Sound

Sound is a vibration that travels. A guitar string, a drumhead or your vocal folds move back and forth, push on the air next to them, and set off a chain of tiny pressure changes: squeezed air (compression), then stretched air (rarefaction), again and again. These pressure waves spread out in every direction. When they reach your eardrum, it vibrates in the same pattern, and your inner ear and brain turn that motion into what you hear.

What sound needs

Sound needs something to travel through: air, water, wood, metal. In a vacuum there is nothing to carry the pressure changes, so there is no sound. In air at room temperature (20 °C) sound travels at about 343 meters per second, roughly a kilometer every three seconds. That is why you see lightning before you hear the thunder. Sound moves faster in warmer air, much faster in water, and faster still in solids such as steel.

The properties of a musical sound

Four properties describe most musical sounds, and each has a physical side:

• Pitch: how high or low it is. It follows the frequency, the number of vibrations per second, measured in hertz.
• Loudness: how strong it is. It follows the amplitude, the size of the pressure changes, usually measured in decibels.
• Tone color, or timbre: why a flute and a violin sound different on the same note. It depends on the mix of overtones and on the envelope, how the sound starts, holds and fades.
• Duration: how long it lasts, which is what rhythm organizes.

Musical tones and noise

When a vibration repeats regularly, as a string or an air column does, you hear a definite pitch: a musical tone made of a fundamental and its harmonics. When the vibration is irregular, as with rain, a snare drum or a whispered "shhh", you hear noise. Most instruments make a little of both: the breath in a flute tone, the scrape of a bow, the thump of a piano hammer.

The range of hearing

Young listeners hear roughly from 20 Hz to 20,000 Hz. The upper limit drops gradually with age and with noise damage, so protecting your hearing at loud rehearsals and concerts is worth the trouble. The piano covers a good slice of that range, from 27.5 Hz (its lowest A) to about 4,186 Hz (its top C), and the overtones of its notes reach much higher.

To see how one instrument turns a key press into sound, read How the Piano Works. Synth Waveforms and Their Harmonics shows how a sound's harmonics shape its color.