I think it’d be useful to gather and summarize the wisdom into a wiki “bracing: theory & practise”.
I have a mix of knowledge and questions, gleaned solely from searching here, most from dave, some from GM and others.
A 1st draft, probably with a number of misconceptions, hopefully not too many, and IMO lacking some bits to be in a wiki just yet(?):
The goal of bracing is to push panel resonances higher. Ideally the natural frequency of the panel is pushed above and outside the passband of the box, eg above a dedicated mid box’s passband.
Where this is not possible, next best is simply to reduce its audibility, as the higher the frequency, the greater the amount of energy needed to excite it.
Also damping materials attenuate higher frequencies better.
The Hz of a panel resonance is inversely proportional to the square (?) of the panel size, eg if a panel size is doubled, the frequency of a resonance increases by a factor of 4(?).
If the frequency of a resonance increases by a factor of 4, the energy needed to excite it falls to 1/ 4 (or 1/ 16??).
As mass stores energy, the releases it with a time delay, a well braced single layer box is better than a non-braced two layer box.
Theory
From "Vibration Analysis for Electronic Equipment", by David S. Steinberg (p 267):
f = (pi/ 2) * SQRT{D/ p[(1/a^2) + (1/ b^2)]}
where
f = natural frequency
D = plate stiffness factor = [E(h^3)] / [12(1-u^2)]
E = modulus of elasticity in lb/ in^2
Note 3/4" no-void ply has an MOE of 1.8 mill psi,
while typical MDF is 0.53 mill psi.
h = plate thickness, inches
u = Poisson ratio
p = mass per unit area = v * h / g
v = material density in lb/in
h = plate thickness, inches
g = acceleration of gravity, 386 inch/sec^2
a = length of the plate, in inches
b = width of the plate, in inches.
The frequency is proportional to the thickness to the 3/2 power, the SQRT(MOE) and to 1/SQRT(mass).
Design
In setting the box dimensions, allow 5-10% for the volume of the braces.
Spacing
· The largest panels resonate at the lowest frequencies, so they are the most important to divide up
· Linkwitz’ rule of thumb from years ago: no unbraced distances should be greater than 4" ie 100 mm.
· GM for void free plywood suggests the points where bracing around the driver is required:
- for 5/8" ply: 0.75 ft^2 CSA (cross sectional area), ie equivalent to 10” ^2, ie 25 mm^2.
- For 3/4" ply: for 1.0 ft^2 ie 30 mm^2.
Shape
· the strongest (bracing) structure is a triangle . .
·
·
Placement
· Bracing the short dimension creates a larger number of panels that are closer to square, with same or similar resonances. It’s better to run them along the long dimension of a panel.
· the middle is the worst place, as energy at divisor frequencies can feed panel resonances - the panel can actually be worse after bracing. To spread resonances, bracing should never be at equidistant intervals, eg instead 1 ft intervals - 11", 13" & 12" would be better.
· So that the 1st resonance is killed instead of enhanced - place them offset and/ or oblique ~ by at least X? degrees
· brace so that you move the dimensions away from each other
· endeavour to brace the driver magnets to the box.
Materials
· plywood or hardwood, because they are stiff and not heavy (mass absorbing energy, then releasing it later).
· A good size is 3/4" x 1 1/2" ie 18 * 36 mm.
· mount edge-on, ie deeper side attached to the box.
· metal braces or steel rods are stiffer so even better
Tips for attaching braces
· Butt joints with glue & screws is fine.
· Ideally each bracing sheet is connected to other bracing sheets(?)
· If the bracing also ties the box walls together, that is better
· Best - run rods all the way thru, and add compression to the panels (ie try to bend the walls into a dish shape).
Shape
www.diyaudio.com/forums/showthread.php?s=&postid=832740&highlight=#post832740
· holes in the braces - are to . . . ?
· Approx 30 - 60% is a good proportion of holes
· Shelf braces ( . . . . . ) are not effective because . .
· Instead, braces should divide the panels into long thin unequal, preferably trapezoidal sub-panels
Boxes for full-range or midrange-&-above :
. . . .
