>roofing felt or dynamat will pull the resonant frequencies of the panels back down so to some extent negates what you are trying to do with the bracing.
====
If you increase the thickness of a low resonance panel by using the same/similar material, you are 'chasing your tail' since to get it really well damped requires its resonance to be pushed below the design's Fb. Might as well use concrete cause that's where you're going to wind up WRT the amount of net mass required if Fb is below ~125Hz.
OTOH, if the panel resonance is raised, its amplitude falls due to its energy falling and the mass of the panel further attenuates it as it appears increasingly massive with increasing frequency. The damping material continues it rather than lowering its resonance by increasing mass/other 'lossy' properties through the impedance mismatch of very dissimilar materials, like when you (highly damped) grab a resonating tuning fork.
I long ago came to the conclusion that there's no such thing as too massive/rigid a cab until its resonance is either below or above the speaker's BW for excellent acoustic efficiency/damping cab colorations to below the -35dB S/N ratio of the average person's hearing acuity.
Since it's more practical in most cases to make a rigid cab than a massive one, using the most practical rigid material (13ply Baltic Birch in most designs) combined with good structural design and bracing it through triangulation to further raise its resonance will yield sufficient mechanical/acoustic efficiency for HIFI/HT apps. All that's left then is to use a very dissimilar material (high DF) to attenuate any audible HF 'ringing' that occurs due to any standing waves.
In average size cabs, lining one wall, back, and top with fiberglass is normally sufficient, and cheap self stick linoleum tiles or similar on larger ones that will have standing waves down in the lower mids/midbass BW. Using non parallel wall designs further reduces standing wave amplitudes, making the need for additional damping material moot in some designs.
Bottom line is that unless I want the cab to 'enhance' the driver's output by 'singing along' over a narrow BW, then before any damping material is added I don't consider it stiff enough until it rings ~uniformly at a pitch similar to one made from sheetmetal when tapped all over with a ballpeen hammer.
GM
====
If you increase the thickness of a low resonance panel by using the same/similar material, you are 'chasing your tail' since to get it really well damped requires its resonance to be pushed below the design's Fb. Might as well use concrete cause that's where you're going to wind up WRT the amount of net mass required if Fb is below ~125Hz.
OTOH, if the panel resonance is raised, its amplitude falls due to its energy falling and the mass of the panel further attenuates it as it appears increasingly massive with increasing frequency. The damping material continues it rather than lowering its resonance by increasing mass/other 'lossy' properties through the impedance mismatch of very dissimilar materials, like when you (highly damped) grab a resonating tuning fork.
I long ago came to the conclusion that there's no such thing as too massive/rigid a cab until its resonance is either below or above the speaker's BW for excellent acoustic efficiency/damping cab colorations to below the -35dB S/N ratio of the average person's hearing acuity.
Since it's more practical in most cases to make a rigid cab than a massive one, using the most practical rigid material (13ply Baltic Birch in most designs) combined with good structural design and bracing it through triangulation to further raise its resonance will yield sufficient mechanical/acoustic efficiency for HIFI/HT apps. All that's left then is to use a very dissimilar material (high DF) to attenuate any audible HF 'ringing' that occurs due to any standing waves.
In average size cabs, lining one wall, back, and top with fiberglass is normally sufficient, and cheap self stick linoleum tiles or similar on larger ones that will have standing waves down in the lower mids/midbass BW. Using non parallel wall designs further reduces standing wave amplitudes, making the need for additional damping material moot in some designs.
Bottom line is that unless I want the cab to 'enhance' the driver's output by 'singing along' over a narrow BW, then before any damping material is added I don't consider it stiff enough until it rings ~uniformly at a pitch similar to one made from sheetmetal when tapped all over with a ballpeen hammer.
GM
GM, thanks for helping to clear up my ideas about cabinet damping, and I'm sure others needed to know too 
btw, I did try the soft foam/rubber type of carpet underlay in some speakers and it sounded much better than the white foam wadding stuff. I won't embarass myself by guessing why...
-Simon
btw, I did try the soft foam/rubber type of carpet underlay in some speakers and it sounded much better than the white foam wadding stuff. I won't embarass myself by guessing why...
-Simon
I'm obviously not so confident in this area of knowledge but...
