loudness

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(n.) The sound pressure level (SPL) of a sound, measured in phons (units of apparent loudness) and equal to the equivalent SPL in dB. A sound deemed as loud as a 40 dB-SPL 1 kHz tone has a loudness level of 40 phons. An increase of 10 phons, or 10 dB-SPL, is deemed twice as loud.