diff --git a/lrftubes.lyx b/lrftubes.lyx
index 2d9ff05..8ca5117 100644
--- a/lrftubes.lyx
+++ b/lrftubes.lyx
@@ -5553,7 +5553,7 @@ p=\frac{C_{1}\exp\left(-i\Gamma x\right)+C_{1}\exp\left(-i\Gamma x\right)}{r_{0}
 
 \begin_layout Standard
 \begin_inset Note Note
-status collapsed
+status open
 
 \begin_layout Plain Layout
 If we assume 
@@ -6258,6 +6258,33 @@ Z_{c,0}=\frac{\omega\rho_{0}}{\left(1-f_{\nu}\right)S_{f,0}\Gamma_{0}}
 
 \end_layout
 
+\begin_layout Standard
+and 
+\begin_inset Formula $\Gamma$
+\end_inset
+
+ is defined in eq.
+ 
+\begin_inset CommandInset ref
+LatexCommand ref
+reference "eq:Gamma"
+plural "false"
+caps "false"
+noprefix "false"
+
+\end_inset
+
+ and can be approximated as 
+\begin_inset Formula $\Gamma\approx k$
+\end_inset
+
+ under some undefined and guessed circumstances AANVULLEN, in which 
+\begin_inset Formula $k=\frac{\omega}{c_{0}}$
+\end_inset
+
+ .
+\end_layout
+
 \begin_layout Section
 Prismatic lined circular duct
 \end_layout