Final answer:
The loudness of a sound is determined by the greater movement of the basilar membrane leading to more significant deflection of the hair cells. The volume of sound is thus related to the number of hair cells stimulated at a particular location on the basilar membrane.
Step-by-step explanation:
The sensation of loudness or the volume of a sound is detected by greater movement of the basilar membrane resulting in greater deflection of the hair cells. In the cochlea, sound waves cause fluid movement, which, in turn, flexes the basilar membrane. This flexing bends the stereocilia attached to the tectorial membrane, which are part of the hair cells. As the stereocilia bend, they open mechano-sensitive channels that allow for depolarization and the generation of a signal transmitted to the cochlear nerve. The intensity or volume of a sound is thus determined by how many hair cells at a particular location along the basilar membrane are stimulated.
These hair cells are precisely arranged along the membrane, tuned to respond optimally to certain frequencies, but with greater intensity, they can respond to a broader range of frequencies. The hair cells at the base of the cochlea are activated by higher frequencies, whereas those at the apex detect lower frequencies. Each hair cell is responsible for a particular frequency range, and the place theory describes how pitch perception is influenced by the location along the cochlea where hair cells are stimulated.
Collectively, this mechanism allows the auditory system to accurately encode the wide variety of sounds we hear, segregating them by both frequency and intensity, much like a prism separates light into its component colors.