How the Ear Works Including Pictures and Graphics

Intensity (Loudness, Amplitude, Pressure) - measured in Pascals (Pa, µPa)
Sound waves also have intensity (loudness) and, when the comparison is made with ripples on the pond, this equates to the volume of the wave. In real life, it is easier to measure the pressure of the wave rather than its intensity. This pressure is measured in units called pascals. One pascal is rather large for sound pressure measurements so  micropascals (µPa), that is, one-millionth of a pascal are used to measure sounds related to the ear.

The following are examples:

Hearing A Clap

We hear sounds because our ears are sensitive to pressure waves. Perhaps the easiest type of sound wave to understand is a short, sudden event like a clap. When you clap the hands, the air that was between the hands is pushed aside. This increases the air pressure in the space near the hands, because more air molecules are temporarily compressed into less space. The high pressure pushes the air molecules outwards in all directions at the speed of sound. When the pressure wave reaches the ear, it pushes on the eardrum slightly, causing you to hear the clap.



A hand clap is a short event that causes a single pressure wave that quickly dies out. The image above shows the waveform for a typical hand clap. In the waveform, the horizontal axis represents time, and the vertical axis is for pressure. The initial high pressure is followed by low pressure, but the oscillation quickly dies out.


Hearing a bell
The other common type of sound wave is a periodic wave. When you ring a bell, after the initial strike (which is a little like a hand clap), the sound comes from the vibration of the bell. While the bell is still ringing, it vibrates at a particular frequency, depending on the size and shape of the bell, and this causes the nearby air to vibrate with the same frequency. This causes pressure waves of air to travel outwards from the bell, again at the speed of sound. Pressure waves from continuous vibration such as a bell look more like this:



When these sounds strike the ear drum, they cause the ear drum to vibrate at that frequency.

Range Of the Human Ear
For convenience pressure levels of sound are recorded as decibels (dB) which are logarithmic measurements. Response to increasing sound intensity is a "power of ten" or logarithmic relationship. The logarithmic unit of measurement means, for example. that 80dB is 10 times louder than 79dB. This is one of the motivations for using the decibel scale to measure sound intensity. A general 'rule of thumb' for loudness is that the power must be increased by about a factor of ten to sound twice as loud.

The range of pressures that the ear can hear is enormous. The quietest, just detectable sound may be 20 µPa, but a jet engine heard close by has a level of 20,000,000 µPa. The quietest sound that the average healthy 18 year old, without previous ear problems and with normal eardrums, can hear has a pressure of 20 micropascals (20 µPa). This level forms the basis for measuring the pressure of other commonly heard sounds in our environment. 

Many young, healthy humans (through teens and early twenties) can hear frequencies from about 20 Hz to 20,000 Hz, and can detect frequency differences as small as 0.2%. That is, we can tell the difference between a sound of 1000 Hz, and one of 1002 Hz.

The Range of Pressure That Can Be Heard By An Undamaged Ear
The ear is capable of hearing a wide range of sounds. The ratio of the sound pressure of the lowest limit that undamaged ears can hear to that of sounds that can cause permanent damage is more than a million. The decibel scale is logarithmic; it can be used to describe very large ratios
Decibels	Micropascals	Typical Perception
0 dB	20µPa	The quietest sound an 18 year-old  healthy ear can hear
20 dB	200µPa	A very soft whisper
45 dB	300 - 800 µPa	A softly spoken voice
60 dB	5,000 µPa	An average spoken voice
70 dB	20,000 µPa	A shout
80 dB	100,000 µPa	A noisy motorcycle on a narrow street
90 dB	500,000 µPa	Jackhammers within 50 feet
100-120 dB	5.000.000 µPa	A heavy metal rock concert
120 - 140 dB	20,000,000 µPa	The noise of a jet engine within 250 yds.

Severity of hearing loss
Hearing loss is also measured in decibels (dB). Conversational speech is around 60dB. The degrees of hearing loss include:

Hearing Loss	Need for a hearing Aid
Mild (cannot hear below 45dB) - soft sounds may be difficult to distinguish.	None
Moderate (cannot hear below 60dB) - conversational speech is hard to hear, especially if there is background noise (such as a television or radio).	Yes- Our hearing aids are excellent for helping you if you have this range of hearing loss, especially if you cannot hear in the 55-60DB range
Moderately severe (cannot hear below 75dB) - it is very difficult to hear ordinary speech.	Yes- Our hearing aids are excellent for helping you if you have this range of hearing loss.
Severe (cannot hear 76-90dB) - conversational speech can't be heard.	Yes- However, our hearing aids will only help in the hearing range below 2000Hz. You will need to see an audiologist for testing.
Profound (cannot hear 91dB+) - almost all sounds are inaudible. Some people with such a profound hearing loss can benefit from a hearing aid, but only to a small extentt.	Yes- However, our hearing aids cannot help you if the hearing loss is this severe. You will need to see an audiologist or an ENT physician (Ear, Nose and Throat specialist

