NEW ANGLES IN GARDENS JUST A MITER MATTER OF DEGREE - Chicago Tribune
The angles used for mitering three decorative shapes that work well as borders are:
Pentagon (five sides): 54 degrees.
Hexagon (six sides): 60 degrees.
Octagon (eight sides): 67 1/2 degrees.
There are other websites but this is just a simple example of the miter angle cuts required...
The angles used for mitering three decorative shapes that work well as borders are:
Pentagon (five sides): 54 degrees.
Hexagon (six sides): 60 degrees.
Octagon (eight sides): 67 1/2 degrees.
There are other websites but this is just a simple example of the miter angle cuts required...
As does a true monopole (omni).
Problem is we're waving bits of cardboard around with magnets, so those assumptions don't always hold.
With regards to ideal OB speakers, placed in a real acoustic environment, of course they'll introduce comb filtering. However, comb filtering occurs because of delayed sources, so any reflection at all will introduce those effects. Controlled-directivity monopoles will clearly interact with the room the least, while OBs will send half their energy backwards (ie, reduced side reflections), and omni speakers will have the most reflections.
In my listening room, OBs can't work. The wall is too close to the back of the speaker, so there'll be lots of near reflections. The speakers I've come up with control directivity (via HF horn) for the mid-high range, and use a fairly large woofer to provide some additional control. Lower down, the cabinet is shallow enough as to acoustically integrate with the wall. The 3-500Hz range isn't perfect, but it's a reasonable compromise in the space I had available.
With regards to the internal reflections of a cabinet, as the wavelength becomes much greater than the reflection distance, that reflection can safely be ignored. Consider a sealed box subwoofer. There's no need for a volume fill (unless we're looking to marginally increase the effective volume), since all the wavelengths are much larger than the cabinet, so no standing waves or discrete reflections can form.
A useful test for the internal treatment of a cabinet is this: do an impedance sweep. Standing waves from the driver's come to various surfaces show up as small resonances, and applying absorption in the reflection path will flatten out those ripples in the impedance curve. You can see a comparison of two impedance curves in the attachments.
You can see a couple of photos of the speakers here: New Speakers
Both curves were taken with the speaker in the same position (on the floor, facing up), and I'd added some internal absorption in the form of 1/2" thick (approx) recycled wool underlay. 3x layers at the top and between the ports, 2x layers down the sides, 1x layer on the back.
As can be seen from the graphs, the resonances in the hundreds-of-Hz range have been damped and flattened. They haven't disappeared completely, but that's probably because I didn't use enough damping. Applying this much lost a couple of dB of output in the 70Hz range, because the port tuning also dropped and became slightly lower-Q. I didn't want to lose any more LF output because these are PA speakers.
Needless to say, my (sealed, floorstanding) HiFi speakers have lots of polyester filling in the bottom half, and lots of layers of recycled wool in the top half. I should probably take an impedance curve of those.
I plan on adding a bit of internal bracing to the PA cabinet in the form of a few batons connecting opposite faces, and seeing how that affects the impedance curve, too.
Chris
Attachments
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How does this type of comb filtering compare to stereo comb filtering and that between our ears?
I believe all absorption graphs i have seen for varios porous materials shows that material thickness has great effect for the low freq absorption, to some degree you can compensate thickness with an air gap behind the absorber
Yes Chris, I doubt it matters much either Comb Filtering, Acoustical Interference, & Power Response in Loudspeakers | Audioholics.
Initially I was referring to the comb filtering caused by baffle diffraction in an OB speaker
Initially I was referring to the comb filtering caused by baffle diffraction in an OB speaker
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...and therefore I don't understand what 8mm - 1cm felt would achieve
. As the velocity is zero at the wall there is no speed to kill at the last mm and therefore an absorber is more effective if it is mounted with a gap instead of direct contact. Do you use the melamine foam with a thickness like the usual felt or more like room absorbers or even like stuffing ?
If the melamine foam is effective as a thin cover of the panels what is the explanation for the effectivness ? The usual room absorber theory says it doesn't work as the thickness is way too short in relation to the wave length (assuming the wave is hitting the wall orthogonal). Is it because of the friction in a tangential movement (like wall friction in a pipe) ? Wouldn't the same amount of material be more effective as a crosssection 'wall' ?
Yes, I guess this is why omni/dipole can't be a major player in real world. The commercial recorded materials are intended to be played back from cardioid speakers. More room interaction can make music sound more natural and open if the music is dynamically uncompressed like string quartet, but most of the recorded instruments are heavily compressed and the transient has been completely killed. When we playback those unnaturally compressed materials, we want to keep the transient as much as possible, but dipole blur the transient with room ambience. Many of us rather choose clear transient with a cardioid speaker even we are impressed with the natural presentation of good OB. Major complains about planner/dipole are, they have no punch, percussive instruments have no dynamics, etc., but it is exactly what the recorded materials are.
Then someone may ask why they have to compress. If we playback uncompressed natural sound at our average listing level, many instruments including vocal are way too dull, far and lifeless, so it is absolutely necessary to do it. We are listening the recorded materials, which is nothing but a miniature or a diorama where the deformation is necessary.
90-54= 36 degrees. The angle tilt on the blade of a common table saw can not exceed 45 degrees, therefor we must work inverted. We are working with the exterior angle. If the internal angle is 108 degrees, the {exterior}working angle must be the difference between it, and 180 degrees. 180-108=72. Each of the 5 sides compromises 1/2 of the angle, therefor 72 divided by 2= 36.
