When do you have to use C weighting and when A weighting? :dontknow:
A-weighting rolls off response below 1000 Hz. That is, it filters off or removes bass frequencies from what registers on the meter. A-weighting is commonly used in industrial settings, as the upper frequencies more readily contribute to hearing loss.
C-weighting adds bass frequencies to what the meter registers, down to ~40 Hz, at which point it begins to roll off. In a typical sound system, particularly a domestic one, that subjectively has linear frequency response, bass levels tend to be higher than the mid and upper frequencies. As such, switching the meter from A to C will result in a higher SPL reading.
Hope this helps.
Regards,
Wayne