A Sense of Sound

Author:  Varun, Learner, Aged 14 years

This is an assigned project researched independently by the learner. A broad conceptual framework ((bare bones) was provided to the learner who then developed additional questions as he went along.

We hear things so often, every second, that we never even stop to think about it. We take it for granted that we have an ear which somehow detects sounds. Yes that is true, but it is way more complex. Just like every other part of the human body, our ear’s ability to detect sounds is an extremely interesting function. To understand how we hear, we first have to understand what sounds are. So what are sounds? Where do they come from? How do we hear? Let’s find out now.

 Contents

 How sounds are created and how they travel

  • Speed of sound through different mediums
  • Characteristics of a sound wave – Frequency and Amplitude
  • Why we hear echoes
  • Structure of the ear
  • How we detect and transmit sounds
  • The brain – How we understand sounds
  • Why people have hearing loss
  • How sound is used to communicate today 

How sounds are created and how they travel

First of all, for a sound to reach you, it has to start from somewhere. A sound has to be created. A sound is created when something vibrates (move rapidly to and fro). When something moves, it causes a disturbance in the air (or any other medium), and pushes the air particles. Let’s take an example of a speaker. When you are listening to something, the speaker is moving back and forth (vibrating). When the speaker moves forward, it is pushing particles or compresses the air and when it moves backward it stretches the air. Thus a series of compressions and rarefactions (stretches) are passed on through the air.  In detail, the particles move forward and bump into other particles. Now, these particles go and bump other particles, and this process goes on. Speed up this process and you see that the particles are also vibrating. In simple words, sound is air in motion.

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This motion of particles is known as a sound wave. Sound is a longitudinal wave, where particles move in the direction of motion. It is also called a pressure wave because there are areas of low pressure (compressions) and high pressure (rarefactions), and it’s a mechanical wave, because the particles are physically moving.

Sound needs a medium to travel through. It needs particles to vibrate. If there are no particles, nothing would be there to vibrate and that’s why sound cannot travel through vacuum (a space without matter).

Sound waves move in all directions, just like a water ripple. When you throw a stone in the water, ripples form around the area of impact, and slowly disappear. That’s exactly how sound moves. Imagine the area of impact is a vibrating object. Then your ripples will be your particles in motion, or sound waves. Sound moves in all directions from its source, which means it is constantly spreading. That’s why you can hear sounds around corners.Sounds are louder when you are close to the source, but as you move further away, the sound becomes softer. For example, when a car passes by, the sound of the engine is loud but when it moves away, the sound becomes fainter. Why does this happen. As mentioned before, sound is constantly spreading. Which means if you are closer to the object, the particles will be more compressed and have more energy.

As the sound waves spread as they move, the waves become less intense and the particles will be spread out. You can only hear sound for a certain distance. Sometimes sounds don’t reach you. One reason is that it will keep on spreading and soon not have enough energy to move. Sound loses energy through heat too, because when particles bump into each other, heat is created.

 Speed of sound through different mediums

Sound can travel through any medium. But it travels at different speeds through different materials. Sound travels at 330 meters per second through air at a temperature of 0 degrees Celsius and at 350 meters per second through air at 30 degrees Celsius. This means that sound travels faster through warmer air. It travels four times faster through water (1400 meters per second) and at 5000 meters per second through concrete. So sound travels fastest in solids, and slowest through gasses.

 Characterization of a sound wave – frequency and amplitude

We all hear different kinds of sounds – High, low, loud, and soft. How do we hear these different sounds? Sound is characterized by its pitch and loudness.

The pitch of a sound is how high or low it is. Pitch is measured in frequency. The frequency of a wave is the number of wavelengths (the distance between one crest or trough to the other) per second. The more number of waves per second, the higher the frequency or pitch. The frequency of a wave is measured in Hertz. The frequency is equal to the number of hertz. 1oo hertz is equal to 100 waves per second. The human ear can hear sounds ranging from 20 hertz to 20,000 hertz. Sounds exceeding this limit are known as ultrasounds.

