August 14, 2012

Waveguide damping

This is how I add damping to plastic waveguides to improve their performance. I use bitumin rubber which is mainly used for waterproofing applications. It has mass, adhesion and flexibility which make it work here also. 

Raw plastic waveguide:

To ensure good adhesion, the plastic surface is scratched up with coarse sand paper:

Then the bitumin rubber is diluted and applied as a primer:

When it dries, the surface is slightly rough and has the kind of texture that easily bonds to successive layers. Masking tape is applied.

After a few hours, a thick first layer is applied with a metal paint scraper and a brush.

Then a second layer includes fine screenings, which add greater mass. The screenings are cheaper than the bitumin rubber, ensuring that it goes further, but they also improve the performance. 

A third layer seals up the mix of screenings and bitumin rubber. This locks them in the middle. Without a layer underneath, they can tend to lack adhestion. Without a layer on top, they can tend to come off.


  1. "improve the performance"

    Got any measurements?

    Not because I think your claim is invalid without them .... Just because I'm interested, and am a sucker for a pretty graph ;-)

  2. Dave, at the moment, R&D on this one consists of the very scientific knock test, which suits the budget for now. It's not ideal but it does give a window into the sonics. You hear the expected plastic sound, lacking in stiffness but having a degree of damping. After about 3 layers as shown, the weight is increase from about 240 grams to 540 and now subjectively it sounds more inert than 18mm MDF. So the point of the exercise is to avoid the waveguide becoming a bottleneck. I believe it has been achieved.

    Down the track I'd like to do some proper measurements - one of many things on my to do list!

  3. ad my vote to doing some measurements of treated vs non treated waveguides. just to get some idea of what parts of the spectrum they effect and how. I'd like to know what 'plastic' and 'inert' mean in terms of the response.


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