A loudspeaker emits sound vibrations on both sides of its membrane. Vibrations are transmitted inside your speaker.
These vibrations can resonate and provide a mask in the low and midrange. Some will like this warmth, others will find it rough. From this will depend the amount of damping to put in the enclosure.
There are two types of damping: one that will reduce the vibrations of the air, and one that will reduce the vibrations of the body. The first is a foam more or less dense, the second is a material that will weigh down your cash.

The minimum is to have no internal surfaces facing each other without damping (foam). It is therefore necessary to at least cover three internal faces of your enclosure damping foam

Internal speaker damping: carpet, mouse,