"For example, if an enclosure...(Cube)...then along each axis, the resonant frequency would be...even more resonances are formed by integral multiples of the resonant frequency....each panel has the same second, third, fourth, etc. harmonics, which leads to a huge "pile-up" or amplification, of resonance frequencies." -Page 7, Ray Alden, 1995, Advanced Speaker Systems, Master Publishing, Inc.
______________________________________________________________Rick....
______________________________________________________________Rick....
P10 if you'r going to swipe my stuff ya might as well swipe the finished one
Thanx
dave
Do you have any data showing a problem at subwoofer bandwidths (to be conservative, let's say sub 200Hz)?
OK, miss use of words on my part regarding damping.
But anyway, it wouldn't be smart to use just one dowel. One dowel is as good as the X brace, according to the circle sizes in the diagram. So, multiple dowels (like 5 in an asymmetric pattern) are both easy to install and allow the entire box to be smaller. The dowels do more with less and I like that. It's a smart use of materials. If you don't like that, then that's your choice. The other braces are just brute force and to me that's dumb.
It's entirely a function of the size of the cube, the bandwidth of the sub, and the distortion products from the driver. Obviously, a cube will have much stronger resonance than other shapes. If the distortion from the driver produces strong enough harmonics in the area of the resonant frequency of the enclosure, then obviously it's going to stimulate it.
P10 if you'r going to swipe my stuff ya might as well swipe the finished one
I don't think these diagrams are accurate representations of real resonant modes. Panels do not behave like that.
The first image what the fundamental resonant mode of a rectangular panel looks like (mode 1,1). If you put an X brace on the panel, then the fundamental mode becomes the second one. (mode 2,2) It appears that the frequency of mode 2,2 is four times the frequency of mode 1,1. I'm not sure about the dowel in the center.
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I think also that a dowel brace requires a much more complex analysis than a standard brace. A standard brace fixes a line on the panel as stationary, whereas a dowel only provides for one point. However, what is not clear to me is how the panel resonates with a dowel in place, since there are no stationary lines produced by the dowel. If we draw an imaginary line from the dowel to the nearest edge, that line is not rigidly held in place but allows some movement. So, I don't think that any resonant modes would be clearly defined with a dowel(s). They would be messy and damped I think.
It's also about effort to build. It takes a lot more effort to cut all of those braces, with all those funny cut outs, than it does to install a hand full of dowels. For dowels, just put the two opposite panels together, drill some holes through both panels, cut the dowels to length, and you're basically done.
I think there is a confusion between stiffness and resonance.
It is true that cutting up the panels into different sized sub-panels results in different resonances and these may be inside or outside the woofer band.
But the ultimate purpose is to get the amount of panel flexing to propagate any frequency into the air very tiny. That's what mechanically sound bracing (like with dowels across and triangular braces) is all about.
Also, there have been arguments about the acoustic sense of using a cube, even for a woofer enclosure, and before needing to add felt. But the important issue of shape is a different one. I think the cube has the biggest unsupported sides (read: drum head) of the solids, perhaps triangular pyramids excepted. THAT's what is unwise about cubes, unless pretty shapes matter more to you than optimal design.
And if you are building a cube, you sure do need bracing or brute thickness if the enclosure is sealed.
I'll say again, those fancy pieces of woodwork in the pictures are producing little benefit given their scope in size, weight, and effort... and in one case pictured, their BR application.
Ben
It is true that cutting up the panels into different sized sub-panels results in different resonances and these may be inside or outside the woofer band.
But the ultimate purpose is to get the amount of panel flexing to propagate any frequency into the air very tiny. That's what mechanically sound bracing (like with dowels across and triangular braces) is all about.
Also, there have been arguments about the acoustic sense of using a cube, even for a woofer enclosure, and before needing to add felt. But the important issue of shape is a different one. I think the cube has the biggest unsupported sides (read: drum head) of the solids, perhaps triangular pyramids excepted. THAT's what is unwise about cubes, unless pretty shapes matter more to you than optimal design.
And if you are building a cube, you sure do need bracing or brute thickness if the enclosure is sealed.
I'll say again, those fancy pieces of woodwork in the pictures are producing little benefit given their scope in size, weight, and effort... and in one case pictured, their BR application.
Ben
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I agree.
I don't think anyone would use just one dowel in the center of a panel. If you put three dowels in a row from one panel to the opposite panel, for example, it would probably have the same effect as a solid brace along the same line formed by the row of three dowels that takes a lot more work to produce. I have read that a dowel spacing of 6" to 10" is more than adequate.
hmmm....:
"Most books on loudspeaker construction really emphasize bracing. What I found is that bracing helps only at the lowest frequencies, below 100 Hz. Another result that really surprised me is that when the cabinet vibrates, the whole thing vibrates. I had expected the panels to exhibit modal resonance patterns. Specifically, at the corners I expected the vibration to be quite low. Not so. All of my expectations were true at certain specific frequencies, but sweeping over the full frequency band shows remarkably similar behavior, regardless of where the pickup is located. This is not to say that bracing should not be used. In fact, I liberally used 2"x2" braces. But don't expect braces to eliminate vibration, and be aware that damping is a lot more effective than bracing."
http://www.silcom.com/~aludwig/Loudspeaker_construction.html#Panel_vibration_damping
I don't think anyone would use just one dowel in the center of a panel. If you put three dowels in a row from one panel to the opposite panel, for example, it would probably have the same effect as a solid brace along the same line formed by the row of three dowels that takes a lot more work to produce. I have read that a dowel spacing of 6" to 10" is more than adequate.
hmmm....:
"Most books on loudspeaker construction really emphasize bracing. What I found is that bracing helps only at the lowest frequencies, below 100 Hz. Another result that really surprised me is that when the cabinet vibrates, the whole thing vibrates. I had expected the panels to exhibit modal resonance patterns. Specifically, at the corners I expected the vibration to be quite low. Not so. All of my expectations were true at certain specific frequencies, but sweeping over the full frequency band shows remarkably similar behavior, regardless of where the pickup is located. This is not to say that bracing should not be used. In fact, I liberally used 2"x2" braces. But don't expect braces to eliminate vibration, and be aware that damping is a lot more effective than bracing."
http://www.silcom.com/~aludwig/Loudspeaker_construction.html#Panel_vibration_damping
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So, then, the answer to this question:
Is "no."
Why not? After years going through lots of stupidity with bracing (and wall thickness) that made cabinets needlessly large and heavy, my SOP now is three dowels ( one joining top and bottom, one joining left and right, one joining front and back, all connected at a "star point") and a small shelf to support the woofers' motor structure.
The 2nd picture has the brace running in the wrong (least effective) direction and is placed on the centre line. A good illustration of why the brace placement in exact centre is not a good idea.
dave
As a dowel advocate, I believe that the enclosure is acting like a balloon. A hardwood dowel or two, even 3/8 inch, has all the tensile and compressive strength anybody could possibly need, provided it was anchored securely and straight. Strength in these senses, is far stronger than using flat reinforcement lumber that bends.
Whatever the resonant frequency of the panel and sub-divisions, if the panel isn't moving, there is no loss.
Ben
It's not wrong. Why are you so judgmental about this? The panel area is cut in half by placing the brace there, increasing the resonant frequency by some factor. The math is here:
Lowering Mechanical Noise Floor in Speakers - page 2 — Reviews and News from Audioholics
The scanned picture picture came from a paper that was citing (and verifying) the older paper. One does not take lightly AES articles that have been 1st vetted for the Journal and then hand pciked for the Anthologies.
dave
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