Physics, visually · Waves, Sound & Light · Concept 09
Sound and Resonance
Why can a singer break a glass, and why does a bridge or a machine sometimes shake violently at one particular speed?
Pitch is frequency, loudness is amplitude. And every object has a favourite frequency that makes it shake wildly.
- Tone tab. Press Play (turn your volume down first). Slide the pitch up and down. Higher frequency, higher pitch.
- Raise the loudness: the wave gets taller, the pitch does not change.
- Resonance tab. Slowly slide the push frequency towards the natural frequency (the gold line). The mass swings wildly right at it. Lower the damping to make it dramatic.
Pitch = how fast. Loud = how tall. Resonance = push at the right beat.
Frequency (Hz) sets the pitch; amplitude sets the loudness. Every object has a natural frequency. If you push it at exactly that rate, each push adds to the motion, like pushing a child on a swing at the right moment: resonance.
Motor and fan vibration
If a motor runs at a speed where its turning frequency equals the frame's natural frequency, the whole machine shakes. VFDs have "skip frequencies" to jump past these speeds.
Bridges and buildings
Wind or marching feet at the natural frequency can build up huge swings. Dampers are fitted to take the energy away.
Ultrasonic cleaners and sensors
They drive a piezo crystal exactly at its resonance to get a strong signal from a tiny voltage.