Finally I've found the answer:
Given the normal vibration velocity of an elementary surface, it is possible to evaluate the acoustic power with the following relation:
W=1/2*(
*c*v^2*S*
)
where:
In the case of the Equivalent Radiated Power model, the elementary surface is assumed to radiate like a plate with a unit efficiency:
From the equation is not to difficult to calculate how much dBA will be radiated in given resonant frequency....
Given the normal vibration velocity of an elementary surface, it is possible to evaluate the acoustic power with the following relation:
W=1/2*(
where:
- is the overall acoustic power [W] radiated by the system under study
- is the air volumetric mass density [kg/m^3]
- is the sound velocity in air [m/s]
- is the normal vibration velocity [m/s2] of the surface
- is the surface area [m^2]
- is the radiation factor []
In the case of the Equivalent Radiated Power model, the elementary surface is assumed to radiate like a plate with a unit efficiency:
From the equation is not to difficult to calculate how much dBA will be radiated in given resonant frequency....
I keep being stunned by those in Sweden having trouble getting really good plywood from next door in Sweden.
MDF is really good at energy storage and slowly letting it out as a time-smeared signal.
Consider that 22mm MDF is structurally about the same as 15mm quality plywood. Also tends to have lower Q, more audible, resonances/For a box as big as yours you will need more than 1 brace across the box. Add more, until you push resonances up,up… 500Hz plus is a decent target.
dave
MDF is really good at energy storage and slowly letting it out as a time-smeared signal.
Consider that 22mm MDF is structurally about the same as 15mm quality plywood. Also tends to have lower Q, more audible, resonances/For a box as big as yours you will need more than 1 brace across the box. Add more, until you push resonances up,up… 500Hz plus is a decent target.
dave
\W=1/2*(*c*v^2*S*)
:^)
clever use of graphiks to put that equation together. Thumbs up (i think many of those symbols are availble in the noral keyset.
Does thsi equation clamp the edges?
dave
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Don't be, it's no different here. All the good stuff get shipped to other countries. I used to watch truckload after truckload of solid 12" x 12" x 24' beams going to the dock in Kitimat when I worked at the mill.
I would love to see the difference between MDF and quality plywood for these tests.
jeff
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The equation doesn't care about the edges. The FEA model handling all those problem. I'm calculating the whole cabinet, with all of the braces, and I can directly compare the radiated sound power between the different each design variations.
Dave, thanks it look much better. Coul you please correct it and add the missing S into it?
It is hard to compare, because for the comparison exact mechanical properties are needed. Orthotrop Young modulus, Poisson's ratio, Shear modulus, visoelastic damping coefficient for both material.
In my case it doesn't care too much, because I'm using same material, and comparing the affect of the geometrical change. I've found a study what contained the measured properties of the MDF, and it is more than enough for comparison. The results are not absolute values, but clearly shows the differences between the geometrical change.
Hey, I didn't say it was going to be easy. 😉 Finding the mechanical properties for any wood is hard, never mind plywood.
jeff
Regarding material mechanical properties. I suggest matweb.com. They have wood and plywood. best to use the search in the upper right corner.
Hi gaszto, and all
Is the height of the speaker is not creating a main internal resonance which frequency is twice the wave length of this heigth ? The bigger in heigth is the cab the worse the lower in frequency this main resonance ?
On topic ? How experts figth this ? brass and damping felt at the top ?
I would use dense mdf (>700 kg/m3) and perhaps two layers of 15 mm for the front baffle with a visco elastic thin foint glue in between à la Sikaflex. My 2 cents.
Is the height of the speaker is not creating a main internal resonance which frequency is twice the wave length of this heigth ? The bigger in heigth is the cab the worse the lower in frequency this main resonance ?
On topic ? How experts figth this ? brass and damping felt at the top ?
I would use dense mdf (>700 kg/m3) and perhaps two layers of 15 mm for the front baffle with a visco elastic thin foint glue in between à la Sikaflex. My 2 cents.
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In a rectagukar enclosure there is the possibility of 3 primary standig wave resonances. One across the width, one across the depth, one across the height.
Does the box house a driver that will produce frequencies of any potential standing wave? If not, not a problem.
Damping will have an effect at reducing these waves, they all have less effectiveness at lower frequencies. One can compensate with thicker damping. The best place to actually damp the standing wave is right in the middle.
dave
Does the box house a driver that will produce frequencies of any potential standing wave? If not, not a problem.
Damping will have an effect at reducing these waves, they all have less effectiveness at lower frequencies. One can compensate with thicker damping. The best place to actually damp the standing wave is right in the middle.
dave
Thanks Dave.
I noticed most of the bracing made boulevard path to the wave length cause the braces have their holes in line from the top to the base.
Do you think it could be usefull to break that heigth distance by offsetting that brace holes? Or it doesn't matter as after all at those low frequency, the wave length are not beaming and are basicly 3D directional in the cab ?
Also a last that gaszto should know the answer as a mechanical engineer : what is the main source of vibration and coloration with those mid and bass drivers, please ?
- vibration of the basket transmitted trough the front baffle to the others wall (that sing in concert) due the speeder and surround excitation ?
- back wave doing the same, plus making pressure on the rear of the cone time offseted because the bounces ?
I ask because I read sometimes than braces inside should not be glued to the walls but just at the base : to avoid to reenforce and couple mechanichal vibration making the outside wall to "sing" more ? While of course a piece of wood stays on each internal wall to make it more rigid and break main resonance into several more little resonance (in spl) and higher in frequencies ?
Finally, about the damping bitumen : is it not there just to damp the main walls avoiding them to vibrate exertaly more and not at least, due to the big wave lengths, prroduce a damping of the internal cabinet energy ?
I am wondering about the spring mass spring effect and also wonder about the damping of the wood with the said constrained layers technics some noticed as used in submarines or planes. Does a very thin sheet of aluminium (non magnetic) : say less than 1 mm thick on a very thin viscoelastic glued on the internal pannels, the best way to make the walls quieter (at the risk to make this sheets inside reflectors towards the back of the cone) ?
In my book, while not able to make and understand the math behind, I thougth about very thin double tape made of a sort of fabric and very thicky to attach this thin metal layer which needs to vibrate to elimate heat. Aluminium seen on thick bitumen to proof the roofs being useless in hifi imho !
So I wonder to make it short, how to balance the 3 factors triangle of the trilogy : mass, damping, rigidity.
I noticed most of the bracing made boulevard path to the wave length cause the braces have their holes in line from the top to the base.
Do you think it could be usefull to break that heigth distance by offsetting that brace holes? Or it doesn't matter as after all at those low frequency, the wave length are not beaming and are basicly 3D directional in the cab ?
Also a last that gaszto should know the answer as a mechanical engineer : what is the main source of vibration and coloration with those mid and bass drivers, please ?
- vibration of the basket transmitted trough the front baffle to the others wall (that sing in concert) due the speeder and surround excitation ?
- back wave doing the same, plus making pressure on the rear of the cone time offseted because the bounces ?
I ask because I read sometimes than braces inside should not be glued to the walls but just at the base : to avoid to reenforce and couple mechanichal vibration making the outside wall to "sing" more ? While of course a piece of wood stays on each internal wall to make it more rigid and break main resonance into several more little resonance (in spl) and higher in frequencies ?
Finally, about the damping bitumen : is it not there just to damp the main walls avoiding them to vibrate exertaly more and not at least, due to the big wave lengths, prroduce a damping of the internal cabinet energy ?
I am wondering about the spring mass spring effect and also wonder about the damping of the wood with the said constrained layers technics some noticed as used in submarines or planes. Does a very thin sheet of aluminium (non magnetic) : say less than 1 mm thick on a very thin viscoelastic glued on the internal pannels, the best way to make the walls quieter (at the risk to make this sheets inside reflectors towards the back of the cone) ?
In my book, while not able to make and understand the math behind, I thougth about very thin double tape made of a sort of fabric and very thicky to attach this thin metal layer which needs to vibrate to elimate heat. Aluminium seen on thick bitumen to proof the roofs being useless in hifi imho !
So I wonder to make it short, how to balance the 3 factors triangle of the trilogy : mass, damping, rigidity.
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Any thoughts on this as a front baffle material? Perhaps for a Pensil 10.3 or FHXL? Pros. Cons.
Home Depot has it on sale for half price ($94)
Home Depot has it on sale for half price ($94)
Yes : not a good idea to source acacia from Vietnam when you are in a country with so much good wood for your hobby. Global warming makes sense as the local incomes.
Gotcha. Maybe that's why it was half price.
They had other versions (made of other wood I think)
Conceptually, then? With Canadian wood???
They had other versions (made of other wood I think)
Conceptually, then? With Canadian wood???
I do not want polluting gaszto thread, but short answer is the trees that grow slowly in the North makes longer fiber that are good for damping and make exceleent ply-wood which should be better than accacia. Now dense equal heavy which is good for a front baffle but I believe the best strategy is to make a damped front baffle in two sheets à la Dynaudio (not clear to me if the driver should be attached to the first or the second layer as the two layers has to be decoupled by a viscoelastic glue. There are also several good others trees in Canada with very good stifness and Young modulus ratio for damping. End of the off topic.