Yes for home audio you have a small amount of acoustical dampening. because in home audio they usually have very little power and are designed to play in a large non acoustical environment. We are talking about car audio. With a ported box its designed to allow for the back wave to be utilized, so if you put something that removes the rear wave you have just made a sealed box with a hole... as for the fiberglass comment. MDF the should be used wood for building boxes isn't airtight so air will pass thru the box. adding a small amount of fiberglass will seal up the boxes porous wood and also allow the air movement to be channeled to the port. with having the vibrations being forced thru the port and not the wood, will allow the " fiberglass" to be used to create a better airflow.