In this article, we'll look at the mechanical systems that make hearing possible. We'll trace the path of a sound, from its original source all the way to your brain, to see how all the parts of the ear work together. When you understand everything they do, it's clear that your ears are one of the most incredible parts of your body! 

To understand how your ears hear sound, you first need to understand just what sound is. 

An object produces sound when it vibrates in matter. This could be a solid, such as earth; a liquid, such as water; or a gas, such as air. Most of the time, we hear sounds traveling through the air in our atmosphere. 

When something vibrates in the atmosphere, it moves the air particles around it. Those air particles in turn move the air particles around them, carrying the pulse of the vibration through the air.

To see how this works, let's look at a simple vibrating object: a bell. When you hit a bell, the metal vibrates -- flexes in and out. When it flexes out on one side, it pushes on the surrounding air particles on that side. These air particles then collide with the particles in front of them, which collide with the particles in front of them, and so on. This is called compression. 

When the bell flexes away, it pulls in on the surrounding air particles. This creates a drop in pressure, which pulls in more surrounding air particles, creating another drop in pressure, which pulls in particles even farther out. This pressure decrease is called rarefaction. 

In this way, a vibrating object sends a wave of pressure fluctuation through the atmosphere. We hear different sounds from different vibrating objects because of variations in the sound wave frequency. A higher wave frequency simply means that the air pressure fluctuation switches back and forth more quickly. We hear this as a higher pitch. When there are fewer fluctuations in a period of time, the pitch is lower. The level of air pressure in each fluctuation, the wave's amplitude, determines how loud the sound is. In the next section, we'll look at how the ear is able to capture sound waves.

Main article: Middle ear

The middle ear

The middle ear lies between the outer ear and the inner ear. It consists of an air-filled cavity called the tympanic cavity and includes the three ossicles and their attaching ligaments; the auditory tube; and the round and oval windows. The ossicles are three small bones that function together to receive, amplify, and transmit the sound from the eardrum to the inner ear. The ossicles are the malleus (hammer), incus (anvil), and the stapes (stirrup). The stapes is the smallest named bone in the body. The middle ear also connects to the upper throat at the nasopharynx via the pharyngeal opening of the Eustachian tube.[3][7]

The three ossicles transmit sound from the outer ear to the inner ear. The malleus receives vibrations from sound pressure on the eardrum, where it is connected at its longest part (the manubrium or handle) by a ligament. It transmits vibrations to the incus, which in turn transmits the vibrations to the small stapes bone. The wide base of the stapes rests on the oval window. As the stapes vibrates, vibrations are transmitted through the oval window, causing movement of fluid within the cochlea.[3]

The round window allows for the fluid within the inner ear to move. As the stapes pushes the secondary tympanic membrane, fluid in the inner ear moves and pushes the membrane of the round window out by a corresponding amount into the middle ear. The ossicles help amplify sound waves by nearly 15–20 times

Hearing threshold and the ability to localize sound sources are reduced underwater, in which the speed of sound is faster than in air. Underwater hearing is by bone conduction, and localization of sound appears to depend on differences in amplitude detected by bone conduction.[10] Aquatic animals such as fish, however, have a more specialized hearing apparatus that is effective underwater.[11]

In vertebrates[edit]

A cat can hear high-frequency sounds up to two octaves higher than a human.

Not all sounds are normally audible to all animals. Each species has a range of normal hearing for both amplitude and frequency. Many animals use sound to communicate with each other, and hearing in these species is particularly important for survival and reproduction. In species that use sound as a primary means of communication, hearing is typically most acute for the range of pitches produced in calls and speech.

Frequency range[edit]

Frequencies capable of being heard by humans are called audio or sonic. The range is typically considered to be between 20 Hz and 20,000 Hz.[12] Frequencies higher than audio are referred to as ultrasonic, while frequencies below audio are referred to as infrasonic. Some bats use ultrasound for echolocation while in flight. Dogs are able to hear ultrasound, which is the principle of 'silent' dog whistles. Snakes sense infrasound through their jaws, and baleen whales, giraffes, dolphins and elephants use it for communication. Some fish have the ability to hear more sensitively due to a well-developed, bony connection between the ear and their swim bladder. The "aid to the deaf" of fishes appears in some species such as carp and herring.[13]

In invertebrates[edit]

Vertebrates aren't the only group of animals that have hearing. Some insects have hearing organs as well (e.g. the long-horned grasshopper, lubber grasshopper and the cicada); they use sound as a form of communication. Many mantises can even perceive ultrasound.[citation needed].

