How does an instrument make sound?
Sound is a vibration that travels through air. Every instrument has three parts:
- a vibrator (string, air column, skin, bar or speaker cone),
- a way to start it (pluck, bow, blow, hit, or an electric signal),
- a resonator that makes it louder (the hollow body of a guitar or sitar, the shell of a drum).
Pitch (high or low) depends on how fast the part vibrates: its frequency, measured in hertz (Hz). Shorter, tighter, thinner or smaller parts vibrate faster, so the note is higher. Halve the length of a string and the frequency doubles: the note goes up one octave.
Instrument families
Musicians in many cultures group instruments by what vibrates. A common world system (Hornbostel–Sachs) has five groups. India's old Natya Shastra used four similar groups.
- Chordophones – strings (tat): violin, cello, guitar, harp, sitar, veena, sarod. Bowed, plucked or struck (piano).
- Aerophones – wind (sushir): woodwinds (flute, bansuri, clarinet, oboe, shehnai) and brass (trumpet, horn, trombone, tuba), where the player's lips buzz.
- Membranophones – skins (avanaddha): tabla, mridangam, dhol, djembe, timpani.
- Idiophones – solid bodies (ghan): bells, cymbals, xylophone, triangle, ghatam, manjira.
- Electrophones – electronic: synthesiser, electric keyboard, electric guitar with amplifier, drum machine.
Timbre: why a flute and a violin sound different
When an instrument plays one note, it also sounds weaker, higher notes at the same time. These are harmonics (overtones). Each instrument has its own recipe of harmonics, plus its own way of starting and fading a note (attack and decay). This gives its timbre, or tone colour.
A flute has few harmonics, so it sounds pure and soft. A violin or a shehnai has many, so it sounds bright or reedy. Electronic instruments build sound from electric signals. A synthesiser can copy a real instrument or invent new timbres. Digital tools (keyboards, apps, MIDI) let one player control many sounds.
Instruments together: orchestra and ensembles
Orchestration means choosing which instruments play which part. A Western symphony orchestra has four sections: strings (the biggest), woodwind, brass and percussion. Strings usually sit at the front, percussion at the back. An Indian classical ensemble might have a melody instrument (sitar or sarangi), a drone (tanpura) and rhythm (tabla). A rock band uses guitars, bass, drums and keyboards.
Composers mix families for colour: strings for warmth, brass for power, woodwinds for light solos, percussion for rhythm and drama.
Try it
Stretch a rubber band over an open box. Pluck it. Now pull it tighter and pluck again. Then press it halfway along. Which is higher each time? Also blow across bottles with different water levels: more water = shorter air column = higher note.
Key formulas and definitions
- Frequency (pitch) – vibrations per second, unit hertz (Hz)
- String or open pipe: f = v ÷ 2L (shorter L → higher f)
- Halve the length → double the frequency (one octave up)
- Chordophone – strings (tat)
- Aerophone – wind (sushir)
- Membranophone – stretched skin (avanaddha)
- Idiophone – solid body vibrates (ghan)
- Electrophone – electronic sound
- Timbre – tone colour, set by the mix of harmonics
Worked examples
1. Classify: sitar, tabla, bansuri, manjira, synthesiser.
Sitar – chordophone (string). Tabla – membranophone (skin). Bansuri – aerophone (wind). Manjira – idiophone (solid). Synthesiser – electrophone.
2. A guitar string 0.65 m long plays 110 Hz. The player presses a fret halfway along. What note frequency now?
Half the length → double the frequency = 220 Hz (one octave higher).
3. A wave on a string moves at 300 m/s. The string is 0.6 m long. Find the basic frequency.
f = v ÷ 2L = 300 ÷ (2 × 0.6) = 300 ÷ 1.2 = 250 Hz.
4. Why does a trumpet and a flute playing the same A (440 Hz) sound different?
Same main frequency, but each adds a different mix of harmonics and starts the note differently, so their timbre differs.
Common mistakes
- Thinking loudness and pitch are the same. Loudness depends on how big the vibration is; pitch on how fast.
- Calling the piano a percussion-only instrument. Its hammers strike strings, so it is also a chordophone.
- Thinking a saxophone is brass because it is metal. It uses a reed, so it is a woodwind.
- Thinking a longer string gives a higher note. Longer means slower vibration and a lower note.