The science behind the sound
In our day-to-day life, silence doesn’t exist. We always are surrounded by sound. But that’s a positive thing. It helps us communicate, warns us of possible threats, or brings us joy. Music is by far the most intriguing and fascinating sort of sound. In this article, we will dive into the world of sound in a way that everyone can understand.
When we think of a wave we often think of this:
This wave can be easily created by periodically moving a rope up and down. The rope wave is a nice example of a wave, but know that not all waves are sinus functions. A wave is simply a disturbance traveling through a medium such as air or water. This disturbance can be generated by a radio, a musical instrument, or really anything that moves up and down. Therefore waves exist in numerous forms.
Imagen, we periodically move a rod from side to side in the air. The following happens:
As you can see, the rod moves the air particles from left to right. Or does it? If you focus on the movement of an individual particle, you will notice that it travels back and forth over its starting position. Why it does this has all to do with pressure.
The denser the particles, the higher the pressure. The more uncrowded a region is, the lower the pressure. A particle will always travel from high to low pressure. If you reexamine the moving particles, you will notice that every particle travels back and forth between two regions that are in turn high in pressure. It is this movement that creates a wave. More specifically a sound wave, since sound waves are a disturbance of pressure in a medium.
The seemingly left-to-right movement of the particles is the propagation of the wave itself. There is no transportation of matter, only of energy. It is this energy that our ears can convert to sound. If the sound wave created in this example could reach our ears, we would hear a sound of a specific frequency(high tones have a high frequency, lower tones have a low frequency). This frequency depends on the speed of every individual particle, which is the same speed as the speed of the rod.
To analyze sound waves we can graph the pressure of the medium for every given position. For the above example that would look something like this:
The height of this wave determines the loudness of the sound, the taller the waves, the louder the sound, and vice versa. The width of the wave determines the frequency. The Gif below shows another example, this time the wavelength is also indicated.
The smaller the wavelength, the higher the frequency of the tone.
If we translate this back to our “physical” wave, the loudness of the sound would be how far every particle travels from its equilibrium point. The speed with which the particles travel from left to right determines the frequency of the wave.
Something you might not know, soundwaves are quieter when the frequency is lower and are the loudest when the wave has a frequency of 4000 Hz. So, if you don’t want to be overheard, speak in a lower register.
When music is played, the generated pressure waves are way more complex than the above example. Even a single note played by a violin results in some interesting-looking pressure changes. The image underneath shows the generated wave when a violin plays a G(196 Hz).
Believe it or not, this wave is the result of the summation of numbers of sines and cosines, each with a different height (or amplitude) and width (or wavelength).
The French mathematician and physicist Fourier was the founder of the Fourier series, which later developed into Fourier analysis. Basically, mathematics that enables us to find all the sines and conines that construct a given wave. To illustrate this a bit better take a look at the GIF below:
The bottom wave is the summation of all the above waves.
Being able to determine out of which sines and cosines a given signal exist has many applications. Being able to easily remove disturbances from a recording is one of them.
Fourier analysis is a pretty advanced math topic. The YouTube channel “3Blue1Brown” has excellent introduction videos.
To hear the incoming wave, our body needs to convert the pressure wave into an electric signal our brain can understand. That’s where the cochlea comes in.
The cochlea is a spiral-shaped cavity with inside over 15,000 tiny hair cells. About 3,500 of these hairs are so-called inner hair cells. The other 11,500 hairs are outer hairs. The outer hairs generate and amplify sounds if necessary, while the inner hairs convert the incoming sound wave to an electrical signal. The reason we need so many hears is pretty straightforward. Every hair cell is “tuned” to a specific frequency. The first cells of the cochlea are tuned to the highest frequencies. The further you travel inside the cochlea, the lower the frequency of every individual hair will get. When a sound wave enters the cochlea, only hairs tuned to the frequency of the wave will vibrate. As a result, only the given frequency will be converted into an electric signal our brain can understand, resulting in us hearing sound.
There is a reason why the cochlea is coiled. Studies have found that this shape enhances low-frequency vibrations allowing us to hear down to only 20 Hz, or 20 changes of pressure per second. Keep in mind a normal conversation is generally between 500 Hz and 3000 Hz.
Although it doesn’t always feel like it, sound is in fact something physical. Sound sources create a pressure wave in the air. If the wave hits our ear, the physical wave gets converted to an electric signal. This electric signal is then interpreted by the brain and we hear sound.