Sound theory: What is sound?

One definition we could give is that what we perceive as being sound is a variation in relation to a constant value, of air pressure. When this variation repeats itself cyclically a certain amount of times within a certain time interval, which we shall better define hereinafter, we perceive sound. For sound to travel it requires a means to transport it; air is one means in that its particles, as we shall see, transmit one to another the vibrations generated by the sound source, and diffuse it into the surrounding space. This means that any means, be it solid, liquid or gaseous, can transport sound, affecting the speed at which it travels depending upon its density. In our case we'll consider air as our "sound transportation means" seeing that our practical examples shall involve the latter.