PLYWOOD vs MDF For Speaker Building - Tests Show Surprising Results

NOTE: This is to clear up some confusion over what I meant about bracing and damping:
Speaker building was always bracing and damping. But at some point the idea started that bracing alone was better, just using a lot more of it. Guys were still doing the traditional bracing and damping (like me), but you also started seeing these bracing only builds happening. The idea was to drive up the panel resonance high enough so that it wouldn't be excited and therefore wouldn't be a problem
I'm speaking 100% from a DIY standpoint, not from what the major manufacturers were doing.

Will making the entire box from the same material make a difference? Possibly, but I did what I could to take the rest of the box out of the measurement. For example the box was clamped down to my very heavy and stable workbench to keep it as stationary as possible, and help restrict the measurement to the panel itself. Yes the other panels were still vibrating and in turn that can affect the panel being tested, but it's the same for each of the different panels.
Each panel was exactly the same size - 8-1/2" x 12" with the screw holes in exactly the same places.
Why 30Hz to 2000Hz? I did some "warmup" testing prior to the measurements I made to determine the best range that shows the most relevant data. 30Hz because the speaker I used doesn't have significant output below 30Hz.
I used my ACH-01 accelerometer connected to REW to run the measurements. I made a special preamp for that accelerometer in this video:    • Making a Preamp for an Accelerometer ...  

My conclusions are that the material used is not a significant factor in how the speaker will sound. As long as what you are working with is of reasonable quality, you should not be able to hear the difference.
You may be able to imagine you hear the difference, though, and the human psyche is a powerful thing to overcome. If you believe MDF sounds dull and sluggish, and Baltic birch plywood sounds lively and fast, then it'll be very difficult to get past that and you should use the one you think will sound best.
As for bracing vs damping, I think both are valid. Use bracing to reduce panel size so that the damping will be more effective. The rationale of those that prefer bracing alone is that the damping "smears" the response. Much like above, it'll be hard to get past this idea if it's what you believe, so best to go with what you think is better way.

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