if i run 2 8ohm 4x10 cabs (4 ohms) into 300 watts......how much louder would it be than a single 4 ohm 4x10 and 300 watts????? same amp, same cabs, same settings on the head

There would be twice as much surface area pushing air. I can't tell you how much louder it would be though.

well you gain way more volume by adding speakers then you would by increasing power. it takes 10 times the power to double your volume.

You really can't make that comparison, because an 8 ohm 4x10 would have speakers with different design parameters than a 4 ohm 4x10, so you can't have the same cabs .

You'll be louder if you put the 300 watts into the speakers rather than trying to somehow force the speakers into the watts. Randy

I don't think I've ever seen someone force a speaker into watts... Basically the principle is this: When you run 300 watts in a 4x10 4 ohm cab you run 75 watts through each speaker. That will result in a specific dB reading. Running the same amount of power in 8 speakers you will get 37.5 watts for each speaker. Add into the mix the 10 times the power 2 times the volume factor and tada, an 8x10 might not produce as much sound as a 4x10. The big advantage of an 8x10 (or 2x4x10) is that you can hear yourself way better on stage because of the hight of your stack

Without any techie stuff to back it up I've heard that if You place 2 identical amps+cabs next to each other the result will be 3dB louder than just one of them. If that's true it should apply to this problem, assuming the speakers have the same specs other than in one case they're 2x8 Ohms in parallel and in the other 4 Ohms single speaker... Double volume is 10dB, so 3dB is plenty louder. Anyone else heard this theory, or have I been misinformed? /Ff

...and I seem to remember that it is below the threshold of perception...meaning you can barely tell the difference in a 3dB increase/decrease ...of course, I think it's 3db. If not, I'm sure we'll all get the answer real soon here's a chart I found: Threshold of Hearing (TOH) 0 dB Rustling Leaves 10 dB Whisper 20 dB Normal Conversation 60 dB Busy Street Traffic 70 dB Vacuum Cleaner 80 dB Large Orchestra 98 dB Walkman at Maximum Level 100 dB Front Rows of Rock Concert 110 dB Threshold of Pain 130 dB Military Jet Takeoff 140 dB Instant Perforation of Eardrum 160 dB

No, 3 dB is generally perceptible by most people. It's nowhere near a doubling in volume, but most people can probably hear it.

Adding speakers without adding power to drive them doesn't do much. Othewise people could use 10 watt amps to drive a wall of 4x10's at concerts. Volume is the result of moving air, which requires cone surface area and power to do the work. Sometimes you're lacking mainly one or the other, but usually to get significantly more volume you need to add both power and speakers. You're talking about a 4 ohm 4x10 vs 2 8 ohm 4x10s, you get about 3db boost from acoustic coupling of the cabs. Both 4 ohm loads, so the amp will actually put out the same 300w to both scenarios. If you had an 8 ohm 4x10 and added another 8 ohm 4x10, you'd get more wattage out of the amp, plus the acoustic coupling. Something approaching 6db depending on how close the amp comes to putting out double the power into half the impedance. Randy

Exactly 6dB to be precise, because SPL is based on the amp's voltage output, not wattage, and the voltage is constant no matter how many cabs. But before everyone runs out to buy four cabs in the hope of getting 12dB you can only double up on the cabs with parallel wiring until you go below the minimum impedance load that the amp will handle, which is usually 2 cabs. That's why you can't push a wall of cabs with a 10 watt amp.

But it IS a doubling of power - in fact it is an exact doubling. In other words doubling the power is an increase of 3dB - which is just noticable. 10 log (P2/P1) = 10 log 2 = 3 dB.

If the drivers are identical save for impedance, four 16-ohm Beta tens in the 4x10 as opposed to eight 8-ohm Beta tens in the 8x10 for instance, then the difference will be only 3dB unless you power the 8x10 with twice the amp size. But you also can outpower a mediocre 8x10 with a high-efficiency 2x10. Before you buy you must try.