Sound and Acoustics: An Overview

The middle ear is primarily responsible for preventing the loss of acoustic energy that would result from direct contact between the low-impedance air in the ear canal and the high-impedance cochlear fluid. Much of the acoustic energy that would otherwise travel through a medium with low impedance (like air) is reflected off the liquid and never reaches its destination (like water). Without the middle ear, the cochlea would only receive 0.1% of the acoustic wave energy that travels through the air, while 99.9% would be reflected. To compensate for the difference in impedance between water and air, a cochlear amplification system is required for physiological hearing. A well-functioning middle and external ear with a healthy tympanic membrane, an ossicular chain, and an adequately ventilated tympanic cavity are necessary for accurate impedance matching. A person’s clinical inability to hear sounds correctly, known as conductive hearing loss, is the outcome of any malfunction or illness of these components. The human ear has a peak sensitivity between 2 and 5 kHz, although this sensitivity does not extend across the audible spectrum. Sensitivity varies with age as well. Most people’s highest audible frequency drops with age, while the decibel level required to hear it rises. Exposure to loud noises over long periods accelerates the natural decline of hearing.