Phon is a dimensionless unit for the subjectively perceived loudness, the volume level, specified in decibels( dB). The perceived loudness is frequency-dependent, therefore a sinusoidal signal of 1 kHz is used for the evaluation.
At 1 kHz, the perceived loudness of 1 phon corresponds to 1 decibel (dB). Since the phon corresponds to the loudness perception according to the A-weighting, the dB(A) is used instead of the obsolete unit phon.
The loudness perception is related to the sinus tone of 1 kHz for different frequencies. Thus, the loudness perception of different frequencies is put in relation to the reference frequency of 1 kHz. This results in lines of equal perceived loudness at different frequencies.
Such line diagrams represent, for example, the hearing characteristics.
The term phon and its definition go back to the German physicist Barkhausen. The specification in phon corresponds to the value in Decibel Sound Pressure Level( dBSPL): 40 phon corresponds to 40 dBSPL. In addition, the phon is directly related to the loudness specified in sone.