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How the ear works Go to patient awareness booklet
How The Ear Works
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The ear is extremely sensitive; with the softest detectable sound, the eardrum only moves approximately one-millionth of an inch. Even this soft vibration is transferred to the inner ear for processing by the brain.  Our ability to detect sounds from the softest to the loudest covers an intensity range of approximately 100,000,000 to 1. 

Sound travels as small waves of pressure through the air at a speed of about 740 miles per hour. What we hear are sound waves provided by vibrations of air molecules. We don't actually measure waves, but the calculated pressure that a wave makes against an object (ear drum). The measurements are shown in a logarithmic scales called Pascals or Micropascals.

The waves of sound act like ripples on the surface of a pond spreading out after a stone has been thrown in.

The "wave" itself consists of small pressure fluctuations in the air about the ambient (atmospheric) pressure. At some points along the sound wave, the air pressure is slightly above the ambient level (the air iscompressed), and at others it is below (the air is rarefied). These compressions and rarefactions are generated by the source of the sound wave, usually a vibrating object such as a violin string, a loudspeaker, or a motor in a machine. When the pressure fluctuations in the wave reach the ear, the eardrum vibrates in direct response, and the pressure fluctuations are heard as sound.

How Sound is measured

Waves have a number of measurable attributes:

  • Pitch (frequency) is the measurement of the pressure fluctuations of a wave i.e. how rapidly the waves change from above to below the ambient pressure (ambient pressure is the normal barometric pressure where the listener is hearing the sound). Another way  of looking at pitch is the number of crests that pass a point in a given second. 
  • Intensity (loudness, amplitude, pressure) determines how loud the sound is. Amplitude is the measurement of pressure fluctuations in the sound wave i.e. how far the waves are above and below the ambient pressure. 

Pitch (Frequency) - measured in Hz/kHz

Pitch is measured as "cycles per second- (cps)" which is now more commonly written as hertz (Hz); 261 Hz is equivalent to middle C on the piano. One thousand cycles per second (1,000 cps) is one kilohertz (1 kHz). The number of vibrations or cycles per second makes up frequency - the more vibrations, the higher the pitch of the sound.

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 twice as loud as 70dB. 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. 

The buzzing of a mosquito is less than one quadrillionth of a watt.   Pressure movement less than the diameter of a hydrogen molecule can still cause the ear-drum to vibrate and can be heard.  Even sound 10 million million (1,000,000,000,000) times larger (short duration) will not damage the hearing mechanism (there is a limit however, shown in the chart, below).  We can discriminate 400,000 approx sounds and recognize a voice blurred by telephone.  Hearing extends over a frequency spectrum of 10 octaves.

Softly speaking Indigenous people living in remote areas isolated from traffic and amplified sound where the majority background sound, except for birds and periodic thunder, is about one-tenth of a refrigerator. They can hear a soft murmur across a clearing the size of a football-sized field, and locate its source. They suffer little to no loss of acuity with age.

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, they 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 (µPa) 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.

How sensitive can one really hear? Vibrations of the ear-drum at the threshold of hearing can correspond to about the diameter of a hydrogen atom! As stated before, some people, especially those living in the countryside away from machinery and big city sounds can actually hear random motion of air molecules bouncing against their eardrums!

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- 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- hearing aids are excellent for helping you if you have this range of hearing loss.
Severe (cannot hear 76-95dB) - conversational speech can't be heard. Yes- hearing aid will in most cases help with this severe hearing loss
Profound (cannot hear 95dB+) - almost all sounds are inaudible. Some people with such a profound hearing loss can benefit from a hearing aid, but only to a small extent. No- Most hearing aids can only provide small hearing assistance at these profound hearing loss levels.

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

The ear is made up of three basic structures: the outer ear, the middle ear, and the inner ear. Connecting the middle ear to the throat is a canal called the Eustachian tube. 

How the Outer Ear Works

The outer ear consists of:

  • The ear lobe (pinna or auricle)
  • The ear canal, through which sound waves pass to the ear drum
  • The ear drum (Tympanic membrane that 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 Ear Drum 

The ear drum is a thin, semitransparent, oval-shaped membrane that separates the middle ear from the outer ear.  It's purpose is to vibrate according to the frequency and amplitude of sounds that strike it. On the middle ear side of the ear drum is attached a horseshoes shaped platform that rests on the ear drum itself. Attached to it is the first of the bones of the middle ear (malleus), and the tensor tympani muscle that dampens and amplifies sounds. 

The Mechanics of the Ear - How the Middle Ear Works

The purpose of the middle ear is to:

  • Transmit and amplify sounds from the eardrum to the oval window
  • Act as a dampener on loud sounds that may damage the inner ear (cochlea)

The middle ear consists of:

  • The inner part of the ear drum to which one end of the hammer is attached
  • The hammer (malleus) (a bone)
  • The anvil (incus) (a bone) which is connected on one end to the hammer and the other end to the stirrup
  • The stirrup (stapes) (a bone) which is connected on one end to the incus, and on the other end to the footplate that rests on the face of the oval window.

All three bones are known as the ossicular chain and are encased in a jelly-like mucous membrane. 

The hammer is attached to the lining of the eardrum, the anvil (middle bone) is attached to the hammer on one end and to the stapes on the other. The other end of the stapes attached to  the oval window with what is called the "foot-plate". These three tiny bones transmit sound from the ear drum to the oval window.


The oval window is the demarcation between the middle ear and the inner ear functions. It provides a platform for the foot-plate to vibrate on. Except for some low frequencies that can be transmitted through the mastoid bone, the footplate and oval window are  the only means by which sounds from the outer ear get transmitted through the middle ear to the inner ear.

The eardrum is some 13 times larger than the oval window, giving an amplification of about 13 compared to the oval window.

There are two tendons attached to the ossicles:

  • The tensor tympani (tensorial) tendon runs from the middle ear side of the eardrum (next to where the hammer is attached) and passes through the top of the middle ear cavity (sometimes called the tympanic cavity) and is secured to tissue on the mastoid bone.
  • The stapedius tendon runs from the stapes to the middle ear cavity wall.

The ossicles as transmitters and amplifiers
As sound transmitters, the ossicles achieve a multiplication of force due to the actions of :

  • The tensor timpani tendon twisting the malleus relative to the incus. This is a lever effect, and enhances or dampen sounds. 
  • The stapedius tendon attached to the stapes and the oval window provides a force multiplier on the oval window.

Because the oval window of the cochlea is smaller than the eardrum, when the stirrup (stapes) vibrates from the other bones, it causes a further amplification of the sound vibration - up to 20 times at some frequencies. It is this attribute that provides the breadth of frequencies we can hear and the sensitivity to sounds.

How is this simple mechanism of three bones with membranes on each side, able to work so well?

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