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What is Sound and How do we Hear it?

Sound waves

Sound waves (jop_pop, istockphoto)

Sound waves

Sound waves (jop_pop, istockphoto)

Harleen Saini

Learn how sound travels in waves and how it is measured in this backgrounder.

When you think about sound, what’s the first thing that pops into your head? A song you heard on the radio this morning? Or maybe something less pleasant, like a school bell?

But, have you ever thought about what sound actually is?  

Anatomy of the Ear

Before you can understand sound, you have to understand how your ears work. After all, what goes on inside your ears is what allows you to hear. 

For example, when a person talks, the movement of their mouth creates waves of moving air. These sound waves travel into your ear canal and hit your eardrum. This causes the ossicles to vibrate. These three small bones are called malleus, the incus and the stapes. They are also known as the hammer, the anvil, and the stirrup. 

The vibrating ossicles transmit the sound waves to the cochlea. It’s a small, snail-shaped structure inside your head. The cochlea contains small cells called hair cells that convert sound waves into signals. The signals then get sent to your brain. And that is what allows you to hear someone’s voice!

Parts of the human ear
The parts of the human ear, including the ear canal, eardrum, cochlea and ossicles (Let’s Talk Science using an image by Lars Chittka; Axel Brockmann [CC BY 2.5] via Wikimedia Commons).

What are Sound Waves?

As you just learned, you hear when your ears process sound waves produced by an object. The object produces waves by vibrating. When an object vibrates, it pushes against the surrounding air. The vibrations cause the air to expand (called rarefaction) and compress. This causes areas of high and low pressure as the sound moves towards your ears in the form of waves.

Sound waves showing how air molecules compress and expand
Sound waves showing how air molecules compress and expand (Source: ttsz via iStockphoto).

As an example, picture a metal spring, like a Slinky. When you push on one end of the spring, it bunches up. The bunched-up area slowly travels the length of spring until it reaches the other end. This is like the areas of high pressure in sound waves. As the spring continues to move, the bunched-up area will start to stretch out. This is like the areas of low pressure in sound waves. In other words, some areas bunch up and others stretch out as the wave travels from a vibrating object to your ear.

Waves demonstrated on a slinky (2016) by Cambridge Volcano Seismology (0:21 min.).

Did you know?

Sound waves travel through the air at a speed of about 1 235 km/h at sea level! 

What is Pitch and Frequency?

Wavelength determines a sound’s frequency. Wavelength is the distance between waves. For sound waves, wavelength refers to one complete cycle of rarefaction. The shorter the wavelength, the higher the frequency of the sound. 

Frequency is measured in Hertz (Hz). One Hertz corresponds to one cycle—in this case, one sound wave—per second.

Frequency determines the pitch of a sound. High-pitched sounds have a high frequency. Low-pitched sounds have a low frequency. 

ow-pitched sounds have long wavelengths as shown in the top image and high-pitched sounds have shorter wavelengths as shown in the bottom image
Low-pitched sounds have long wavelengths as shown in the top image and high-pitched sounds have shorter wavelengths as shown in the bottom image (Source: Let's Talk Science based on an image by ttsz via iStockphoto)

For example, imagine a very deep note - one that would come from a bass guitar or a key at the lower end of a piano. This note has low frequency. That means it’s low-pitched.

Now, imagine you hear a very bright note. One that might come from a violin or the higher end of a piano. This high note has a higher frequency. That means it’s high-pitched.

Did you know?

Frequencies too low for people to hear (lower than 20 Hz) are called infrasonic. Frequencies too high for us to hear (above 20 000 Hz) are called ultrasonic.

Most humans can hear sounds between 65 Hz and 23 000 Hz. But that doesn’t necessarily mean that ultrasonic sounds are comfortable or even safe to listen to!

Did you know?

Most dogs can hear sounds between 65 Hz to 45 000 Hz. Dog whistles have very high frequencies. When you blow one, dogs go crazy, but people can’t hear a thing!

What is Loudness?

Amplitude determines the loudness of a sound. Amplitude refers to a sound wave’s size, or height. 

Volume is directly related to amplitude. Volume refers to a sound wave’s intensity

Quiet sounds have small amplitudes as shown in the top image and loud sounds have large amplitudes as shown in the bottom image
Quiet sounds have small amplitudes as shown in the top image and loud sounds have large amplitudes as shown in the bottom image (Source: Let's Talk Science based on an image by ttsz via iStockphoto).

For example, the harder you pluck a guitar string, the louder the sound it will make. That’s because you’re transferring more energy to the string and producing larger sound waves. When you pluck a string very lightly, you are transferring much less energy. As a result, you produce much smaller sound waves. The effect is similar to dropping a large rock or a small pebble into the water. The louder sound is like the larger splash!  

Remember that song you were listening to the radio this morning? Now you have a better idea of how vibrations from the radio created sound waves. These were captured by your ears, which sent signals to your brain. And the pitch, loudness and volume of the sounds you heard depended on the frequency, amplitude and intensity of the sound waves.


The Science of Hearing (2018) 

Ted-ED video (5:16 min.) explaining how we hear sounds, and on the mechanics of the ear.

Virtual Oscilloscope 

This page from Academo features an interactive virtual oscilloscope - a tool that shows a visual representation of sound waves and how they can be manipulated. 

Can “Distracting” Noise Actually Help You Study Better? (2017)

This article by Kaitlin Goodrich for BrainScape compares the ability to study in quiet places vs. places with ambient or “background” noise.

Frequency, Wave Shape and Pitch (2017)

Video (7:05 min.) from Walk That Bass that explains the different components of a sound wave and how a wave can be manipulated to create different sounds. 


Dangerous Decibels. (n.d.). How do we measure sound waves?

National Centre for Initial Teacher Training in Primary School Science. (n.d.). The speed of sound in air. University of Leicester.

Silver, J. (n.d.). Dog whistles: When and how to use them safely. LittleThings.

The Physics Classroom. (n.d.). Sound is a pressure wave.

University of Maryland Medical Center. (n.d.). How the ear works.

Wonderopolis. (n.d.). What makes sounds louder? National Center for Families Learning.

Harleen Saini

Harleen is a premedical student from the United States who is hoping to become a physician one day. Her love for science began with watching shows such as Bill Nye the Science Guy and The Magic School Bus series. These helped make science fun and exciting, and eventually led Harleen to pursue an undergraduate degree in Biological Sciences which was completed in 2016. She enjoys being able to share her passion for STEM topics, especially health science and biology, and hopes to help others fall in love with the sciences as well. Outside of this, Harleen is also a musician, enjoys dancing and loves to learn foreign languages!

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