Metal rings, because it has poor 'self-damping', and I think has more than one area of resonance. This is partly why metal racks tend to introduce a nasty and un-natural character to the sound.
The best practical material may be some sort of hard wood, which resonates over a broader band than stuff like metal, and MDF. Also, some people seem to think using two or more types of wood/material is better than one, becuase the two characters help to null the effects of each other.
The 'best' dynamic speakers all seem to be large and heavy. When a cabinet is super-stiff and heavy, the waterfall/time response plot of the speaker is good.
However, from reading the previous replys there seems to be a slight lack of clarity in identifying what is ideal. There must be a goal, if you were making, say, an 'ultimate speaker', like the andromeda for example - that is heavy. The guy making that used bitumen between two layers of MDF I believe! Why is that (not?) a good idea?
-Simon
planet10 said:
lets start from teh very begining.....
lets say you need an effective volume of 3 cu. ft. (~80 liters) for your cabinet. the volume of teh box will be caculated so the volume using the inner dimensions of the boxis volume used by drivers, braces, ports, etc. so for a 3 cu. ft box you really need 3.5 cu. ft (~100 liters) intrenal volume. for small boxes the % increase is more. 3.5cu. ft. for a tower speaker is say 36"Hx12"Wx14"D internal. using 1" MDF which is often what DIYers use mean the external dimensions are 38"x14"x16". now keeping external dimensions teh same if we used 1/2" MDF we'd get internal dimensions of 37"x13"x15". we could then brace teh cabinet using about 1100 cu in of wood (1"x1"x1100" for example) and get the same net internal volume yet get a cabinet that is more rigid that a unbraced cabient using 1" MDF. once teh cabinet is rigid and the resonsces are in teh 400Hz range instead of in teh 100Hz range there are many damping amterials that can absorb those resonsces.
lucky you.....if you follow my previous posts i had to sandwich layers of 3mm MDF and 4mm Ply to achieve this result.planet10 said:
>GM, thanks for helping to clear up my ideas about cabinet damping.....
====
You're welcome! Year in and year out this seems to be one of the most misunderstood facets of speaker building. The kind of sad part is even after they grasp it, most folks will continue to use MDF anyway because it's so prevalent among manufacturers/cheap/its ease of routing compared to plywood. Or maybe they're just hooked on snorting formaldehyde loaded fumes.
====
>This is quite interesting...So having super thick MDF is not necessarily a positive thing.
====
Not if you have a bad back, like me. I mean it works if thick enough, but it has to be pretty thick to match a minimally braced 3/4" Baltic Birch plywood construction.
====
> And how about using 5mm thick steel? or is the lack of metal cabinets around related to stored energy or something?
====
Just like wood, its thickness will be dependent on the desired BW, size of cab and/or bracing. For instance, early theater horns only had to reproduce ~250-6kHz so they were made out of fairly thin sheetmetal with no damping since the 'cab's Fs was above the intended BW and it didn't go low enough to require extreme rigidity. As the LF BW increased, they switched to 3/4" marine grade plywood for separate multiple driver tower or folded horn cabs.
Since the HF was extending also, most of the various HF horns were designed as multiple small horns that when ganged together had a large enough mouth area to support down to ~100-150Hz. Most designs had each horn cell made from thin sheetmetal which were attached to a cast metal adapter/throat then damped with tar.
This is really about as good as it gets, no ringing in the audible BW, period. I was around one that had the tar removed so it could be repaired, and it was about as resonant as a tuning fork. Some used very dense wood cut in thin sheets, then damped, and worked just as well in a theater app, but I don't know if they're as 'dead' in a HIFI app where the S/N level is much less. Later, they went with thicker metal and a spray on asbestos loaded auto undercoating used by the auto industry at the time. These have a very slight metallic sound to them if used in a HIFI app without a supertweeter.
Anyway, 5mm is a little thicker than 3/16" thick so is extreme overkill for most speaker apps. I used to have occasional access to industrial electrical cabs, some as large as 30ft^3. These were made from 10ga (~0.125"/3.18mm) HRSPO sheet, and with some angles spot welded on diagonally to break up the large panels/damping, could handle four HE15" without any audible coloration that we could tell, though they were never measured so can't swear on a stack of bibles that there wasn't any.