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The loudness of a sound is measured in amplitude. The amplitude of a wave is the distance from the wave’s equilibrium to its trough or crest. The more energy a wave has, the higher its amplitude. A wave can be of the same frequency, but the amplitude can be more. It is like playing the same note on the piano twice, but the second time you increased the volume. Loudness is measured in decibels. We can hear sounds from 0 decibels and sounds above 120 decibels can be extremely painful.

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Why we hear echoes

Sometimes when you say something, you here the same thing again. When this happens, it is called an echo. When sound waves hit hard surfaces, the sound is bounced back. So when you say something, the sound waves from your mouth move towards the wall, hits it, comes back and enters your ear. Echoes are mostly softer than the original sound because some of the waves get absorbed as well. Softer materials like carpets absorb sounds and harder materials like walls reflect sound.When you hear multiple echoes, it means that the sound wave is bouncing off something many times and entering you ear. If the repetition of echoes are fast, then your room is small, because the waves are hitting the walls fast. If you hear echoes slowly, it means that your room is large and the waves are taking time to hit the wall and come back.

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Echoes are used to measure the depth of water. Here’s how. A ship sends a sound down into the water. The sound moves down hits the ocean bed/ground and bounces back up. When the sound bounces back up, the echo is heard in the ship. Let’s say that the sound took 1 second to come back up from the time it was sent. If the sound took 1 second to hit the ground and come back up, it means that it took 0.5 seconds (half the time) to hit the ground. If the speed of sound is 1400 meters per second in water, the sound has traveled the sound that went and came back traveled 1400 meters (because it took 1 second). So the sound took half the time to reach the ground so the depth of the water is half of the total distance, which is 700 meters.

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Structure of the ear

Now we know how sounds are created and how they travel, but what is hidden inside our ear? What is our ear made of?

The outer ear is called the pinna. The pinna is shaped in a specific way so that it catches sound waves. The hole that you see on the outside of the ear is the ear canal or auditory canal, a curved passage that runs along for a short distance. This is the outer ear. A membrane (a flexible sheet of material forming a barrier) called the tympanic membrane or ear drum. Connected to the ear drum are three small bones called the Malleus (hammer), Incus (anvil) and Stapes (stirrup). That’s the middle ear. Now let’s move on to the inner ear. The stapes is connected to the oval window; a membrane covered opening which leads to the inner ear. The inner ear is made up of a spiral shaped bony structure called the cochlea and its body is called the vestibule (a chamber full of liquid connected to the chochlea). The cochlea is filled with fluid. The organ of corti is a sensitive receptor (an organ or cell able to respond to light, heat, sound etc. and transmit a signal through a nerve) which holds tiny hair cells, the receptors for hearing. The auditory nerve is connected to the cochlea and to the brain.

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How we detect and transmit sounds

Now let’s look at what happens inside our ear and how we detect sounds. When our pinna catches sound waves, they travel through the ear canal. When the sound waves hit the ear drum, they cause it to vibrate. These vibrations are passed on to the malleus, which passes the vibrations to the incus which makes the stapes vibrate. When the stapes vibrates, it pushes on the oval window. This makes the fluid inside the vestibule to move up into the cochlea. When the fluid moves, it passes these vibrations to the tiny hair cells in the cochlea. Different hairs in the cochlea pick up certain vibrations depending on their frequency. When these hair cells vibrate they send an electric signal through the auditory nerve and to a part of the brain called the auditory cortex, which interprets these sounds. The auditory cortex has a bunch of neurons that translates or decodes these sounds into a language that we understand.

