On my Shure headphones, is the power rating 500mW per side at 1Khz (L=500mW, R=500mW) or 1000 mW for each side? And is that a sustained (A.C. rms) level?
The 1000 mW rating is per side. We use broadband noise for this specification, not a 1 kHz tone.
We do subject the headphones to a prolonged 1000 mW signal as part of the verification process, but the main intent of the specification is to ascertain that the headphones can survive short-term exposure to excessive transient signals.