What is sound?
First take a medium in which sound can exists. One of such media is air that we breathe - tiny particles of gases, such as oxygen, water vapor, etc. There might be more or less such particles in some place. We call this density and this might be variable.
These particles usually make some tiny random motions but if we let some air closed and do not affect it from outsite on average the density doesn't change. Since no energy is transported via the medium, in this case air, we can hear nothing - it's silence.
Now what happens if something disturbs the quiet air? Let's say someone claps his hands. Or similarly we can have a box with one side movable (imagine a loudspeaker with a membrane) and we move the membrane very quickly outside. The particles of air are pushed away and their local density increases. Each particle affects its neighbors with some force and they want to eventually achieve stability again. It is similar to a spring. So the this change in density moves away and also fades away. What you could hear is a click or clap. Some energy were transported via the medium.
Then what happens if this movable side of the box (the membrane) is moving periodically back and forth in time? Then the particles would also move back and forth, and thus the density of the air would fluctuate between high and low around some steady-state densisty. These sound waves would travel away from the source. The back and forth movements of particles are called vibration.
Frequency and amplitude
Let's take one point through which the sound wave is propagating. The density varies there up and down in a similar way as the membrane is moving back and forth.
The number of times there's a maximum of this simple wave in this point per second is called the frequency. If the membrane is moving slow the frequency is low, if its moving faster the frequency is higher. It is measured in Hertz (Hz). It is connected with how low or high we perceive a tone.
Also we might be interested how loud the wave is. The amount of how the membrane gets displaced is affecting the amount of displacement of the air particles. This is called amplitude of the wave.
Attenuation with distance
Since the energy in the waves propagate in all directions it gets spread into the space thus farther from the source the amplitude gets lower and lower. This is the reason why we can hear whispered words from close distance while we can't hear loud speak from several kilometers.
In practice the typical sounds are not just clicks or waves of a single frequency. Rather many simple sound waves are mixed together. Many waves with various frequencies and amplitudes, continuously changing in time.
Practically any music uses some kind of complex sounds. And our aim will be to analyze and understand these complex sounds in music.
We have talked about air as of the media in which sound waves can propagate. However, it is not the only one. Roughly speaking sound can exist in just any matter other than vacuum (which is almost absent of matter), ie. gas (eg. air), fluid (eg. water), solid (eg. a steel rail), even plasma (eg. the Sun). It is sufficient that the medium has some movable particles that interact together the usual way. This is in contrast to electromagnetic radiation (eg. light or WiFi signal) which doesn't require any matter to propagate in.
The various media differ in one important thing - how densely the particles are spaced. In air and other gases they're very sparse, in fluids their much more together and in solid they're very closely spaced.
This affects the speed the sound waves travel in the medium. Since the particles in steel are much closer the sound is faster there than in the air. This is the reason why you can hear a train approaching the station first from the rails then via the air.
We have outlined the basic principle of sound - the vibration of particles in a medium creates waves that propagate through the medium. The vibrations are typically introduced by movements of some object in the medium and they carry some energy. They might be periodic which result in tones and we can measure the frequency of such waves. Or they might be non-periodic, eg. one-time pulses such as clicks or claps. The amount of energy the sound waves carry corresponds to the amplitude of the sound waves. In case no vibrations propagate through the medium there is no sound, ie. there is silence.
Still we are going to explore how the sounds are created in practice and how they are perceived. And also we will delve more deeper in exploring sound from simple ones to much more complex ones.
You can read an in depth treatment of Sound at Wikipedia.
A very nice and illustrative video lesson about transmisson of sound via sound waves in various media.
Real-world amazing examples of sound sources - strings of guitar/bass/violin vibrate and transfer the vibrations into the air which results in sound waves. The strings actually vibrate much more quicker than in the video. The reason why the motion looks to slow is that the shutter speed of the camera is very short and captures the strings in different phases. This effect is called aliasing.
This violin video is captured on a real high-speed camera so the slow motion is not a result of aliasing.
Stunning bass-string shot:
So what do you think? Did I miss something? Is any part unclear? Leave your comments below.