How Musical Instruments Turn Vibration into Sound

When an object vibrates, it pushes and pulls on the surrounding air. This creates changing regions of higher and lower pressure that travel outward as sound waves. When those waves reach your ears, they make parts of the ear vibrate, and your brain interprets the pattern as sound.

Faster vibrations usually create a higher pitch, while slower vibrations create a lower pitch. A stronger vibration often produces a louder sound because it creates larger pressure changes. Pitch and loudness are different features, so a note can be high and quiet, low and loud, or any other combination.

Most instrument vibrations are too small to move much air on their own. The body of the instrument, a hollow tube or a stretched surface helps transfer more of the vibration to the air. This is why an unplugged electric guitar string sounds much quieter than the same note on an acoustic guitar.

String instruments use vibrating strings. A player may bow, pluck or strike them. Shorter, tighter or lighter strings usually vibrate faster and produce higher pitches. The wooden body and enclosed air help strengthen the sound.

Wind and brass instruments use vibrating air columns. In a flute, air moves across an edge. In a clarinet, a reed vibrates. In a trumpet, the player's lips begin the vibration. Opening holes, pressing valves or changing the tube length changes which air patterns fit inside, and that changes pitch.

Percussion instruments often use a stretched membrane or a solid body. A drumhead vibrates after being struck, while a xylophone bar vibrates as a solid piece. Some percussion instruments have clear pitches, and others produce a broad mixture of frequencies that your ear hears more as a beat, crash or shake.

Two instruments can play the same note at the same loudness and still sound different. This quality is called timbre, or tone colour. It comes from the mixture of the main vibration with additional frequencies called overtones, along with how the sound begins, changes and fades.

To listen for how an instrument works:

• •Identify what vibrates first.
• •Notice what part strengthens the sound.
• •Listen for a clear pitch or a broad noise.
• •Compare how the note begins and fades.
• •Describe the timbre with simple words.

A string fixed at both ends cannot vibrate at any frequency it likes. Only patterns that fit neatly between the two ends survive, forming standing waves. The simplest pattern, with the whole string moving as one arc, gives the fundamental, the note you name. The string can also vibrate in halves, thirds and so on, producing harmonics at two, three and more times the fundamental frequency. A real string does many of these at once, which is where overtones come from. Touching a string lightly at its midpoint suppresses the fundamental and leaves a note an octave higher.

Air columns follow similar rules, but the ends of the tube matter. A flute is open at both ends and supports the full series of harmonics, so blowing harder makes it jump up an octave. A clarinet behaves more like a tube closed at one end, because the mouthpiece and reed act as a closed end. That shape favours odd-numbered harmonics, which contributes to the clarinet's hollow tone in its low register, and when the player overblows it jumps up by an octave and a fifth instead. Players use these jumps, together with finger holes and keys, to reach every note.

The instrument you use most is your own voice, and it follows the same principles. Air pushed up from the lungs passes between the vocal folds, two small bands of tissue in the larynx. They open and close rapidly, chopping the airflow into pulses that set the pitch. Muscles change their length and tension, and stretching them raises the note. Adult male vocal folds are usually longer and thicker than female ones, which is the main reason most men speak at a lower pitch. The raw buzz from the folds sounds thin on its own.

The throat, mouth and nose then act like the body of an instrument. Their shape creates resonances called formants, which boost some overtones and weaken others. Moving your tongue and lips changes those resonances, and that is how you make different vowels at the same pitch. This explains the helium voice. Many people think helium raises the pitch, but the vocal folds vibrate at much the same rate. Sound travels faster in helium, so the formants shift upwards, giving the squeaky, cartoon-like tone colour while the underlying note barely changes.

The rules for strings can be made exact. If everything else stays the same, halving a string's vibrating length doubles its frequency, which raises the note by one octave. The A string on a guitar, tuned to 110 vibrations per second, sounds at 220 when pressed at the twelfth fret, which sits at the midpoint of the string. Tension works more gently. Because frequency rises with the square root of tension, you would need to pull a string four times as tight to double its pitch, which is why turning a tuning peg nudges the note rather than leaping by octaves.

Western music divides the octave into twelve equal semitones, so each fret must raise the frequency by the same ratio, about 1.06. That means each fret shortens the vibrating string by about 5.6 per cent of whatever length is left, not by a fixed distance. The result is the familiar pattern of frets that crowd closer together as you move up the neck. Mass matters too. Making a string thicker and heavier lowers its frequency, so the lowest strings on a guitar or piano are wound with extra wire, letting them sound deep notes without being impractically long or slack.

School music books usually divide instruments into strings, woodwind, brass and percussion, but those labels contain oddities. A modern flute is normally made of metal yet counts as woodwind, and so does the saxophone, despite its shining brass body. In 1914 the scholars Erich von Hornbostel and Curt Sachs published a system that avoids this confusion by asking one question: what vibrates first to make the sound? Idiophones vibrate as a whole solid body, like a xylophone bar, cymbal or rattle. Membranophones use a stretched skin, as drums do. Chordophones use strings, and aerophones use a vibrating column or stream of air.

A fifth group, electrophones, was added later for instruments whose sound is produced electronically, such as synthesisers and the theremin. The system gives some surprising answers. A piano's strings are struck by hammers, which might suggest percussion, but in this scheme it is a chordophone. Brass instruments become aerophones sounded by the player's buzzing lips, while the saxophone, with its single reed, sits beside the clarinet rather than the trumpet. Museums and researchers use expanded versions of the scheme to catalogue instruments from every culture, because it relies on the physics of sound rather than on any one musical tradition.

Musical instruments create sound by starting a vibration and transferring it to the air. Strings, reeds, lips, air columns, membranes and solid bars each produce different patterns. Follow the vibration from its source, and an instrument's sound becomes easier to understand.