The Complete Guide to Sound: What is Sound?

The Complete Guide to Sound: What is Sound?

For us humans sound is a pressure wave. Everybody has some sense of what a wave is, but when you stop and think about it it means “energy is moving through air, but the air is stationary (or hardly moving).”

When you snap your fingers, it makes the air molecules near your hand vibrate back and forth, and bump into the molecules next to them. Eventually this cascade of vibrating molecules reaches your ear and vibrates your ear drum, so you hear a sound.

As sound move through air it means these molecules are moving at the speed of sound, and they bump into each other and produce these changes in momentum which is manifested also as a change in pressure which your ear can sense. Its a TINY, TINY amount of pressure change and yet we animals are set up to detect it.

Another way to describe this chain reaction is a pressure wave, since the vibrations cause tiny changes in air pressure that your ears detects and processes as sound.

To reach your ears, sound has to have something to travel through, which is called a Medium. It can be air, water, or even a solid like metal. In places where there aren’t any molecules to create pressure, sound cannot travel AT ALL!

That is why sound can’t travel through space - and as some people like pointing out, in that case the space battles in Star Wars should really have been silent! But then that probably would have been boring to watch..!

We measure sound waves by their frequency - which means how many times the waves repeat every second. The scale we use to measure frequency is called Hertz - named after a German scientist called Heinrich Hertz.

If a sound has a frequency of 500 Hertz, it means that there are 500 waves per second. If we raise it to 1000 waves per second, or 1000 Hertz, the Pitch gets higher.

The lowest range of human hearing is about 20 hertz, the upper range is around 20,000 HERTZ! That means 20,000 wave cycles per second. Cor. Thats right at the top end for most of us, except for small children, who actually have a higher range, before their ears get beaten down by years of listening to heavy metal!!

So Kids have better hearing than Adults, but some Animals can hear even higher than we can. For instance dogs can hear up to 40,000 hertz. Dog whistles have a very sharp edge, and what this does is essentially causes vortices to occur which spin off at an exceptionally high frequency - and that is the frequency dogs can hear that we cannot.

Elephants and Whales can hear much lower frequencies than we can. They both use these low frequencies to communicate with each other - over 6 miles apart for Elephants, and thousands of miles for Whales!

This is because sound travels so well through water. But no matter what medium the sound travels through, lower pitches will always end up traveling further than higher ones.

There is a simple test for this - put something hollow over your ear, like an empty Pringles can, if you tap your finger on the end of the can, it will sound quite high pitched. But if you stuff the can full of tissue and tap, it will now sound very low pitched - most of the high frequencies are no longer present!

This is because the different frequencies have different Wavelengths.

High frequency sounds have short wavelengths, and these tiny frequencies bounce around and loose a lot of energy in whatever they are travelling through.

Low frequency sounds have longer sound waves, this means they can travel farther without bumping into as many particles and loosing energy. This is why we use low frequencies when we need to make sound travel as far as possible - like a Fog Horn, or a Siren, or even your neighbours Music (or if your reading this YOUR MUSIC!). But when they open their door, all the high frequencies escape and can be heard - remember the Jim Carry Ace Ventura moment?

Some materials are really good at adsorbing sound, like the foam tiles you see in recording studios. They absorb the sonic energy, so there is no reverb bouncing back at you.

Other times the sound waves get reflected back causing an echo.

So you make a sound and the pressure wave moves through the air, bounces off some distant surface (a cliff, a valley, the stands on the other side of a field) and then bounces back towards you.

When an echo happens, you have a single sound bouncing in many directions. Another remarkable thing about waves is that they can pass through each other. When sound waves meet, they can have all kinds of interactions. One extreme is that they can cancel each other out - so there is no sound at all! The other end is that they can add up to create double the volume!

