Sound is a mechanical wave caused by the vibration of particles in a certain medium, and therefore requires an elastic medium to create it. The simplest example of such a medium is air, as it consists of particles that, when vibrated, will pass them on to each other, causing sound to spread. In a vacuum, however, no sound can be heard.
Imagine you are sitting in a quiet classroom and there is a large metal bell at the front with a string attached to it. Because the room is filled with air made up of countless tiny particles, it can be thought of as an elastic medium. These particles are constantly moving, but are invisible to our eyes. The vibrating bell causes the air particles around it to vibrate too. As the bell moves outwards, it presses on the nearest air molecules, squeezing them together. When the bell moves back in, it creates space, causing the particles to spread out. This action creates a pattern of compression (where the particles are close together) and dilation (where the particles are further apart), moving outwards in all directions from the bell.
What is sound propagation?
The constant densification and dilution of the elementary particles of a medium causes a wave of disturbance in the medium, and therefore the propagation of sound. But why are there so many different types of sound when the same medium vibrates? How does this happen in musical instruments? By vibrating a string of e.g. a violin, we create a standing wave (which oscillates up and down). The frequency of vibration of the string depends on the length of the string, but also on the speed of sound propagation in the string (which, by the way, depends on the string's tensile strength and linear density), which means: on the type of string. A well-known school principle also states that the shorter the string is, the higher the sound is heard.
Nevertheless, a string put into vibration does not emit just one sound, it is only a fundamental tone. At the same time, other tones, called overtones or higher harmonics, are produced. The intensity of the sound, or loudness, is determined by the amplitude. In our case, the fundamental tone is the tone with the greatest amplitude, i.e. the loudest one. Each overtone gets weaker and weaker, so they have smaller amplitudes and are consequently quieter.
Let’s consider a scenario in which a musician tunes his guitar in a quiet room. He begins by gently plucking the guitar's E string. This string vibrates at its fundamental frequency, producing the lowest pitch the string can produce, just known as the fundamental tone. In the case of the low E string on a guitar, this fundamental frequency is approximately 82.41 Hz, meaning that the string vibrates 82.41 times per second.
When the musician plucks the string, it vibrates in specific segments: halves, thirds, quarters... these are what constitute the overtones. The frequency of each is an integer multiple of the fundamental frequency. For example, the second harmonic would have a frequency of 2 × 82.41 Hz = 164.82 Hz, the third: 3 × 82.41 Hz = 247.23 Hz, etc. Aliquots add richness and complexity to the sound we hear.
What is timbre then?
The timbre of a sound is the mixture of all the harmonic tones, in other words: timbre is the spectrum of sound. An important component of many instruments is the sound box, whose main task is to amplify the sound. Different sound boxes create different timbres and can be influenced by the type and thickness of the wood, the distribution of the grain, the size and shape of the sound box, the way the wood is varnished, the way the wood is joined (plywood, nailed,bolting), the size of the holes etc.
English (UK) 
Polski 