Resonance

Everything that can vibrate has one or more natural frequencies: the rates at which it prefers to swing. Push a playground swing once per swing and, even with small pushes, it goes higher and higher; push at a random rhythm and it barely moves. Resonance is the same effect in sound: an object driven at one of its natural frequencies vibrates far more strongly than at any other.

Resonance in instruments

A vibrating string on its own moves very little air and is surprisingly quiet. Instruments use resonance to make it heard:

• Soundboards and bodies. A piano's strings pass their vibration through a bridge to a large wooden soundboard; a guitar's or violin's strings drive the top of a hollow body. The wood and the air inside respond strongly in some frequency regions and less in others, which makes the instrument louder and shapes its timbre.
• Air columns. In a flute, clarinet or trumpet, the air inside the tube is the resonator. Its natural frequencies, set by the tube's length and shape, decide which notes the instrument can play. Tone holes and valves change the length of the column, and so its resonances.
• The voice. Your vocal folds produce a buzz full of overtones. Your throat and mouth act as resonators, strengthening certain frequency regions called formants. Changing the shape of your mouth moves them, which is how you sing different vowels on the same pitch.
• Resonator tubes. The metal tubes under the bars of a marimba or vibraphone are tuned to each bar's pitch, making the notes louder and longer.

Sympathetic resonance

Resonance can also let one object set another in motion through the air. Sing a loud note into a piano with the dampers raised and some strings will answer, because their natural frequencies match frequencies in your voice. That case has its own name: sympathetic vibration.

Unwanted resonance

Resonance is not always welcome. A room can boom on certain bass notes because its dimensions match those wavelengths, a snare drum's wires can buzz whenever the bass plays one particular note, and a loose music stand can rattle on a single pitch. In recording, EQ is often used to tame a resonant frequency that sticks out.

Resonance in a teacher's vocabulary

Teachers also use "resonance" loosely for a full, ringing tone, as in "more resonance in your low notes". That usage describes quality, but it rests on the same physics: a tone that makes the instrument's body, or the singer's throat and mouth, respond well.

See how a piano's strings and soundboard work together in How the Piano Works.