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 is compressed), 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 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. 

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.

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!

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

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 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 Mechanics of the Ear - How the Middle Ear Works

The cochlea is the second part 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 frequency 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 (vestibular labyrinth) 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 pressure waves of a certain frequency move the basilar membrane under them (inner hairs)
     • Stiffer hairs (outer hairs) that run parallel to the inner hairs and act as inner hair and basilar membrane inhibitors
     • The basilar membrane in which the inner hairs are embedded that transmits fluid movement to the inner hairs
     • The tectorial membrane in which the tops of the outer hairs are lightly embedded
     • The auditory nerves that run through the middle of the cochlea from the opening to close proximity to the helicotrema.  These nerves from the inner hair roots link to the 8th nerve that transmit information 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 and the associated fluids they contain.

  
   
  Graphic courtesy of NYU medical Center
   

Endolymph in the scala media (Organ of Corti) consists of a liquid solution similar to cerebro-spinal fluid and is made up of sodium, potassium, chloride, bicarbonate, glucose and protein. Along the length scala media, the actual ratios of potassium and sodium vary depending on the local frequency attributes of the tectorial and basilar membranes.

Perilymph, the liquid in the scala vestibuli and the scala tympani also contains sodium, potassium, chloride, bicarbonate, glucose and protein. However, it has higher concentration of glucose and total protein.

The following graphic shows a horizontal cross-section of the scala tympani (containing perilymph) and the scala vestibuli (containing perilymph) as they spiral parallel to each other, the length of the cochlea. It is through these liquid-filled chambers that vibrations are transmitted.

Image of the cross-section slice below >>>>>>>>>>>

The organ of Corti (center chamber)  of the cochlea (contains endolymph) Scala vestibuli contains perilymph Scala tympani contains perilymph Auditory nerve Auditory nerve fibers The red arrow shows vibrations entering the scala vestibuli half of the cochlea by way of the perilymph medium from the stapes (middle ear) and the oval window. The blue arrow shows vibrations exiting from the helicotrema at the top of the the cochlea into the scala tympani by way of the perilymph medium. The vibration(s) will progress to the bottom of the cochlea where they will be amplified by the round window.

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

The cochlea as sound wave interpreter and converter

When sound waves from the outside world strike the eardrum, it vibrates. These vibrations from the eardrum are transmitted through the ossicles into the inner ear through the oval window. Action of the oval window causes fluids in the cochlea to create pressure waves, where as they travel the length of the cochlea and back, they disturb the fluids in the organ of Corti. These waves are known as intra-cochlear pressure waves.

Wave Action in the Cochlea

From the oval windows, waves first travel the length of the scala vestibuli to the small hole at the end of the cochlea (helicotrema) affecting the tectorial membrane, then the waves squeeze through the helicotrema and travel back through the cochlea in the scala tympani fluids affecting the basilar membrane until they bounce off the round window which provides an amplification factor on the order of 20X. As sound waves travel through both fluid-filled canals and disturb the fluids in the organ of Corti (scala media), the basilar membrane will vibrate at the precise location where a wave pressure (frequency) is cancelled out. Inner hairs attached to the basilar membrane at that specific location react to the basilar membrane vibrating to the frequency and convert the wave into electrical impulses that are transmitted to the brain by the auditory nerve. This conversion is known as the cochlear microphonic. This action occurs almost concurrently thousands of times a second as multiple pressure wave frequencies disturb the fluids of the cochlea from sounds hitting the eardrum and transmitted to the oval window.

The Round Window

The round window is a flexible membrane at the opposite end of the fluid-filled channels from the oval window. There has to be flexibility in the cochlear fluids for the the oval window to vibrate and create the pressure waves for the cochlear fluids. The round window provides that flexibility plus other functions:

• It keeps the cochlear fluids contained within the scala vestibuli and scala tympani
• It functions as a sound multiplier, moving outward or inward at a 20X multiple as a result of pressure waves. The waves are generated from the oval window membrane as it is pushed in or sucked out by the stapes.

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; one row of inner hairs and 2-4 rows of outer hairs, all running the length of the basilar membrane at the "roots", and the tectorial membrane along the tops of the outer hairs. In a healthy young human there are about 3,500 of these inner hair cells and about 12,000 outer hair cells. They are sometimes called stereocilia.

Electron Microscope view of hair cells embedded in the basilar membrane

Electron Microscope view of one clump of inner hair cells embedded in the basilar membrane

Inner hair cell operations

Each inner hair cell cluster is also made up of two types of cells: those that bend to a specific sound frequency when the liquid in the organ of Corti disturbs the basilar membrane under them, and a group of small rigid hairs which act as inhibitors/dampeners.  When the basilar membrane reacts to a pressure wave of a certain frequency, it pushes up at a specific location along its length. The distortion of the basilar membrane forces the inner hair cells to bend or "shear", and they squirt potassium (K+) into the lower part of themselves at  their roots. This action causes a small electric current to flow through the inner hair roots across a synapse (gap) and release a neurotransmitter that creates a response in the auditory neurons. This message travels through the auditory nerve (also called the 8th nerve) to the brain.

The hair cells are critical to hearing; it is the inner hairs that move in the basilar membrane in reaction to pressure waves in the Organ of Corti fluids. It is the inner hair cells that translate the fluid movements to chemical messengers that can in are converted to electrical impulses that the brain can understand.

