When panels resonate, they produce their own sound, which interferes with the sound from the drivers.
�No matter how good the drivers are, no matter how good the crossover blends the drivers, if the cabinet resonates, it will destroy the clarity and imaging of the speaker you have built� (Ref 1).
Purpose
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 less the amount of energy available to excite it.
Also damping materials attenuate higher frequencies better.
The part of excitation that originates from the sound pressure inside the box is largely proportional to 1/f� if the radiated sound is flat. Besides the direct mechanical excitation from the driver, the sound pressure inside the box is the dominating source of excitation. The pressure outside the box is typically neglectible.
So pushing the a resonance upwards by two octaves, using the numbers above should decrease excitation by 24+4 =28 dB.
In the simulation below it can be seen that the pressure inside the box (dashed) is tilted by approximately -12 dB/oct (=1/f�) relative to sound pressure at 1m (solid).
[ img
https://www.tolvan.com/boxpressure.gif]
The Hz of a panel resonance is inversely proportional to the square of the panel size, eg if a panel size is halved, 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
When setting the box volume and dimensions, allow 5-10% for the volume of the braces.
There is a limit to how effective even Herculean efforts can be. Note the effects of matrix bracing in the Stereophile review of the B&W 801. While the amplitude is low, the box still resonates at a low frequency:
https://stereophile.com/floorloudspe...506/index7.html
When comparing the plots, note where the accelerometer was placed.
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, which can be used bracing across corners in large boxes.
In most boxes,
�
Some comparisons
Measurements of cabinet wall decay times for 5 materials from a German website, with Thorsten�s summary translation; followed by comparison of 3 bracing and damping methods.
Cabinet materials: MDF, plywood, coarse fibre board, sandwich of ply/ sandfill & ply, and marble:
Bracing and damping: bitumen damping mats, board plate, and bitumen and plywood against opposite walls (not clear which material is brace and which is plate, can any German readers help . . .).
www.diyaudio.com/forums/showthread.php?s=&postid=1201165&highlight=#post1201165
He concludes by recommending that tying the braces together in the centre, so all the walls connect together �matrix style� �things get even better�.
As this was unlike the test of the rest of the material apparently without data (can�t be certain without reading German) supporting data or a reference is desirable . .
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.
[ img
https://www.diyaudio.com/forums/attachment.php?s=&postid=638823&stamp=1115687614]
- 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 the unbraced dimensions move away from each other.
� Endeavour to brace the driver magnets to the box.
[ img
https://www.diyaudio.com/forums/attachment.php?s=&postid=1171325&stamp=1175208948]
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
Discussion on materials: www.diyaudio.com/forums/showthread.php?s=&threadid=23443&highlight=
For attaching braces, maximum strength is the aim. Tips:
- Butt joints with both glue and screws; preferably two part epoxy (Ref 1).
- For maximum strength, ideally each brace is connected to other braces.
- 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 subtle concave shape).
Shape
www.diyaudio.com/forums/showthread....ght=#post832740
- holes in the braces - are to allow air movement (?)
- Approx 30 - 50% is a good proportion of holes (holes only needed to allow air flow if necessary although adding holes will have an affect on the brace's resonant signature)
[ img
https://www.diyaudio.com/forums/attachment.php?s=&postid=1181336&stamp=1176365485]
[ img
https://www.diyaudio.com/forums/attachment.php?s=&postid=832740&stamp=1138741492]
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....&threadid=96902 and this wiki
www.diyaudio.com/wiki/index.php?pag...driver+mounting
and experience with Nick at Lambda suggesting that round metal is better than square wood:
https://tech.groups.yahoo.com/group/LambdaDrivers/message/3045
. and where Nick put the bracing
https://tech.groups.yahoo.com/group/LambdaDrivers/message/3053
Acknowledgments
Inspirations and sources include: dave (planet10) GM, jeff mai and John Bonham. Also paul spencer, Zarathu, Ron E, Ginger Baker, Sonusthree and CeramicMan
?.
Further information
1.
https://bellsouthpwp.net/l/j/ljfrank/Bracing.html which amongst other things, summarizes:
- An article by James K. Iverson, The Theory of Loudspeaker Cabinet Resonances in the Journal of the Audio Engineering Society, April 1973, Volume 21, Number 3
- An article in audioXpress in February 2002
"Panel Damping Studies: Reducing Loudspeaker Enclosure Vibrations".
2. audioXpress by Jim Moriyasu in January - February 2006
�Lowering a Loudspeaker's Mechanical Noise Floor�, Pts 1 & 2