Damaged hair cells and deafness

There are a series of hair cells contained in the cochlea (inner ear) that are key to most people's hearing. They are called the "inner hairs" (more on this later). It is damage to, or lack of the inner hair cells that cause most deafness. High decibels i.e. loud music or sounds above 140 db. will cause some of these hairs to die, as will some serious infections. Once an inner hair dies, it cannot be replaced. Because we initially are born with only about 3500 of these hairs, loss of a few can make a big difference in our hearing capacity. (The cochlea and the role of these hairs are discussed in detail in this document)

The Mechanics of the Ear - How the Ear Works

• The ear lobe (pinna or auricle)
• The ear canal, through which sound waves pass to the ear drum
• The ear drum (which separates the outer ear from the middle ear)

The ear lobe and the outer ear canal, which delivers sound to the middle ear, make up the outer ear, the part that we see. Within the outer ear canal are wax-producing glands and hairs that protect the middle ear.

The middle ear consists of:

• The inner part of the ear drum
• The hammer (malleus)
• The anvil (incus)
• The stirrup (stapes)

The middle ear is a small cavity with the eardrum (tympanic membrane) on one side and the entrance to The inner ear on the other. Within The middle ear are 3 small bones (the ossicles) known as the hammer (malleus), anvil (incus), and stirrup (stapes) because of their shapes.



The function of the middle ear is to deliver sound to the inner ear where it is processed into a signal that the brain can recognize.

Within the middle ear, vibrations travel through three small bones (first the hammer, then the anvil, and finally the stirrup) to the cochlea. The hammer is attached to the lining of the eardrum, the anvil is attached to the hammer, and the stirrup links the anvil to the oval window, the opening to the inner ear. These three tiny bones amplify the sound from the ear drum by the time the vibration is transferred to the stirrup and the oval window.

The oval window as amplifier

The stirrup is connected to what is called the oval window on the outside of the cochlea. Because the oval window of the cochlea is smaller than the eardrum, when the stirrup vibrates from vibrations transmitted by the other bones, it causes a further amplification of the sound vibration - up to 20 times at some frequencies.

Normally, when sound hits the surface of a liquid, 99.5 per cent or more is reflected. However, the operation of the middle-ear mechanism results in about 50 per cent of the sound to be transferred to the inner ear - a very efficient mechanism!

The Eustachian tube

The middle ear is also connected by a narrow channel to the throat called the Eustachian tube. Th Eustachian tube is a pressure equalizer and drainage pipe.  Ordinarily, the Eustachian tube is closed because the air in the middle ear chamber must be completely still for the optimal vibration of the middle ear bones.

When you swallow or yawn, it opens briefly to allow an exchange of air, equalizing the air pressure within the middle ear and the air pressure outside. When you yawn or sneeze, or hold your nose and push out on your ear drums with your breath when descending in an airplane, and your ears "pop", the Eustachian tube is forced open and  the air pressure between the inner ear and the outside is balanced.

The Eustachian tube is also used to drain accumulations in the middle ear such as mucous or bacterial detritus.

The inner ear

The inner ear contains the most important parts of the hearing mechanism - two chambers called the vestibular labyrinth and the cochlea.

The vestibular labyrinth consists of elaborately formed canals (3 semicircular tubes that connect to one another), which are largely responsible for the sense of balance.

The cochlea, which begins at the oval window, curves into a shape that resembles a snail shell. Tiny hairs line the curves of the cochlea. Both the labyrinth and cochlea are filled with various fluids.

Vestibular labyrinth (semi-circular canals)

The semi-circular canals as balancers

The semicircular canals help to maintain a sense of balance by responding to gravity and changes in acceleration of the head (up/down, forward/back, and side to side) as shown in the following graphics.



The body senses its different positions and controls it's balance through fluid dynamics in the 3 semicircular canals providing separate "readings"; The semicircular canals act as miniature accelerometers.



