Editing Stimulus modality (section)

===Pitch, loudness and timbre===
There are many different qualities in sound stimuli including [[loudness]], [[pitch (music)|pitch]] and [[timbre]].<ref name=Carlson />

The human ear is able to detect differences in pitch through the movement of auditory hair cells found on the [[basilar membrane]]. High frequency sounds will stimulate the auditory hair cells at the base of the basilar membrane while medium frequency sounds cause vibrations of auditory hair cells located at the middle of the basilar membrane. For frequencies that are lower than 200&nbsp;Hz, the tip of the basilar membrane vibrates in sync with the sound waves. In turn, neurons are fired at the same rate as the vibrations. The brain is able to measure the vibrations and is then aware of any low frequency pitches.<ref name=Carlson />

When a louder sound is heard, more hair cells are stimulated and the intensity of firing of [[axons]] in the [[cochlear nerve]] is increased. However, because the rate of firing also defines low pitch the brain has an alternate way of encoding for loudness of low frequency sounds. The number of hair cells that are stimulated is thought to communicate loudness in low pitch frequencies.<ref name=Carlson />

Aside from pitch and loudness, another quality that distinguishes sound stimuli is timbre. Timbre allows us to hear the difference between two instruments that are playing at the same frequency and loudness, for example. When two simple tones are put together they create a complex tone. The simple tones of an instrument are called [[harmonics]] or [[overtones]]. Timbre is created by putting the harmonics together with the [[fundamental frequency]] (a sound's basic pitch). When a complex sound is heard, it causes different parts in the basilar membrane to become simultaneously stimulated and flex. In this way, different timbres can be distinguished.<ref name=Carlson />