And for subs (XO < 100 Hz):
At 100 Hz, as long as the biggest box dimension is < 1.7 m (5.6’), there is no benefit to bracing, as potential panel resonances will be above the panel size.
See also this short thread on push-push:
www.diyaudio.com/forums/showthread.php?s=&threadid=96902 and this wiki
www.diyaudio.com/wiki/index.php?page=Push+pull+driver+mounting
Please feel free to correct me, let me know if you think it’s ready to be wiki-ed
Cheers
I have a mix of knowledge and questions, gleaned solely from searching here, most from dave, some from GM and others.
A 1st draft, probably with a number of misconceptions, hopefully not too many, and IMO lacking some bits to be in a wiki just yet(?):
The goal of bracing is to push panel resonances higher. Ideally the natural frequency of the panel is pushed above and outside the passband of the box, eg above a dedicated mid box’s passband.
Where this is not possible, next best is simply to reduce its audibility, as the higher the frequency, the greater the amount of energy needed to excite it.
Also damping materials attenuate higher frequencies better.
The Hz of a panel resonance is inversely proportional to the square (?) of the panel size, eg if a panel size is doubled, the frequency of a resonance increases by a factor of 4(?).
If the frequency of a resonance increases by a factor of 4, the energy needed to excite it falls to 1/ 4 (or 1/ 16??).
As mass stores energy, the releases it with a time delay, a well braced single layer box is better than a non-braced two layer box.
Theory
From "Vibration Analysis for Electronic Equipment", by David S. Steinberg (p 267):
f = (pi/ 2) * SQRT{D/ p[(1/a^2) + (1/ b^2)]}
where
f = natural frequency
D = plate stiffness factor = [E(h^3)] / [12(1-u^2)]
E = modulus of elasticity in lb/ in^2
Note 3/4" no-void ply has an MOE of 1.8 mill psi,
while typical MDF is 0.53 mill psi.
h = plate thickness, inches
u = Poisson ratio
p = mass per unit area = v * h / g
v = material density in lb/in
h = plate thickness, inches
g = acceleration of gravity, 386 inch/sec^2
a = length of the plate, in inches
b = width of the plate, in inches.
The frequency is proportional to the thickness to the 3/2 power, the SQRT(MOE) and to 1/SQRT(mass).
Design
In setting the box dimensions, allow 5-10% for the volume of the braces.
Spacing
· The largest panels resonate at the lowest frequencies, so they are the most important to divide up
· Linkwitz’ rule of thumb from years ago: no unbraced distances should be greater than 4" ie 100 mm.
· GM for void free plywood suggests the points where bracing around the driver is required:
- for 5/8" ply: 0.75 ft^2 CSA (cross sectional area), ie equivalent to 10” ^2, ie 25 mm^2.
- For 3/4" ply: for 1.0 ft^2 ie 30 mm^2.
Shape
· the strongest (bracing) structure is a triangle . .
·
·
Placement
· Bracing the short dimension creates a larger number of panels that are closer to square, with same or similar resonances. It’s better to run them along the long dimension of a panel.
· the middle is the worst place, as energy at divisor frequencies can feed panel resonances - the panel can actually be worse after bracing. To spread resonances, bracing should never be at equidistant intervals, eg instead 1 ft intervals - 11", 13" & 12" would be better.
· So that the 1st resonance is killed instead of enhanced - place them offset and/ or oblique ~ by at least X? degrees
· brace so that you move the dimensions away from each other
· endeavour to brace the driver magnets to the box.
Materials
· plywood or hardwood, because they are stiff and not heavy (mass absorbing energy, then releasing it later).
· A good size is 3/4" x 1 1/2" ie 18 * 36 mm.
· mount edge-on, ie deeper side attached to the box.
· metal braces or steel rods are stiffer so even better
Tips for attaching braces
· Butt joints with glue & screws is fine.
· Ideally each bracing sheet is connected to other bracing sheets(?)
· If the bracing also ties the box walls together, that is better
· Best - run rods all the way thru, and add compression to the panels (ie try to bend the walls into a dish shape).
Shape
www.diyaudio.com/forums/showthread.php?s=&postid=832740&highlight=#post832740
· holes in the braces - are to . . . ?
