Acoustic Phenomenon
Acoustic phenomenon describes the variation in the velocity of ultrasonic waves as they travel through a thin material of a specific thickness. In the testing of paper and board, lamb wave dispersion reveals the elastic stiffness of the sheet without damaging the sample. The waves propagate along the plane of the paper, and their speed changes based on the frequency of the pulse and the mechanical properties of the fibers.
This relationship allows for the calculation of the modulus of elasticity in both machine and cross directions.
Sensor Application
Sensors placed at a set distance apart capture the time of flight for the acoustic energy to traverse the web. Because the signal follows the principles of lamb wave dispersion, the resulting data provides a high resolution view of the internal bond and the sheet density. This non destructive method replaces traditional tensile tests in some high speed production environments to allow for real time monitoring of quality.
The setup uses transducers to inject energy and receivers to pick up the resulting vibration.
Thickness Sensitivity
Thickness sensitivity makes the effect highly dependent on the ratio between the wavelength and the thickness of the board. For very thin papers, the wave behavior simplifies, but thick multi-ply boards require complex mathematical models to extract the correct stiffness values.
