RMS
This takes no account of the nature of the signal content, and determines the power using a calculation based upon each sample value. The value returned will often be higher than that calculated using the weighted functions below.
ITU 1770
Introduced in 2006, this was designed specifically for use in digital broadcasting. It has the advantage of greater accuracy than earlier methods when the programme content includes music as well as dialogue. It may be less suitable for normalising signals that comprise largely speech.
ITU 1770-2 and ITU 1770-3
These standards, which are identical for Power Normalising, use a 'gated loudness' approach that omits very quiet passages (such as the gaps between utterances) from the gain calculations., making them particularly suitable for signals that largely comprise speech.
A- B- C- and M- weighted measurements
Because the human ear is more sensitive to sounds in the range 1kHz - 4 kHz than those outside it, and because this response changes with overall loudness, audio that predominantly occupies these frequencies may seem louder than audio of the same loudness that lies predominantly above or below this range. Power calculations can be been carried out using weighting curves that compensate for the ear's response, and the most common of these are called A-, B- C- and M- weighted.
A-weighted:
This corresponds roughly to the equal-loudness curve for the human ear at moderate levels, so signals at the extremes of the audible range contribute less to the calculation than those in the mid-band. Wide-band signals will often return lower values using A-weighting than when the power is calculated using other methods.
B-weighted:
This lies between the A- and C- curves.
C-weighted:
This has a flat response over a much wider range that the A and B curves, and is suitable for measurements at very high signal levels, when the ear's response is closer to flat.
M-weighted:
Adopted by the film industry, this is used to assess the power when audio is heard at cinema playback levels.