the speed of sound

When you see a lightning flash and then count the seconds before the thunder arrives, you are using the speed of sound, the pace at which a sound wave travels through the air. The question it answers is: how fast does sound get from where it is made to where it is heard?

The speed of sound is how quickly a pressure disturbance propagates through a medium. In dry air at 20 degrees C it is about 343 m/s (roughly 1235 km/h). It depends on the medium's stiffness and density, not on how loud or how high-pitched the sound is. For an ideal gas, v = sqrt(gamma R T / M), where T is the absolute temperature, M the molar mass, R the gas constant, and gamma a property of the gas. The useful consequence is that sound speeds up as the air warms: near everyday temperatures, roughly v is about 331 + 0.6 times (temperature in degrees C) m/s.

Sound travels faster in liquids and faster still in solids (about 1500 m/s in water, about 5000 m/s in steel), because they resist compression more strongly. An honest note: the speed does not depend on the loudness or the frequency, a whisper and a shout cross the room at the same speed. Only when speeds approach the speed of sound (breaking the sound barrier) do dramatic effects like shock waves appear.