Question for Dr. Geddes about his WG foam plug

[NV&H]

I've read on your website a brief description and purpose of the open cell foam that you use in your wave guides. I'm looking for a little more information.

Are the foam cells random in size/distribution or are they ordered in such a way as to scatter (or perhaps absorb?) sound at certain angles to combat unwanted reflections and NOT scatter at other, more favorable, angles?

Is this discussed in any of your publications? I'm on the verge of buying your waveguide book. If this is discussed there, just might push me over the edge :-)

[gedlee]

There is no published information on the foam plugs and my book pre-dates its use.

Basically the pores in the foam are a random distribution of sizes arround a certain porosity usually defined as PPI or Pores Per Inch. I use 30 PPI, but it ranges from about 10 to about 100. The pores are indeed random as far as the wavefront is concerned so there is no "spectral" diffraction effect like you are talking about. There is some viscous loss in the material, but it is really pretty low, especially at LFs. If you hold a piece of it up to your ear it is very difficult to hear any effect. It is also difficult to measure its effect, but there are some results posted on my web site in one of the ALMA papers.

Its an interesting phenomina that something that has such a small measureable effect can have such a pronounced audible effect. This has interested me for years and I have some ideas, some of which we have published (also on my web site) and some which I have not talked about in public yet. But I am convinced that this fundamental lack of obvious correlation between the objective effects and the subjective ones is going to be a key issue for loudspeaker design in the future.