Or does the second speaker just make it sound, i dunno, better?
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Schecter C-1 Elite EMG 81/85
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not noticeably louder, no. better... in short, most likely.
Gibson SG Standard + 18volt EMG-81 & 85
Mesa/Boogie Mark IV + Recto 2x12
Keeley Modded BD-2
Vox V847a
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Assuming everything else is the same, the larger cab size and and speaker surface area of the 2x12 will make it sound a little bit louder.
The biggest thing you'll notice though is headroom. Usually with an 18W amp you'll be using a 25W speaker like a greenback, so if you use two speakers the handling doubles to 50 watts and you'll have a lot more headroom since you're not creating any speaker distortion like you would with a single speaker.
Not really louder, it will sound fuller, less boxy, which may give the impression of more volume.
in addition to the extra headroom and change in tonal dynamics the real loudness will come from the speaker efficiency aka the dbs/watt of the speaker. you can have a 112 with a really efficient speaker that will be much louder than a 212 that contains 2 inefficient speakers
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Quote by Dempsey68
get a cheap marshall... my MG15DFX has a button that simulates the sound of one of the expensive tube marshall amps.
18watts and 18watts are the same volume, but with 2 speakers you will get more air pushed, giving the effect that the 2x12 might be slightly louder