Something widely spread among insects is body hair, that can be made to swing by sonar waves. Due to the resonance phenomenon certain hairs swing stronger when exposed to a specific sonar-frequency. This specificity depends on the stiffness and the length of the hairs. That is why certain caterpillar species have evolved hair that would resonate with the sound of buzzing wasps, thus warning them of the presence of natural enemies. Moreover, mosquitoes have hair on their antennae that resonate with the flying sound of homogeneous females, enabling the males the ability to detect potential sexual partners.

Some insects possess a tympanal organ. These are "eardrums", that cover air filled chambers on the legs. Similar to the hearing process with vertebrates, the eardrums react to sonar waves. Receptors that are placed on the inside translate the oscillation into electric signals and send them to the brain.

Mathematics[edit]

The basilar membrane of the inner ear spreads out different frequencies: high frequencies produce a large vibration at the end near the middle ear (the "base"), and low frequencies a large vibration at the distant end (the "apex"). Thus the ear performs a sort of frequency analysis, roughly similar to a Fourier transform.[14][15] However, the nerve pulses delivered to the brain contain both rate-versus-place and fine temporal structure information, so the similarity is not strong.

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We saw in the last section that sound travels through the air as vibrations in air pressure. To hear sound, your ear has to do three basic things: 

The pinna, the outer part of the ear, serves to "catch" the sound waves. Your outer ear is pointed forward and it has a number of curves. This structure helps you determine the direction of a sound. If a sound is coming from behind you or above you, it will bounce off the pinna in a different way than if it is coming from in front of you or below you. This sound reflection alters the pattern of the sound wave. Your brain recognizes distinctive patterns and determines whether the sound is in front of you, behind you, above you or below you. 

Ear diagram courtesy NASA

Your brain determines the horizontal position of a sound by comparing the information coming from your two ears. If the sound is to your left, it will arrive at your left ear a little bit sooner than it arrives at your right ear. It will also be a little bit louder in your left ear than your right ear. 

Bodily Sensations
The nervous system determines the countless sensations we feel all over our bodies every day. How does this work? What causes your leg to feel tingly when it falls asleep? How do you know when you're about to sneeze? This activity from Discovery Channel explains how sensations are produced in the body.

­Since the pinnae face forward, you can hear sounds in front of you better than you can hear sounds behind you. Many mammals, such as dogs, have large, movable pinnae that let them focus on sounds from a particular direction. Human pinnae are not so adept at focusing on sound. They lay fairly flat against the head and don't have the necessary muscles for significant movement. But you can easily supplement your natural pinnae by cupping your hands behind your ears. By doing this, you create a larger surface area that can capture sound waves better. In the next section, we'll see what happens as a sound wave travels down the ear canal and interacts with the eardrum. 

The eardrum is rigid, and very sensitive. Even the slightest air-pressure fluctuations will move it back and forth. It is attached to the tensor tympani muscle, which constantly pulls it inward. This keeps the entire membrane taut so it will vibrate no matter which part of it is hit by a sound wave. 

Ear illustration courtesy NIDCD
Normal ear anatomy

This tiny flap of skin acts just like the diaphragm in a microphone. The compressions and rarefactions of sound waves push the drum back and forth. Higher-pitch sound waves move the drum more rapidly, and louder sound moves the drum a greater distance. 

The eardrum can also serve to protect the inner ear from prolonged exposure to loud, low-pitch noises. When the brain receives a signal that indicates this sort of noise, a reflex occurs at the eardrum. The tensor tympani muscle and the stapedius muscle suddenly contract. This pulls the eardrum and the connected bones in two different directions, so the drum becomes more rigid. When this happens, the ear does not pick up as much noise at the low end of the audible spectrum, so the loud noise is dampened. 

In addition to protecting the ear, this reflex helps you concentrate your hearing. It masks loud, low-pitch background noise so you can focus on higher-pitch sounds. Among other things, this helps you carry on a conversation when you're in a very noisy environment, like a rock concert. The reflex also kicks in whenever you start talking -- otherwise, the sound of your own voice would drown out a lot of the other sounds around you. 

The eardrum is the entire sensory element in your ear. As we'll see in the coming sections, the rest of the ear serves only to pass along the information gathered at the eardrum. 

As we'll see in the next section, the cochlea in the inner ear conducts sound through a fluid, instead of through air. This fluid has a much higher inertia than air -- that is, it is harder to move (think of pushing air versus pushing water). The small force felt at the eardrum is not strong enough to move this fluid. Before the sound passes on to the inner ear, the total pressure (force per unit of area) must be amplified. 

This is the job of the ossicles, a group of tiny bones in the middle ear. The ossicles are actually the smallest bones in your body. They include: 

 The malleus is connected to the center of the eardrum, on the inner side. When the eardrum vibrates, it moves the malleus from side to side like a lever. The other end of the malleus is connected to the incus, which is attached to the stapes. The other end of the stapes -- its faceplate -- rests against the cochlea, through the oval window. 

When air-pressure compression pushes in on the eardrum, the ossicles move so that the faceplate of the stapes pushes in on the cochlear fluid. When air-pressure rarefaction pulls out on the eardrum, the ossicles move so that the faceplate of the stapes pulls in on the fluid. Essentially, the stapes acts as a piston, creating waves in the inner-ear fluid to represent the air-pressure fluctuations of the sound wave. 

The ossicles amplify the force from the eardrum in two ways. The main amplification comes from the size difference between the eardrum and the stirrup. The eardrum has a surface area of approximately 55 square millimeters, while the faceplate of the stapes has a surface area of about 3.2 square millimeters. Sound waves apply force to every square inch of the eardrum, and the eardrum transfers all this energy to the stapes. When you concentrate this energy over a smaller surface area, the pressure (force per unit of volume) is much greater. To learn more about this hydraulic multiplication, check out How Hydraulic Machines Work. 

The configuration of ossicles provides additional amplification. The malleus is longer than the incus, forming a basic lever between the eardrum and the stapes. The malleus moves a greater distance, and the incus moves with greater force (energy = force x distance). 

This amplification system is extremely effective. The pressure applied to the cochlear fluid is about 22 times the pressure felt at the eardrum. This pressure amplification is enough to pass the sound information on to the inner ear, where it is translated into nerve impulses the brain can understand. 

1The smallest bones are the ossicles in the middle ear: the incus, the malleus, and the stapes (also called the anvil, hammer, and stirrup). 

2The inner ear is no larger than a pencil eraser in circumference. 

3Your sense of hearing is dependent upon tiny hairs deep inside your ear. If you lose these hairs, you lose your hearing. 

4You do not need to clean wax out of your ears unless you have an abnormal condition. Ears push excess wax out as needed. 

5The majority of individuals suffering from hearing loss are under the age of 65. 

6The number one cause of hearing loss is exposure to excessively loud sounds (85 decibels or higher). 

7Your hearing can be damaged permanently even after a single incident of exposure to extremely loud noise (shotgun blast, explosion, etc.). 

8Your ears never stop hearing, even when you sleep. Your brain just ignores incoming sounds. 

9Your ears are more than just necessary for hearing; they also help you keep your balance. 

10Not all living creatures hear with ears. Snakes use jawbones, fish respond to pressure changes, and male mosquitoes use antennae. 

If you or a family member is struggling with hearing loss, help is available. Call 855-355-9064 or contact us online to  schedule a no-obligation appointment today and preserve this life-enriching sense.

 

Action on Hearing Loss estimates that there are more than 10 million (about 1 in 6) people in the UK with some degree of hearing impairment or deafness.

Hearing loss can occur suddenly, but usually develops gradually. General signs of hearing loss can include:

Read more about the symptoms of hearing loss.

When to see your GP

See your GP if you're having problems with your hearing, or your child is showing signs of hearing difficulty. If you lose your hearing suddenly, in one or both ears, you must see your GP as soon as possible.

Your GP can check for any problems and may refer you to an audiologist (hearing specialist) or an ENT surgeon for further tests.

You can also visit the Action on Hearing Loss website for an online hearing test.

Read more about diagnosing hearing loss.

Why it happens

Hearing loss is the result of sound signals not reaching the brain. There are two main types of hearing loss, depending on where the problem lies:

It's also possible to have both these types of hearing loss. This is known as mixed hearing loss.

Some people are born with hearing loss, but most cases develop as you get older. 

Read more about causes of hearing loss.

Preventing hearing loss

It isn't always possible to prevent hearing loss if you have an underlying condition that causes you to lose your hearing.

However, there are several things you can do to reduce the risk of hearing loss from long-term exposure to loud noise. This includes not having music or the television on at a loud volume at home and using ear protection at loud music events or in noisy work environments.

You should also see your GP if you have signs of an ear infection, such as flu-like symptoms, severe earache, discharge or hearing loss.

Read more about preventing hearing loss.

Treating hearing loss 

The way hearing loss is treated depends on the cause and how severe it is. 

In cases of sensorineural hearing loss, there are several options that may help to improve a person’s ability to hear and communicate. These include:

Conductive hearing loss is sometimes temporary and can be treated with medication or minor surgery, if necessary. However, more major surgery may be required to fix the ear drum or hearing bones. If conventional hearing aids don't work, there are also some implantable devices for this type of hearing loss, such as a Bone Anchored Hearing Aids (BAHAs).

Read more about treating hearing loss.

How hearing works

Sound waves enter your ear and cause your eardrum to vibrate. These vibrations are passed to the three small bones (ossicles) inside your middle ear.

The ossicles amplify the vibrations and pass them on to your inner ear where tiny hair cells inside the cochlea (the coiled, spiral tube inside the inner ear) move in response to the vibrations and send a signal through a nerve called the auditory nerve to the brain.

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Whether you are worried about your hearing, know you have hearing loss, or are profoundly deaf this section will give you the practical information and advice you need. We know hearing loss can be a big problem, but there is lots you can do about it. 

We will help you take action on your own hearing loss, or help a friend or family member. Not sure if you're at risk of losing your hearing? You can find out how to look after your hearing here too.

Why not check your hearing today? This can be the first step in getting the help and support you need. We give you expert advice about deafness and hearing loss, whether hearing aids could help you, and the truth about tinnitus.

There's also information on the latest advances in biomedical research that could help to cure hearing loss and tinnitus in the future. And discover how technology could help you.

Once the sound waves travel into the ear canal, they vibrate the tympanic membrane, commonly called the eardrum. The eardrum is a thin, cone-shaped piece of skin, about 10 millimeters (0.4 inches) wide. It is positioned between the ear canal and the middle ear. The middle ear is connected to the throat via the eustachian tube. Since air from the atmosphere flows in from your outer ear as well as your mouth, the air pressure on both sides of the eardrum remains equal. This pressure balance lets your eardrum move freely back and forth 

The eardrum is rigid, and very sensitive. Even the slightest air-pressure fluctuations will move it back and forth. It is attached to the tensor tympani muscle, which constantly pulls it inward. This keeps the entire membrane taut so it will vibrate no matter which part of it is hit by a sound wave. 

Anyone can develop a middle ear infection but infants between six and 15 months old are most commonly affected.

It's estimated that around one in every four children experience at least one middle ear infection by the time they're 10 years old.

Symptoms of a middle ear infection

In most cases, the symptoms of a middle ear infection (otitis media) develop quickly and resolve in a few days. This is known as acute otitis media. The main symptoms include:

In some cases, a hole may develop in the eardrum (perforated eardrum) and pus may run out of the ear. The earache, which is caused by the build-up of fluid stretching the eardrum, then resolves.

Signs in young children

As babies are unable to communicate the source of their discomfort, it can be difficult to tell what's wrong with them. Signs that a young child might have an ear infection include:

When to seek medical advice

Most cases of otitis media pass within a few days, so there's usually no need to see your GP.

However, see your GP if you or your child have: 

Read more about diagnosing middle ear infections.

How middle ear infections are treated

Most ear infections clear up within three to five days and don't need any specific treatment. If necessary, paracetamol or ibuprofen should be used to relieve pain and a high temperature.

Make sure any painkillers you give to your child are appropriate for their age. Read more about giving your child painkillers.

Antibiotics aren't routinely used to treat middle ear infections, although they may occasionally be prescribed if symptoms persist or are particularly severe.

Read more about treating middle ear infections.

What causes middle ear infections?

Most middle ear infections occur when an infection such as a cold, leads to a build-up of mucus in the middle ear and causes the Eustachian tube (a thin tube that runs from the middle ear to the back of the nose) to become swollen or blocked.

This mean mucus can't drain away properly, making it easier for an infection to spread into the middle ear.

An enlarged adenoid (soft tissue at the back of the throat) can also block the Eustachian tube. The adenoid can be removed if it causes persistent or frequent ear infections. Read more about removing adenoids.

Younger children are particularly vulnerable to middle ear infections as:

Certain conditions can also increase the risk of middle ear infections, including:

Can middle ear infections be prevented?

It's not possible to prevent middle ear infections, but there are some things you can do that may reduce your child's risk of developing the condition. These include:

Avoiding contact with other children who are unwell may also help reduce your child's chances of catching an infection that could lead to a middle ear infection.

Further problems

Complications of middle ear infections are fairly rare, but can be serious if they do occur.

Most complications are the result of the infection spreading to another part of the ear or head, including: 

If complications do develop, they often need to be treated immediately with antibiotics in hospital.

Read more about the complications of middle ear infections.