Table 5.

Impedance properties of LEP candidate materials. Reflected and transmitted waves as a function of the incident stress at the surface (liquid–coating) and at the interface (coating–substrate).

Material Combination: Coating–Substrate	ZL (kg/m2s)	ZC (kg/m2s)	ZS (kg/m2s)	$\frac{{\mathit{\sigma }}_{{\mathit{R}}_{\mathit{L}\mathit{C}}}}{{\mathit{\sigma }}_{{\mathit{I}}_{\mathit{L}\mathit{C}}}}$	$\frac{{\mathit{\sigma }}_{{\mathit{T}}_{\mathit{L}\mathit{C}}}}{{\mathit{\sigma }}_{{\mathit{I}}_{\mathit{L}\mathit{C}}}}$	$\frac{{\mathit{\sigma }}_{{\mathit{R}}_{\mathit{C}\mathit{S}}}}{{\mathit{\sigma }}_{{\mathit{I}}_{\mathit{C}\mathit{S}}}}$	$\frac{{\mathit{\sigma }}_{{\mathit{T}}_{\mathit{C}\mathit{S}}}}{{\mathit{\sigma }}_{{\mathit{I}}_{\mathit{C}\mathit{S}}}}$
LEP–GFRP	1.48M	0.09M	5.65M	0.882	0.118	−0.968	1.968
LEP–Filler	1.48M	0.09M	2.83M	0.882	0.118	−0.936	1.936
LEP–Primer	1.48M	0.09M	2.28M	0.882	0.118	−0.922	1.922
Primer–Filler *		2.28M	2.83M			−0.107	1.107

* The relative acoustic properties are not considered on the liquid–coating surface and the primer acts as a first substrate layer instead of the coating layer and the filler as the second substrate layer.