Closed back combo: Bad idea if the cabinet is small?

[Ptron]

Like really small. It's a 12" speaker and it will probably be 15"h x 17"w 10"d at most. That's the speaker compartment, not the whole amp. What if it was ported or had a thin open strip along the bottom or something? My instinct is the resonant frequencies of such a small box would not be the ones I would want or result in the "typical closed back sound", if there is such a thing.

[black_labb]

depends on the speaker. if you have teh speakers t/s specs plug them into winisd to check. otherwise compare speakers that are likely somewhat similar. marshall 4x12's were designed to fit the 4 speakers into as small a cab as possible (without tipping backwards from being too shallow). it does effect the sound a fair bit, but not neccesarily in a bad way

if its a tube amp i'd be concerned with ventilation for the tubes. make sure you have the tubes in a sepatate ventilated compartment. otherwise having the amp on for an hour could mean alot of heat trapped in the sealed section. plywood and tolex dont conduct heat very well.

[cbarrow7625]

Ptron, you do not want to make it into a ported cabinet. Ported cabinets do not work well for guitar in most cases. The added group delay and port resonance tend to add up to make the woofer sound unnatural. A typical 4 x 12" cabinet is about 30" x 30" x 14". 1/4 of that would be 15" x 15" x 14". Sounds like you are right in the ballpark for a closed back cabinet that is almost exactly 1/4 the volume of a 4 x 12". You might be a littel small but not too terribly.

I would try it in the closed back cab first. If that goes horribly wrong then I would recommend cutting a large enough slot in the rear of the cabinet that the "port" resonance is well below woofer resonance so that the cab acts more like a partial open back than a ported cab. Do not put this slot on the front. You will get too much out of phase cancellation from the rear of the driver combining with the output from the front of the driver at useful frequencies. Putting the slot on the rear of the cabinet will force the output from the rear of the driver to take a longer path around the cabinet and be out of phase at lower, less useful frequencies (not up in the lower mid-band). It will also attenuate the output from the rear slightly compared to the front.

[cbarrow7625]

Good thought but I've found that guitar speakers do not always represent well in cabinet simulation programs. The T/S parameters are so far off from those usually required for a good "hi-fi" or "professional" speaker that the results spit out by the program can be way different than anything you've ever seen from a guitar cabinet - in a bad way. I once had the program tell me that I needed a negative cabinet volume based on the T/S specs for a Jensen Alnico 25 watt speaker. I never did figure out how to build that cabinet.

Guitar speakers & cabinets are made to influence the tone not to be neutral to it. Pro & Hi-fi speakers are just the opposite and these programs (and T/S parameters in general) are dialed in to help you neutralize the contribution from the speaker, not emphasize it as we usually want.

[Steve Conner]

Index of /speaker voila!

This was inspired by the EV TL806, which is really not that big volume-wise. High efficiency speakers like the EV (and probably many other guitar speakers too) end up needing a smallish cab and a high port tuning, and making mostly midbass output with very little actual bottom end. But that's the way of the world. It's a case of "deep, loud, and portable: choose two", and for this project I took loud and portable.

The port tubes only cost a few bucks each, so I bought a spare set to try a lower tuning frequency. But I liked it better with the original 89Hz tuning.

Once you start driving guitar speakers heavily, they go non-linear and all the math and CAD goes out of the window. That's probably why Celestion won't even tell you the T-S parameters for their guitar speakers.

[Ptron]

Rough Sketch

My proportions are completely whack in this sketch, it looks way deeper than it will be, but something like this:



cbarrow, you are absolutely right. I can always do it this way and take out the back pieces or cut a hole later if it doesn't work out. The only slight downside is the overall cab has to be taller than my original design so the txfrmers clear the enclosure.

Sort of a side question: I've been playing/testing this amp through the speaker (Jensen P12Q) unmounted and it's very honky. Is it typical for unmounted speakers to sound like that? I don't really feel like going and pulling speakers out of my other cabs to find out.

[cbarrow7625]

It is 100% normal for an unmounted speaker to sound honky. First, all of the lower frequencies are cancelled becasue the front & rear of the speaker just cancel each other out at the longer wavelengths. Second, as the wavelengths get shorter they start cancelling at only specific frequencies creating a "comb filter". Last, the frequencies that aren't cancelled are actually adding together from the front & back of the cone exacerbating the comb filter effect.

That will all clear up once mounted.

[Ptron]

So birch ply seems to be the going thing around here for baffles. I used 5/8" on the last cab I made. Does the size of the cab affect the choice of baffle/back/top thickness? Like would I lean towards 1/2" for this guy and 5/8" for the similar but larger version I plan to build next?

[Ptron]

Ok then. 1/2" baltic birch ply it is for both projects 'cuz I just dropped forty on a 5x5 sheet. Time to start cutting some wood. I'll post some pics when I have something.

[Guitarist]

I second that, porting in the back can sound fine. I have a pair of 1x12 cabs roughly the same as yours and they can PUMP low end. They've Swamp Thangs and two ports. The portability is rather handy.

I wonder about the 5150 combo that had sealed 12s and if anyone added some ports.

Ear Candy are the forerunners of this approach it appears.