diff --git a/sources/user_problem.F90 b/sources/user_problem.F90
index 0398383..7263773 100644
--- a/sources/user_problem.F90
+++ b/sources/user_problem.F90
@@ -211,7 +211,7 @@ module user_problem
     ptot  = pres + pmag
     csnd2 = pres / dens
     csnd  = sqrt(csnd2)
-    valf  = sqrt(2.0d+00 * pmag / dens)
+    valf  = sqrt(bamp**2 / dens)
     lund  = valf / max(tiny(eta), eta)
     dlta  = lund**(-alpha)