheets and got them cut to 2.5cm X 120cm. I'm just curious now, is it important what thickness you use for well tops?

[Jens Eklund]

Originally Posted by Rairun

My final plan:




I purchased 4mm plywood sheets and got them cut to 2.5cm X 120cm. I'm just curious now, is it important what thickness you use for well tops?

Unless way too thin and nothing underneath to stabilize; no.