Frequencies below 20 Hz are infrasonic , and frequencies above 20,000 Hz are ultrasonic . These condensations and rarefactions in the medium will move through it as the disturbed air particles collide with others, creating another disturbance in the medium further away from the source. This process is called sound propagation . The speed of sound is approximately 343 meters per second in standard Earth atmosphere, but it can vary greatly depending on the properties of the medium the sound is traveling through.
Steel is much stiffer than water, which is in turn much stiffer than air.
Sound travels faster through a hot medium than a cold one. When two or more sound waves are brought into contact with one another, a new resulting wave is created. This new wave can then interfere with the continued motion of the previously-existing sound waves, and hence, is called interference. Mathematically, interference is the phase-matched sum of waveforms involved. The extreme cases can be demonstrated with two identical waveforms.