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Old 16th October 2011, 07:38 AM   #1
akis is offline akis  United Kingdom
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Default Guitar amp loudspeaker(s) enclosure

Hi, I have a little Vox guitar amp, and it uses a sealed box for its loudspeaker. When you take the back cover off the sound loses a lot of bass, and if you remove the speaker from the box completely there is almost no bass at all.

Is there a quick and easy formula that gives the size of the enclosure, maybe in volume, or maybe there are other dimensions that have to be observed?

Edit: I just looked in another thread:
Quote:
as long as you give the 12" about 1.5-2 cubic feet volume itll be fine

say 24" wide by 18" high and 12" deep? sealed or open back...its down to preference. i myself, would prefer closed back/sealed. but then again im using my guitar with a 120Watt dean markley valve head and a Hartke 4x10 bass guitar cab its damned loud and heavy as hell but due to the aluminium drivers, still nice and bright when i want it to be

Last edited by akis; 16th October 2011 at 07:41 AM.
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Old 16th October 2011, 08:17 AM   #2
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For optimal frequency response you can use various measured driver parameters to calculate the appropriate box size. However, guitarists often don't want optimal frequency response, but prefer a bit of tone shaping, so really it's down to experiment. Sealed boxes also tend to get very big if you want better LF response, and that leads on to practicality problems.

Do you have any information on the driver?
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Old 16th October 2011, 09:34 AM   #4
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And what do you want to achieve?
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Old 16th October 2011, 10:09 AM   #5
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Boxes control the bass of a speaker through two means.

One, you've already experienced - they keep the negative wave from the back of the speaker from cancelling the positive wave from the front. By taking the back off, you're letting waves longer than the distance around the side of the box cancel out. By taking the box away, the cancellation goes way up into the midrange (ie. 'no bass at all').

The other thing boxes do is trap air that can be compressed and add to the "spring" of the speaker. In simplest terms, ignoring the "motor", a speaker is a weight (speaker cone) on the end of a spring (the surround and spider). The weight will bounce at a certain frequency, depending on the weight of the weight and the stiffness of the spring. By adding stiffness to the spring with a closed box, we can raise the lowest frequency the speaker can produce.

The problem with guitar speakers is the 'spring' is already so stiff, to keep the speaker from bottoming out when you pluck a string, that to make the suspension stiffer, you'd need a box so small the speaker wouldn't fit. Not that you'd want to raise the lowest frequency anyway - most guitar speakers are weak on bass and depend on a huge EQ boost from the amp to get any bottom end.

And then there's materials and construction to consider..... guitar cabinets make noise. A lot of it, when they're cranked. Just as the right tonewoods and bracing separate good acoustic guitars from mediocre, the way you build the box will affect the sound.

If you just want a new cabinet and aren't looking to turn this into a separate hobby, my best advise is simply copy a known-to-sound-good cabinet as closely as possible.

If you want to mess around, then make a cab a bit bigger than you have now (you shouldn't lose "a lot of bass" taking the back off). Use 3/4" plywood for the baffle and back, and solid pine boards for the sides. Try a back panel that only encloses 2/3s of the back, as 'open back' combos have a cool sound that many prefer, sort of a "folded-back open baffle".

In sum, guitar speakers are direct descendants of console radio speakers - they are meant to be used in open backed cabinets. Messrs Thiele and Small have nothing to say about them. You don't want "flat" and you don't want "dead". Hifi doesn't apply here. In fact, the only thing worth measuring is the smile on your face. If you like it, it's good.


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Last edited by Keriwena; 16th October 2011 at 10:17 AM.
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Old 16th October 2011, 12:52 PM   #6
akis is offline akis  United Kingdom
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Thanks for the comments, very educational. Ok in this case I will make a cabinet for size, 2 x 12" mounted side to side, I suppose since the back is open we do not care about cabinet volume, but we depend on the construction materials to add harmonics to the sound, much like the soundboard of an acoustic piano. My Fender amp has two 10" side by side and open backed. My VOX has a 12" in a smallish fully sealed cabinet. Both made of chipboard.
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Old 16th October 2011, 05:40 PM   #7
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most guitar cabs, IF NOT ALL arent designed for a flat bass response. A good reason being that they are usually used in an open area on stage, so its maybe better to design for 4Pi rather than 2Pi radiation.

Also many are open backed since the drivers CAN have a very large Qts, this is an advantage on stage as the drummer hears the guitar better, but a downside as far as eliminating feedback.

Many work well in boxes AROUND 1-2 cubic feet/driver sealed, AS A ROUGH approximation, If the back is open, or box too large, then power handling MAY be tested to its limit if you play LOUD.

Something like the G10 greenback, which is what i replaced the Hartke speakers with in the cab the OP mentioned, need alot more volume than the bass drivers (Q~.4) the G10s are good in open back OR a BIGGG sealed box, and these need more like 2 cubic feet each, with 4 in around 5 cubic feet, the bass is bloated. I can actually turn the bass knob down to 2 and it sounds right.

Get the specs if you want to check the drivers, but as long as the Q isnt really high, and your not using reissue or vintage drivers, id assert that 1-2cu ft per driver would be fine.

EDIT: if its a Vox ac 15/30 I think the drivers are blue back celestions, which MAY need quite a large box, or to run open back. THough from the OP description I guess it cannot be this amp.
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Last edited by mondogenerator; 16th October 2011 at 05:45 PM.
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Old 24th October 2011, 07:00 AM   #8
Ian444 is offline Ian444  Australia
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I built a near exact copy of a Vox AC30 amplifier cabinet (and left the amplifier out, but the dimensions were identical) about 20 years ago and its still my only speaker box to this day, with 2 x 12" Celestion G50, made of 1/2" ply. If you can't think of anything better, its a great all-rounder, puts out a lot of volume, not too heavy, and easy to build. At gigs I would sit it on a milk crate to get it off the ground a bit, sounded best that way. It is a little bit sensitive to where it is placed, same as any open-backed combo amp I guess.
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Old 24th October 2011, 08:02 AM   #9
akis is offline akis  United Kingdom
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is there a formula to translate "Q" to volume of air needed for any particular driver? Or is that already shown, eg:

Eminence Tonker specs:
Mechanical Q (Qms) = 10.02
Electromagnetic Q (Qes) = 0.49
Total Q (Qts) = 0.47
Compliance Equivalent Volume (Vas) = 34.1 liters / 1.2 cu.ft.

Is the answer right there? Does this speaker need 1.2 cubic feet of enclosed volume to play well? And if I have two of these, would I then need 2.4 cubic feet in total?
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Old 24th October 2011, 10:33 AM   #10
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Yep, the answer is right there. 'Vas' is a measure of what size box will equal the compliance of the spider/surround. In other words, the air in the box will cushion the cone to the same extent the speaker's suspension will.

There are several factors involved, so it's hard to make predictions based on one spec, but basically, with guitar speakers, you want a box about twice the size of the Vas. A small box does two things - it raises the resonant frequency and the resonant peak. In short, it makes the speaker "boomy". Since many guitar speakers are boomy in free air, it's unlikely you'll make the box too big. (though it's possible, as mondo relates)

And yes, two speakers require twice the volume.
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