 The brain – How we understand sounds

Before sounds reach the cortex they are actually already half decoded, because they take quite a long path to come to the cortex. The first stop is the brain stem, where basic decodes such us frequency, amplitude and duration takes place. The next stop is the thalamus. The thalamus integrates several other sensory systems of the brain such as parts that process memory and the motor cortex. This why you know familiar sound because you have heard it before and it was stored in your memory. Since even the motor cortex is used, sounds may cause you to physically react, like turning your head, or jumping. Sound trigger different types of reflexes and can also cause you to vocally react, like screaming when you are frightened by a sound. As you see, the sound is largely already decoded. The thalamus sends the sound to the brain and the sound is moreover recognized and stored. Many times music can make you dance, or sleep or cry. Music is basically sounds, but organized sounds. They are not random. They have a certain pitch, frequency and rhythm. This is for the same reason. The music can trigger several parts of the brain and can cause any emotional or physical change.

 Why people have hearing loss

Sometimes when sounds are too loud, the ear drum or hairs can get damaged. This is why people have hearing loss. When sound vibrations are too much for the hairs to handle, they just bend and die. These hairs never grow back. Sometimes the ear drum can swell and stop vibrating properly and doesn’t transmit sounds properly which can also lead to hearing impairment. Old age hearing loss is natural and can be caused without any infection. As you grow old, some cilia get tired of detecting sounds and they can die.  Now days, children with hearing impairment are getting hearing aids. Hearing aids work in a very simple way. They have three basic parts. A microphone, amplifier and speaker. When worn around the ear, the microphone picks up sounds, converts it into electric signals and sends it to the amplifier. The amplifier increases the strength and intensity of the sound and sends it out through the speaker enabling the person to hear. Depending upon the level of deafness, different kinds of amplifiers are used.

 How sound is used to communicate today

Earlier, there were no telephones or internet to communicate. People started by speaking in their own language or with sign language. Even we, now, communicate through speech. But how would people communicate over long distances? People used messengers or pigeons, to carry messages or letters. The first fast way of communicating was the telegraph. Later on the telephone was invented, by Alexander Graham Bell. Everyone uses a telephone in everyday life, so let’s see how it works. A telephone handset has two basic parts – the microphone and the speaker. When we speak, sound waves travel into the microphone, and cause a thin sheet plastic called the diaphragm to vibrate. There the sound waves are converted into electric signals. Imagine there are two telephones, connected directly by wires. This electric signal is passed on through a telephone wire and into the other person’s handset. In the other handset, the opposite happens. The electric signals are converted back into sound waves and move through the loud speaker, which amplifies the sound.

In olden days, wires ran beneath the earth to other countries. When people would call, there would be a few seconds of lag, because the electric signal would take time to reach. But why don’t we have any lag nowadays? Today the electric waves are quickly converted into radio waves, which move through the air, reach a satellite and are directed to the other telephone. That’s how mobile phones work, except that they directly get converted into radio waves.

You must have heard about a string telephone. It uses the same basic concept of an actual telephone. When you speak into the cup, the sound waves make the bottom of the cup vibrate. This vibration is passed on to the tied string. The vibrations are carried through the string until it reaches the other cup. Here the cup vibrates and the sound waves are passed on through the air and into the other person’s ear. So you see, it’s pretty much the same as a normal telephone, except that the waves are transmitted as sound waves only.

All of us hear sounds every day, and everywhere.  We never really think about what sound actually is. We talk, listen to music make and hear thousands of sounds every day. There are so many sounds we hear in everyday life, and each and every sound goes through the long process to reach our brain. When you understand what sound is and how we hear, it really is interesting that so many things are going on every second. Hopefully after reading this, you will realize that sound and the way the human ear works, is truly amazing.

Websites used for images in this project:

http://www.passmyexams.co.uk/GCSE/physics/images/long_waves.jpg

https://whybecausescience.files.wordpress.com/2014/01/high-and-low-frequency-waves.png

https://musicandcomputerscience.files.wordpress.com/2010/11/amplitude-comparison.jpg

http://res.cloudinary.com/dk-find-out/image/upload/q_80,w_1440/Sound-3_qrd2ed.jpg

http://noc.ac.uk/f/content/research-at-sea/ships/equipment/EchoSounder-290.png

http://images.tutorvista.com/content/sound/human-ear-structure.jpeg

 

By Varun