Structural Resonance

Inspiration

It all began with playing guitar and exploring harmonics—how frequencies play a role and the physics behind them. Then came the discovery of dancing bridges, and even more beyond that…

Structural resonance

According to physics, everything in the universe vibrates at some level. This is rooted in the fact that all matter is made up of atoms and subatomic particles, which are in constant motion. Even seemingly solid objects have atoms that vibrate due to thermal energy.

Structural resonance happens when a structure vibrates in response to external forces at its natural frequency. This match causes the structure to vibrate more strongly, which can lead to damage.

Resonance also occurs at multiples of the structure's natural frequency, called harmonics. For example, if the natural frequency is "f," the structure can also resonate at 2f, 3f, and so on, though usually with less intensity.

The reason of the resonance coming from the transfer of energy. When external vibrations match the natural frequency, energy is transferred efficiently to the structure, making it vibrate more. Each vibration cycle aligns with the structure's natural oscillations, reinforcing them, this builds up over time. As a result, even small forces, when applied at the natural frequency, can cause large vibrations, sometimes leading to damage.

Real-World Examples

The Tacoma Narrows Bridge in the gif above shows how wind caused the bridge to vibrate at its natural frequency. This resonance amplified the motion, making the bridge twist wildly until it eventually collapsed.

During an earthquake, the ground moves in waves, creating seismic waves that travel through the Earth's crust. These waves have different frequencies depending on the earthquake and the area's geology. If a building's natural frequency is similar to that of the seismic waves, the shaking can become amplified, causing more intense vibrations.

In instruments

In musical instruments, the body or structure vibrates at certain frequencies, amplifying the sound created by vibrating parts like strings or air columns. This helps shape the tone and volume of the sound. For example, when a guitar string is plucked, it vibrates at a frequency based on its tension, length, and mass. The guitar’s body - the soundboard and hollow interior resonates at frequencies that match the string’s vibrations, making the sound louder. This resonance also adds richness and affects the guitar’s tonal quality. The same principle applies to other stringed instruments like the piano and violin, as well as wind instruments (e.g., flute, trumpet, saxophone) and percussion instruments (e.g., drums).

In addition to the design of the instrument's body, harmonics (multiples of the fundamental frequency) are used to create unique sound effects. Take the guitar as an example: when you pluck a guitar string, it doesn’t just vibrate as a whole—it also vibrates in segments, producing higher-pitched overtones or harmonics. By lightly touching the string at specific points without pressing it down fully, you can isolate these higher frequencies. For instance, touching the string at the halfway point (the 12th fret) produces the octave harmonic, which vibrates at twice the fundamental frequency. Similarly, touching the string at one-third or two-thirds of its length yields other harmonics, adding texture and depth to the sound.