March 9, 2011

Waveguide foam mod

Do you have horn or waveguide based speakers?

I'm sure you've noticed how they can at times hit you with a harsh sound. It isn't related to over-driving them, but this problem occurs almost randomly, even at levels well below their maximum output. Dr Earl Geddes has put forward that this effect can be described as higher order modes, or HOMs. They are reflections within a horn or waveguide. The solution he has proposed involves filling the device with reticulated foam, often called "filter foam." It differs from the more common open cell foam in that it has larger pores so there is less attenuation, but the concept is to selectively attenuate the HOMs without too much loss of efficiency. Having now tried this modification to waveguides I can say quite emphatically that it works. Once modified in this way, a waveguide with an obnoxious sting is tamed to the extent that it rivals a dome tweeter in smoothness and lack of listening fatigue. However, the efficiency is about 20 db greater and this combination can achieve far better controlled dispersion characteristics.

Three steps are involved:
  1. Determine the horn/waveguide shape.
  2. Cut the foam to shape.
  3. Measure the response and modify the crossover to compensate for the loss

How to cut the foam

Various methods have been proposed, however the most effective appears to be using an electric hot knife to sculpt the foam into shape. Axi-symmetric units are the simplest, as they can be placed on a turntable. In my first attempt, I derived the contour with a process of measuring, trial and error. I ended up with a template for the contour. I have used a soldering iron set to about 420 degrees. First, I cut the ellipse for the end that is seen and that must be neat. I had a template made to ensure it was neat, then traced with the iron around the curve. It was a slow process and since I don't plan to make a large number of these, I have used patience and a certain amount of skill rather than building a cutter. Most of the foam will not be seen.

Here the foam is inserted into an 18 Sound waveguide in what is a deluxe version Econowave. 

Front view

Rear view seen by the compression driver.

Confused about the foam? What is the difference between open, closed and articulated foam? There is a good description here:


  1. Hey Paul

    Can I ask where you got your foam from, as I'd like to get some myself.

    Phil (LD)

  2. I have another page about where to get the foam and I've added the link to the end of this post.


All comments are moderated.