What unit is used to measure the loudness of sound?

There are two units used to measure the loudness of sound, phon and sone. Loudness is considered a subjective measurement which is influenced not only by sound pressure, but the length of the sound.

Phon is based on sounds heard at a 1000 hertz level. The sound is then rated by the listeners of the sound. If a person says that the sound seems to be around 50 decibles, then it is expressed as 50 phon, which means a 50 decibel sound at 1000 hertz. Sone is based on orchestral sound, where the lowest sound is assigned 40 phons. Each sone doubles in the level, so 40 phons is one sone, 50 phones is two sones and 60 is four sones, continuing all the way up to 100 phons.