Loudness contours. A subjective impression of equal loudness by listeners as a function of frequency and sound level (dB). An increase in low frequency sound will be perceived as being much louder than an equivalent high frequency increase.<br><br>1) A unit of equal loudness for all audio frequencies. 2) The phon is numerically equal to dBspl at 1000 Hz but varies at other frequencies according to ear sensitivity to frequency.