We want to protect people against damaging their hearing. Since the one and only weighting filter does not exist, which reflects the properties of the hearing with all volume levels, we decided to assume the worst case, which means measuring the sound pressures unweighted in dB (SPL). The procedure automatically meets all weighting filters which are provided for low sound pressures (A-, B-, C-weighting). Although passive limiters which operate with an A-weighting allows higher sound pressure level with normal, deep music material before they respond, but the question arises if this really makes sense talking about limiters for high sound pressure levels. In our opinion a limiter should operate to protect the human hearing against damages and operate more conservatively than optimistically. This means the use of a limiter without a weighting filter. 

More information: Spot on Technology - Passive Limiters in Headphones