diff --git a/src/Envelope.cpp b/src/Envelope.cpp
index 0ba1a76..39de929 100644
--- a/src/Envelope.cpp
+++ b/src/Envelope.cpp
@@ -13,6 +13,7 @@
 /***************************************************/
 
 #include "Envelope.h"
+#include <cmath>
 
 namespace stk {