If the speaker is already mounted in a box (closed box) then its back is also closed automatically. You need to paste some sponge (sound absorbing materials) on the inside walls of the box.
If the speaker is not mounted at all, it will sound horrible because of the interference (front and back) of the sound waves. The speaker cone separates the front and back effectively and must be mounted with air seal.
Everytime the cone moves forward, it creates a compression wave in front and a rarefaction wave in the back. If these two different waves are allowed to interfere, they will cancel (phase permitting) each other.