I haven't used dynamat, mainly because I could not source any. I also knew it's pretty expensive, so I used a different but similar material.
I found that several of my KEF loudspeaker enclosures have pieces of bitumen like sheets covering parts of the inside panels. Small rectangular pieces, the size of an 8" woofer each. Of course, after 30+ years they had been altered to a form more brittle than elastic, so I consider them gone.
I found nice rolls of bitumen sheets in construction suppliers. There are two kinds of sheets. The ones with small grain gravel encased on one side (these are no good) and the more suitable ones with plain bitumen material. They are heavy and somewhat elastic, more plastic I should say.
I made an experiment enclosure, around 200lt big by external dimensions, using 3/4" plywood. I enclosed a small enclosure in that with a 12" woofer in that. I then did experiments of measuring the output at various frequencies. I should not be getting any output (as the outer enclosure was fully closed) but of course I did get a slightly muffled version of the output of the woofer.
I made three main tests by applying up to three layers of 3" thick sheets along all internal sides of the panels. Not on some places but all around.
The results was very good. Each new layer decreased the "parasitic" output by a couple of dB, more at the resonance frequencies of the enclosure. The third layer decreased the overall output by about 20dB, more at the resonance points that showed a slight drop in frequency as well as new layers were added. I attribute the drop in resonance frequencies to the increased mass of the enclosure.
I stopped at the third layer as it showed a decreased effect compared to the second layer which in turn showed a relatively smaller effect compared to the first layer. And three layers increased the weight by a lot so I had to stop, really.
This lining is not meant to replace the fiberglass or mineral wool stuffing. The lining is there to reduce panel resonance and emission through the panels. The stuffing is there to tame standing waves (which also helps reduce parasitic emissions) as well as increase the effective volume of the enclosure as a load to the woofer. Enough stuffing can achieve up to 25-30% increase of the effective load volume. However, when treating a product loudspeaker, you should observe the stuffing added by the manufacturer as they probably have considered how much was needed.
Apart from the test enclosure mentioned above, I treated my pair of KEF R105, a pair of Calinda and Carlton III. Of course after removing the original few pieces of sheets that were mounted there that broke into pieces.
One thing I should mention is that I didn't use just staples, (like KEF originally used) but applied contact glue to the entire surfaces to be glued and then mounted the sheets, applied pressure with a roller and finally staples using a staple gun. That was done to ensure full contact between the two and to make the wooden panel and the bitumen sheet act together as one lump.
The results have been great in terms of sound quality. The sound is clear, transient response (kick drum and upright bass strings) sounds cleaner and I also realized the loudspeakers in their original form had some element of boxiness in their sound. Nothing I noticed before but once you listen to the difference it's something that's easy to identify as ... missing.
There is a caveat however. Three layers of bitumen sheets on the 200lt box made for a really heavy box. The 105s have become a lot heavier as well, but manageable. The 50lt Calinda / Carlton are much more easy to handle after being treated. Also, measuring and cutting the bitumen sheets to properly fit into the "compartments" created by asymetric and diagonal braces is a small challenge but I used paper to create templates so it was doable.
If you add three layers (i.e. 9mm thick) you should also consider they take up some space that was meant for loading the woofer, so in turn I increased the stuffing density with some more baf material to compensate.