diff --git a/Appendix-D:-Hardware.md b/Appendix-D:-Hardware.md
index ac5ac51..a1a497b 100644
--- a/Appendix-D:-Hardware.md
+++ b/Appendix-D:-Hardware.md
@@ -57,13 +57,15 @@ Details:
 Details:
 
 - 3/4" thick
-- Lowest possible pressure to compress you can get
+- Lowest possible pressure to compress you can get (a PSI of 1.5 to compress 25% or better is recommended)
+- Polyurethane or similar material (different materials may result in different acoustic properties)
 
 ## 1/8" Foam
 
 Details:
 
 - 1/8" thick
+- Polyurethane or similar material (different materials will result in different acoustic properties)
 
 ## Magnets