Felt or foam walled waveguide?

[dabbler]

Having successfully tamed the sometimes bright response of the Scanspeak HDS tweeter using a thick elliptical wool-felt ring has perked my interest in the idea of using foam or felt waveguides.

Lipinski uses such a device in their Sound 707 speaker and the measurements indicate that it works quite well at controlling dispersion and improving decay times. I think the latter has to do with the absorption effect of the acoustic foam walls.

I have some 100% wool felt sheets (5mm) and access to a laser cutter to experiment with my own version of this. The proposed elliptical waveguide is shown in yellow surrounding the recessed tweeter in the attached sketch. The “contour” lines are the 5mm steps that form the waveguide.

I’d appreciate some comments from those who have tried something similar. Im not quite sure if this will work but It should be a fun experiment.

[dabbler]

Sketch attached

[bear]

Measure.

Try.

Measure.

Compare.

Listen.

Report back to the mother ship. Post pix.

_-_-bear

[speaker dave]

Looking at the Lipinski picture and curve I wouldn't call that a waveguide by any stretch. It is a recessed tweeter in a well damped cavity.

It seems to work well and the overall system design is commendable with good blending between units and great lateral curves. My guess is that he wanted the recess to get best phase allignment through crossover, rather than to go for any particular directivity.

I think your sketch falls into the same category of a recessed tweeter in a damped cavity. As such the design challenge is the absorption of the surrounding material. Get that right and the contour of the recess becomes irrelevant. Energy radiating at wide angles will be absorbed rather than constrained along the contours of a waveguide.

David S.

[weltersys]

The design probably requires that "specially designed Belgian foam" ;^).

[gedlee]

Conceptually the idea has merit and it was patented decades ago in Japan. The problem is the size. Thin layers of felt or foam don;t really do much especially at lower frequencies, so to be really correct the device has to be pretty big. The Lipinski picture is just a different form of using baffle foam. Although

I wouldn't call that a waveguide by any stretch

its as much of a waveguide as other examples that I have seen called "waveguides".

[speaker dave]

Gee, Earl, I thought I was on your side for once, trying to stop the errant overuse of the term "waveguide".

I remember a couple of systems back in the 80s, during the great time alligned loudspeaker scare, that used the same approach.

If the absorption is high does it change the axial response? It seems like the tweeter would think that it was in less than 2 pi if lateral energy was immediately absorbed.

David

[_Wim_]

Wouldn't it be possible to make a waveguide out of foam completely? Ideally maybe varying the foam density from inner to outer layers?

[speaker dave]

I'm still questioning whether that should be considered a waveguide in any sense.

If the notion of a waveguide is that it is a boundary of a particular profile, that waveforms propogate through it and their wavefronts are (more or less) perpendicular to its rigid boundary surface, how do we make the jump from that to a primarily absorptive device?

If the walls are sufficiently absorptive, do the contours matter at all? Isn't it a well damped recess with radiation only through the unobstructed tunnel of the particular shape and absorption outside those boundaries?

Not the same.

David S.

[_Wim_]

I can see that shape would be less strict (which is good, especially for a DIY device), but I would think it still matters. I also think that the absorption would have a positive effect on horn resonances (HOM) because they are more absorbed then the direct wave fronts. Efficiency would probably go down a lot, but for a home environment with a modern compression drives this is not a problem.

Off coarse these are all assumptions from me, I cannot prove them mathematically. That is why I wanted to hear what Earl though about this.