Headphone sound pressure level?

[Gemma Rachael Hutchinson <gemma@xxxxxxxxxxxxx>]

Dear all,

We have a set of Sennheiser HD 280 pro headphones with a "sound  
pressure level" of 102 dB "as per IEC 268-7". Since it also states a  
sound pressure level of 113dB/1V rms, we are unsure exactly what the  
maximum output level of these headphones actually is. The impedance is  
64ohm. Would anyone know how to calculate this, or does anyone have  
access to the full international standard for headphones (e.g., IEC  
60268-7), a preview of which can be found at the link below?

http://webstore.iec.ch/preview/info_iec60268-7%7Bed3.0%7Den.pdf

Thanks, Gemma