Which material sounds better for speakers construction? Wood, Ply or MDF?

It helps to hear, literally, what is going on, by listening to the cabinet’s walls through a stethoscope, playing music and test tones. You find two things:

• Flexing of the cabinet walls allows low bass to come through. Out in your room, this adds in phase to the direct sounds coming from your woofer, making the overall presentation warmer, and adding bass ’ambience’ below 80 Hz. The bass is less tight, less defined, of course. Probably makes the hifi sound ’better’ at soft volumes.
  
  For low-bass flexing, braces help, granite helps, thick materials help, thin plywoods do not. Thin carbon fiber does not. Solid hardwood will split given time. Think here about maximizing panel ’stiffness’ or rigidity, not its ’strength’. Cement, concrete? Sure! FYI, paint the inside of wood cabinets with thinned-out wood glue-- as the surfaces of MDF and plywoods are porous, absorbing bass pressure whenever the woofer fires off.
  
  
• The other stethoscope discovery is that a cabinet lets voice range sounds, in the 200 to 300 Hz range, come right through. This makes for ’scratchy’ sounds in wood boxes and for ringing sounds in granite and metal. These vibrations do not come from the SPL inside the cabinet (unless there is a large, undamped (loud) standing wave inside). Those 200-300Hz vibrations come from the direct excitation of the cabinet material via the screws mounting ’that’ driver. When you loosen all its screws, all of a sudden, the walls go silent!
  
  One fix is to use rubber-mounted screws, but this makes bass impacts ’rubbery’. KEF and others tried this in the 80’s, giving it up after sales tanked from all that loose bass. The driver could be mounted to a regular inner cabinet with a vibration-isolated cabinet wrapped around it. Unfortunately, this leaves those inner-cabinet vibrations undamped, which get back to the driver, making its cone vibrate (= noise).
  
  When these tones get into the cabinet material via those screws, there is no way to damp them, and no way to brace against them, because they travel inside the cabinet material, not on the surface. If those screws cannot be rubber-isolated, the cabinet material needs to have ’high internal damping’, which is not a property of metal, cement, nor most woods. Cabinet thickness does not matter here, at least for making something to fit in a home.
  

For a home constructor, I recommend 3/4" Baltic birch plywood cabinets, or at least for its front panel, with braces inlaid 6-8 inches apart, center-to-center. But BB plywood is such a tough wood to work with and to make pretty! Two 1/2" BB layers glued up with Elmer’s Carpenter’s Glue made a front panel that was a bit more dead to those midrange tones getting into it from the screws, but not enough to justify the extra work. It made far more difference to put 1/4" wool felt on the front baffle to suck up the tweeter’s reflections before getting on with designing the tweeter’s crossover.

Roy Johnson
Green Mountain Audio

MDF can be affected by relative humidity, which in turn affects any attached veneer. One must totally seal all exposed MDF surfaces to render it immune to the affects of humidity. Kind of a pain. 

^Solid wood and plywood are even more susceptible. Solid wood, depending on treatment, will swell, or in the case of low humidity - crack. Plywood will begin to bend in as little as 60% RH. It takes very high humidity (>80%) to warp MDF - the level that would make for an unhealthy household.

I laugh when reading threads like this.  It's as though there is an absolute or holy grail.  All materials have different properties.  All speakers have compromises.  

Designers figure out what they want for their sound and build/design accordingly.  They do a ton of R&D if they are large enough and will use all different materials.  They can do modeling and figure out most of it on a computer.  I think this is one reason the DIY groups started to grow again after they were kind of losing steam a bit in the early 2000's based only on what I've been told (I may be wrong).

Folks will find fault with all speakers.  We all hear differently.  I'm glad that folks who have Magico's love them, but to many they do ring and are fatiguing.  I feel that Wilson's and their polymers and choice of drivers lose much of the micro and macro details, but boy are they dynamic.  I love listening to Harbeths, but again, they too lack much detail, but man are they enjoyable.  All use different cabinet materials.  

I own Vandersteen's and I choice them over nearly everything I could possibly audition that was under 35k or so.  He went the carbon fiber way years ago from house made cones to the cabinet.  I also know that he 'auditioned' all the different weaves of carbon fiber that he could get and choice a specific on based on it's sound quality. It's a very expensive cloth as it's teh same one used in my ultra high end walking poles (I have MS and have poles to get around inside buildings when I can't use my rollator).  

the bottom line is that some will find fault wiht the sound of the Vandersteen 7's as they will focus on his compromises, just like I get fatigued listening to Magico's as he's lifted the treble to make them sound more open on top etc... (if you measure speakers, many of them are lifted up to 3db in the high end to give them a more 'detailed and or larger soundstage".)

Again, it's all implementation and compromises.  There is personal preference, which makes this hobby a blast, but no 1 correct answer.