This chapter discusses loudness by asking whether the same amount of intensity results in the same amount of loudness for tones of different frequencies. The use of loudness levels represents a significant improvement over vague concepts such as "more intense sounds are louder"; however, the phon itself does not provide a direct measure of loudness, per se. Loudness scales or functions show how the loudness percept is related to the level of the sound stimulus. Since we are interested not only in the loudness of a particular sound, but also in how much louder one sound is than another, the relationship between loudness and sound level is best determined with direct ratio scaling techniques. Loudness adaptation refers to a decrease in the loudness of a signal that is continuously presented at a fixed level for a reasonably long period of time.