Thiel Owners

If I were going to try to reinforce an mdf box the first thing I'd look at is porcelain floor tiles.  They're incredibly stiff and tough.  They don't resonate much but what little they do is at a high frequency.  I'd think combining them with mdf would be an ideal combination.

jon - you're on it. When developing the CS5 cabinet I explored custom ceramic tiles for the interior, attached with a researched mastic adhesive - it worked extremely well. There were two big problems. 1: it was too heavy to ship and handle. Dealer home delivery was the deal breaker. 2: the tiles were not always flat and the ratio and durometer of the adhesive layer would change the tuning of the enclosure depending on adhesive thickness variability, thereby changing the final tweak voicing of the system.

For your in-place situation, #1 may not matter. #2 is a small issue (and no, I don't remember adhesives options or winner 30+ years on.) It's worth exploring. Also, you will change the acoustic volume of the enclosure, but it won't matter a lot. I suggest finding the most active surfaces with your fingers or stethoscope and treating those. High likelihood includes the top and halfway up the sides. Please let us know what you do and how it works.

A friend of mine has a patent on the selection of travertine stone to be used as an acoustic  sink for the endpin of cellos. The right tile removes the wolf tones present in almost every cello..You may research travertine stones and cut as Tom suggested. Debbie’s website is Cellostone.com

With the absence of the original Cascade Vbloc. I will experiment with quick set cement and mix in a quart or more of Micro Bearing steel and trough on like thin set. Careful to measure and match the amount laid out so the stereo pair have the same volume. Let thoroughly dry for a few days and then carefully vacuum out what may have fallen. Tom

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I will experiment with the mix on some plywood first and see how the coating hangs on. After drying I may have to spray coat with a low shear acoustic paint I have been working with as a sealer over the Micro Bearing. Tom

The most interesting German Physiks speaker line (I'd love to hear a pair!) use interesting cabinets, damping and drivers(!).
 
German Physiks Carbon Mk IV Loudspeakers — German Physiks (german-physiks.com)