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.
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.
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.
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.
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
its as much of a waveguide as other examples that I have seen called "waveguides".
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
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
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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.
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.
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.
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.
Ive tried this once or twice with just a single ring of felt 5mm thick, the inside diameter was the same as the faceplate of the tweeter (100mm), or slightly larger. WHen used with a 5" woofer, it did seem to help the 2 drivers integrate, however using a smaller diameter ring than 100mm, with this particular tweeter, started to absorb so much of the HF from the (dome) tweeter, as to make it sound a little strange. Left the sound a little limp and soft, whereas the faceplate diameter ring made little difference on axis or 30° off axis, but did seem to constrain the radiation pattern at higher angles.
whether better or worse than a real waveguide is anyones guess, although this absorbtion method doesnt focus the radiation in any way.
with a ring 100mm in internal diameter, and 1" thick and even only 5mm thick felt the absorption at close to 90° is quite heavy, but rapidly becomes less effective. too many layers of 5mm seemed to constrain the sound, in my modest listening tests
whether better or worse than a real waveguide is anyones guess, although this absorbtion method doesnt focus the radiation in any way.
with a ring 100mm in internal diameter, and 1" thick and even only 5mm thick felt the absorption at close to 90° is quite heavy, but rapidly becomes less effective. too many layers of 5mm seemed to constrain the sound, in my modest listening tests
I've played with soaking up peripheral radiation from drivers with felt and foam for years. It's just another tool in the box. I always recommend to people with hard-termination (read- rectangular with angles creating the junction to the flange, a la just about every rectangular horn out there) to use some felt or foam to extend the horn profile outwards- it works similarly to a proper rounded termination, but can be somewhat more flexible. It removes the horrible, axially inconsistent, artifacts at the bottom end of a horn's response, if done properly and makes a smoother, better imaging sound.
When using a thick felt ring the tweeter does not need to padded down as much, relative to the woofer. I usually prefer a tipped down frequency response but with the Felt I notice that a flat response sounds perfectly balanced, because there is less total energy being radiated into the room. Still, there is a dryness and focus to the sound that is different to the usual wide dispersion sound of a dome and different again to a constant directivity horn / waveguide sound. I tend to prefer it, especially at higher SPL but each to their own.
However I have noticed sometimes that there seems to be a strange effect in the soundstage which may have to do with the mismatch in dispersion between the woofer and tweeter, especially when using a 2khz crossover. I’d like to try a lower crossover point, closer to 1khz (the “invisible crossover”) and take some off-axis measurements to check the integration.
An externally hosted image should be here but it was not working when we last tested it.
On the big horn, covering the sides made double the difference to covering the top bottom. Ideally it would be a smooth (no breaks) in the horn path, and no corners. That's why I've been looking at the jbl newer horn, the 2384 used in the 3622 and the 4622 (double 15" plus horn, like my setup). And I've also been looking at the stereolabs 400hz horns, but I don't think I could live with the lack of dispersion past 3-4khz (tractrix horn). I like my 90 degree dispersion setup.
It is well worth the time and ugliness to add the foam. It makes everything "sharper" as my wife says, less reverby, revealing detail that was obscurred before. A friend suggested trying small weatherstripping in all the corners, I'd imagine it would help, but only add a little.
There is a company making horns where the last 1/4 of the horn is a foam, softer than the stuff used in the urei coaxial (can't remember the name), more large format stuff (huge 3'x3' horns, etc).
Also check out link 2046.
http://www.diyaudio.com/forums/multi-way/103872-geddes-waveguides-205.html
Norman
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A lot of tweeter response raggedness in the two upper octaves is due to diffraction. Geddes' solution is a good way of dealing with it; waveguides may also help integration between woofer and tweeter by better aligning the radiation patterns of the two drivers. I don't think adding absorbing material on the walls of a waveguide will be beneficial, because the whole point of a waveguide is that it is calculated to manipulate precisely the natural radiation pattern of the tweeter to accomplish a desired effect. Absorbing material would negate this effort.
A tweeter mounted on a baffle will show effects that are difficult to predict from a purely theoretical standpoint, since diffraction around tweeter surrounds, mounting plates, other components on the baffle, rims might wreak havoc on the FR. Foam is one way of absorbing reflected waves that may cause peaks and valeys in the FR when they combine (de)(con)structively. You really need to measure 1) to see if there is a problem and, if there is a problem, 2) see if modifications, such as adding absorbtion, will lead to improvements.
vac
A tweeter mounted on a baffle will show effects that are difficult to predict from a purely theoretical standpoint, since diffraction around tweeter surrounds, mounting plates, other components on the baffle, rims might wreak havoc on the FR. Foam is one way of absorbing reflected waves that may cause peaks and valeys in the FR when they combine (de)(con)structively. You really need to measure 1) to see if there is a problem and, if there is a problem, 2) see if modifications, such as adding absorbtion, will lead to improvements.
vac
I would think we should make a distinction between horns with foam at the termination and devices made totally with absorptive materials (as shown in the original post: "out of foam completely"). If you make it totally with foam or felt, it isn't a waveguide as we know it and (I believe) the particular shape contour becomes fairly irrelevant.
David S.
David S.
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