The tapping of footsteps on floorboards. The growl of a car out on the street. The distant rumble of an aeroplane.
Each of these is a sound. A sound of which you can’t see the source, but which you recognise immediately. Maybe – like the plane – it is a sound which is produced literally miles away. Or maybe – like the footsteps of your neighbour in the flat upstairs – it is a sound which comes from the most accidental of things.
Even if you put a glass of water down on a table, there is a sound. If you scratch your head, you can hear it. And if you type away on a keyboard, a sound is produced there too.
Sound is literally everywhere. It is almost impossible to avoid. The whole world, in fact, seems to endlessly vibrating with different noises and disturbances – the birds or the rain, people’s voices, the wind. We are constantly surrounded with sounds – sounds that we hear and sounds that go unheard.
But what is sound? What is actually happening when you can hear something? And how on earth does that sound actually arrive however far to get into your ear?
These are the sorts of questions that we will be discussing here. Along with the rather more cool-sounding thing known as ultrasound – with all its different uses.
Let’s take a look.
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What is Sound?
Sound is a type of wave, a disturbance in a medium that transfers energy. If you want to know more about the science of waves, check out our articles on the properties of waves and on transverse waves and longitudinal waves.
Sound waves are a type of longitudinal wave – meaning that the way in which they disturb their material medium is parallel to the direction of the energy transfer. (Transverse waves, on the other hand, create displacements that are perpendicular to the energy’s direction.)
So, imagine that plane in the sky. Although it is fairly far away, you can hear it. That’s because its engine produces vibrations which propagate energy all around them. And given that that energy meets no resistance, it can come all the way to your ear.
That, in short, is the science of sound: movements of energy that make particles in the air vibrate. However, let’s go into a bit more detail.
What is a Sound Source?
A sound source is where the sound begins – whether that be the pulsing diaphragm of a speaker, a human voice box, or the mechanical whirring and grinding of a machine. These places – i.e. everywhere – are the sources of the sound waves that then begin to propagate through the medium.
How these work depends on the specific nature of the source. So, with a speaker, the technology within the speaker converts electrical energy into vibrations of particles, which cause all the particles around them to vibrate too. Meanwhile, the air rushing through your throat causes your voice box to vibrate, again making everything else around it vibrate too.
As you can see, then, sounds all start with vibrations – a particular type of energy being converted into this kinetic energy. And once this vibration starts, it can spread through lots of different media to your ears.
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How do Sound Waves Travel?
When a sound source vibrates, it makes everything else around it vibrate too – as the sound energy propagates outwards from the source in waves of rarefactions and compressions.
A sound wave is a type of mechanical wave, meaning that it has to have a medium through which to pass. Sound cannot travel through a vacuum – as it needs the vibration of atoms to transfer its energy. Given this, we might see that sound travels at a different pace depending on the medium through which it is travelling.
Sound travels through the air at a rate of 330 metres per second. This is known as the speed of sound. However, the air – given that gas is a state in which the atoms are actually least densely arranged – is the medium at which sound travels the slowest.
Waves of sound travel much more quickly through solids than both liquids and gases. For comparison, the speed sound travels through aluminium is 6320 m/s. That’s twenty times faster than through air. You know this intuitively actually – because when you put your head to a table and someone knocks on it, it is much louder than it would be if you were sat up straight.
This is because the molecules in solids are generally much closer together than in air – and so the energy is much more easily transferred from one molecule to the next.
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However, when there is a change of medium through which the sound is travelling, some of the sound will be reflected – in something known as an echo.
So, if you shout into a tunnel, the sound will travel through the air – and some of it will return to your ears after contact with the interface between the two media, the air and the solid wall.
If you want to learn more about reflection, we have an article on reflection and refraction.
What Changes a Sound’s Pitch and Tone?
So, we know what a sound is by now: it’s a vibration in a medium. Yet, an important question remains: how come we hear different sounds with different levels of loudness? And how come, when we listen to music, we can hear all sorts of different pitches and tones?
To understand the answer, you need to remember that ‘sound waves’ are not singular, self-identical things. Rather, there is a whole spectrum of different sound waves, of an infinite variety of different sizes and speeds.
Frequency is the word that refers to the number of times a wave oscillates – or goes from peak to trough to peak, or from compression to rarefaction and back – in a given time. The higher the frequency – i.e., the faster the wave oscillates – the more high-pitched the sound we hear.
Amplitude, meanwhile, is the word for the size of the displacement that the wave creates. The larger the amplitude – meaning the more energy transmitted by the wave – the louder the sound.
This, however, doesn’t explain the tone of a sound – or the way that a guitar sounds different to a piano, or the way your voice sounds different to someone else’s.
This particular fact owes its explanation again to the fact that sound waves are not singular.
If you play a guitar string, it is not just one type of sound wave that is produced – not one wave with one set amplitude or frequency. Rather, that string will produce lots of different sizes and speeds of wave – and it is the particular combination of these that produces that specific tone.
What is Ultrasound?
Ultrasound might sound like it is something a bit more interesting. However, the ultrasound and sound are essentially the same things.
The difference is that what we refer to as ultrasound is the sound waves with frequencies that the human ear cannot hear. If the range of human hearing stretches from twenty Hertz to twenty thousand Hertz (Hertz being the measurement of a wave’s frequency), ultrasound is anything above a frequency of twenty thousand.
There is, quite honestly, an awful lot of sound that classifies as ‘ultra’. As in, there is actually more ultrasound than sound itself. But, given that we are humans, our categories tend to be determined by specifically human concerns.
Dogs, on the other hand, whilst they are not categorising anything scientifically – at least, as far as we know – can hear a much larger range of sounds than humans. Their range is actually twice as large as ours – which makes the distinction between sound and ultrasound rather arbitrary.
What do We Use Ultrasound for? And How does it Work?
The most common use of ultrasound is for creating images of babies in the womb. This technology is made possible by the process of reflection.
Ultrasound works because there are lots of different sorts of material in your body: fat, muscle, bone.
By using a tool that can both emit and detect sound waves, the process of ultrasound can essentially build images by receiving reflections – or echoes – from the interfaces between the different layers of material.
So, at the interface between fat and muscle, some of the emitted sound waves are reflected and detected. At the interface between muscle and bone, the same thing happens. All of this can then be compiled onto a computer and an image can be created from the detection.