Explanation:
If one exterior angle is taken at each vertex of any polygon, and their measures are added, the sum is 360∘. Each exterior angle of a regular pentagon has the same measure, so if we let t be that common measure, then
5t=360∘
Solve for t:
5t÷5=360∘÷5
t=72∘
I have been studying speaker design for 50 years, and building them for 48. Having stated that, my introduction to a pentagonal enclosure has only been within the last 12 years. I'm 62, but I do have one (and only mentor) who is 76. He's the one who introduced me to the 5 sided box. But even a *cheap* 5 sided box will have problems. Must be built with panel damping in mind. The internal structure/shape only deals with a reduction of standing waves, but not address panel vibration, especially those transmitted from the driver.
A proper built 5 sided box (pentagon) has an extreme OPEN sound, without the detriments of that foolish open baffle concept.
The open baffle will forever be dealing with the acoustical short circuit. It takes away much more than it gives you. Except, it's relatively easy. Cut a hole in a plank and call it a day.
I know, I know, it's confusing between the 108 degrees and the 72.
Scroll down to example question #8:
How to find an angle in a pentagon - Intermediate Geometry
I also have been doing speakers for over 50 years! My few penta-boxes were decades ago. 90 - 54 = 36. Likewise 180 - 108 = 72 then divide by 2 = 36. So; we are saying the same thing in different ways I guess. I remember setting the saw angle and cutting several scrap pieces until I measured exactly 108 degrees with 2 panels edge to edge.
Pentagon Properties
Some properties of the pentagon are as follows:
In the pentagon, the sum of the interior angles is equal to 540°.
If all the sides are equal and all the angles are of equal measure, then it is a regular polygon. Otherwise, it is irregular.
In the regular pentagon, the interior angle is 108°, and the exterior angle is of 72°.
An equilateral pentagon has 5 equal sides.
The sum of the interior angles of a rectangular pentagon is 540°.
the above from this:
Pentagon - Definition, Properties, Types, Formula & Example
Pentagon Properties
Some properties of the pentagon are as follows:
In the pentagon, the sum of the interior angles is equal to 540°.
If all the sides are equal and all the angles are of equal measure, then it is a regular polygon. Otherwise, it is irregular.
In the regular pentagon, the interior angle is 108°, and the exterior angle is of 72°.
An equilateral pentagon has 5 equal sides.
The sum of the interior angles of a rectangular pentagon is 540°.
the above from this:
Pentagon - Definition, Properties, Types, Formula & Example
That's funny because, of course, a good open baffle speaker is anything but easy.
...and therefore I don't understand what 8mm - 1cm felt would achieve
Only applies to standing waves (which are, of course, important). Doesn't apply to reflections in general. Think of waves hitting a breakwater or sea-wall.
Softening the air/wall interface is useful.
I think you must consider the wavelength also. Waves that are much larger then the absorber or diffractor will not be altered.
Would like to see more information regarding that. Do you have anything which relates reflection coefficients versus thickness and types of damping material? An appreciation of "much larger" would be helpful.
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– SoundAssured
Acoustic Foam Pyramid Panels
– SoundAssured
I use some of these; especially the foam pyramids, 4 inch and 1 inch sizes in the volume directly behind the driver. Not touching the driver, not filling up the volume 100% either, This is on top of the felt composite dampening glued to the internal walls.
If you look through the data; they have frequencies vs absorption charts. This is sold to be used on recording studio walls. I find that it helps; I haven't done any scientific or sophisticated measurements; just basing it on the sound quality.
Noise Reduction Coefficient Ratings (NRC)
Range 1" Pyramid 2" Pyramid 3" Pyramid 4" Pyramid Bass Traps
125 Hz 0.14 0.19 0.24 0.28 1.18
250 Hz 0.13 0.22 0.29 0.39 1.27
500 Hz 0.31 0.45 0.55 0.74 1.26
1000 Hz 0.43 0.55 0.56 0.69 1.19
2000 Hz 0.46 0.6 0.61 0.75 1.16
4000 Hz 0.54 0.76 0.85 0.96 1.16
Overall 0.35 0.45 0.5 0.65 1.2
sorry, this chart is in the data, a simple copy and paste obviously didn't go so well here but you get the idea.
– SoundAssured
Acoustic Foam Pyramid Panels
– SoundAssured
I use some of these; especially the foam pyramids, 4 inch and 1 inch sizes in the volume directly behind the driver. Not touching the driver, not filling up the volume 100% either, This is on top of the felt composite dampening glued to the internal walls.
If you look through the data; they have frequencies vs absorption charts. This is sold to be used on recording studio walls. I find that it helps; I haven't done any scientific or sophisticated measurements; just basing it on the sound quality.
Noise Reduction Coefficient Ratings (NRC)
Range 1" Pyramid 2" Pyramid 3" Pyramid 4" Pyramid Bass Traps
125 Hz 0.14 0.19 0.24 0.28 1.18
250 Hz 0.13 0.22 0.29 0.39 1.27
500 Hz 0.31 0.45 0.55 0.74 1.26
1000 Hz 0.43 0.55 0.56 0.69 1.19
2000 Hz 0.46 0.6 0.61 0.75 1.16
4000 Hz 0.54 0.76 0.85 0.96 1.16
Overall 0.35 0.45 0.5 0.65 1.2
sorry, this chart is in the data, a simple copy and paste obviously didn't go so well here but you get the idea.
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