· Approx 30 - 60% is a good proportion of holes
· Shelf braces ( . . . . . ) are not effective because . .
· Instead, braces should divide the panels into long thin unequal, preferably trapezoidal sub-panels
Boxes for full-range or midrange-&-above :
. . . .
And for subs (XO < 100 Hz):
At 100 Hz, as long as the biggest box dimension is < 1.7 m (5.6’), there is no benefit to bracing, as potential panel resonances will be above the panel size.
See also this short thread on push-push:
www.diyaudio.com/forums/showthread.php?s=&threadid=96902 and this wiki
www.diyaudio.com/wiki/index.php?page=Push+pull+driver+mounting
Please feel free to correct me, let me know if you think it’s ready to be wiki-ed
Cheers
Guys,
Please note, “gleaned solely from searching here” ie I have 0 claim to expertise.
I started simply to gather info for my own understanding and use; then thought - why not share it. I know nothing beyond what I posted.
. . also “if you think it’s ready to be wiki-ed”
Dave, as a moderator – wiki it now?
Cheers
Please note, “gleaned solely from searching here” ie I have 0 claim to expertise.
I started simply to gather info for my own understanding and use; then thought - why not share it. I know nothing beyond what I posted.
. . also “if you think it’s ready to be wiki-ed”
Dave, as a moderator – wiki it now?
Cheers
I noted your discussion on the parts Express forum and a number of the EE types they had comments.
You could read the comments.
Here: http://www.pesupport.com/cgi-bin/config.pl?read=350291
Zarathu
(I post as Marlboro everywhere but here)
You could read the comments.
Here: http://www.pesupport.com/cgi-bin/config.pl?read=350291
Zarathu
(I post as Marlboro everywhere but here)
While I admire the enthusiasm, this is definitely not ready to be wiki'd. One could go through and pick this all apart, but I won't.
I think it is good to have a plate resonance equation. Make sure to work though an example and make sure everything works correctly. You might want to put a membrane resonance equation there and discuss the frequency region where it is applicable.
There is a limit to how effective even herculean efforts can be. Take a look at B&W 801 review in stereophile and note what the effects of matrix bracing are. The box still resonates at a surprisingly low frequency. The amplitude is satisfactorily low, though.
http://stereophile.com/floorloudspeakers/506/index7.html
When looking at (comparing) these plots you have to be very careful to understand where the accelerometer was placed. I wonder how JA chooses a location?
A (box) speaker is more than just 6 panels, there is complicated 3D motion going on (twisting, etc) that requires FEA to analyse.
Point bracing is nowhere near as effective as shelves or ribs.
IMO complicated bracing structures are probably more likely to rattle.
There are too many unsubstantiated general statements in your text. You would do well to track down supporting literature, etc.
Cheers.
I think it is good to have a plate resonance equation. Make sure to work though an example and make sure everything works correctly. You might want to put a membrane resonance equation there and discuss the frequency region where it is applicable.
There is a limit to how effective even herculean efforts can be. Take a look at B&W 801 review in stereophile and note what the effects of matrix bracing are. The box still resonates at a surprisingly low frequency. The amplitude is satisfactorily low, though.
http://stereophile.com/floorloudspeakers/506/index7.html
When looking at (comparing) these plots you have to be very careful to understand where the accelerometer was placed. I wonder how JA chooses a location?
A (box) speaker is more than just 6 panels, there is complicated 3D motion going on (twisting, etc) that requires FEA to analyse.
Point bracing is nowhere near as effective as shelves or ribs.
IMO complicated bracing structures are probably more likely to rattle.
There are too many unsubstantiated general statements in your text. You would do well to track down supporting literature, etc.
Cheers.
rick57 said:
Originally posted by rick57
Where this is not possible, next best is simply to reduce its audibility, as the higher the frequency, the greater the amount of energy needed to excite it.
posted by dave
No time to go thru the whole yet, but this should be "there is less energy available to excite the resonances"
dave
Is it mostly your point about energy available, but also that a greater amount of energy is needed to excite it?