The semicircular canals are arranged roughly at right angles with each other so that they represent all three planes in three-dimensional space. The horizontal canal lies in a plane pitched up approximately 30 degrees from the horizontal plane of the earth-erect head. The anterior (front) canals are located in vertical planes that project forward and outward by approximately 45 degrees. The posterior (rear) canals are located in vertical planes that project backward and outward by approximately 45 degrees.



If you could take the upper portion of each inner ear on each side of the head, they would be symmetrical: the front canal on one side of the head is parallel to the rear canal on the other as shown in the above diagram

Communicating balance to the brain

Messages to the brain as to the head's changes in position are generated by the fluid in the semicircular canals acting on calcium carbonate crystals that shift on their bed of sensory hairs in two fluid-filled cavities at the base of the semicircular canals: the utriculus and the sacculus. The changes as to which hairs are being stimulated by the presence of crystals are reported through the vestibular nerve to the cerebellum (a part of the brain) which in turn, translates the information into knowledge of the position of the head relative to gravity.
 

The Cochlea

The cochlea is the bottom portion of the inner ear and is the actual organ of hearing. It is embedded in the skull in what is called the mastoid area, a spongy part of the skull just behind where the jaw hinges. The mastoid bone acts as an amplifier for some sounds, especially those in the lower decibel ranges.

The cochlea is made up of:

• The oval window to which the stirrup from the middle ear is attached
• The round window which acts as a sound magnifier
• Three fluid-filled canals that run the length of the cochlea and separated by thin membranes.
  1. The vestibular canal (scala vestibuli)
  2. The tympanic canal (scala tympani)
  3. The cochlear canal (scala media) that contains the Organ of Corti which in turn contains:
     • The hairs that move when certain wave lengths hit them (inner hairs)
     • Different hairs (outer hairs) that act as inner hair inhibitors
     • The basilar membrane that transmits fluid movement to the hairs
• The auditory nerves that transmit information from the inner ear to the brain

       


How the Various Canals Work

The cochlea begins at the oval window where the middle ear stirrup is attached and curves into a shape that resembles a snail shell, where the chambers get narrower towards the end.  The coiled tube inside the snail-like apparatus contains three parallel canals discussed above, that run the length of the cochlear envelope.

The following diagram is a longitudinal cross-section of a cochlear showing the three major canals or ducts.

 
Graphic courtesy of NYU medical Center
 

The following is a cross-section of the same diagram above.


 

The cochlea as microphone

When sound waves from the world outside strike the eardrum, it vibrates. These vibrations from the eardrum are transmitted through the three bones of the middle ear and into the inner ear through the oval window. Action of the oval window causes fluids in the cochlea to create waves where they disturb the basilar membrane. Inner hairs attached to the basilar membrane convert the waves into electrical impulses that are transmitted to the brain by the auditory nerve.

The hair cells - keys to hearing

There are approximately 24,000 fibers or hairs in the Organ of Corti, organized into "inner" and "outer" hairs. These hairs are arranged in four long rows the length of the basilar membrane in the cochlea. In a healthy young human there are about 3,500 inner hair cells of varying lengths and about 12,000 outer hair cells. Each hair cell also has a cluster of small rigid hairs, which project from the thicker upper surface of the cell into the fluids that fills the cochlear duct. The tips of the hairs of the outer hair cells are lightly embedded in the under-surface of the basilar membrane whereas the tips of the inner hair cells do not reach the basilar membrane and stand free in the endolymph.

Inner hair cell operations

The inner hair cells consist of a single row running the length of the cochlea and lie closer to the core of the cochlea. Sensor neurons are in close contact with the top surface of each receptor hair cell and they transmit electrical changes to the brain through the basilar membrane when sound waves disturb the fluids in the vestibular and tympanic canals.  The inner hair cells are critical to hearing; it is the inner hairs that move in the Organ of Corti fluids, and translate the fluid movements to chemical messengers that can in turn be converted to electrical impulses that the brain understands.

Outer hair cell operations

There are three or four rows of outer hair cells, which are further away (sideways to the length of the cochlea, through which the brain sends messages back to the cochlear to mediate activity that the inner hairs have transmitted.  The outer hair cells complete the feedback loop from the brain back to the inner ear. The returning messages refine the sensitivity and frequency selectivity of the mechanical vibrations of the fluids in the cochlea by causing the outer hairs to inject inhibitor chemicals to counteract the action of the inner hair fluids. It is believed that this feedback mechanism and resultant injection of inhibitor chemicals allows the brain to "shut off" loud sounds that may damage the ear.

Basilar membrane