According to my HD598 box, the spl is stated as being 112 dB @ 1kHz and 1Vrms. However in several places including a review, I see the spl rated as 112 dB /mW for the HD598. Obviously they can't both be correct with the nominal 1kHz impedance being ~ 50ohm. If 112 dB/1Vrms is correct that means 99 dB/mW (for 50ohm nominal). I contacted Sennheiser but they didn't answer the question.