diff --git a/docs/source/api_Acous.rst b/docs/source/api_Acous.rst
index 30749a9..f11063d 100644
--- a/docs/source/api_Acous.rst
+++ b/docs/source/api_Acous.rst
@@ -171,7 +171,7 @@ By parameterizing the physical coordinate as
 
     x<0 \ : \ \text{real axis}
 
-    x>0 \ : \ \Im[\tilde{x}] < 0 \text{decaying term}
+    x>0 \ : \ \Im[\tilde{x}] < 0  \ (\text{decaying term})
 
 Another core treatment of PML is decomposing \\(\\phi \\) as: