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# Physics of the Ear

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The physics of the ear depends on its structure as this is the one able to capture the sound vibrations and convert them into a signal that travels through the nerves to the brain. The structure of the ear is divided into three main parts, the outer, the middle and the inner ear. Some features of the ear owe their shape or functions to evolution. It was one of the most important organs for our survival.

Figure 1.- Visual representation of the earing system. Source: Wikipedia (CC by 2.5).

## The ear system

We can divide the ear into three parts, these parts are defined by their position and functions and are known as, the outer ear (the part visible to us), the middle ear which is the apparatus that focus and transforms the vibrations, and the inner ear that connects our system to the nerves and brain.

### The outer ear

The outer ear is the visible part. It is made up of the pinna, the ear canal that ends where the middle ear begins, at the outer part of the tympanic membrane. The pinna is shaped to catch the sound from the front and has ridges to direct it towards the tympanic membrane. This structure helped to focus on the sounds emitted by a prey.

### Middle ear

The middle ear is where the sound is transformed from a pressure wave to an electric signal. In this section, three small bones act as mechanical transmitters of the sound, they are called: the malleus, incus, and stapes. This is an important function because it acts as an impedance adapter, allowing the ear to have an extensive amplitude range of perception with the lower limit of 20μPa being absolute silence. As a comparison, around 60μPa is the sound pressure of normal breathing.

Two thresholds determine the upper limit of our hearing range, the first is where we start to feel pain, and the second is where the intensity of the sound causes harm to the ear.

### Inner ear

This part of the ear is hidden from sight, enclosed in a bony labyrinth where the cochlea, semicircular canals and the vestibule are located. When it arrives here, the sound has been attenuated by the middle ear and is ready to be analyzed, transformed and transmitted to the brain, where it will be processed.

Figure 2.- Visual representation of the inner ear system. Source: Wikipedia (CC by 2.5)

## Physics of the Ear: Measuring pressure and sound

Our ears are very sophisticated instruments with incredible sensitivity. We can carry out calculations on this range of perception.

### Physics of the Ear: From air pressure to decibels

Sound waves are pressure waves, which means that information travels thanks to the alternation of rarefaction and concentration zones of particles in the air.

We measure pressure in pascals. Our hearing covers a range from a millionth of a pascal to hundreds of pascals, and sometimes we need to describe even louder sounds. It isn't convenient to use this unit, so we apply a logarithmic scale to this quantity to reduce the excursion range from millions. The decibel is an intensity scale and is used to represent a sound's pressure level; this is defined with respect to a reference that is the softest sound we can hear. For definition:

${L}_{p}=20\mathrm{log}\left(p/{p}_{0}\right),{P}_{0}=20\mu {P}_{a}$

Where Lp is the pressure level relative to the pressure p, and p0 is the reference pressure. Let's take an example: we want to know the pressure level corresponding to 20Pa:

$p=20{P}_{a}$

${L}_{p}=?$

${L}_{p}=20\mathrm{log}\left(p/{p}_{0}\right)=20\mathrm{log}\left(20/20·{10}^{-6}\right)=20\mathrm{log}{10}^{6}=120dB$

${L}_{p}=20\mathrm{log}\frac{p}{{p}_{0}}=20\mathrm{log}\frac{20}{20·{10}^{-6}}=20\mathrm{log}{10}^{6}=120dB$

We say that 20Pa corresponds to 120dB SPL. SPL stands for sound pressure level to indicate we're talking about sound since the logarithmic scale has many uses.

Figure 3.- Decibel scale showing the noises we usually hear and their normal range. It also shows the sensations we have for every scale, Camacho - Study Smart Originals

The same evolutionary processes that shape the human hearing system are the ones that constrain the range of frequencies we can capture. The range of sound we can hear is the result of this process.

## Consequences of damage to the ear

The ear is a complex, delicate system, even though it is very adaptive. As with every other part of the body, it can be subject to damage. Here are some potential consequences.

### Physics of the Ear: Loss of hearing

The most common issue is the loss of hearing. This happens naturally with age - a child has a spectrum of hearing that goes from 20Hz to 20KHz, while an adult won't hear sounds above approximately 15KHz. Human hearing is limited by its physical characteristics (you won't be able to hear any sound too low to cause vibrations in your ear) but also by evolution as most of the human senses work in the range that is useful for us to receive and analyze information.

After aging, loud sounds are the leading cause of hearing loss. These can cause damage instantly or are partially harmful, leading to more severe damage with exposure over time. The threshold of pain begins at 130dB. Damage occurs immediately when the sound pressure is 140dB or more. For instance, it takes just one rifle shot to cause a gap in hearing of around 5KHz. Hunters often experience this.

Another malfunction of hearing is tinnitus. This is described as hearing sounds such as ringing or hissing where there is no cause for the sound. Tinnitus can be caused by exposure to loud noises.

### Physics of the Ear: Balance

The inner ear is responsible for another important function: balance. We have three semicircular canals that perceive movement from the three dimensions of space, while otoliths are structures that detect linear acceleration. A well-known malfunction of the vestibular system is vertigo. This is defined as the perception of movement when there's none.

## Physics of the Ear - Key takeaways

• The ear is made up of three parts: outer, middle and inner ear.
• The inner ear is responsible for balance as well as the perception of sounds.
• Our range of hearing is from 0 to 130dB without pain, from 130dB - 140dB is the danger zone, and from 140dB, sounds are loud enough to cause instant harm.
• The human ear can hear frequencies from 20Hz to 20KHz in childhood, and this range decreases because of aging and hearing damage.

The human hearing process converts the mechanical waves of sound into an electrical signal by using sensitive cells inside the ear.

