The Ear

In the world of audio a lot has been said about the sound recording and reproduction, while not so often there is talk about a human factor: the taste of music, the taste of sound and a physical factor – the ear. 

We all roughly know how the ear works, so let's refresh our knowledge in the matter.

The ear consists of three basic parts, where each part has its purpose:

  • the outer ear collects and channels the sound to the middle ear.

  • the middle ear transforms the energy of a sound wave into the internal vibrations of the bone structure of the middle ear and ultimately transforms these vibrations into a compressional wave in the inner ear.

  • the inner ear transforms the energy of a compressional wave within the inner ear fluid into nerve impulses that can be transmitted to the brain.

  • The earflap and about 2 cm long ear canal are part of the outer ear. The earflap provides protection for the middle ear in order to prevent damage to the eardrum and at the same time channels sound waves that reach the ear through the ear canal to the eardrum of the middle ear. Because of the length of the ear canal it is capable of amplifying sounds with frequencies of approximately 3 kHz. As the sound travels through the outer ear, the sound is still in the form of a pressure signal, with an alternating pattern of high and low pressure regions. When the sound reaches the eardrum at the interface of the outer and the middle ear, the energy of the mechanical signal becomes converted into vibrations of the inner bone structure of the ear.

    Above: Some in-ear headphones are custom shaped upon customer's ear canal - this way they reach middle ear directly. On picture: Ultimate Ears 4 Pro Custom In-Ear Monitors.

    The middle ear is an air-filled cavity that consists of an eardrum and three tiny, interconnected bones - the hammer, anvil and stirrup. The eardrum is a very durable and tightly stretched membrane that vibrates as the incoming pressure waves reach it. A compression forces the eardrum inward and a rarefaction forces the eardrum outward, making the eardrum vibrate at the same frequency as the sound wave.

    Being connected to the hammer, the movements of the eardrum set the hammer, anvil and stirrup into motion at the same frequency as the sound wave. The stirrup is connected to the inner ear - vibrations of the stirrup are transmitted to the fluid of the inner ear and create a compression wave within the fluid. The three tiny bones of the middle ear act as levers to amplify the vibrations of the sound wave. Due to a mechanical advantage, the displacements of the stirrup are greater than that of the hammer. Furthermore, since the pressure wave striking the large area of the eardrum is concentrated into the smaller area of the stirrup, the force of the vibrating stirrup is nearly 15 times larger than that of the eardrum. This feature enhances our ability of hear the faintest of the sounds. 

    The middle ear is an air-filled cavity that is connected by the Eustachian tube to the mouth. This connection allows for the equalization of pressure within the air-filled cavities of the ear. When this tube becomes clogged during a cold, the ear cavity is unable to equalize its pressure; this will often lead to earaches and other pains. 

    Above: B & W Nautilus speakers are shaped in the form of cochlea.

    The inner ear consists of a cochlea, the semicircular canals, and the auditory nerve. The cochlea and the semicircular canals are filled with water-like fluid. The fluid and nerve cells of the semicircular canals provide no role in the task of hearing; they merely serve as accelerometers for detecting accelerated movements and assisting in the task of maintaining the balance. The cochlea is a snail-shaped organ that would stretch to approximately 3 cm. In addition to being filled with fluid, the inner surface of the cochlea is lined with over 20,000 hair-like nerve cells that perform one of the most critical roles in our ability to hear. These nerve cells differ in length by minuscule amounts; they also have different degrees of resiliency to the fluid that passes over them. As a compressional wave moves from the interface between the hammer of the middle ear and the oval window of the inner ear through the cochlea, the small hair-like nerve cells will be set in motion. Each hair cell has a natural sensitivity to a particular frequency of vibration. When the frequency of the compressional wave matches the natural frequency of the nerve cell, that nerve cell will resonate with a larger amplitude of vibration. This increased vibrational amplitude induces the cell to release an electrical impulse that passes along the auditory nerve towards the brain. In a process that is not clearly understood, the brain is capable of interpreting the qualities of the sound upon reception of these electric nerve impulses.

    Source: The Physics Classroom (www.physicsclassroom.com)

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