January 18, 2011

Geddes foam experiment

Here is a quick test of Geddes-style foam. Above you can see my "Miniwave" speaker which is a test of using a waveguide loaded compression driver in a small format. It looks even less pretty with some dacron added. Before foam:

Why foam?

The idea behind this is to damp what Earl Geddes calls HOMs (higher order modes). It's a simple concept. In a horn or waveguide there is the intended axial sound as well as waves which reflect off the walls. Designs that use diffraction to achieve constant directivity create even more HOMs. If we insert foam, the HOMs will be attenuated more than the axial wave because they travel a longer path. In this way the foam is selective. It also has an impact on the response. I noticed a fair amount of top end roll off as a result, but it isn't difficult to correct. The end result is shown here:

The shaded area shows a +/- 3 dB window which has been achieved with the response at 1m. In this case I measured slightly off axis to show the kind of angle that I listen to. The green plot is measured in the listening position. As it's a room plot, it's not as smooth and you can see the treble response above the crossover (2k) is shelved down.

I have moved them closer to the wall and used some damping behind them which acts as a bass trap as well as a mdirange absorber.

My impression has been that the sound has improved as a result of the foam, although it's hard to isolate as I did some other changes. The speakers have a different sound stage due to increased distance. The damping has an obvious effect, particularly in the bass. The top end is very revealing and detailed. I'm starting to get the impression that dome tweeters as a general rule are veiled in comparison. One surprise has been that simply swapping the tweeter from a dome to a waveguide loaded compression driver has created a much more dynamic speaker. The mid driver isn't efficient and the subs are the same and yet the sound has become effortless.


  1. Is that the proper foam?

  2. Far from it - this is dacron but the proper foam is reticulated 30 ppi foam. I used it because I had it on hand and it didn't require shaping. I do plan to use the proper stuff.


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