Although 0 db is often referred to as the lower threshold of human hearing, it is important to realize that the human ear is not equally sensitive to all frequencies of sound. In other words, a particular noise may sound louder or softer depending on the frequency of the sound wave being transmitted. Because of this variation, scientists have defined a unit of loudness, called a phon, to represent the intensity of sound waves with a frequency of 1000 hz: a 60-phon sound is one that is perceived by the human ear to have the same loudness as a sound wave with an intensity of 60 db and a frequency of 1000 hz.