Overview

What is sound?

A travelling wave which is an oscillation of pressure transmitted through medium such as solid, gas and liquid generated by vibration. 

The number of compressed or rarefied patches of air that are received by our ears each second is called the frequency (Hz). Another important feature perceived by our ears is the intensity, which is the pressure difference between compressed and rarefied patches of air measured in decibels (dB).

The change of sound amplitude and frequency at the ear roughly corresponds to loudness and pitch, respectively. 

The speed of sound is constant for all frequencies, although it does vary with the medium through which it travels. In air, sound travels at a speed of roughly 340 meters per second.

 Sine waves different frequecies. Image above courtesy of en.wikipedia.org/wiki/File:Sine_waves_different_frequencies.svg This image is in the public domain and thus free of any copyright restrictions.

What is the structure of ear?

Anatomy of human ear. Image courtesy of en.wikipedia.org/wiki/File:HumanEar.jpg  This image is in the public domain and thus free of any copyright restrictions.

 External ear    

In the external ear, the pinna enhances high-frequency sounds and funnel sound waves into the middle and inner ear. The unique structure of the pinna plays important role in determining the direction from which a sound originates.

The ear canal is about 2.5cm long and leads to the tympanic membrane (eardrum) of the middle ear.

The outer two-thirds of the canal contains glands secrete a wax-like substance which serves to keep dust, insects and other foreign materials from going deeper into the ear, and helps maintain a constant humidity and temperature of the middle ear.

Middle ear

The eardrum separates the outer ear from the air-filled middle ear. Incoming sound waves vibrate the eardrum, which in turn vibrate three tiny bones in the middle ear collectively called ossicles: malleus, incus and stapes. The stapes can move in and out to vibrate a small membrane at the base of the cochlea, the oval window, which separates the middle ear from the inner ear.

The elegance of the middle ear system lies in its ability to greatly amplify vibrational energy to the fluids of the cochlea. 

Inner ear

Conversion of auditory signal into neural activity occurs in the cochlea, a coiled structure as auditory portion of the inner ear. The structure not only amplifies sound waves but also acts as a mechanical frequency analyzer, decomposing complex acoustical waveforms into simpler elements.

The region nearest to the oval-window membrane is the base of the cochlear spiral, the other end is apex.

Cholea cross-section. Image courtesy of en.wikipedia.org/wiki/File:Cochlea-crosssection.png under the terms of the GNU Free Documentation License. 

The structure is filled with fluids within three parallel canals: the scala tympani, scala vestibuli and the scala media. The principal elements for converting sound into neural activity are found on the basilar membrane, a flexible structure that separates the scala tympani from the scala media.

Within the scala media and atop the basilar membrane is the organ of corti, which is the collective term for all the elements invovled in the transduction of sounds. The organ of corti includes three main structures: the sensory hair cells, an elaborate framework of supporting cells and the terminations of the auditory nerve fibers.

The stereocilia of the outer hair cells extend into indentations in the bottom of the tectorial membrane. Hair cells synapse onto neurons that project along the auditory nerve into the brain.