diff --git a/watpy/wave/wave.py b/watpy/wave/wave.py
index 9ad911de75b0aff1875402f0da831336c209733c..9511ebc05069cd02cbf82a338a7b65df24f4f0c6 100644
--- a/watpy/wave/wave.py
+++ b/watpy/wave/wave.py
@@ -618,7 +618,7 @@ class wave(object):
         """
         if self.prop['var']=='Psi4':
             if fcut < 0. :
-                fcut = 2 * self.prop['init.frequency'] / max(1,abs(self.prop['mmode']))
+                fcut = max(1,abs(self.prop['mmode'])) * self.prop['init.frequency'] / 2
             dt = self.time[1] - self.time[0]
             return win * fixed_freq_int_2( win * self.p4, fcut, dt = dt)
         else: