APA Dictionary of Psychology

the function relating decibels sound-pressure level (dB SPL) to frequency for pure tones that have a loudness equal to that of a 1-kHz tone at a fixed level. For example, a 100 Hz tone presented at 70 dB SPL is as loud as a 1 kHz tone presented at 60 dB SPL. This is a point on the equal loudness contour for a 60-phon reference. This function has been incorporated in audio circuitry as “loudness compensation,” primarily in an attempt to correct for the decreased loudness of low frequencies when listening at low levels. See loudness.