Home Theater Forum and Systems banner

Damping sheets (CFD) for loudspeaker panels

1841 Views 5 Replies 2 Participants Last post by  deepthought
:dontknow:Apologies if this has already been covered in other discussions. I did a search for related terms and couldn't find anything in the results - so here goes.:sweat:

I am wondering if anyone has tried using any of the damping sheets (Dynamat, Megamat etc etc) on in their speaker projects and if so, has anyone attempted to quantify the "before and after" panel vibrations once their enclosure has been treated?
I am putting together a 3 way design at the moment using damping sheets from an Australian manufacturer that is a heavy polymer with a sound absorbing internally facing material. These guys supply to heavy industry and are involved in fixing issues with heavy transport vehicles so they test their products extensively. The sheet adds mass to the panel and of course reduces reflections in the enclosure though of course I doubt this effect will extend in any way to lower frequencies.

There are a lot of car-sound oriented suppliers of damping material out their that offer little quantitative figures on the performance of their products and perhaps rely on empirical data to design their product - though I am by no means stating that they don't work! The great majority of these products seem to rely on constrained layer damping (CLD) which consists of a dense polymer (some use bitumen but this best avoided) and a sheet of "damp" aluminium. I am interested in working out how effective the CLD products stack up against the D10 damping sheet. In fact I am interested in finding out whether the implementation of CLD in these sheets is really effective as their intended use is on metal panels. How do they stack up when placed on a thicker substrate such as MDF or Plywood for example which has different mechanical properties?

Some high end speaker manufacturers have used CLD but their implementation was quite different in that the layer of dense polymer or bitumen is sandwiched between layers of MDF or plywood. I would instinctively think that this would be a massively more effective way to affect damping the panels as the "stiff" layer is much more substantial that the very thin aluminium layer used in the above mentioned sheeting products. I don't have the necessary motivation to embark on a project like that at the moment so I'm looking at a sheet based solution.

I was thinking about getting a piezo instrument pickup then running it into my Behringer UMC204 and using REW to get an idea of the waterfall plots of a panel before and after the damping material has been applied. I know the pickup with inherently have an uneven sensitivity over the frequency range but I am probably going to use a subtractive process i.e. measure a reference panel with no damping, then remeasure with damping applied and subtract the 2nd measurement to see what dB attenuation has occurred and at what frequency.
In the time domain, REW allows you to overlay 2 waterfall plots to see the difference at a given time from the original signal.
So does anyone have any experience with this? I'm sure someone out there has used one of the CLD type products and if so, have you tried to quantify the before and after? Did you try different manufacturer's products to compare them? Or is it all just marketing hogwash! :dontknow:
See less See more
1 - 6 of 6 Posts
You just might be pioneering on this one, at least among our group. Your approach sounds well-thought-through, and we would love to hear your results.

I thought I'd ask here first. Even though I am member of some other DIY Audio forums, unfortunately the "tone" on them is not nearly so friendly as it is on this forum. I'll keep you guys posted. I am in the throws of preparing for an important presentation at work but once that's out of the way, I'm going to work out some course of action. :spend:
We are glad you think of it as a safe place. That is our goal. We all trip up once in awhile as we experiment, and it is nice to know we will be supported and assisted when that happens, not devoured or insulted.

I like the piezo pickup for comparative measures. An accelerometer of sorts? Should be fun.
So I finally got around to doing something about this. I bought an electric peizo pickup - a cheap unit with dual pickups that is used on acoustic guitars to amplify their sound. Not terribly scientific in that the units have a notoriously rising response, would not be anywhere near reference quality and as you'll note in the graphs, seems to have a nasty 50hz hump (self resonance?). Anyway the measurements are RELATIVE i.e. they are not intended to display absolutes - not that you could claim that for any measurement but less so in this case. It's about comparisons. To explain, I had a small cube printed up in PLA plastic as a test enclosure for my scanspeak 10F 4ohm midrange drivers. It also served as a perfect opportunity to see (empirically) what effects dynamat sheeting might have on plastic. Dynamat is of course intended to damping ringing metal panels I wanted to see whether they would have a quantifiable affect on other materials. Here is the pickup - I took care to arrange the pickup in the same locations on each test. I fixed them using a semi-permanent tape.

Measurements were done using my Rotel amp and a Behringer UMC 202. I set levels to 1volt using my true RMS multimeter.
I used a sweep pattern so that I could get impulse data.
The enclosure has a net volume of 0.9 litres. It consists of 8mm thick sides. The scanspeak driver was fixed securely using self-tapping screws.
Dynamat was fixed inside the enclosure to cover 5 internal faces but not the front as there was a very small gap between the opening and the enclosure inside faces.

These are the mildly smoothed response graphs. There was a lot of "fuzz" at higher frequencies so 1/48th cleared that up:

Note that the lower trace was taken under the same conditions as the top one. Above 300hz I am seeing anywhere between 0dB to 20dB less on the "damped" trace. I tried to get REW to give me a graph showing A-B but couldn't get the software to give me a meaningful result - perhaps I need some input here. Interestingly there seem to be higher levels of output from the damped test.
See less See more
Next up is a waterfall plot. I honestly don't have much experience with these but I think that they don't reveal much - perhaps someone can get more out of these than I could!

I also used REW to show me the total THD of the 2 scenarios. I couldn't get REW to overlay the distortion graphs but hopefully and A/B comparison will reveal the differences:

I guessed a median for the THD in both tests. The un-damped enclosure read about 12db higher than the damped one.

In conclusion, through my very unscientific test I have at least demonstrated to myself that the dynamat is of use in non-metallic panels. Will this be as good as CLD? No, probably not. But it does suggest a good method to tame resonances in speakers that is less complex than CLD is. BTW, I am very happy to take suggestions about ways to improve my measurement techniques.
See less See more
1 - 6 of 6 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.