different materials for bracing and walls?

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morbo

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Joined 2004
#1
I just came across this quote searching through some BASSlist digests:

Brace Material

Bracing material must possess qualities very different from side
wall material. The purpose of a brace is to prevent the side
walls from bowing sypathetically with the woofer. Therefore, the
job of a brace is to remain dimensionally stable under the forces
of tension and compression.

It was shown by Tappan in 1962 that solid soft wood is not
suitable along its width for bracing material (ref. 1). Modern
composite boards, such as particle board and MDF, possess far LESS
tension and compression strength than solid soft wood, and
therefore make far POORER braces.

The correct material to use for internal bracing is
multidirectional plywood. Plywood is strong in any direction
under tension, because wood does not stretch along the grain.
Although not as strong under compression, it is still superior to
soft wood and far superior to composite board.

Ref. 1: Tappan, "Loudspeaker Enclosure Walls", JAES, July 1962
Click to expand...


It sounds like the authors are talking about fullrange enclosures (as opposed to bass cabinets, where my understanding is that plywood is preferable anyway), and suggesting MDF walls with ply braces. I personally have been a bit skeptical of the strength of MDF 'windowpane' braces I made, I was able to get them to bend fairly easily. Does anyone use different material for braces and walls?
 
#2
IMO, it's always a good idea to mix'n'match your materials in a speaker cabinet - but don't pass off MDF too lightly. While it's modulus of elasticity is generally half (or less) than that of wood or plywood, it's actual strength can be the same or greater. It all depends on the manufacturer.

If you're using 'windows' (apologies to Bill), then you might try 2 x 1-inch dowels instead. You'll get more strength, and less reflections. They're a pain to fit sometimes - but that's one of the joys of DIY. If you're planning to take off one wall of the cabinet (regularly) while developing - then use a substitute with rubber 'feet' until you're ready to seal it.

I tend to use 1 inch mdf with 2/3 inch holes drilled in it occasionally - sometimes braced 2 ways, somtimes 3. And I generally use a scupltured piece of the 'spine brace' to support the magnet of the speaker.
 
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