Sound travels to the outer part of the ear where it is focused into the ear canal and then transmitted to the inner ear. The inner ear then provides a way to convert these mechanical movements into electrical signals that are sent to the brain.

The same evolutionary processes that shape the human hearing system are the ones that constrain the range of frequencies we can capture. The range of sound we can hear is the result of this process.

The three bones located in the ear for hearing are Malleus, incus and stapes

## Final Physics of the Ear Quiz

Question

What does decibel SPL mean?

SPL stands for sound pressure level.

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Question

Why do we use decibels to represent sounds?

It is the most convenient scale. It allows us to convert a huge range to a smaller one, thus allowing us to deal with smaller amounts without a loss of information.

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Question

What are the parts that constitute the outer ear?

The pinna and the ear canal.

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Question

What are the parts of the middle ear?

The tympanic membrane and three small bones that act as an impedance adapter.

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Question

What is the softest sound we can hear?

The softest sound we can hear is 0dB. That corresponds to a pressure of 20μPa.

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Question

What is the loudest sound we can hear without damage?

140dB

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Question

When does the volume of sound become dangerous?

Above 80dB.

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Question

Is it possible to have a gap in the range of hearing?

Yes. Hunters have a gap around 5KHz due to the loud noise of a rifle.

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Question

Why is our pinna shaped that way?

The unique shape of the pinna focuses sounds coming from the front to the ear canal.

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Question

What is the tympanic membrane?

It is a membrane that a little bone hits to make it vibrate. This mechanism transfers the information from the world outside to our inner ear

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Question

What causes tinnitus?

Tinnitus can be caused by several things, but the most common cause is loud noise exposure.

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Is there a difference between human noises and natural sounds?

Yes, even if very loud, natural noises hardly ever become disturbing, while we have lower tolerance towards artificial sounds.

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Question

What is vertigo?

Vertigo is sensing movement where there isn't any.

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What causes vertigo?

Vertigo is caused by the vestibular system malfunctioning.

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Question

What is the effect of ageing on hearing?

We lose our hearing with age, so adults have a narrower range of audible frequencies.

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Question

What is the cochlea?

It is a fluid-filled spiral-shaped organ that can be tuned into different sound pitches based on the sound's locations along the cochlea length.

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Question

What is the vestibulocochlear nerve?

A cranial nerve which sends information to the brain concerning balance and sound.

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Question

What is the endolymph?

The semicircular ducts, which detect changes in speed and direction depending on the motion of the head.

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Question

List the main parts that the ear is composed of.

It has an outer, inner, and middle ear.

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What are the main parts of the inner ear?

The cochlea, vestibular system and semicircular canals.

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What are the main parts of the middle ear?

The bones (hammer, anvil, and stapes), the oval window, and the Eustachian tube.

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What are the main parts of the outer ear?

The pinna, the ear canal, and the ear drum.

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What is the function of the inner ear?

To convert sound waves into electrical waves which are then sent to the brain.

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Question

What is the function of the ear besides hearing?

Balance

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Question

Which part of the ear is responsible for providing balance-specific information to the brain?

The inner ear, through the vestibular system.

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Question

How is balance achieved?

The vestibulae senses altitude, rotational, and linear motion signals and sends them to the brain via the cranial nerve, which responds accordingly to maintain balance.

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Question

What is the seventh facial nerve?

It provides hearing signals to the brain and is also responsible for facial muscle movement.

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Question

What is the corti?

It is the main organ of the cochlea filled with fluid and cilia.

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What are the utricle and saccule?

They are part of the vestibular system and detect vertical and horizontal movement.

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What are the semicircular ducts?

They are part of the inner ear comprised of sensory receptors that detect changes in speed and direction.

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Where is the Eustachian tube located and what is its function?

It is located in the middle ear. Its function is to equalise pressure on both sides of the eardrum.

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Question

What are hearing defects?

They are defects that cause partial or total hearing impairment.

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What are the main types of hearing loss?

Sensorineural, conductive, and mixed.

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What is sensorineural hearing loss?

It is the hearing loss associated with damage to the tiny hair located in the cochlea, also called cilia.

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Question

List the most common hearing defects.

Ageing, injuries, noise exposure, genetics, viral diseases.

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Question

What is vestibular neuritis and labyrinthitis?

Vestibular neuritis occurs when the cranial nerve is inflamed or swollen. Labyrinthitis occurs when the labyrinth is filled with fluid and the in the inner ear gets inflamed.

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Question

What  is BPPV?

It is the most common form of vertigo caused from injury or dust in the inner ear.

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Question

What is the treatment for acoustic neuroma?

Surgery.

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Question

What are the most common signs of hearing loss?

Difficulty hearing consonants and high-pitched sounds, and to have conversations in loud environments.

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Question

What are the two types of hearing loss tests ?

Pure-tone and tuning fork test.

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What is vestibular testing?

It is a test used to examine the extent of functionality of the vestibular system using electronystagmography or videonystagmography.

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Question

What are the different levels of hearing loss?

Normal, slight, mild, moderate, severe, and profound.

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Question

What level of hearing loss is a loss in the range of 26-40 dB?

Mild.

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Question

What level of hearing loss is a loss in the range of -10 to 15?

Normal.

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Question

Which of the following is true?

The highest dB hearing loss range indicates the most severe hearing loss.

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Question

Which of the following is not true?

The treatment of hearing loss caused by down syndrome defect is surgical.

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Question

List some hearing defects.

Meniere’s disease, anatomical defects, congenital hearing defect, SSCD, BPPV, etc.

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Question

What is the hearing range of the human ear in Hz?

20 to 20,000 Hz.

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Question

What is sound intensity?

Sound intensity is the power carried by the mechanical waves.

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Question

What units are used for sound intensity?

Watts per square metre.

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