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Set to “A,” it’s an A-weighted noise meter (A-weighting is the most commonly used of a family of curves relating to the measurement of sound pressure level, as opposed to actual sound pressure).
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Set to “R-D,” it’s a Robinson-Dadson equal-loudness meter (An equal-loudness contour is a measure of sound pressure, over the frequency spectrum, for which a listener perceives a constant loudness).
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Set to “None,” it’s a VU meter with 100 DB of visual range (Volume Unit metering averages out peaks and troughs of short duration to reflect the overall perceived loudness).
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