The smaller cabs I liked to use were ~1ft^3 12ga stainless steel Nema 4 cabs. All I did to them was add some cheap rubber tiles to the panels. Again, no measurements, but they sounded pretty much like what the drivers did on an open baffle so I consider this as having no audible coloration.
I assume that few manufacturers use metal due to the extra skill required, hence higher labor costs, and more expensive tooling.
====
>Stainless or alumium would be better. Just imagine the interference on a TV when you have steel speakers
====
Actually, cold rolled sheet steel (CRS) makes an excellent shielding material, and what I use when needed.
====
>I've read that laminated boxes are better than single sheet, like 2x10mm instead of 20mm. Hardwork though...
====
Yes, and no. As I noted, if it's the same material...... for instance, around here, the quality of MDF is pretty good since we have several processing plants in the region, with an average MOE (modulus of elasticity) of ~527k psi.. 13 ply 3/4" Baltic Birch is 1.8m.. quite a disparity in stiffness. Since stiffness goes up at the cube of thickness, with these values it takes 1.125" thick MDF to match the BB ply's.
Now because the BB's MOE is higher than the MDF's, it takes less bracing to reach a given stiffness than for the MDF. Doubling the BB's stiffness to 3.6m through some bracing requires the MDF to be the equivalent thickness of 1.414", so the rule-of-thumb I use is that doubling up 3/4" MDF is ~ equal to bracing up a single layer of 3/4" 13 ply BB.
The former weighs in at ~100lbs/ft^3 of material plus glue Vs ~36lbs/ft^3 plus much less glue, not to mention the extra work, wear n' tear on tools, greater potential health hazzard with MDF, etc..
====
>The best practical material may be some sort of hard wood, which resonates over a broader band than stuff like metal, and MDF.
====
No-void plywood with 11 plys minimum, i.e. Baltic Birch, Appleply, some marine grades, gets my vote. High MOE, reasonable weight, not too expensive.
====
>Also, some people seem to think using two or more types of wood/material is better than one, becuase the two characters help to null the effects of each other.
====
This is constrained layer construction. Damping is about impedance mismatching, so the greater the difference in the material's properties, the better, such as MDF/thin alum sheeting bonded together with a non-hardening/lossy glue.
====
>The 'best' dynamic speakers all seem to be large and heavy. When a cabinet is super-stiff and heavy, the waterfall/time response plot of the speaker is good.
====
Yep, though due to acoustic energy falling with increasing frequency, once you get above ~125Hz, much of the need for mass loading starts going away and is down fully 50% at 632Hz based on a 20-20kHz BW, so a cardboard box just strong enough to not collapse under the driver's weight is plenty sufficient for a super tweeter.
====
>However, from reading the previous replys there seems to be a slight lack of clarity in identifying what is ideal. There must be a goal, if you were making, say, an 'ultimate speaker', like the andromeda for example - that is heavy. The guy making that used bitumen between two layers of MDF I believe! Why is that (not?) a good idea?
====
Hopefully, I've clarified this subject enough for you to answer these now. If not, I've wasted a lot of time.
GM
====
You're welcome! Year in and year out this seems to be one of the most misunderstood facets of speaker building. The kind of sad part is even after they grasp it, most folks will continue to use MDF anyway because it's so prevalent among manufacturers/cheap/its ease of routing compared to plywood. Or maybe they're just hooked on snorting formaldehyde loaded fumes.
====
>This is quite interesting...So having super thick MDF is not necessarily a positive thing.
====
Not if you have a bad back, like me. I mean it works if thick enough, but it has to be pretty thick to match a minimally braced 3/4" Baltic Birch plywood construction.
====
> And how about using 5mm thick steel? or is the lack of metal cabinets around related to stored energy or something?
====
Just like wood, its thickness will be dependent on the desired BW, size of cab and/or bracing. For instance, early theater horns only had to reproduce ~250-6kHz so they were made out of fairly thin sheetmetal with no damping since the 'cab's Fs was above the intended BW and it didn't go low enough to require extreme rigidity. As the LF BW increased, they switched to 3/4" marine grade plywood for separate multiple driver tower or folded horn cabs.
