diff --git a/sources/user_problem.F90 b/sources/user_problem.F90
index c2233cf..2cb2f2c 100644
--- a/sources/user_problem.F90
+++ b/sources/user_problem.F90
@@ -145,7 +145,7 @@ module user_problem
       csnd2 = pres / dens
     end if
     csnd  = sqrt(csnd2)
-    valf  = sqrt(2.0d+00 * pmag / dens)
+    valf  = bamp / sqrt(dens)
     lund  = valf / max(tiny(eta), eta)
 
 ! determine if to append or create another file