A unit of noisiness, perceived noise level, or perceived loudness. It is calculated from the sound pressure level in phons of a reference sound subjectively judged to be as noisy as the sound being measured. The reference sound is spread over 1 octave, centered at 1000 Hz.

formula: 10 times log to the base 2 N prime equals N double prime minus 40

where N' is the perceived noise level in noy and N'' is the sound pressure level in phons.

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