Maryaguirre

Foundations

How Instruments Make Sound

From a trembling string to a buzzing lip, every instrument turns motion into music through one of five routes, and its body decides how that sound is heard.

Every musical sound starts as a vibration, but a vibration alone is faint. This guide follows the five routes used across the library: vibrating string, air column, membrane, solid body and free reed. It then explains how a resonant body amplifies and colours the result, and how harmonics and overtones, the hidden layers inside every note, give instruments their individual character.

A shiver that becomes a note

Every musical sound begins as a motion too small and too quick to follow by eye. Something elastic is pushed from its resting position, springs back, overshoots, and keeps trading energy back and forth. The number of complete cycles in one second is the frequency, measured in hertz, and the ear translates it into pitch. A string that shivers 220 times a second is heard as the A below middle C; one that shivers 440 times a second is heard as the A above it, exactly an octave higher.

Vibration by itself is seldom audible across a room. A thin string moves very little air, which is why a wire stretched over a bare block of wood sounds faint. Instruments solve the problem by coupling the vibrating part to something larger: a wooden plate, a column of air, a stretched skin, a metal tube. That larger partner moves more air, and moving air carries the sound to a listener. Much of what makes an instrument recognisable lies in the design of this partner.

Five routes from motion to music

This library sorts instruments by the route they use to start the vibration. There are five. A vibrating string is stretched between two fixed points and set moving by plucking, striking or bowing. A vibrating air column is the air inside a tube, set moving by breath directed across an edge, past a reed, or through buzzing lips. A vibrating membrane is a stretched skin or film, usually struck. A vibrating solid body is a rigid object, such as a bar, that rings when hit. A free reed is a thin metal tongue that swings through a slot as air flows past it.

The routes overlap in practice. The Piano sounds through strings, yet they are struck by hammers that a keyboard controls. The Timpani are membrane instruments that add a mechanism for changing pitch. Many instruments also have a secondary vibrator: a Violin has a resonant wooden body, but the string is what starts the sound. Classifying by the original source of vibration, rather than by shape or by how the player touches the instrument, keeps the question manageable.

What the string and the pipe have in common

A string fixed at both ends can vibrate only in certain patterns called standing waves, in which points named nodes stay still while the stretches between them swing. Three things set the pitch: a shorter, tighter or lighter string sounds higher. That is why a Double Bass string is thick and long while the top string of a violin is thin and short, and why a player of the Acoustic Guitar raises a note by pressing a string against a fret, shortening the part that is free to move.

Air in a tube behaves in a comparable way. A Flute player splits a stream of breath against an edge; a Clarinet player sets a single reed buzzing; a Trumpet player buzzes the lips into a cup-shaped mouthpiece. In each case the air column responds at particular frequencies, and the player coaxes the vibration onto one of them. Opening holes or adding tubing changes the effective length of the column, and therefore the note.

Skins, bars and metal tongues

A stretched membrane vibrates in two dimensions, which complicates matters. When a drumhead is struck, it moves in patterns whose frequencies are not neat whole-number multiples of one another, so most drums sound indefinite in pitch, as the Drum Kit shows. Kettledrums are a partial exception: a tightly tuned head, a bowl-shaped shell and the air enclosed beneath combine to give a clearer note, which the timpanist adjusts by changing the tension of the head, on modern instruments with a pedal.

Solid bodies work differently again. On a Xylophone or Marimba, each bar is shaped, with wood carved from its underside, so that its higher vibrations fall at chosen multiples of the main one, and a tube beneath reinforces the fundamental. A Vibraphone uses metal bars and adds rotating discs in its tubes that vary the sound periodically.

A free reed is a metal tongue fixed over a slot. Air makes it swing through the slot, chopping the airflow into rapid pulses. Unlike the reed of a clarinet, it is not governed by a long tube; the tongue’s own length and mass largely decide the pitch. The Harmonica is driven by the player’s breath, the Accordion by bellows.

How the body amplifies and colours sound

Resonance is the tendency of an object to respond strongly at certain frequencies when it is pushed by a vibration. A violin body is a resonant structure: the string’s motion passes through the bridge into the top plate, the back and the air inside, and the whole body radiates sound far more effectively than the string could alone. The same principle appears in the soundbox of a Harp, in the stretched head of a Banjo, and in the tubes under a marimba’s bars.

Amplifying is only half the job. A body does not respond equally to every frequency; it has peaks and dips, and it boosts some ranges more than others. That uneven response colours the tone, and it is one reason two violins, or two clarinets, can sound different on the same note. Wind instruments show a related effect, since the bore and the flare of the bell influence which parts of the sound reach the listener. The Electric Guitar is an instructive contrast: its solid body usually adds little acoustic volume, because pickups sense the string and an amplifier does the rest.

Harmonics and overtones in plain terms

Bow a string and you hear one note, yet the string vibrates in several ways at once: as a whole, in halves, in thirds, and so on. Each way adds a faint tone at a whole-number multiple of the lowest frequency. The lowest is the fundamental; the rest are overtones. If the fundamental is 200 hertz, the harmonic series continues at 400, 600, 800 and upward. By convention the fundamental counts as the first harmonic, so the first overtone is the second harmonic.

The ear fuses these components into a single note whose pitch follows the fundamental, while their relative strengths shape its character. Real instruments bend the tidy picture. Stiff strings push the upper components slightly above exact multiples, and bars and drumheads can have overtones that are not harmonic at all, which is part of why bells and most drums sound less clearly pitched. Bowed strings and winds, by contrast, tend to lock their vibrations into near-perfect harmonic order.

