The A-weighting filter is one of the oldest ones used in audio analysis – providing a measure of loudness as perceived by the human ear from unweighted measurements. Yet the Audio Precision machines do not come with one built in as standard.
The plug-in architecture allows for adding an A-weighting card, such as the one offered here.
This filter implements the A-weighting frequency transfer function.
When measured using the inverse of the A-weighting standard used as measurement equalisation, the frequency transfer function is flat: