Off Topic

Each week I plan to provide some outside the box thinking and ideas. This week I will be covering the Human Ear and how it works. Sometimes its nice to understand how things work just because, and then other times it may benefit you.

As a producer / engineer, I personally feel having an understanding of how the ear works is an important part of what we do. Why do you ask? Well, it becomes a tool in a producer / engineers arsenal to understand what frequencies the average human being hears (usually between 16hz and 16 kHz and degrades as we age) and what instruments fall in a particular frequency.

Another great reason to understand how the hear works it so help you get a better overall mix. If you place different instruments in different frequency ranges and pan them and adjust the levels accordingly you will be well on your way to creating great separation in
your mix. Below is some information on the ear, how it works and parts of the ear. Please forward this to anyone you think may be interested and enjoy the read…

How does the Human Ear Work

The ear is a major organ of the auditory system that helps in detecting and perceiving sound waves. Ear may be used to define the visible portion (pinna or auricle) or the entire organ. The position of ear (two on opposite sides of the head) and the parts contribute to
localizing and perception of sound. Before we discuss the functioning of
the human ear, let’s understand the anatomy of human ear.

Anatomy of the Human Ear

The human ear comprises three main parts viz. outer ear, middle ear and
inner ear. The outer ear is the first receiver of sound waves. It
consists of the ear flap and the ear canal (2 cm long). The middle ear,
as the name suggests, lies between the outer and inner ear. It is
basically an air-filled cavity (tympanic cavity). It is connected to the
mouth via the Eustachian tube and helps in balancing pressure. The
middle ear is separated from the outer ear by the eardrum and consists
of three tiny bones viz. hammer (malleus), anvil (incus) and stirrup
(stapes). After the middle ear, comes the inner ear, which is made up of
a cochlea (spiral tube), the semicircular canals and the auditory
nerve. Any damage in the ear parts may result in different ear problems.

How does the Human Ear Work

The ear parts function in coordination for sensing, collecting and
interpreting the sound waves. Each ear component is responsible for
carrying out specific functions. The external ear or the outer ear
collects sound waves and channelizes them to the middle ear. In this
process, the ear flap first receives the sound waves, transfer them to
eardrum (tympanic membrane) via the ear canal (meatus). The length of
the ear canal also contributes in amplifying the sound. In the ear
canal, the sweat glands secrete ear wax. If ear wax is produced in
excessive amount, then it hampers the hearing ability of the individual.
It can also lead to ear infections and at times, hearing loss.
The sound waves that are collected by the outer ear remain as pressure
waves, until they reach the eardrum. The eardrum is a flexible membrane
that vibrates after receiving the pressure waves. As the sound waves
reach the eardrum in the middle ear, the three tiny bones or the
auditory ossicles vibrate in response to the sound waves and convert the
sound energy into a compressional wave. The middle ear is connected
with the inner ear by the stirrup bone. It is the transmission of
vibrations by the stirrup bone into the inner ear that the compressional
wave is created. The main function of the auditory ossicles is to
amplify the sound.

The inner ear components, namely, the cochlea and the semicircular canals are fluid-filled structures. In the inner lining of the cochlea, there are over 20,000 minute nerve cells, which differ in length and resiliency. As soon as the compressional wave
reaches these nerve cells, they resonate at certain frequencies and
release an electrical impulse. This electrical impulse is perceived by
the auditory nerve and transmitted to the brain. In the brain, the
impulses are interpreted as ‘sound’. Thus, the human ear is a complex
structure that performs important functions like hearing, head
positioning and/or maintaining the balance.

Parts of the Ear

Temporal Bone

The temporal bones are situated at the sides and base of the skull, and lateral to the temporal lobes of the cerebrum.

The temporal bone supports that part of the face known as the temple.

Pinna

The pinna (Latin for feather) is the visible part of the ear that resides outside of the head (this may also be referred to as the auricle or auricula).

The function of the pinna is to collect sound. It does so by acting as a funnel, amplifying the sound and directing it to the auditory canal. While reflecting from the pinna, sound also goes through a filtering process which adds directional information to the sound (see sound localization, head-related transfer function, pinna notch). The filtering effect of the human pinna preferentially selects sounds in the frequency range of human speech.

In various species, the pinna can also signal mood and radiate heat.

Ear Canal

The ear canal (external auditory meatus, external acoustic meatus), is a tube running from the outer ear to the middle ear. The human ear canal extends from the pinna to the eardrum and is about 26 mm in length and 7 mm in diameter.

Structure: The human ear canal is divided into two parts. The cartilaginous part forms the outer third of the canal and contains the cartilage and the continuation of the cartilage
framework of pinna. The bony part forms the inner two thirds. The canal is approximately 26 mm long and 7 mm in diameter. Size and shape of the anal vary among individuals. This is an important factor to consider when fitting earplugs.

Tympanic Membrane (Eardrum)

The tympanic membrane, or eardrum, is a thin membrane that separates the external ear from the middle ear. Its function is to transmit sound from
the air to the ossicles inside the middle ear. The malleus bone bridges
the gap between the eardrum and the other ossicles. Rupture or
perforation of the eardrum can lead to conductive hearing loss.

Development: The eardrum forms from the joining of the expanding first pharyngeal
pouch and groove. Around day 30 of gestation, the endoderm-lined first
pharyngeal pouch expands to form the tympanic cavity, which subsequently
envelops the inner ear ossicles. Simultaneously, the first pharyngeal
groove, which is lined with ectoderm, expands to form the developing
external auditory meatus. Separated by a thin layer of splanchnic
mesoderm, the tympanic cavity and external auditory meatus join to form
the tympanic membrane. As a result, the tympanic membrane is one of very
few adult structures that is derived from all three germ layers.

Tympanic Cavity

The tympanic cavity is a small cavity surrounding the bones of the middle ear.
It is formed from the tubotympanic sulcus, an expansion of the first pharyngeal pouch.
It is mostly surrounded by the bony labyrinth, except on its lateral
surface, where it abuts the external auditory meatus from which it is
separated by the tympanic membrane (eardrum).

Malleus

The malleus or hammer is a hammer-shaped small bone or ossicle of the
middle ear which connects with the incus and is attached to the inner
surface of the eardrum. The word is Latin for hammer.

It transmits the sound vibrations from the eardrum to the incus.
The malleus is unique to mammals, and evolved from a lower jaw bone in
basal amniotes called the articular, which still forms part of the jaw
joint in reptiles and birds. Embryologically it is derived from the
first pharyngeal arch along with the rest of the bones of mastication,
such as the maxilla and mandible.

Incus

The incus or anvil is the anvil-shaped small bone or ossicle in the middle
ear. It connects the malleus to the stapes. It was first described by
Alessandro Achillini of Bologna.

The incus transmits sound vibrations from the malleus to the stapes. The incus only exists in mammals, and is derived from a reptilian upper jaw bone, the quadrate
bone. Embryologically it is derived from the first pharyngeal arch along
with the rest of the bones of mastication, such as the maxilla and
mandible.

In meteorology, the term incus is applied to the characteristic cirriform anvil top of a cumulonimbus cloud at its most
mature stage of development.

Stapes

The stapes or stirrup is the stirrup-shaped small bone or ossicle in the
middle ear which is attached to the incus laterally and to the fenestra
ovalis, the “oval window”, medially. The oval window is adjacent to the
vestibule of the inner ear. The stapes is the smallest and lightest bone
in the human body.

Semicircular Canal

The semicircular canals are three half-circular, interconnected tubes
located inside each ear. The three canals are the horizontal
semicircular canal (also known as the lateral semicircular canal),
superior semicircular canal (also known as the anterior semicircular
canal), and the posterior semicircular canal.

The canals are aligned approximately orthogonally to one another. The horizontal canal
is aligned roughly horizontally in the head. The superior and posterior canals are aligned roughly at a 45 degree angle to a vertical plane drawn from the nose to the back of the skull. Thus, the horizontal canal detects horizontal head movements (such as when doing a pirouette), while the superior and posterior canals detect vertical head movements.

Each canal is filled with a fluid called endolymph and contains a motion
sensor with little hairs (cilia) whose ends are embedded in a gelatinous
structure called the cupula. As the skull twists in any direction, the
endolymph is thrown into different sections of the canals. The cilia
detect when the endolymph rushes past, and a signal is then sent to the
brain.

The semicircular canals are a component of the bony labyrinth.
Among species of mammals, the size of the semicircular canals is correlated
with their type of locomotion. Specifically, species that are agile and
have fast, jerky locomotion have larger canals relative to their body
size than those that move more cautiously.

Vestibular Nerve

The vestibular nerve is one of the two branches of the Vestibulocochlear
nerve (the cochlear nerve being the other). It goes to the semicircular
canals via the vestibular ganglion. It receives positional information.
Axons of the vestibular nerve synapse in the vestibular nucleus on the
lateral floor and wall of the fourth ventricle in the pons and medulla.
It arises from bipolar cells in the vestibular ganglion, ganglion of
Scarpa, which is situated in the upper part of the outer end of the
internal auditory meatus.

Facial Nerve

The facial nerve is the seventh (VII) of twelve paired cranial nerves. It
emerges from the brainstem between the pons and the medulla, and
controls the muscles of facial expression, and functions in the
conveyance of taste sensations from the anterior two-thirds of the
tongue and oral cavity. It also supplies preganglionic parasympathetic
fibers to several head and neck ganglia.

Internal Auditory Meatus

The internal auditory meatus (also internal acoustic meatus, internal
auditory canal, and internal acoustic canal) is a canal in the petrous
part of the temporal bone of the skull that carries nerves from inside
the cranium towards the middle and inner ear compartments namely cranial
nerve VII and cranial nerve VIII.

Structure: The opening to the internal acoustic meatus is located inside the cranial cavity, near the center of the posterior surface of the petrous bone. The size varies
considerably; its margins are smooth and rounded. The canal is short
(about 1 cm) and runs laterally into the bone. At its end are the
openings for three different canals, one of which is the facial canal.
The internal acoustic meatus transmits the facial and vestibulocochlear
nerves and the labyrinthine artery (an internal auditory branch of the
basilar artery). The facial nerve travels through the facial canal,
eventually exiting the skull at the stylomastoid foramen.

The opening of the meatus is called the porus acusticus internus, or its English translation, the internal acoustic opening.

The antero-superior part transmits the facial nerve and nervus intermedius
and is separated from the postero-superior section, which transmits the
superior vestibular nerve, by Bill’s bar (named by William House who
founded the House Ear Institute). The falciform, or transverse, crest
separates the superior part from the inferior part. The cochlear nerve
runs antero-inferiorly and the inferior vestibular nerve runs
postero-inferiorly.

Cochlear Nerve

The cochlear nerve (also auditory or acoustic nerve) is a nerve in the head
that carries signals from the cochlea of the inner ear to the brain. It
is part of the vestibulocochlear nerve, the 8th cranial nerve which is
found in higher vertebrates; the other portion of the 8th cranial nerve
is the vestibular nerve which carries spatial orientation information
from the semicircular canals. The cochlear nerve is a sensory nerve, one
which conducts to the brain information about the environment, in this
case acoustic energy impinging on the tympanic membrane. The cochlear
nerve arises from within the cochlea and extends to the brainstem, where
its fibers make contact with the cochlear nucleus, the next stage of
neural processing in the auditory system.

Cochlea

The cochlea is the auditory portion of the inner ear. Its core component is the Organ of Corti, the sensory organ of hearing, which is distributed along the partition separating fluid chambers in the coiled tapered tube of the cochlea.

The name is from the Latin for snail, which is from the Greek ?????a? kokhlias (“snail, screw”), from ???? kokhlos (“spiral shell”) in reference to its coiled shape; the
cochlea is coiled in most mammals, monotremes being the exceptions.

The Eustachian tube (or auditory tube or pharyngotympanic tube) is a tube
that links the pharynx to the middle ear. In adults the Eustachian tube
is approximately 35 mm long. It is named after the sixteenth century
anatomist Eustachius.[1] Some modern medical books call this the
pharyngotympanic tube.[2]

Location: The Eustachian tube extends from the anterior wall of the middle ear to the lateral wall of the nasopharynx, approximately at the level of the inferior nasal concha. A
portion of the tube (~1/3) proximal to the middle ear is made of bone;
the rest is composed of cartilage[3] and raises a tubal elevation, the
torus tubarius, in the nasopharynx where it opens.

In the equids (horses) and some rodent-like species such as the desert hyrax, an
evagination of the eustachian tube is known as the guttural pouch and is
divided into medial and lateral compartments by the stylohyoid bone of
the hyoid apparatus. This is of great importance in equine medicine as
the pouches are prone to infections, and due to their intimate
relationship to the cranial nerves (VII, IX, X, XI) and the internal and
external carotid artery, various syndromes may arise relating to which
is damaged. Epistaxis (nosebleed) is a very common presentation to
veterinary surgeons and this may often be fatal unless a balloon
catheter can be placed in time to suppress bleeding.
(Source: http://en.wikipedia.org/)