"Efficiency = ( B^2 * L^2 ) / ( R * Sd^2 * Mms^2 )
B = magnetic field strength
L = length of wire
R = resistance
Sd = surface area
Mms = mass
So for your 8 ohm voice coil, using the same wire as a 4 ohm voice coil, you would need twice the L or length to get an 8 ohm impedance. That makes...