Two instruments, one pitch

Play the same A4 on a flute and on a clarinet and the fundamental is identical, yet nobody confuses them. Part of the difference is the mix of harmonics. A clarinet’s cylindrical tube, closed at one end by the reed, favours odd-numbered harmonics in its low register, which gives that register a hollow colour. A flute’s tube supports a fuller set of harmonics and sounds purer and lighter.

The rest of the difference lies in time. The first few hundredths of a second, when the string is plucked or the air starts to speak, contain noise and rapid change that the ear uses to identify the source. The way the sound then fades or sustains completes the picture. Sound production, body resonance and harmonic content together explain why instruments are so distinct, and why they can blend so well.

Terms used in this article

Free reed
A thin metal tongue that swings through a slot in a frame, chopping airflow into pulses without being governed by an attached tube.
Frequency
The number of vibration cycles per second, measured in hertz. Higher frequency is heard as higher pitch.
Fundamental
The lowest frequency in a note, which usually decides the pitch we hear.
Membrane
A thin stretched sheet, such as a drumhead, that vibrates when struck.
Overtone
Any component of a sound above the fundamental. In a harmonic series, overtones sit at whole-number multiples of the fundamental.
Resonance
The strong response of an object when driven at or near a frequency it naturally favours.
Standing wave
A stable vibration pattern in a string or air column, with fixed still points (nodes) and points of greatest motion between them.

References

  1. UNSW Sydney, School of Physics (Music Acoustics). How harmonic are 'harmonics'?. newt.phys.unsw.edu.au/jw/harmonics.html
  2. UNSW Sydney, School of Physics (Music Acoustics). Strings, standing waves and harmonics. newt.phys.unsw.edu.au/jw/strings.html
  3. UNSW Sydney, School of Physics (Music Acoustics). What is a Sound Spectrum?. newt.phys.unsw.edu.au/jw/sound.spectrum.html
  4. Philharmonia Orchestra. Instruments | Philharmonia. philharmonia.co.uk/explore/instruments
  5. Philharmonia Orchestra. Timpani | Philharmonia. philharmonia.co.uk/explore/instruments/timpani
Published Last reviewed 1,194 words · 5 min read

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Instruments mentioned

Follow the ideas into the entries.

Front view of a varnished golden-brown violin with two f-holes, an ebony tailpiece and fingerboard and four strings, photographed upright against a soft grey background.
Strings

Violin

The violin is the smallest and highest-pitched member of the modern violin family, with four strings tuned a fifth apart and set into vibration by a horsehair bow. It took its recognisable form in northern Italy in the sixteenth century and was refined in Cremona over the next two hundred years. It leads the orchestra, anchors the string quartet, stars in the concerto and, under the name fiddle, drives dance and folk music in many regions.

Renaissance–Baroque (1400–1750)
Dark wooden transverse flute with gold rings and gold keys lying horizontally below its matching rosewood case with a gold inlaid cartouche, on a light grey background.
Woodwinds

Flute

The modern concert flute is a long, narrow metal tube played sideways, sounded by directing a stream of air across a hole near one end. It has no reed, which is why it is classed as an edge-blown flute and not a reed woodwind. Its form comes from Theobald Boehm’s designs of the 1830s and 1840s, which reorganised holes and keys for accuracy of pitch. Related side-blown flutes of bamboo, wood and bone appear in musical traditions across Asia, Europe and beyond.

Classical–Romantic (1750–1900)
Four copper-bowled timpani with pale calfskin heads, pedals and tuning rods on a wood-panelled stage, with mallets on a tray, a music stand and a chair behind.
Percussion

Timpani

Timpani are large kettledrums: a calfskin or plastic head stretched over a bowl of copper or fibreglass and tuned to a definite pitch, today usually with a foot pedal. Descended from cavalry drums brought into Europe, they joined orchestras in the seventeenth century and became the percussion section’s principal pitched drums. Their sound depends on the interplay of membrane, air and shell.

Renaissance–Baroque (1400–1750)
Ten-hole diatonic harmonica with a chrome cover plate engraved with the Marine Band name, and wooden comb and reed plate visible from the front edge, on a black background.
Free-reed

Harmonica

The harmonica is a small free-reed instrument played by breathing in and out across a row of holes, each leading to a pair of metal reeds. It grew from a crowd of early nineteenth-century experiments in German-speaking Europe, then spread across the world as a cheap, pocket-sized instrument. Diatonic, chromatic and tremolo models serve blues, folk, country, jazz and concert repertoire.

Classical–Romantic (1750–1900)
Concert marimba on a wheeled black frame, with rosewood bars along the top and graduated silver resonator tubes hanging beneath, photographed from the side on a stage.
Percussion

Marimba

The marimba is a keyboard-shaped set of tuned wooden bars struck with mallets, each bar backed by a resonator that amplifies its note. Its name and its basic design trace to African traditions, and it took its best-known forms in Guatemala and southern Mexico, where gourd resonators gave way to wooden and later metal tubes. Today it serves village celebrations, marimba bands and concert halls alike, with a solo repertoire built around four-mallet playing.

Classical–Romantic (1750–1900)
Rosewood grand piano with carved scroll cheek blocks, tapered turned legs on brass casters and a lyre-shaped pedal assembly, shown from the keyboard end with the lid and music desk raised against a plain white background.
Keyboard

Piano

The piano is a keyboard instrument in which felt-covered hammers strike tensioned metal strings, and in which the player shapes loudness through the speed of the touch. Devised in Italy around 1700, it grew through the eighteenth and nineteenth centuries into an iron-framed machine with a standard compass of 88 keys. It serves as solo voice, accompanist, chamber partner and orchestral colour, and it remains a working tool for composing, teaching and rehearsing in many musical traditions.

Renaissance–Baroque (1400–1750)

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