Is damping material necessary for a speaker having such a small cabinet volume?

I am confused.

Are you talking about in-wall speakers? or wall mounted speakers?

Well,it's an in -wall speaker mounted on my garage wall... this wall is framed with 2" by 4" at 16" on center...  so it gives me a 14-1/2" by 18-1/4 by 3-1/2" deep enclosure inside measurement...

My question is : is it necessary to add some damping material in an enclosure that small.. there are many opinions saying that it could help to improve the sound quality while some others would say that a small speaker enclosure does not need any damping material.. where should i draw the line between a small and a BIG speaker

Wait.. Is this something you bought designed to be wall mounted? if so, follow manufacturer's recommendations. 

In a speaker enclosure, damping material lessens reflections, but also increases internal volume, so it depends.  I have seen some delightful, tiny speakers use internal wadding, to good effect. 

Best,

E

GR Research sells a cabinet wall damping material named NoRez. It has a thin layer of mechanical damping material, with a 1" layer of acoustical foam, and attaches to the interior cabinet walls with self-sticking adhesive.

Generally speaking most speakers have way too much fiberglass or polyester, whatever stuffed into them, who knows why? It might be a case of monkey see, monkey do. Excellent results, in terms of bass performance, dynamics, and open transparent sound, can be obtained by removing all stuffing save for a small grapefruit size ball, depending on volume of the speakers. For small speakers I would give consideration to removing all (rpt all) stuffing. Hollow fiber wool is the best material for such applications.

A-did you glue or caulk the 2x4 studs so no air leaks?
B-is the drywall double layer and taped on the seams?
C-is the drywall glued or caulked?
D-What's on the outside wall
E-Personally I would have used double layer drywall, liquid nails and decking screws to build the wall before even trying it out.

Box construction is vital.  You may have just made a big bass drum.  Also, if it's leaking, that will wreak havoc on the sound and probably leak noise to the rest of the house.  This is why most quality in-wall speakers come built into aluminum enclosures.  The manufacturers can easily design a speaker if the cabinet is a known source.