Outer hair cell operations

The outer hair cells provide two functions:

• Inner hair enhancer: When the basilar membrane distorts and causes inner hair bending/shearing activity, the outer hairs stretch (push downward) into the basilar membrane at the same time and enhance the wave that is already generated (called motility). This stretching forces a change in amplification of the wave at that point and increases the mechanical input to the inner hair cells. i.e. it helps the inner hairs become more sensitive to the wave. This phenomenon is called the "cochlear amplifier".
  
  
• Feedback suppression: The outer hair cells are also the means through which the brain sends messages back to the cochlea to mediate activity based on messages the inner hairs have transmitted.  The outer hair cells complete the feedback loop from the brain back to the inner ear. If warranted, messages to the outer hairs instruct them to inject inhibitor chemicals into the organ of Corti fluids resulting in a dampening to inner hair activity.
It is believed that this feedback mechanism is another way for the body's sound sensing system to "shut off" loud sounds that may damage the ear, except that this method uses the brain itself (in conjunction with the outer hairs) to perform the task. Because the tops of the outer hair cells are lightly embedded in the tectorial membrane (discussed below), it is posited, but not yet proven, that the feedback messages from the brain also cause the outer hair cells to stiffen and suppress the tectorial and basilar membrane wave actions. However, this is just a theory at this point. There is also another viewpoint on how and why the tectorial membrane reacts to loud sounds.

Tectorial membrane

The tectorial membrane is a stiff membrane that lies over the hair cells of the organ of Corti. Much like a group of people holding a long carpet runner over their heads, the outer hair cells are lightly embedded in the tectorial membrane along its length. The tectorial membrane, like the basilar membrane, runs the length of the cochlea.

The tectorial membrane is the first of the cochlear components to react to wave pressures from the oval window as this membrane sits inside the fluids of the organ of Corti next to the scala vestibuli. Like the basilar membrane in the scala tympani, it runs the length of the cochlea to the helicotrema.  The tectorial membrane reacts to pressure waves from the scala vestibuli and the fluids in the organ of Corti by vibrating sideways. The pressure waves first affect the tectorial membrane as they move through the organ of Corti fluids to the top of the cochlea, then when they travel through the helicotrema and back through the scala tympani, the waves affect the basilar membrane.  In other words, a specific pressure wave will first cause the tectorial membrane to vibrate laterally, then a split second later, it will cause the basilar membrane to vibrate up and down.  The combination of the relatively stiff tectorial membrane and flexible basilar membrane and outer hair motility act to stimulate the inner hair cells. This combination (along with the ossicular chain amplification) explains the incredible sensitivity of the human ear. Note: at this time the electrochemical processes that coordinate the time delays of the vibrations between the two membranes for presentation to the brain are unknown.

Basilar membrane

One edge of the basilar membrane is attached to the bony core at the center of the cochlea (the modiolus); the other is loosely attached to the organ of Corti outside the outermost outer hair cell.

The properties of the basilar membrane change as its shape changes; just as with guitar strings, thin things vibrate to high pitches, and thick things vibrate to low pitches. The basilar membrane vibrates to high frequencies at the base (opening) of the cochlea and to low frequencies at the apex (towards the helicotrema). This phenomenon is counter-intuitive as one would expect the opposite. But the answer lies in the properties of the basilar membrane itself. Even though wider at the mouth of the cochlea, the basilar membrane is actually at its thinnest and it is this property that allows the basilar membrane to be affected by very high notes at this location. Towards the helicotrema, the basilar membrane is much narrower then at the opening, but it is also much thicker. It is the thickness that causes the basilar membrane to only be affected by pressure waves created from low bass notes (less than 100 herz).

The key concept here is that hair cells in and of themselves are no different from each other, whether they are located at the mouth of the cochlea (high frequencies) or next to the helicotrema (low frequencies). It is the properties of the basilar membrane to react to wave pressure at a specific location in the cochlea that define whether hair cells will move or not, not the properties of the hair cells themselves.

Simulation of a wave displacing the basilar membrane

Messaging to the Brain

Both the inner and outer hairs are all connected to sections of the auditory nerve (8th nerve) through the basilar membrane and, depending on the nature of the wave movements in the cochlear fluids, different inner hairs are put into motion. When the hairs move they send electrical signals through the 8th nerve which is connected to the auditory center of the brain. In the brain the electrical impulses are translated into sounds which we recognize and understand. As a consequence, these hair fibers are essential to our hearing ability. Should these hair fibers become damaged or die, our hearing ability deteriorates.

The image below shows one inner hair cell cross section. It is this cell multiplies thousands of times (3500+/-) that translates the outer world vibrations by way of the eardrum, ossicles, and inner ear fluids, into a chemical/electrical signal that the brain can interpret.

Opposite the base of each "root" of a hair cell (across synapses) are the nerve endings of nerve bundles that carry the hair-cell generated impulses to the brain. At least 90 per cent of these nerves are associated with inner hair cells, despite their smaller number, because an inner hair cell has about 10 nerve endings associated with it. This means that in a healthy young person there are about 30,000 - 35,000 associated nerve fiber endings across each synapse. The nerve fibers make up the acoustic nerve or "8th nerve"  that transmits the electrical/ chemical equivalents of the sound waves to the brain and back.

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Putting it all together - How the Ear Works - Steps to how we hear sounds (click here)