Phon: With regard to the perception of sound, the human ear does not have a constant frequency response sensitivity. Since the perception of loudness depends not only on the sound level but also on the frequency, the phon was introduced. In relation to the sound level and the frequency, the phon curve shows the pairs of values at which the same volume is perceived. It is therefore not a purely physical measurement, but the course of the curve is determined by the human perception of volume.
