What Internal Volume for a guitar cab?

[Mind Riot]

Hello all. I just registered on here. I find this stuff to be fascinating and informative. I'm not too heavily into building cabs and enclosures, but what I am interested in is building a 1x12 guitar cab. I know about the Golden Ratio and will design the cab with proper dimensions, even though it seems to be generally thought that the dimensions are less vital for guitar cabs.

What I'm wondering is what is the recommended internal volume for a 1x12 cab? I know more specifics are needed, so here I go,

The driver is a Celestion G12H-80. It is a discontinued model so the TS parameters on it are somewhat difficult to locate. It is a very clean and neutral speaker as guitar speakers go, but it tends to have a bit more midrange and treble than bass, so I would like to design the cab to fill out the low end a bit, but not to an extreme.

Currently the driver is in a very cheaply made 2x12 with the other 12" disconnected. I only need to record in my home, so 1x12 is more practical, plus I get the fun and satisfaction of building it myself. The ratio is fairly easy to calculate, I just need a volume recommendation.

Oh, and do you guys recommend caulking the inside joints of a guitar cab? Or using any kind of filler material? If so, do you have any recommendations as to what types and where to acquire them?

Thanks in advance for any responses!!

[kelticwizard]

The Thiele-Small parameters are difficult to locate because even in present models, Celestion doesn't seem to publish them.

I can give you a good guess, though.

Here is a pdf which gives you some data plus a frequency response chart.
http://www.mojotone.com/G12H-80.pdf

You already know the SPL: 99 dB, as backed up by the chart.

You already know the resonant frequency-90 Hz, although the chart seems to indicate 83 Hz, (certainly within acceptable variation).

Qts? Under the formula that output at resonance = 20 log Qts, we see that the SPL at resonance is between 9 and 10 dB down from the SPL in the midband.

Well, if the Qts =.4, then the SPL is -8 dB at resonance, (Fs).

If the Qts = .3, the SPL is -10.5 at resonance, (Fs).

So a good estimate is the Qts of the woofer in the chart is about .35.

Vas? Well, looking at some brands that do publish the figures like Eminence, Precison Devices and McKenzie, we can assume that a 12 inch guitar driver with a 90 Hz resonant frequency and a 99 dB sensitivity is likely to have a Vas about 1.0 cubic feet.. Call it 1.2 cubic feet because it is better to have a little more than a little less.

The speaker itself should occupy no more than 4 liters, or .14 cu ft.

So here is a good guess of your Thiele-Small parameters for your Celestion G12H-80:

Fs=90 Hz
Qts=0.35
Vas=1.2 cu ft, (34 liters)
SPL=99 dB@1M/1W

I have never designed a guitar speaker, so I will let someone else who has play with those numbers for a little while to see what he comes up with.

As far as having not much on the low end. You said that you have a 2x12 cab with one speaker disconnected. Well that second speaker is acting like a Passive Radiator, (a variation on a port) and is definitely tuning the cab one way or another. My guess, if you are complaining about lack of low end, is that the second speaker is tuning the cab above 150 Hz, maybe higher, and as a result you are getting big output in the 150 Hz range or above but very little below it. That is how ported and Passive Radiator boxes work-increased output above the port/Passive Radiator tuning frequency, fast dropoff beneath it.

For a quick suggestion, I would advise removing the disconnected driver and then plugging the hole. You can either cut an old piece of plywood if you have a small saber saw handy, or you can just take a couple of pillows, squeeze them together real tight, and stuff them into the hole. I'm serious. That will make the enclosure act similar to a completely closed box. Then see how it sounds.

Like I said, I would prefer to have an experienced guitar cab maker design your box with the Thiele-Small parameters I estimated. I can come back later with a couple of suggestions, but understand that they will be coming from someone who never built a guitar cab.

[leadbelly]

Originally posted by Mind Riot
What I'm wondering is what is the recommended internal volume for a 1x12 cab

What sound are you looking for? Why won't a standard open back cabinet meet your needs?

[sreten]

The simple answer here is is its not critical for guitar,
you can use a sealed box that looks the right size
and there is no point making it bigger than this.

Giutar cabs usually have no stuffing, but you can
get a darker / cleaner sound with stuffing added.

You sometimes see 12's in quite large (frontally)
boxes, this is an advantage with open backed
designs.

(Bass guitar is an entirely different matter)

sreten.

[PeteMcK]

whether you like the sound of open back or closed back cabs;
Closed will give you more volume and more low end;

If you want a cube shaped box, I'd measure up a marshall quad box and make your box 1/4 of the size, then you can try open & closed by removing the back

I've made a wedge shaped box, so the speaker is facing towards my ears rather than my knees; lets you play at a lower volume in those noisy jam sessions...

Pete McK