I found a very interesting paper on omni directional sound sources.
Paper title is: “An experimental evaluation of regular polyhedron loudspeakers as omni directional sources of sound” from T.M. Leishman e.a.
The paper can be downloaded from: https://scholar.google.nl/scholar?q...sound+pdf&hl=nl&as_sdt=0&as_vis=1&oi=scholart
Tests were done in an anechoic chamber with an circular microphone array on regular polyhedra with 4 to 20 drivers. The sound sources were meant for acoustic room measurements but I think that some results are also valid for sound reproduction applications.
Especially, the “balloon” presentation of the results was an eye-opener for me.
For me, the measurements at 2 and 4 kHz suggest the following:
Paper title is: “An experimental evaluation of regular polyhedron loudspeakers as omni directional sources of sound” from T.M. Leishman e.a.
The paper can be downloaded from: https://scholar.google.nl/scholar?q...sound+pdf&hl=nl&as_sdt=0&as_vis=1&oi=scholart
Tests were done in an anechoic chamber with an circular microphone array on regular polyhedra with 4 to 20 drivers. The sound sources were meant for acoustic room measurements but I think that some results are also valid for sound reproduction applications.
Especially, the “balloon” presentation of the results was an eye-opener for me.
For me, the measurements at 2 and 4 kHz suggest the following:
- The radiation pattern continuously changes when frequency changes. I suppose we don’t notice this because the many room reflections will cancel out these effects.
- The prickly shape of the radiation patterns are caused by diffraction on the polyhedral edges. I assume that the different (truncated) polyhedra will all have different patterns. The size act as a scale factor. These diffraction effects are not always taken into account in theoretical analysis.
- The differences between the different polyhedrons, and with that the number of drivers, are not significant. All are appr. equal good (or bad).
- The diffraction effects cannot be eliminated by EQ methods. At the most, the spl can be make more or less flat in one direction.
It is already established that EQ can do nothing about diffraction.
If you consider that a polyhedron is an approximation of a sphere with flat bits “glued” together. Use smaller panels, and add as much roudnover as you can. A sphere with flat spots only to mount the driver would be the resulkt if you took the concept to the limit. Edge diffraction would then be one driver interfering with another.
Now i’ll go read the paper, you make it seem quite enticing.
dave
I often heard experienced listeners say about omni-directional speakers that the sound was sparkling but that the stereo image seemed a bit woolly.
Some time ago I argued in this topic that diffraction results in a poor spl, leading to a less optimal stereo reproduction for my omni’s.
I now think that it should be possible to greatly reduce the diffraction effects by placing damping material on the outside of the omni’s. By sheer luck, this is possible with the shape I chose for my omni’s.
The omni’s have the shape of a truncated octahedron, consisting of eight hexagons and six squares. The drivers are mounted in four of the hexagons.
Due to this arrangement, there is an empty plane (hexahedron or square) between all the drivers. Damping material can now be placed on these empty surfaces to reduce diffraction.
I took a sheet of melamine foam (4 cm thick) and made the slices from it. This turned out to be quite difficult due to the strange angles and double and triple miter. However, in the end I managed more or less to do so.
Halfway through I realized that only the areas visible from the sweet spot had to be covered. Both speakers were treated in this way.
I didn't bother to take SPL measurements, I was only concerned with the stereo behavior. Because I know of no objective method to measure stereo quality, I just started listening to music.
The result exceeded my expectations. The improvement was immense. The stereo image was sparkling and very detailed. Better than I'd ever heard.
To be honest, I would like to emphasize that I am not an expert in qualitative assessment of loudspeaker systems, nor an experienced stereo listener, but I liked what I heart.
In short, I am now very happy with my omni directional speakers.
My only remaining concern are the WAF aspects.
Jaap
Some time ago I argued in this topic that diffraction results in a poor spl, leading to a less optimal stereo reproduction for my omni’s.
I now think that it should be possible to greatly reduce the diffraction effects by placing damping material on the outside of the omni’s. By sheer luck, this is possible with the shape I chose for my omni’s.
The omni’s have the shape of a truncated octahedron, consisting of eight hexagons and six squares. The drivers are mounted in four of the hexagons.
Due to this arrangement, there is an empty plane (hexahedron or square) between all the drivers. Damping material can now be placed on these empty surfaces to reduce diffraction.
I took a sheet of melamine foam (4 cm thick) and made the slices from it. This turned out to be quite difficult due to the strange angles and double and triple miter. However, in the end I managed more or less to do so.
Halfway through I realized that only the areas visible from the sweet spot had to be covered. Both speakers were treated in this way.
I didn't bother to take SPL measurements, I was only concerned with the stereo behavior. Because I know of no objective method to measure stereo quality, I just started listening to music.
The result exceeded my expectations. The improvement was immense. The stereo image was sparkling and very detailed. Better than I'd ever heard.
To be honest, I would like to emphasize that I am not an expert in qualitative assessment of loudspeaker systems, nor an experienced stereo listener, but I liked what I heart.
In short, I am now very happy with my omni directional speakers.
My only remaining concern are the WAF aspects.
Jaap
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How porous is that material? Looking at the basic shape now you have sharper corners which should imply more nasty diffraction... but if that whits stuff is really absorbing/dampening it can work. Is it https://se.rs-online.com/web/p/acoustic-insulation/1034063 ?
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Apparently it works. I don't know how porous this material is. It helps that damping is only really needed where the problems are, namely above approximately 1 kHz.
I bought the material from https://www.akoestiekwinkel.nl/flamex-basic-akoestische-platen
This site also provides some information about absorption. It looks similar to the material on the website you mentioned.
I bought the material from https://www.akoestiekwinkel.nl/flamex-basic-akoestische-platen
This site also provides some information about absorption. It looks similar to the material on the website you mentioned.
Some carefully crafted grilles would greatly help that.
You may also be able to get by with thinner melamine. With thinner materials you get less absorption at lower frequencies, but you may still get the result you want.
That is great work. A few off axis measurements of one speaker 0 deg, 30 deg, 60 deg should show the effect that lead to the improved stereo image. I expect you could create a cylindrical lamp shade using some wire and acoustically transparent cloth and metal screen, add a light bulb above and below the speaker and tell your wife it is a lamp!
I have planned to take measurements. However this requires foam slices on all free surfaces, not just the front. So I will first complete the foam slicing and also take WAF aspects into account as much as possible. Luckily, I have already done some REW measurements on the speakers without foam, so a good comparison should be possible.
I have a feeling that camouflage is not the right way to go. I like functional shapes and think that they are more likely accepted. I am thinking about accentuate the foam slices by, for example, using two types of foam with different shades of gray.
I have a feeling that camouflage is not the right way to go. I like functional shapes and think that they are more likely accepted. I am thinking about accentuate the foam slices by, for example, using two types of foam with different shades of gray.
Interesting suggestion. I will think about it