Trigonometric location techniques such as triangulation is based on the Time of Arrival (ToA) of the waves. The difference in arrival time of an acoustic-emission wave at a pair of sensors represents the difference in the distance of the emission source from the two sensors. The emission source location can then be represented by a hyperbola defined by the difference in the ToA with the two sensors as foci. By having an array of three sensors, the intersection of the pair of hyperbolae defines the location of the emission source.
In isotropic structures the velocity of the wave is constant in all directions, and triangulation is achieved with three sensors. For anisotropic material, it is possible to obtain velocity characteristics as function of the angle of the wave propagation to estimate the location of the impact combined with triangulation techniques. However, to use this approach it is required to reduce the number of unknown variables to the number of equations such that each pair of sensors will experience the same group velocity, i.e. placing each pair very close to each other. This is a limitation for complex structures such as fuselage panels where stiffeners and frames are present.
