Why does SPL increase 6 dB for two drivers in parallel, when the electrical power consumed only increases by 3 dB?
A21 – The acoustic power Pa radiated from a small source at long wavelengths is the product of the radiating piston area Ap, the square of the piston velocity vp, and the real part of the piston’s radiation impedance Zp: Pa = (vp)2 Ap Re{ Zp } Since the real part of Zp is also proportional to the radiating area Ap, it follows that the radiated power is proportional to the square of piston velocity and the square of piston area: Pa ~ (vp)2 (Ap)2 When two identical drivers are connected in parallel, each piston moves with the same velocity as the single driver, because the current through each voice coil is the same as before. The total radiating area has doubled, and the radiated acoustic power has increased fourfold (10 log (4) = 6 dB) over that of the single driver. The electrical power consumed by the two drivers has merely doubled (+3 dB). With 4x acoustic power for 2x electric power you have a 3 dB increase in power conversion efficiency. The sound pressure p at some distance from
