There are a couple of things that don't make sense to me.
For example he's saying that the headphone drivers are in parallel. That's not the case, there's usually a single driver per channel.
Then he's saying 10% volume corresponds to a gain of 0.1 which is just a measly -20 dB.
His power formula also seems off.
It's actually quite simple:
P = V * V / R
Let's say the ipod is set to max volume and output 1 V into the headphones which have 32 ohms nominal impedance.
P = 1 * 1 / 32 = 0.03125 watts = about 31 mW
Let's say the headphones have 100 dB @ 1 mW efficiency. To get the SPL:
Sensitivity + 10 * log10(P/0.001) = 100 + 10*log10(0.03125 / 0.001) = 115 dB SPL.
This is with full-scale sine waves of course. For rock music you can subtract about 12 dB (-> 103 dB SPL), compressed metal about the half (-> 109 dB SPL).
Edited by xnor - 2/23/13 at 12:58am