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In Reply to: 6dB gain fron double power? posted by SSL Tech on November 20, 2002 at 11:52:28:
A common misconception, the subject of endless and continuing debate. When you go from one to two cabinets (or drivers for that matter)and double power input you get 6dB additional output- 3dB from doubling of power, 3dB from the increased efficiency obtained in doubling the radiating area. This is subject to frequency limits- once the radiating area is about eight or so wavelengths radiating efficiency ceases to increase and additional doubling gets only 3dB. You can easily test it yourself- put two cabs side by side,first feeding only one,then both with the same signal. Assuming your amp has constant voltage output the power will double when the impedance load is halved, and broadband output will go up 6dB. Try it. It works.
Follow Ups:
Makes perfect sense now that I think of it... The cyclically displaced air will be twice as much from two pistons (drivers) traveling the same distance as from one piston (drive unit) traveling double the distance (equal to a 6dB gain in drive level) assuming no summing losses, of course.-Never thought about it until now... Thanks for stimulating the grey matter Bill!
Keith.
Thanks for not doing the usual when this subject comes up, which is denying the physics of it. Actually, when you double drivers (or cabinets)with a parallel connection while maintaining voltage output two things happen. Halving the impedance doubles the power, giving you 3dB. The other 3dB comes from increasing the radiation efficiency of the system. This holds true up to the point where the radiating area is about 2 times a wavelength squared, beyond which there are no further increases in radiation efficiency and each doubling gives only 3dB. This is why in very large concert systems tweeters require more sensitivity to keep up with the woofers, the opposite of the situation in small systems.
-Right you are.In fact my mental 'model' was basically 'muting' one channel of a stereo system, with both channels fed identically. -The original speaker would still be cyclically pumping the same volume of air (imagining a perfect acoustic 'bridge rectifier') except that the second channel would be doing the same, doubling the work done by doubling the volume pumped through twice the piston surface area.
-Same would be achieved by using one piston and doubling it's range of motion... (assuming perfect linear travel, this would be equivalent to a 6.02dB increase...)
Tanks also for the physics of the radiating surface dimension/frequency limit interplay. -Now that you explain it, I see why.
I had always assumed -double power, therefore +3dB, but I suppose a mild analogy might be like horsepower over torque in cars, if only in the fact that so many people mis-grasp the fact that one (hp) is work done, and not a measure of force or power.
-A question for you... When considering the radiating surface area of a horn hf driver, do you consider the throat size, the flare, size, the diaphragm size... My guess would be that it would be a number smaller than the final flare area, varying with the acoustic gain efficiency or impedance matching ability of the horn... but I'm guessing here... (I'm interested by acoustics, but not a specialist by any stretch! -I do like to grasp how things work though!)
-Thanks,
Keith A.
Well, double the power in and of itself is 3dB. The other side of this coin is if you use 2 drivers/cabinets and wire them in series instead of parallel. The impedance is doubled, so the voltage sensitivity drops by 3dB. But the radiating efficiency still doubles, so the net change is zero. This works very nicely in the case of line array boxes. Two drivers parallel gets 6dB. Add two more in series and the dB stays the same. Add four more drivers in another series/parallel config and you pick up another 6dB. If you started with 8 ohm/88dB drivers you now have 100dB/2.83v at 4ohms,with a bit of smoothing roll-off at the high end to help overcome the diffraction step and great horizontal dispersion to boot. If you used inexpensive 5 inch drivers it would be both a small and inexpensive box as well.
As to the tweeter question, the radiating area in and of itself doesn't determine sensitivity. That is a product of all the parameters of the unit, from the Q and moving mass and coil inductance etc. to the throat size,path length, taper rate, etc. What you can say though is that if all the other factors are equal a larger mouth will usually give higher SPL than a smaller one, up to the point where the mouth is so large that radiating efficiency ceases to improve with further expansion. The radiating surface is the actual dimension where the mouth ends and space begins. You can mentally apply a membrane to that area and that area performs the same function as a driver cone that drives directly into space.
-Oops! -didn't mean to imply that efficiency per se was a function of area, but I think you have answered my question in that the theroetical radiating area needs to be established in other to predict what other issues you will encounter.-Thanks, Bill! -I think I'll investigate some of your posts "over there"!!!
Keith A.
Well, you did hit upon one of the reasons that horn drivers work so well. A very simplistic view of the horn versus direct radiator is that a direct radiators output is radiating area (the cone)X excursion, while a horn is radiating area (the horn mouth) x excursion of the driver cone energizing the horn. If your using the same driver in both cases, one pushing air directly and the other pushing a much larger cross-section of air at the end of the horn, the horn loaded will be a lot louder without using any more energy. That's a simplistic view, again, but is as good an expanation as any. But you don't get something for nothing- the horn takes up a lot of space.
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