Since the HF was extending also, most of the various HF horns were designed as multiple small horns that when ganged together had a large enough mouth area to support down to ~100-150Hz. Most designs had each horn cell made from thin sheetmetal which were attached to a cast metal adapter/throat then damped with tar.
This is really about as good as it gets, no ringing in the audible BW, period. I was around one that had the tar removed so it could be repaired, and it was about as resonant as a tuning fork. Some used very dense wood cut in thin sheets, then damped, and worked just as well in a theater app, but I don't know if they're as 'dead' in a HIFI app where the S/N level is much less. Later, they went with thicker metal and a spray on asbestos loaded auto undercoating used by the auto industry at the time. These have a very slight metallic sound to them if used in a HIFI app without a supertweeter.
Anyway, 5mm is a little thicker than 3/16" thick so is extreme overkill for most speaker apps. I used to have occasional access to industrial electrical cabs, some as large as 30ft^3. These were made from 10ga (~0.125"/3.18mm) HRSPO sheet, and with some angles spot welded on diagonally to break up the large panels/damping, could handle four HE15" without any audible coloration that we could tell, though they were never measured so can't swear on a stack of bibles that there wasn't any.
The smaller cabs I liked to use were ~1ft^3 12ga stainless steel Nema 4 cabs. All I did to them was add some cheap rubber tiles to the panels. Again, no measurements, but they sounded pretty much like what the drivers did on an open baffle so I consider this as having no audible coloration.
I assume that few manufacturers use metal due to the extra skill required, hence higher labor costs, and more expensive tooling.
====
>Stainless or alumium would be better. Just imagine the interference on a TV when you have steel speakers
====
Actually, cold rolled sheet steel (CRS) makes an excellent shielding material, and what I use when needed.
====
>I've read that laminated boxes are better than single sheet, like 2x10mm instead of 20mm. Hardwork though...
====
Yes, and no. As I noted, if it's the same material...... for instance, around here, the quality of MDF is pretty good since we have several processing plants in the region, with an average MOE (modulus of elasticity) of ~527k psi.. 13 ply 3/4" Baltic Birch is 1.8m.. quite a disparity in stiffness. Since stiffness goes up at the cube of thickness, with these values it takes 1.125" thick MDF to match the BB ply's.
Now because the BB's MOE is higher than the MDF's, it takes less bracing to reach a given stiffness than for the MDF. Doubling the BB's stiffness to 3.6m through some bracing requires the MDF to be the equivalent thickness of 1.414", so the rule-of-thumb I use is that doubling up 3/4" MDF is ~ equal to bracing up a single layer of 3/4" 13 ply BB.
The former weighs in at ~100lbs/ft^3 of material plus glue Vs ~36lbs/ft^3 plus much less glue, not to mention the extra work, wear n' tear on tools, greater potential health hazzard with MDF, etc..
====
>The best practical material may be some sort of hard wood, which resonates over a broader band than stuff like metal, and MDF.
====
No-void plywood with 11 plys minimum, i.e. Baltic Birch, Appleply, some marine grades, gets my vote. High MOE, reasonable weight, not too expensive.
====
>Also, some people seem to think using two or more types of wood/material is better than one, becuase the two characters help to null the effects of each other.
====
This is constrained layer construction. Damping is about impedance mismatching, so the greater the difference in the material's properties, the better, such as MDF/thin alum sheeting bonded together with a non-hardening/lossy glue.
====
>The 'best' dynamic speakers all seem to be large and heavy. When a cabinet is super-stiff and heavy, the waterfall/time response plot of the speaker is good.
====
Yep, though due to acoustic energy falling with increasing frequency, once you get above ~125Hz, much of the need for mass loading starts going away and is down fully 50% at 632Hz based on a 20-20kHz BW, so a cardboard box just strong enough to not collapse under the driver's weight is plenty sufficient for a super tweeter.
====
>However, from reading the previous replys there seems to be a slight lack of clarity in identifying what is ideal. There must be a goal, if you were making, say, an 'ultimate speaker', like the andromeda for example - that is heavy. The guy making that used bitumen between two layers of MDF I believe! Why is that (not?) a good idea?
====
Hopefully, I've clarified this subject enough for you to answer these now. If not, I've wasted a lot of time.
