Second, something must disturb this medium in order to create waves. These waves are generally alternations in air pressure, but they behave just as waves in water. Third, a receiver needs to be present in order to perceive the disturbance. The transmitter and receiver might be the same person, or two different individuals, but either way sound is perceived by anyone present. A Complex Phenomenon Simplified All the sounds we hear are a complex mixture of waves, of all different sizes travelling in all different directions.
Frequency is dependent on the length of each wave.
Since the speed of sound is constant within a given medium, then the shorter the wave, the higher the frequency. Amplitude is the literal strength of the wave measured in air pressure. The relationship can be shown easily by drawing a graph of amplitude over time. Since there are 440 cycles of pressure every second, we know the frequency is 440 Hz. If we add a second wave at the same amplitude and frequency and begin both waves at exactly same time, they will reinforce each other and the result will be a doubling of amplitude.