The term UNIT MEASURING SOUND is a four-letter word, PHON. A phon is a unit of measurement for sound intensity, which is the loudness of a sound. It is used to measure the sound pressure level of a sound relative to a reference value. The reference value is usually set at 20 micropascals (µPa). The phon is a logarithmic unit, meaning that a sound with twice the intensity of the reference value is measured as 40 phons.