Earing & Sounds

What is Sound?

Sounds are defined as an auditory sensation generated by an acoustic wave (Larousse). This definition introduces two notions: acoustic waves and human hearing. When an acoustic wave reaches the head of a listener, it is picked up by the pinna and directed toward the ear canal. These two elements form the outer ear. The shape and geometry of the outer ear make it possible to amplify sounds between 1 and 6 kHz.

The sounds thus captured vibrate the tympanum as well as the ossicles of the inner ear (the malleus, the incus and the stapes). These elements of the middle ear make it possible to adapt the mechanical impedance between the outer ear and the inner ear.

The cochlea is a snail-shaped organ filled with fluid. The vibrations of the liquid are transmitted to a serie of cilia. The hair cells located at the beginning of the cochlea are responsible for hearing high-pitched sounds while those located at the end are sensitive to lower-pitched sounds. The semicircular canals are not involved in the hearing process. They are used to detect head tilt and are essential for equilibre.

Understand Acoustic Waves

Acoustic waves are pressure disturbances around the value of atmospheric pressure. The ear is sensitive to disturbances as low as 0.00002 Pa (earing threshold) and up to 30 Pa (pain threshold). Note that these variations are very small compared to atmospheric pressure (about 100,000 Pa). Since acoustic waves are pressure waves, one would expect to measure sound in Pascal, but the linear scale between 0.00005 Pascal and 30 Pa is impractical. Instead, we use a logarithmic scale, the decibel, also noted as dB.

The following graph shows a sound level chart of different sound sources in Pascal or dB.

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