diff --git a/SFS_time_domain/wfs_preequalization.m b/SFS_time_domain/wfs_preequalization.m
index 9af427e0..3815e6b3 100644
--- a/SFS_time_domain/wfs_preequalization.m
+++ b/SFS_time_domain/wfs_preequalization.m
@@ -80,7 +80,7 @@
     % get IIR filter
     hpre = wfs_iir_prefilter(conf);
     % apply filter
-    ir = filter(hpre.b,hpre.a,ir);
+    ir = filter(hpre.b,hpre.a,ir,2);
 else
     error('%s: %s is an unknown filter type.',upper(mfilename),hpretype);
 end