Open-backing a closed-back cab - design suggestions? Rules of thumb?

[Che_Guitarra]

A few days ago I made back panels for a pair of 1x12 cabs I have. Prior, these cabs were completely open-backed - no back paneling of any kind... apart from a wee strip to hold the jack.

I never expected to keep them fully-closed, but I have lived with them like this for a few days to get a feel for the change in response (and just in case it was exactly what the doctor ordered). Tighter bottom end? You bet. Beamier projection? Yes sir. Interestingly, it also killed of a lot of top end richness and complexity.

Anyway, i'm digressing.


I notice most Fender-style combos/cabs are actually 3/4 closed-back (or thereabouts). Is there a best design (or rules of thumb) for me to follow as I turn these from closed-back into some kind of open-back design?

Celestion V30s inside. I want these cabs to suit a 'rock' orientation. Not blues, not metal.

 

[archetype]

Some aftermarket build-to-order cabs are available with convertable backs consisting of 3 panels. Pull out the middle panel and you have a 1/3 open back. Seems to be a good formula that blends a bit of the sealed cab low end with a more open tone. Way better than just removing the whole back.

 

[Che_Guitarra]

Time is of the essence, so I had to knock this project on the head while I could.

Here's my end results. No hard science to the design - just an average of everything I saw via a google search, and what I could manage with the tools I own.

All I can say - unbelievable. That is, without doubt, the best $30 i've ever spent on mods. The improvement has almost left me dumbfounded. All the shortcomings I thought were amp-related - it was actually my cabs.

My bass and mid response is now tight and focused, the top end has retained the opened-back complexity and detail, the speakers are much more punchy and dynamic, and for the first time ever the amp has a useable dimed sound. And I will no longer hesitate to reach for my humbuckered guitars for fear of bad pairing with the amp. Wow.

I'm absolutely thrilled with the results.

 

[jackal]

I've just cut a hole in the back of a couple of closed backs, and was really pleased with the sound. Just guessed as to the size and shape.

 

[Greenstreet]

Kevin O'Connor did lots of research and experimentation on the subject a few decades ago.

If I remember correctly, he concluded that the opening should be roughly the same size as the speaker (π r², so for example with a 12" speaker, you'd want an opening of about 113 square inches, give or take).

Conveniently, if you have a 2X12 closed back cabinet, just taking out one of the speakers will get you there.

 

[Che_Guitarra]

Kevin O'Connor did lots of research and experimentation on the subject a few decades ago.

If I remember correctly, he concluded that the opening should be roughly the same size as the speaker

Funny you should say that. That's exactly what I was aiming for when routing out the holes.