Where this is not possible, next best is simply to reduce its audibility, as the higher the frequency, the greater the amount of energy needed to excite it.
posted by dave
No time to go thru the whole yet, but this should be "there is less energy available to excite the resonances"
dave
Is it mostly your point about energy available, but also that a greater amount of energy is needed to excite it?
Ron,
"There are too many unsubstantiated general statements in your text. You would do well to track down supporting literature, etc."
My statements are drawn from the experience of others, mostly dave. If I had access to any supporting literature I’d look there too.
audioXpress in January - February 2006 had Lowering a Loudspeaker's Mechanical Noise Floor, Pts 1 & 2; and in February 2002 "Panel Damping Studies: Reducing Loudspeaker Enclosure Vibrations", but I don’t have audioXpress . . Anyone who can post a summary or better a PDF?
Cheers
"There are too many unsubstantiated general statements in your text. You would do well to track down supporting literature, etc."
My statements are drawn from the experience of others, mostly dave. If I had access to any supporting literature I’d look there too.
audioXpress in January - February 2006 had Lowering a Loudspeaker's Mechanical Noise Floor, Pts 1 & 2; and in February 2002 "Panel Damping Studies: Reducing Loudspeaker Enclosure Vibrations", but I don’t have audioXpress . . Anyone who can post a summary or better a PDF?
Cheers
Well it certainly sounds like a good start, though it might be better to try a top-down approach and get stuck into the details later. Try sorting out some headings and sub-headings first.
If people are going to make use of the wiki, they'll need to be able to find the information they want, quickly and efficiently. Keeping in mind possible FAQs, some headings might include, eg:
Bracing
-Purpose
--detailed reasoning
--[another sub-sub-heading]
-Materials and techniques
--[sub-sub-heading]
...etc
"Bracing" could itself be grouped with "loudspeaker design techniques" or something along those lines. Then, even if you run out of time, other people will have an easier job of adding stuff.
If people are going to make use of the wiki, they'll need to be able to find the information they want, quickly and efficiently. Keeping in mind possible FAQs, some headings might include, eg:
Bracing
-Purpose
--detailed reasoning
--[another sub-sub-heading]
-Materials and techniques
--[sub-sub-heading]
...etc
"Bracing" could itself be grouped with "loudspeaker design techniques" or something along those lines. Then, even if you run out of time, other people will have an easier job of adding stuff.
"loudspeaker design techniques" is a very worthy and very big topic
The biggest I thought I’d make this is cabinet design & making techniques, but I thought I’d KIS it for now, and it could grow later.
I’ve actually not used the wiki much at all
, though I’ll be looking at the horn wiki real soon . .
I thought the structure was ok, anyone else thinks the structure needs work before the content is worked on?
The biggest I thought I’d make this is cabinet design & making techniques, but I thought I’d KIS it for now, and it could grow later.
I’ve actually not used the wiki much at all
, though I’ll be looking at the horn wiki real soon . .I thought the structure was ok, anyone else thinks the structure needs work before the content is worked on?
rick57 said:
So why should anyone believe your wiki?
http://bellsouthpwp.net/l/j/ljfrank/Bracing.html
Ron E
Thanks for the link, I’ll read it
Jeff
“a moderator has to initiate things”?
"excuse"?
I thought it was clear, I’d make a wiki when I get the feeling that it’s mostly right.
So far no one has said this . .
I don’t believe it’s right to give a bunch of statements wiki status, just because I think they’re probably right, without more positive feedback
Thanks for the link, I’ll read it
Jeff
“a moderator has to initiate things”?
"excuse"?
I thought it was clear, I’d make a wiki when I get the feeling that it’s mostly right.
So far no one has said this . .
I don’t believe it’s right to give a bunch of statements wiki status, just because I think they’re probably right, without more positive feedback
Well, now that you mention it, i've actually started a static page that goes in a similar direction here .
I wrote a bit about sealed speaker boxes, ported, dipole, acoustic labyrinth, and a couple of other things before running out of steam... You guys are welcome to make use of it if you think it's any good (but try not to hot-link the images though).
I think wikis can be a lot better because they're so much more extensible and easier to maintain compared to static web pages. I don't think it matters if a certain piece of information is 'worthy' or not – someone else can always improve on it.
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