The science of human hearing sound is called human acoustics. Human acoustics specifically indicates the different processes that are involved in the perception of some sound by our ears.
Sound waves set up oscillations in the eardrum made up of tympanic membrane in the upper ear. The oscillations in the eardrum induce movements of the three small soft bones in the middle ear behind the eardrum.
The movements or oscillations in the middle ear pass through viscous fluid in the inner ear, creating oscillations of the fluid. These oscillations then reach the auditory nerves and are finally transmitted to the brain.
The oscillations or sounds are identified and interpreted in the brain, which has the capacity to analyze sound into different frequency components.
Human detectable frequency ranges are 2 to 20,000 Hz.
An eighteen year old person, with normal hearing, has audio range between 20 to 20,000 Hz. The audio sense is most sharp in the frequency range 2000 to 5500 Hz.
Sensitivity of the ear varies from person to person. With ageing, hearing power decreases progressively. The ear is susceptible to damage if it receives high intensity noise. However, the ear has some natural protective device for short durations.
Thus, decibel measures how much intense is the sound compared to reference intensity. Human beings can receive a vast range of intensities from around 1 x 10″12 W/m2 to 1000 W/m2.The reference intensity Iq is taken as 1 x 10″12 W/m2, which is considered to be an intensity level just audible to man.
This thus can be said to be threshold of hearing. Unfortunately, there are practical difficulties in accurately measuring intensity.