While teaching multimedia at the university, that was quite a question for me – how to explain to students in a simple and concise way what sound is?.
Sound is a mechanical vibration of a medium in the hearing range.
A medium can be practically anything: a gas, a liquid or a hard material. Most commonly we perceive sound propagation in the air, where it travels with the speed of 343 m/s at the room temperature (20°C). The speed varies with temperature, but in the human habitat's thermal range, that's not a big variation (a couple of percent).
Some aircrafts can reach the speed which is higher than the speed of sound (343 m/s= 1,235 km/h), also called the supersonic speed. Some other phenomena in the nature can reach that speed, such as meteorites entering the Earth's atmosphere, There are also scientific speculations about the fact that some dinosaurs were able to move their tail with the ultrasonic speed, generating intimidating sounds.
The speed of sound is different in different media: for example, in water, this parameter is 1,484 m/s, while for iron it is 5,120 m/s, again, both at the room temperature.
The basic shape of a pure uniform oscillation is a sine wave. This is what we use as a basic signal shape in sound, too, especially in electronics. This is the shape that we use for most measurements to test the behavior of sound equipment.
This signal is defined by two parameters:
amplitude (A), which is directly connected with the loudness, therefore how loud or quiet the sound is, and
frequency (f), which determines the pitch of the sound - it tells us how many times per second a sound vibrates.
The signal makes its full turn when it equally passes positive and negative half-period creating a full cycle.
The time needed for this signal to terminate a full cycle is called a period (t0) or wavelength and it is measured in seconds. Frequency determines how many periods are completed in one second. The unit for frequency is 1/s or, more commonly, Hertz (Hz).
For example: if a signal oscillates with a hundred periods in a second, that's 100 Hz.