Sammas said:
Hey chaps,
Here is a quick question for you.
The going consensus with a valve guitar amp is, volume
doubles every time you increase the output power by a factor of 10.
So 100watt is twice as loud as 10watt which is twice as loud as 1watt.
That is not only for guitar amps, but for anything that produces sound (or noise). But be ware, this is the perceptual side of things; it takes into account the non-linearity of the complete hearing process, from the outer ear to the brains. And in fact, it is an approximation and an average of a panel of testers; you may hear things somewhat differently. The unit is the sone; one sone corresponds to a sensation of doubling.
The question I have is what happens when, if all things are equal (speaker, amplifier, cabinet, etc) you double the entire amplifer setup? IE. Instead of just one guitar combo, you play through two. How does it affect the preceived volume?
Now, let's talk about the objective, measurable side:
Doubling the electrical power applied to the same cabinet will increase the level by 3dB, which may be perceived as an increase of about 0.3 sone
Now, in your example, you have two set-ups, receiving the same power, but the sound pressure does not increase by 3 dB. It may be less, it may be more. Why?
Because the way the sound pressure is distributed in the listening area is completely changed.
If the cabinets are located close to each other, the sound waves will combine in-phase in all the area on the median plane, and they will combine out-of-phase in some other areas. In most of the side area, they will combine erratically, creating lobes of energy, but also holes of energy.
The overall result is that the sound will be "concentrated" in front of the cabinets, with an increase of SPL close to 6dB, which is equivalent to quadrupling the electrical power.
Is there a free meal here? yes and no.
There will be more sound on the front, less on the sides, and of a lesser quality; if you rely on your amp(s) only for hearing yourself play, that may be a problem.
[out of topic] This effect is known in sound reinforcement system design as coupling.
Since most of the times, the desired goal is to increase the SPL in the direction of the audience, it's a good thing. [/out of topic]
To go back to the original subject, if you had put you cabinets in front of each other, they would cancel each other, because they would be out-of-phase.
And if you put them at some distance, like on each side of the stage, the SPL recombination would not be perfect, it would be closer to 4dB than 6.
[out of topic] Ok, some will say it should be 3dB; but it can't be: 3dB is the SPL increase of two completely uncorreleated signals, which would be the case if you played the high strings in one amp and the low strings in the other (that's the way some "stereo" guitars worked). But generally, I think you would experiment with the placement and favour a position where the sound is enhanced, not hindered. [/out of topic]
