This is a request as to how you calculate the SPL of a multiamp system. If you put one watt into a 90db/1watt/metre fullrange speaker you get 90 db out. But what do you get if you have a multi amp system? Take the follwoing example. If the tweeter, mid and woofer are all 90db/w/m what happens if each amp is putting out one watt?
I can vaguely remember the principle of superposition(?) of wave forms from high school physics where the amplitudes add. But I'm not sure how to use that in this case. Do we get 96db because each amp/speaker combination doubles the SPL or what? I mentioned this to a very compertent physicist recently and he went immediately to work in logrithims but we were distrcted by something else and never did finish the calculations.
Any clues anyone?
I can vaguely remember the principle of superposition(?) of wave forms from high school physics where the amplitudes add. But I'm not sure how to use that in this case. Do we get 96db because each amp/speaker combination doubles the SPL or what? I mentioned this to a very compertent physicist recently and he went immediately to work in logrithims but we were distrcted by something else and never did finish the calculations.
Any clues anyone?
You'll get 90 + 20 log 3 = 99.6dB SPL when the signals reach the measurement microphone with identical frequency and phase (this won't be the case) but just 90 + 10 log 3 = 94.8dB SPL when the signals are different uncorrelated signals.
While this might be close to the efficiency of the system at the cross-over point it's not terribly useful.
The maximum output level gains with multi-amplification come from SPL being proportional to the square of excursion which is in turn proportional to voltage. Power is proportional to voltage squared.
So when you feed two signals of the same magnitude into an amplifier and then split them up into separate drivers you need 4X the power rating to reproduce a single signal. Feeding the signals into two separate amplifiers means each amplifier only needs to be big enough to produce the single signal.
Musical power distribution is not uniform; so in practice a bi-amplified system with identical driver efficiencies could have the same maximum output with 60W+30W amplifiers as the same drivers with a passive cross-over driven by a 200W amplifier.
While this might be close to the efficiency of the system at the cross-over point it's not terribly useful.
The maximum output level gains with multi-amplification come from SPL being proportional to the square of excursion which is in turn proportional to voltage. Power is proportional to voltage squared.
So when you feed two signals of the same magnitude into an amplifier and then split them up into separate drivers you need 4X the power rating to reproduce a single signal. Feeding the signals into two separate amplifiers means each amplifier only needs to be big enough to produce the single signal.
Musical power distribution is not uniform; so in practice a bi-amplified system with identical driver efficiencies could have the same maximum output with 60W+30W amplifiers as the same drivers with a passive cross-over driven by a 200W amplifier.
Thank you Drew. While in this area of mathermatics I thought some people might appreciate the following. After the Ark reached dry ground Noah told all the animals to leave the Ark and "go forth and multiply" A year later he went around and inspected them to see how they were getting along. Well, the two sheep had produced some lambs. The elephants had a baby under construction (long term project). There were rabbits everywhere but when he came to the snakes he found there were no baby snakes to be seen. Noah ask them, why no kids? And they replied: "we can't multiply because we're Adders". Another year past by and Noah went around again and this time he found Mr and Mrs snake had a little family of baby snakes. So Noah ask them how this happy situation occured. And they smiled and said; "We did it by logs".
Hi,
never tried to do the numbers (probably not able to) but my guess is that the spl/w figure does not change. Put in a 1 watt narrow band test signal and 90db comes out from any of the three speakers.
However, say you now have three amplifiers and three speakers sharing the load. The total sound power coming from the speakers will be greater when a wideband music signal is sending post crossover signals each less than the power limits of the combined amp and speaker. The total sound power will be higher even though you are not overloading any individual unit (amp or speaker).
never tried to do the numbers (probably not able to) but my guess is that the spl/w figure does not change. Put in a 1 watt narrow band test signal and 90db comes out from any of the three speakers.
However, say you now have three amplifiers and three speakers sharing the load. The total sound power coming from the speakers will be greater when a wideband music signal is sending post crossover signals each less than the power limits of the combined amp and speaker. The total sound power will be higher even though you are not overloading any individual unit (amp or speaker).
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