- To avoid resonance, the natural frequency of a system must differ from the input frequency
- When considering the input frequency it is essential to understand that any non-sinusoidal wave can be deconstructed into a series of harmonics through Fourier’s analysis:
- Each harmonic has increasing frequency and decreasing amplitude
- In general, to avoid resonance, it is vital to ensure that the natural frequency of a system is greater than the 8th harmonic of the input system:
- An invasive arterial blood pressure trace is assumed to have a maximum fundamental frequency of 3 Hz (equivalent to a heart rate of 180 bpm)
- The natural frequency of the measurement system must therefore have a natural frequency of >24Hz (above the frequency of the 8th harmonic) to avoid resonance
- The equation below determines the natural frequency of a system, where k is stiffness and m is mass.
- In practice, using invasive blood pressure monitoring as an example, the natural frequency of a system can be manipulated by changing the
- Stiffness of the system: radius of tubing, elasticity of tubing material
- Mass of the system: length of tubing, density of fluid