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Sound & Vibration

Modal & Resonance 101

Why machines have favourite frequencies — natural frequency, resonance, mode shapes and the FRF.

Sometimes a machine vibrates far more than the forcing on it should explain, and the culprit is resonance — the structure has a favourite frequency and something is exciting it. Modal & Resonance 101 explains why every structure has natural frequencies, set by its mass, stiffness and damping, and what happens when a forcing frequency lands on one of them. This is the concept that separates analysts who fix vibration from those who merely re-balance a machine again and again without success.

You will learn to read mode shapes — the patterns in which a structure prefers to deflect — and the frequency response function that maps how a structure answers an input. The course puts a practical tool in your hands: the bump or impact test, which reveals natural frequencies quickly on site so you can tell a genuine resonance from an ordinary forcing-frequency fault. Finally you will learn the three levers for detuning a resonance — add or remove stiffness, add or remove mass, or add damping — so you can move the problem out of the operating range rather than living with it.

What you'll be able to do
  • Understand natural frequency, resonance and damping
  • Read mode shapes and the frequency response function
  • Run a bump (impact) test to find natural frequencies
  • Recognise resonance versus a forcing-frequency fault
  • Detune a resonance by mass, stiffness or damping
Curriculum
  1. 01 Natural frequency, stiffness, mass and damping
  2. 02 Resonance — what happens when forcing meets a natural frequency
  3. 03 Mode shapes and how a structure deflects
  4. 04 The frequency response function (FRF) and transfer functions
  5. 05 The bump test — practical impact testing
  6. 06 Detuning a resonance — stiffen, add mass or damp
Go deeper

Written for this course's subject, free to read, and none of it behind a form. Each one goes further than a primer can.

What the measurements are taken with

The instruments and rigs this subject is measured on, in case you want to see the hardware behind the worked examples. Nothing here is required to take the course — it is free and self-contained.

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