Iso Box redux

[daav]

I built an ISO box not so long ago, and am very happy with the sound control aspects, but I am starting to wonder if my lack of design forethought might have made me compromise too much on the quality of the recordings themselves.

Basically the box is about 16 inches wide, 28 inches long and 18 inches high. The speaker is mounted in slab of decent quality plywood where the magnet of the speaker is about 4 inches from the back of the box. The space in front of the speaker is pretty small, with auralex along the bottom, both sides and the front wall. With that, there is barely enough room for a SM57 to stretch from a inch or two from the speaker to the back wall.

My original thought was that since i was close micing the speaker, it would not make that much of a difference what the space inside was, but now i am second guessing and considering rebuilding the box to make a couple improvements. I find the recordings to be a bit boomy and maybe muddy right now, but i am not sure if that is the amp (91 or so Fender Blues deluxe) or if a speaker change out might help or if it is the box itself. Building a box is cheap and fun, so that isn't a big deal.

Wondering what ideal dimensions might be, would i be better off lining the whole space with 4 inches of OC 703 and then a space and then the walls? Should i make the wall not parellel, etc. With such a small smace in there, will any of this matter?

Thanks,
Daav

 

[Zaphod B]

My first thought was that you've probably got that speaker in a box that's too small - small enough that you are encountering resonant frequency problems, or boosting the low-mid response of the speaker and getting a boomy sound.

Typically, sealed enclosures for guitar speakers are deliberately oversized. This results in a very gradual low-frequency rolloff wherein the resonant frequency of the speaker is so far down from peak efficiency that you won't trigger a resonance problem.

A smaller enclosure affects the lower frequency response of a speaker by boosting its lower frequency response as it would normally start to roll off (a common trick with low-cost audio speaker design) but this must be done with care to ensure that (1) you don't end up with a "boomy" sound caused by unnaturally boosted low-mids and (2) you don't have the response boosted at the speaker's resonant frequency, which causes voice coil overexcursion and a resulting very nasty speaker flutter.

On re-reading your post, I think I undersand that you actually have two sealed enclosures that share the speaker baffle. To be honest, I don't know what the ramifications are in terms of acoustic damping. But I'd start with the basic premise that the part of the box that contains the back part of the speaker should be sized appropriately for what the speaker would normally be mounted in, and the front part should be big enough to let the speaker have some breathing room. How much, I don't know, but it still seems as if the overall enclosure is way too small and that this is affecting the sound.

 

[tom18222]

is it a closed back amp? ported? that may be the cause.

 

[daav]

Oh-- to clarify, i am not putting the amp in the box, only the speaker. Speaker is mounted in a piece of plywood.

Attached is a quick mock up from Paint.

Daav

 

[Zaphod B]

Right, that's what I thought.

I think your box is too small, particularly if it's even close to scale - particularly the portion containing the speaker basket.

 

[faderbug]

Oh-- to clarify, i am not putting the amp in the box, only the speaker. Speaker is mounted in a piece of plywood.

Attached is a quick mock up from Paint.

Daav

ok is that pic showing exactly where you position the mic, when it is why don't you do center speaker and the recorded tone should improve tremendoulsly.

 

[daav]

First of all thanks for the replies, it is great to get some outside opinions on this.

faderbug- I do move the mic around, shown is one option- I thought that more toward the center was usally more bass heavy, but i haven't really done much serious a/b'ing, I've tried a few spots and decided 'this sounds best'. That doesn't mean with a bit more trial and error i wouldn't be able to improve it.

Also, the amp has not been retubed for years, it is probabaly due. And I am using the 12 inche speaker that came with the amp. I have another 10 inch from David Eden that i have set up to test once, but haven't really tried out much yet. The 10 might help. But I am still interested in a great iso box design, so...

See my design idea below, what do you think? It is box-within-a-box, which is the only way i could really get the volume undercontrol on the first attempt. The outer dark grey recatanlge is the outside box, and the inner cofin shaped dark grey is the inner box. The light grey rectangles will be OC 703 2- 4 inches thick, 2-4 inches away from the inner walls for absorbtion.

The doors will be from the front side in both boxes so I can put stuff on top and still get at the mics and speakers to switch out and change location.

Would love some feedback and input on the proper dimensions of the box. I would use high quality voidless plywood, probably glued and caulked with bisquit joints. The inner box will be sitting on something to try to isolate from the outer box a bit, either the tops of nails or some sort of foam like the commercial speaker isolation things. I may pack moving blankets or some4hting else between the boxes as well.

 

[Zaphod B]

For the proper size of the part that corresponds to a speaker cab, find a closed-back 4X12 cab, measure it, and divide by 4. You don't need any insulation in that part of the cab. Just make it strong, thick, and tight. Use some foam tape on the baffle where you mount the speaker to eliminate the possibility of rattles.

The "mic" side should probably be as large as your situation can comfortably accommodate. I'd recommend playing with insulation placement on this side - you may just want to have some on a couple of walls, or just in the corners, or you may find some equally cool and unique sounds based on your insulation placement.

 

[daav]

For the proper size of the part that corresponds to a speaker cab, find a closed-back 4X12 cab, measure it, and divide by 4. You don't need any insulation in that part of the cab. Just make it strong, thick, and tight. Use some foam tape on the baffle where you mount the speaker to eliminate the possibility of rattles.

The "mic" side should probably be as large as your situation can comfortably accommodate. I'd recommend playing with insulation placement on this side - you may just want to have some on a couple of walls, or just in the corners, or you may find some equally cool and unique sounds based on your insulation placement.

Sounds good, so if i read you right, the part behind the speaker should be a bit longer and in proportion ot a 1/4 sized 4 X 12 cab, like if we made a 4X 4 grid with a speak in each section of the grid?

I think I will need to seal the area from the speaker to that back cab part better as well, the one i have now has about a 2 inch gap from a bit above the top of the speaker to the top of the box.

This go-round i am going to try to make a long, low box, kind of like a long enclosed bench that I can put against a wall and use like furniture and have it fit intot he room a bit better.

Thanks much!

 

[Zaphod B]

Sounds good, so if i read you right, the part behind the speaker should be a bit longer and in proportion ot a 1/4 sized 4 X 12 cab, like if we made a 4X 4 grid with a speak in each section of the grid?

Yep!

I think I will need to seal the area from the speaker to that back cab part better as well, the one i have now has about a 2 inch gap from a bit above the top of the speaker to the top of the box.

For sure, seal it tight. Silicone window caulk in the joints works well and it's cheap. You want to isolate what's happening inside that part of the box from the mic.

This go-round i am going to try to make a long, low box, kind of like a long enclosed bench that I can put against a wall and use like furniture and have it fit intot he room a bit better.

Thanks much!

You're welcome, and good luck!