A pair of identical 10w combos - perceived volume?

Two sound sources with same magnitude would theoretically lead to + 3 dB increase in SPL. In practice the sound waves will be interfering with each other. Depending on phase they'll either reinforce or cancel each other, producing comb pattern response, and this all happens in a three-dimensional space. So where you happen to measure the effect to sound pressure level has great effect on what you will actually measure.

http://zonalandeducation.com/mstm/physics/waves/interference/twoSource/TwoSourceInterference1.html

Those were point sources, radiating evenly to all directions. A real loudspeaker does not even behave in such fashion. It actually has a very distinct pattern of dispersion. Then there are parameters like "stacking" two drivers either vertically or horizontally. The cone area will virtually increase towards one direction and it again has distinct effect on dispersion of the system. Horizontally stacked system has wider dispersion vertically, vertically stacked system horizontally. So obviously in practice the sound energy coming from an actual dynamic loudspeaker (with distinct dispersion characteristics) will not spread all around the three-dimensional space in an uniform manner.

Theoretical assumptions can be made. Yet, real life is probably so complex that you'd need thousands of measurements to "map" what really happens in practice when you start combining speakers / sound sources in a three-dimensional space.
 
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Printer2

Member
2010-04-02 6:34 pm
Sorry, 6dB. Had this question pop up on TDPRI and I set up an experiment to find out what was the real answer. Used a sine wave generator and swept the frequency to make sure I was not just measuring at a node.

Running Two 15 Watt amps. . - Page 2 - Telecaster Guitar Forum

And for those that it was just an interference pattern with peaks and troughs I had the same argument a year or more later than the above thread. The person said I should have used white noise which has the whole frequency band and you will not get an interference pattern as you would sine waves. I repeated the experiment with a FM tuner tuned between stations as a broadband source. Same outcome. I can't seem to find that thread, but considering I have over 5000 posts there that is not unusual.

Have the pictures. Excuse the order, I have to run.

1WFMhissonechannelto15inchAltec_zps85be5f94.jpg

1W FM hiss one channel to 15 inch Altec

SPL1WFMhisstwochannelsto15inchAltecs_zps16849784.jpg

SPL 1W FM hiss two channels to 15 inch Altecs


1WFMhissonechannelto15inchAltec_zps85be5f94.jpg

1W FM hiss one channel to 15 inch Altec

SPL1WFMhissonechannelto15inchAltec_zps7d94da6c.jpg

SPL 1W FM hiss one channel to 15 inch Altec

1WFMhisstwochannelsto15inchAltecs_zpsc0b288e9.jpg

SPL Two Speaker Test With FM Receiver White Noise
 

Printer2

Member
2010-04-02 6:34 pm
This thread is turning out to be fun.

So did you check using smaller speakers?
Are they open back cabinets in front of a wall?

Sealed cabinets. Been too busy learning how to make acoustic guitars so no. If I did not give away my smaller amps I probably would not be venturing in the electronic side of things. Stopped in here and got involved with the $100 amp thread again.
 

Gnobuddy

Member
2016-03-01 4:10 pm
If the speakers were close to each other you would get a 6dB increase. You get 3dB from better acoustical coupling to the air. Really.
I agree with Printer2. Most of the fundamental frequencies emitted by a guitar have very long wavelengths (in air) compared to the diameter of a speaker cone. Under these circumstances, the efficiency of coupling from speaker diaphragm to air is almost directly proportional to the cone area - so doubling the cone area with two speakers gets you a 3 dB increase. Doubling the power (two amps) gets you another 3 dB. Total increase 6 dB, as Printer2 said.

I remember the original thread where Printer2 posted about his 6 dB SPL increase from twin speakers. I also remember searching for an explanation to match his experimental observations at the time, and suggesting he was measuring at an interference peak. But, at the time, I didn't cotton to the increased efficiency of acoustical coupling that goes with more speaker area.

Even though fundamental frequencies from a guitar are pretty low, those acoustical interference effects between two sources do happen, but, with guitar, they mostly affect the harmonics rather than the fundamental notes that are played.

The highest open-string note from a guitar in standard tuning is around 330 Hz, with a corresponding wavelength of a little more than a metre (roughly 3 feet). That's still much bigger than the usual 12" or 10" guitar speaker, so there aren't a whole lot of diffraction effects from two speakers at that frequency, unless the speakers are spaced more than a metre apart.

That's just for open-string notes, of course there are another 21 - 24 frets to go higher in pitch, and then there are harmonics that may go up to 5 KHz or more. So the twin speakers do cause diffraction effects up at those higher frequencies. Some electronic speaker emulators (often built into guitar DI boxes) actually include peaks and dips supposed to emulate the acoustical interference effects between the various speakers in a 4x12 or similar guitar stack.

I have thought for a long time that twin and quad-speaker guitar amps should have the speakers stacked vertically, one above the other, rather than side-by-side, like all the "classic" valve amps. The funny thing is that many musicians today understand that vertically stacked speakers produce better horizontal dispersion (Bose makes a lot of money for themselves using this idea), and yet, that idea doesn't transfer from P.A. systems to guitar amps. Tradition outweighs science most of the time, when it comes to guitar amps!

Also as usual, bass players seem more willing to embrace technological progress: I recently saw a small, light bass combo amp with twin vertically stacked speakers.

-Gnobuddy