Speaker cabinet (guitar) - Air tightness & other stuff

[TinkeringSteve]

Hey,

I'm experimenting with DIY guitar speaker cabinet stuff.
Right now I'm building a closed-back one, and for that to work properly as I understand it, it needs to be air tight, so that there is some resistance by the limited air volume behind the speaker being compressed or expanded.

So I was wondering how to make the front & back plate / frame joint air tight.
The cab has an inner wood frame around the inner circumference of the whole thing, at the back, against which I intend to press the e.g. back plate with some M6 screws. Between the back plate and that frame, I wanted to put some of that black adhesive foam rubber tape, like 1..2mm, which would seal, hopefully, imperfections in the frame/plate joint, which is, by itself, probably not air tight, due to several "hobbyist in the garage corner & minimal tool set, doing this for the 1st time" construction challenges

Now I wonder... would this actually be "correct", I mean - such a film of foam tape vs. wood-on-wood means that the whole thing is less like "one piece" acoustically, the foam dampens higher frequencies I would guess.
(And the front, as planned so far, gets coarse speaker protection cloth wrapped-around the plate, not sure how air tight that can be made, maybe I'll manage to staple it just to the sides of the plate, instead of going fully around and staple it on the inside...)

But I do not understand enough of speaker cabinets' detailed physical workings to know whether that matters or not. If the wood is not supposed to contribute anything by "oscillating" itself, and it's just about making the thing air tight, then it probably won't matter.

But is that so?

[themadhippy]

i tend to run a thickish bead of silicon sealer around the frame then screw the panels to the frame,excess silicon gets squeezed out,but still maintains a good air tight join.
 Also a fair amount of guitar cabs are open backed,even those like marshal that are sealed,aint air tight

[Benta]

"Air tight" might be relevant for frequencies below 1 Hz.
Don't believe everything you read on the Interwebs.

[TinkeringSteve]

"Air tight" might be relevant for frequencies below 1 Hz.
Don't believe everything you read on the Interwebs.

Should I believe you?
(yeah I know, the likelihood of woo here is different than on audiophooles-R-us.com )

I also noticed there are some in-between cabs which have like, both, the upper and lower third of the back "closed" by a plank of wood, and the center third is open.
I wonder at which point it could be considered "closed" with regards to the frequencies of interest (say 60..70Hz).

As for putting in some sealant which will harden - I want the thing to remain open-able.

[SiliconWizard]

Completely closed? That's weird for your application.
Even closed speaker cabinets for HiFi usually have vents (sometimes cleverly designed), or they are simply gigantic (and never completely airtight anyway).
I guess a completely airtight cabinet could give you a number of problems to solve, with only marginally better frequency response (provided the internal volume is sufficient for your speaker(s)...)

For a guitar amp... really? I don't think so.

[tooki]

Whether vented or not, what is critical is that air only move through the intended orifices. So sealing everything else is critical. Even small leaks can cause massive distortion. (In the subwoofer of a set of PC speakers I have, I immediately noticed that I hadn’t quite tightened a screw enough after a repair, just because of the sound. Air was leaking around the gasket, instead of solely through the vent, and it sounded awful.)

[TinkeringSteve]

Completely closed? That's weird for your application.
Even closed speaker cabinets for HiFi usually have vents (sometimes cleverly designed), or they are simply gigantic (and never completely airtight anyway).
I guess a completely airtight cabinet could give you a number of problems to solve, with only marginally better frequency response (provided the internal volume is sufficient for your speaker(s)...)

It's said to have a punchier response, whatever that means exactly.

For a guitar amp... really? I don't think so.

Have a look at the Orange PPC113 then.



That's what the dimensions I used are based off of, and the speaker I have is a knock-off of the one they used, just with the nasty mid spike fixed, so they say, otherwise very similar.

[SiliconWizard]

Closed backs have better low-frequency response. You may or may not like the corresponding sound with a guitar.
I didn't say that closed back cabinets were useless. I was specifically talking about fully airtight ones. So I'd be curious to take a look at those Orange cabinets behind the speaker grille. I'd be surprised if there was no vent.

[TinkeringSteve]

Closed backs have better low-frequency response. You may or may not like the corresponding sound with a guitar.
I didn't say that closed back cabinets were useless. I was specifically talking about fully airtight ones. So I'd be curious to take a look at those Orange cabinets behind the speaker grille. I'd be surprised if there was no vent.

Hrm. I could go to a shop and have a closer look, trying to avoid store personnel looking strange when I touch feel the thing to detect vents

It looks like there is a slit that could work as a vent there behind the cloth, between those 2 wooden parts of the front. No idea whether that's deliberate, and whether the 1x12" has some extra vent somewhere (having only 1 front plate, presumably), that would indeed be interesting.
(UH, the time is &t=2m47s, which this player strips...)

[themadhippy]

So I'd be curious to take a look at those Orange cabinets behind the speaker grille. I'd be surprised if there was no vent.

your in for a surprise then.

[SiliconWizard]

It looks like there is a slit that could work as a vent there behind the cloth, between those 2 wooden parts of the front. No idea whether that's deliberate, and whether the 1x12" has some extra vent somewhere (having only 1 front plate, presumably), that would indeed be interesting.

It's hard to see in the video. What can be seen though, is that there doesn't seem to be any special sealing. Just glue, nails and screws. Pretty basic construction. No way it could be perfectly air tight, at least from what we can see on the video. As to slits, it's really hard to see. But there seems to be one indeed. They show a lot while showing very little. That's the art of filming production without unveiling fabrication secrets. Although again, there frankly doesn't seem to be anything very special here.

[viperidae]

I wouldn't try to make it 100% air tight. You don't want pressure to build up inside when the temperature rises.

[themadhippy]

As to slits, it's really hard to see. But there seems to be one indeed

Its not for tuning,just a left over from making the angled baffle for the top 2 drivers

[TinkeringSteve]

Ok, then I guess I'll try how it sounds without any foamy stuff, and if I hear air hissing through somewhere, I'll try to make changes so that it won't do that, but the air escapes less squeezed.

Say, what is the benefit of some guitar cabs to have only the center 1/3 of the height of the back open?
I.e. upper 1/3 of the back has wood, lower 1/3, but the center 1/3 is missing.
I at first though it might be a compromize, achieving the prevention of waves to the front cancelling with those emitted at the back with less "depth" of the cabinet (front to back measurement), but... the left and right sides are still as "open" (although the whole things is somewhat wider than it is high, hrm...)