How Sound Is Made: Pitch, Loudness and Timbre

Everything on this site is eventually about pitch — which note, how far from which other note. It is worth spending one lesson on where pitch comes from, because pitch is only one of three things a sound has, and the other two get confused with it constantly.

A sound is something vibrating

A sound starts as something moving back and forth: a string, a column of air inside a tube, a drum skin, your vocal cords. That movement pushes on the air around it, the push travels outward as a wave of slightly-higher and slightly-lower pressure, and your eardrum moves in step with it. That is the whole mechanism.

Stop the vibration and the sound stops with it — which is exactly what happens when you put your palm on a ringing cymbal, and it is why a note on a real instrument dies away rather than lasting forever: the vibration loses energy with every cycle.

Pitch: how fast it vibrates

How many times per second the thing moves back and forth is its frequency, measured in hertz (Hz), and frequency is what your ear reports as pitch. Faster vibration, higher note. The A that orchestras tune to is 440 vibrations per second; the A an octave below it is 220, and the A an octave above is 880.

You can change that number on purpose, and a guitar is the clearest place to watch it happen. The open fifth string sounds A. Press it down at the third fret and only the part of the string between your finger and the bridge is still free to move:

That shorter length vibrates faster, so the note is higher: stopped at the third fret, the fifth string sounds C. Every way of changing pitch on every instrument comes down to the same three levers — length, tension and weight. Tighten a string and it rises, which is what tuning is; use a thicker string and it falls, which is why the low strings on a guitar are the fat ones.

Loudness: how hard it vibrates

Loudness is a different measurement of the same movement: not how often it happens, but how far the string travels each time. Pluck the same string harder and it swings wider, pushes the air harder, and arrives louder. The frequency has not changed at all.

This is the distinction that gets muddled most often, and it is worth being blunt about: pitch and loudness are independent. A high note can be barely audible and a low note can be deafening. If a teacher asks for a higher note and you play the same note harder, you have answered a different question.

Timbre: everything else

Play the same note, at the same loudness, on a piano and on a guitar. Nobody would confuse them. Whatever it is that tells them apart is timbre, and it is the property that carries the identity of an instrument or a voice.

Timbre exists because almost nothing in the physical world vibrates in one simple way at a time. A real string vibrates along its whole length and in halves and in thirds, all at once, and the extra tones that come from those simultaneous motions are what your ear reads as “guitar” rather than “piano”. How strong each one is, and how quickly each one dies away, is the instrument’s fingerprint.

You have already heard this site do it. The four instruments here are not recordings; each note you hear is calculated on the spot from a model of how that instrument actually makes sound — a plucked string for the guitar, a bowed one for the violin, a stack of simultaneous tones for the piano. They are built differently because they have to be: one method of generating sound cannot produce four timbres.

Why this course is mostly about pitch

Of the three, pitch is the one that music theory is built on. Melody is a sequence of pitches, harmony is pitches sounding together, and a scale is a selection of them. Loudness and timbre matter enormously to how music feels, and almost not at all to how it is analysed — a chord is the same chord played softly on an organ or hammered on a piano.

So the rest of this site will talk about pitch nearly to the exclusion of the other two, and that is a deliberate narrowing rather than a claim about what matters in music.

Common mistakes to avoid

• Using “higher” to mean “louder”. They are separate properties of one sound, and every instrument can change either without touching the other.
• Expecting pitch to be about size alone. A short thin string is high, but a long string pulled very tight can be higher than a short slack one. Length, tension and weight all count.
• Treating timbre as decoration. It is the reason you can pick one instrument out of an ensemble, and the next lesson shows that it comes from the same physics that decides which intervals sound consonant.