ABOUT HEARING

Hearing is fundamental to stay in touch with the world around us, and enables a multitude of activities including self-defense, communication, and leisure.

Sound is a mechanical wave of variation in pressure, which travels through a transmission medium which is normally air. Each sound has a unique waveform and spectrum of frequencies.

Our auditory system enables us to perceive this sound wave. In essence the cochlea converts the incoming mechanical wave into electrical stimuli which are then transmitted by the acoustic nerve to the brain. The human ear is an incredible organ because of its capability to detect broad ranges of frequencies and amplitudes. Audible frequencies are typically between 20 Hz and 20,000 Hz. But more importantly, the dynamic amplitude range is well above 100 dB. For this reason we can hear clearly the faintest sounds (breathing, 10 dB SPL), but also the loudest without discomfort (jet take-off, 120 dB SPL).

To better understand the complex process of hearing, we have to take a look to the anatomy of the ear. The ear can be divided into three parts.

The outer ear comprises the auricle, the external auditory meatus, and the tympanum. Its function is to capture sound waves and convey them through the ear canal to the tympanic membrane, which begins to vibrate.

The middle ear consists of the space behind the eardrum and the ossicular chain (hammer, anvil, and stirrup). These tiny structures amplify the vibrations generated by the tympanum before they reach the internal fluid of the inner ear. The outer and middle ear together are an efficient device that transmits sounds from the outer ear to the cochlea without acoustic energy loss.

The inner ear transforms these mechanical vibrations into electrical signals. This takes place in the cochlea, a spiral-shaped, fluid-filled tube, which is divided lengthwise by the Corti’s organ. It is also the most complex and fascinating part of the auditory system.

The cochlea first transforms the mechanical signals from the ossicular chain into waves that propagate in the fluid and membranes. This is facilitated by the outer hair cells, which act as an acoustic preamplifier and increase the vibration of the membranes.

Then the inner hair cells within the Corti’s organ convert the displacement of the membranes into particular electrical impulses called action potentials. The inner hair cells connect synaptically to the axons forming the auditory nerve and carrying the electric signals to the brain.

The signals are processed primarily in the temporal cortical area: here the messages are memorized, decoded and integrated as sounds