What is sound?
Here at Chirp, we use sound to send data between devices — for example, allowing your TV to send a digital message to your laptop, simply using the TV’s loudspeaker and the laptop’s microphone.
Over the next few weeks, we’ll be publishing a series of posts detailing why we believe that sound is a powerful medium for sending information, and the unique things that can be done with it.
In this post, we’ll start from the very basics by taking a look at the definition and properties of sound.
Sound is defined broadly as vibrations that travel through a medium.
We’ll look at each part separately here, starting with a look at what we mean by vibrations.
Vibration
A vibration is simply the movement of an object back and forth, repeating at regular intervals. This movement is characterised more generally as an ‘oscillation’, and if we look at this motion over time, we recognise the distinctive wave pattern created from these oscillations in the graphic below. This wave is created by the left-most point simply moving up and down in a regular rhythm. How fast it moves up and down in this way defines its frequency.
The frequency of a sound is measured in “hertz” (Hz), which describes the number of vibrations per second — how often the “back-and-forth” oscillations take place. A sound of 200Hz has a movement that repeats 200 times per second.
We are able to hear these vibrations because air particles trigger a vibration of the same frequency in tiny hairs within our ear canal. The mechanisms within our ears are only able to sense sounds within a particular range of frequencies — typically, between around 20Hz to 20,000Hz.
The audible frequency range varies significantly from individual to individual, and as we get older we lose the ability to hear higher frequencies, due to the deterioration of some of the vibratory hairs in our inner ear.
Now let’s look at what happens when we take our abstracted, isolated concept of an oscillation and start applying this to real-world, physical materials — when we place it inside a physical medium.
Medium
Sound needs a medium, or material, in which to travel. This is why in the vacuum of space, where there is no atmosphere, no vibrations are able to propagate through the void.
The medium itself will affect several characteristics of how these vibrations travel: their speed through the material, how far they can travel before dying out and which frequencies can travel more easily through it than others.
Give our vibrations a material to work with, and they will start to travel. For people, our most common medium for sound is, of course, air. We can also hear sounds underwater, and even through our own bodies via bone-conducting headphones.
Travel
Sound travels within a medium when a force causes the collective compression and rarefaction (that is, decompression) of member particles within a material’s structure. Each particle oscillates in sympathy with its neighbours, but viewed in aggregate a pressure wave emerges and begins to travel through a medium.
Look closely at the diagram below. The red line on the left is our vibrating object, the black dots our particles which together constitute a medium. This could, for example, represent the cone of a loudspeaker (red line) and individual molecules of air (black dots)
Here, the wave travels from left to right radiating from the red oscillating line, but the individual particles also simply oscillate around each of their centre positions, back and forth, back and forth. The important point here is that although the wave travels through the material at a certain speed, this is not the speed of an individual particle.
Sound waves travel through air at approximately 340 metres every second, depending on environmental factors this can fluctuate slightly, though in other materials the speed of travel is significantly different from air. For example, sound travels at over 6,000 metres per second through steel, and 12,000 metres per second through diamond.
To return to our original definition of sound, “Sound can be defined broadly as vibrations that travel through a medium”. We can now add some detail covered in the previous sections, and confine it specifically to sounds that humans are able to hear:
Human-audible sound is created from the oscillation of individual molecules, between the frequencies of 20Hz and 20,000Hz. Together they create a wave travelling through air at ~340m/s.
So now we know a bit more of the physical nuts and bolts involved, let’s look at some of the characteristics that these give sound when it interacts with our shared environment.
Applications of sound
Sound’s unique characteristics afford a wide range of applications, not least allowing us to talk to each other. This is often taken for granted, but it is actually quite remarkable that, thanks to our ability to create and perceive sound, we have the ability to communicate a concept or piece of information. One way of looking at this highlights just how special these abilities are:
- Person A has a concept or piece of information they wish to share
- They ‘encode’ that information by vibrating their vocal chords in a specific pattern
- This vibration travels through the air all around Person A
- The wave reaches Person B’s ear, vibrates their eardrum in turn ‘decoding’ the vibrations into electrical signals that the brain can understand
- Person B receives and understands Person A’s concept or piece of information
This also highlights some additional beneficial characteristics of sound for particular uses. Sound can fill a room from a single source, it can turn corners and go around obstacles, humans can sense it, and it can be used to carry information- even between machines.
In our next post we’ll drill down on these aspects to look at the why sound makes a good medium for sending data, and compare its capabilities with other methods of data transmission.
If you want to get straight to how Chirp uses sound for data transmission, head over to Chirp.io
