Sound

Music from your headphones, your alarm clock ringing, your friend's voice: sounds reach you all day long. But what actually arrives at your ear? Once you understand that, you can even work out how far away a thunderstorm is.

Vibrations need a medium

Sound is created when something vibrates: a guitar string, a speaker membrane, or your vocal cords. The vibration pushes the air particles, they pass the push along, and so a pressure wave travels all the way to your ear. Sound therefore always needs a medium such as air, water, or metal. In space there are almost no particles, so it is completely silent there, no matter how enormous an explosion would be.

Frequency and amplitude

The frequency counts how often something vibrates per second. Its unit is hertz (Hz): 100 Hz means 100 vibrations per second. The higher the frequency, the higher the pitch. Concert pitch A, which orchestras tune their instruments to, is 440 Hz. The amplitude, on the other hand, is the size of the swing, in other words how far the particles get pushed back and forth: the bigger the swing, the louder the sound. So high notes are not automatically loud, pitch and volume are two separate properties.

How fast is sound?

In air at about 20 °C, sound covers roughly 343 m per second. But how fast sound travels depends on the material: in water it is more than four times as fast, in metal faster still, because there the particles hold together more tightly and pass the push along more quickly. 343 m/s sounds like a lot, but light is almost a million times faster. That is why in a thunderstorm you see the lightning first and hear the thunder later. Count the seconds in between: every second means roughly 340 m of distance, so three seconds is about one kilometre.