GM
I'm getting there slowly...
Let me break it down a bit more for my simple mind:
Ideal material properties:
a) Stiff as possible (high MOE)
b) High damping properties
c) Uniform
Human-Usability properties
a) Light-weight
b) Workable
c) Cheap
d) Attractive
What have I missed on the material properties list? How is damping measured?
Let me break it down a bit more for my simple mind:
Ideal material properties:
a) Stiff as possible (high MOE)
b) High damping properties
c) Uniform
Human-Usability properties
a) Light-weight
b) Workable
c) Cheap
d) Attractive
What have I missed on the material properties list? How is damping measured?
Very well clarified thankyou GM!
Your last post really is *the* definitive guide now, as far as I'm concerned
I think I will experiment with some bits of wood (light, with high resonant frequency), lining some of the innards of my (MDF only) speakers. I intend to create a bumpy surface on the side walls and cover them with rubbery carpet underlay, any thoughts on that?? (my fronts/bacsk and tops/bottoms are non-parallel already.)
I realise now that bitumen would only be very useful if sandwiched between sheets of stiff/cabinet-wall material.
Vikash,
I think you've broken it down well, except uniform would probably come under 'human-usability-properties' rather than 'ideal material properties', though maybe it is important for both..?
-Simon
Your last post really is *the* definitive guide now, as far as I'm concerned
I think I will experiment with some bits of wood (light, with high resonant frequency), lining some of the innards of my (MDF only) speakers. I intend to create a bumpy surface on the side walls and cover them with rubbery carpet underlay, any thoughts on that?? (my fronts/bacsk and tops/bottoms are non-parallel already.)
I realise now that bitumen would only be very useful if sandwiched between sheets of stiff/cabinet-wall material.
Vikash,
I think you've broken it down well, except uniform would probably come under 'human-usability-properties' rather than 'ideal material properties', though maybe it is important for both..?
-Simon
>Let me break it down a bit more for my simple mind:
Ideal material properties:
a) Stiff as possible (high MOE)
b) High damping properties
c) Uniform
====
Uniform??
====
>Human-Usability properties
a) Light-weight
b) Workable
c) Cheap
d) Attractive
What have I missed on the material properties list?
====
Nothing jumps out at me.
====
> How is damping measured?
====
Like anything else, the amplitude and Q of the Fs peak.
====
>Very well clarified thankyou GM!
>Your last post really is *the* definitive guide now, as far as I'm concerned
====
You're welcome! Well, I hope it's more definitive anyway.
====
>I think I will experiment with some bits of wood (light, with high resonant frequency), lining some of the innards of my (MDF only) speakers. I intend to create a bumpy surface on the side walls and cover them with rubbery carpet underlay, any thoughts on that?? (my fronts/bacsk and tops/bottoms are non-parallel already.)
====
As walls become increasingly non-parallel, their combined stiffness increases and eigenmodes are lower amplitude, so don't be surprised if it winds up being severely overdamped. Early LF drivers used much deeper diaphragm designs than today for this reason due to the limited materials available to get the 'right' physical properties required for low power tube amps.
====
>I realise now that bitumen would only be very useful if sandwiched between sheets of stiff/cabinet-wall material.
====
Yep.
GM
Ideal material properties:
a) Stiff as possible (high MOE)
b) High damping properties
c) Uniform
====
Uniform??
====
>Human-Usability properties
a) Light-weight
b) Workable
c) Cheap
d) Attractive
What have I missed on the material properties list?
====
Nothing jumps out at me.
====
> How is damping measured?
====
Like anything else, the amplitude and Q of the Fs peak.
====
>Very well clarified thankyou GM!
>Your last post really is *the* definitive guide now, as far as I'm concerned
====
You're welcome!
Well, I hope it's more definitive anyway. ====
>I think I will experiment with some bits of wood (light, with high resonant frequency), lining some of the innards of my (MDF only) speakers. I intend to create a bumpy surface on the side walls and cover them with rubbery carpet underlay, any thoughts on that?? (my fronts/bacsk and tops/bottoms are non-parallel already.)
====
As walls become increasingly non-parallel, their combined stiffness increases and eigenmodes are lower amplitude, so don't be surprised if it winds up being severely overdamped. Early LF drivers used much deeper diaphragm designs than today for this reason due to the limited materials available to get the 'right' physical properties required for low power tube amps.
