I used to own a live sound production touring company, and all our custom boxes were built with MDF , and then an outer shell of baltic birch plywood glued and fastened to the MDF. Effectively this becomes 1.5 inch plywood. Rigidity is important for low frequency enclosures as the Q of the box is lowered considerably with "flappy" enclosure walls. Using this method of construction , you get the advantages of plywood and MDF.
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