Introduction
Our brains and ears are very good at detecting repeating patterns in air pressure. Such repetitions give rise to a perceptual characteristic of sounds called pitch.
Definition: Pitch
Pitch is the subjective characteristic of sounds by which we rank them on a spectrum from low to high.
Frequency
Pitch is affected by multiple factors but by far the most influential is the frequency with which patterns in the change of air pressure repeat.
of these repetitions, i.e. the number times per second the pressure departs from and then returns back to some particular value, is what we perceive as pitch.
Higher frequencies correspond to higher pitches and vice versa.
There are many ways in which pressure can oscillate, but the simplest way is in a sinusoidal wave, starting at some particular value, going up to a maximum, highest, value, then dropping down to a minimum, lowest, value and then returning to the original value. We perceive different pitches depending on how many times per second this cycle occurs. Here is a graphical representation of how pressure changes in a sine wave with a frequency of 220 Hz:
The blue line shows how the pressure of air varies in time around some particular value which has been labeled “0.0”. Note that this is not the actual absolute value of the pressure - we have simply chosen a particular value as a reference and have decided to measure changes relative to it. Whenever the blue line goes below 0.0, this means that the pressure has dropped below the reference value. Whenever it goes above 0.0., the pressure has increased above the reference value. If the pressure of the air around you oscillated in the way illustrated by the graph, you would perceive the following sound:
As you are listening to the above sound, your headphones are causing the pressure of the air inside your ears to oscillate around the pressure of the room you are currently occupying in exactly the same way as illustrated by the graph.
We can also illustrate the dependence of volume on how drastic the changes in pressure are. In the following graph, the pressure of the air still oscillates at the same frequency - if you count the number of peaks in both graphs, you will see that the number is the name, meaning that the frequency is the same. However, the peaks in the following graph are higher than the peaks in the previous graph and the troughs in the following graph are lower than the troughs in the previous graph which means that the pressure changes the same number of times per second but the change itself is more drastic in the second case - it reaches a higher maximum and a lower minimum pressure.
If the pressure of the air inside your ears changed in the way illustrated by the graph, then you would perceive the following sound:
Indeed, it sounds just as before but louder.
Now, lets see what happens when the number of oscillations, i.e. the frequency, changes. Following is the graph of a sine wave with a frequency of 110 Hz and how it sounds:
Therefore, lower frequencies are perceived as lower pitches, i.e. they sound more like a bass, and higher frequencies are perceived as higher pitches.