Newer FFT analyzer input components are equipped with acoustic filters. The characteristics of these filters are specified clearly in sound level meter (noise meter) specifications (JIS C 1509-1 or IEC61672-1).
Frequency weighting A characteristics (A-weighting) are characteristics initially designed to approximate human auditory characteristics (pure tone loudness characteristics). Sound pressure levels with this filter in place (i.e., A-weighted sound pressure levels) are also termed noise levels in Japan and are fundamental values in noise measurement. Frequency weighting C characteristics (C-weighting) have comparatively flat characteristics compared to A-weighting and are used when measuring impact or other such noise.
When inputting AC output (filtered signal output) from a noise meter to an FFT analyzer for analysis, care should be taken to avoid double-filtering before analysis.