Pro Audio Asylum

Posted by Bill Fitzmaurice (R) on December 19, 2002 at 13:13:02

In Reply to: Re: Close-here's the real dope. posted by SSL Tech on December 19, 2002 at 07:52:06:

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.