Perception of pitch

The note A above middle C played on a piano is perceived to be of the same pitch as a pure tone of 440 Hz, but does not necessarily contain a partial having that frequency. Furthermore, a slight change in frequency need not lead to a perceived change in pitch. In fact, the just noticeable difference (the threshold at which a change in pitch is perceived) is about five cents (that is, about five hundredths of a semitone), but varies over the range of hearing and is more precise when the two pitches are played simultaneously. Like other human stimuli, the perception of pitch also can be explained by the Weber-Fechner law.

Pitch may depend on the amplitude of the sound, especially at low frequencies. For instance, a low bass note will sound lower in pitch if it is louder. Like other senses, the relative perception of pitch can be fooled, resulting in "audio illusions". There are several of these, such as the tritone paradox, but most notably the Shepard scale, where a continuous or discrete sequence of specially formed tones can be made to sound as if the sequence continues ascending or descending forever.