If you can imagine sound waves as squiggly lines that move up and down, over and over again. Imagine two sounds played out of your headphones, one in the left ear and one in the right ear. If the two lines are perfectly in sync, or in phase with each other, then they are moving up and down at the exact same time. If these two waves are brought together, they combine into a single sound which is TWICE as loud! Both tones from each ear are still the same volume on their own, but now their sound waves are added together, this makes the sound louder.

We can also do the opposite. If we make one sound wave OUT OF PHASE with the other one. Imagine a wave going up meeting another wave which is going down - they would cancel out, and become completely silent.

This can be done with any sound, even a voice. If you made a recording of your voice and copied it, then panned one copy completely over to your left ear, and one copy completely over to your right, the result would be that you would hear yourself as though you were speaking from the centre of your head. So long as the two samples a kept “in phase” they will combine into a louder voice. But if they were out of phase, as you combined the sound waves they would eventually get quieter and quieter until you could not hear the voice anymore.

Both these processes happen all the in your home studio - it is something to bear in mind if you listen to your mixes on speakers - the rooms shape will naturally cause some sound waves to bounce back out of phase, or in phase, causing certain frequencies to appear LOUDER or QUIETER than they actually are!

This is essentially how noise cancelling headphones work - they pick up the sound from the outside environment via a little microphone, and then super quickly create a sound that is the exact inverse of the environment - canceling it out!

This same principle is also used in modern cars and aeroplanes to give a smoother ride! Going fast creates a lot of noise.

But what happens if you go faster than the speed of sound?

Under normal conditions the speed of sound is about 767mph, and it doesn’t change based on loudness. So if a kitten meows 10 ft to your left, and a foghorn goes off 10 feet to your right, both sounds will be heard simultaneously.

But the speed of sound is effected by what the sound is travelling through. For instance, sound travels over 4x as fast in water as it does in air. This is because water is so much denser than air - in other words molecules bump into each other more readily, as there is much less empty space between them as there is in air. The higher you go in the atmosphere, the less air there is, the molecules are further apart, and so sound travels more slowly.

Temperature also effects the speed of sound. When we measure the temperature, what we are actually measuring is the average energy of the molecules, and more energy means the molecules are bouncing around more. Warmer molecules therefore bounce around faster than cold ones, it means they can transmit sound faster. So the speed of sound in the Sahara Desert is slightly faster than the speed of sound at the North Pole!


Warmer = Molecules move Faster

Denser = Molecules are closer and interact more frequently

Most of the time the speed of sound is the fastest speed molecules travel in nature. Sometimes though they can get pushed faster. When this happens, they bump into other molecules and slow down immediately - A shock wave. EG - the end of a whip.

The sound generated by a shockwave is called a Sonic Boom.

Shock waves are incredibly powerful, they are also TINY. Shockwaves are barely 100th of a millimetre thick - thinner than a piece of paper.

Imagine you are in a jet flying faster than the speed of sound. Then imagine you decided to shout out of your window, in this case how fast is my voice traveling?

Someone standing outside would hear it at the speed of sound, in other words they would hear it as fast as a sound wave can move. (assuming you arn’t smashed by the pressure outside the window… which you would be…)

Another analogy - when you open your window on the freeway, you instantly feel the pressure from outside pushing back at you. In order for your voice to reach the sidewalk, the pressure wave from your voice has to be stronger than the pressure of the air coming through the window.

Hearing developed as a kind of early warning system - think about the times you have spent the night in an unfamiliar place, you are always hyper aware of any noise.

With all of the distractions we have today, it is tempting to shut out the sonic world more and more, especially as we are so over exposed to the visual world.

This is especially true as we no longer have to listen out for a dangerous lion about to make us dinner.

But our ears still play a valuable role in listening to important cues from our surroundings.

Our ears are fantastic receptors of sound, they are able to detect the amazingly small sounds from a pair of headphones, and because of spiral inside the ear, it makes low pressure sounds (weak or quiet sounds) detectable, whilst at the same time not being overwhelmed by very loud sounds. Truly amazing.