====
>I realise now that bitumen would only be very useful if sandwiched between sheets of stiff/cabinet-wall material.
====
Yep.
GM
i.e. MDF is uniform. Real wood is not so much due to knots making it stronger/weaker in places.
I was looking for something that would tell me whether a particular material provides good damping.
I'm interested in some densified laminated wood which has a whole bunch of technical properties (on a spec sheet) that I don't understand. I was wondering whether any of them would give me a clue to its damping properties. Its MOE is greater than the Baltic Birch you mentioned, so if I could also be sure of its damping properties, then I'd pay the premium and give it a go...
Loudspeaker bracing
From other suggestions it appears you are on the right track, my priority is always use hardwood bracing between the baffle to rear and where possible from front/back to side if the area warrants it. This really breaks up the resonant modes. Pine wood is u/s.
I often make large cabinets, 5-7Cuft and Don't be skint with wood glue and screws, use it/them librally with torx screws and drill. (Pilot hole first).
When you complete the system, use amp and run signal genny from 30Hz to a few K's you will never regret proper bracing. Placing the hand on an unbraced panel gives a nice sensation but brace it at once.
In theory you should increase the volume of the cabinet if the amount of bracing is substantial.
A word of warning, make doubly sure that the frontpanel (baffle) is flat as a die (before screwing the drive units) and not warped from screwing in bracing struts. It's too easily done.
Modern day woofer baskets are pretty versatile but none too often the magnets are simply glued on to the back and screwing on to a warped baffle would simply ruin 50 quids worth of driver. I believe my Peerless (which have run for decades without problems) have this manu technique.
I used a <lazy susan> to "check the baffle in flat" with a surplus flat wood and grit paper. If the amount out of "run out" is minimal, it doesn't take long.
A few simplistic construction hints......
Egyptian Golden dimensions rule 0.6-1-1.62 for example,
8"D-13"W- 21"H. ( I know speakers weren't designed those days but mathematically this ratio was already sussed up)
Others; Always place HF units above LF units; tweeters at ear height where directional properties are optimum. LF units near floor for good reinforcement, unless on stand.
Keep drivers in straight line otherwise system may be marred by phase distortion. If you design units off centre then make sure that for stereo the other speaker is a mirror image of the one you have just made.
If one is building a tower speaker (slim narrow enclosure) place driver 1/5 from end, which supresses 5th harmonic, (tube type resonance effects).
Two drive units in an enclosure sound far better than singles and one can relax a bit on the double up volume requirements without a detriment in sound.
Moto.... keep it simple and you will be more than satisfied.
rich
From other suggestions it appears you are on the right track, my priority is always use hardwood bracing between the baffle to rear and where possible from front/back to side if the area warrants it. This really breaks up the resonant modes. Pine wood is u/s.
I often make large cabinets, 5-7Cuft and Don't be skint with wood glue and screws, use it/them librally with torx screws and drill. (Pilot hole first).
When you complete the system, use amp and run signal genny from 30Hz to a few K's you will never regret proper bracing. Placing the hand on an unbraced panel gives a nice sensation but brace it at once.
In theory you should increase the volume of the cabinet if the amount of bracing is substantial.
A word of warning, make doubly sure that the frontpanel (baffle) is flat as a die (before screwing the drive units) and not warped from screwing in bracing struts. It's too easily done.
Modern day woofer baskets are pretty versatile but none too often the magnets are simply glued on to the back and screwing on to a warped baffle would simply ruin 50 quids worth of driver. I believe my Peerless (which have run for decades without problems) have this manu technique.
I used a <lazy susan> to "check the baffle in flat" with a surplus flat wood and grit paper. If the amount out of "run out" is minimal, it doesn't take long.
A few simplistic construction hints......
Egyptian Golden dimensions rule 0.6-1-1.62 for example,
8"D-13"W- 21"H. ( I know speakers weren't designed those days but mathematically this ratio was already sussed up)
Others; Always place HF units above LF units; tweeters at ear height where directional properties are optimum. LF units near floor for good reinforcement, unless on stand.
Keep drivers in straight line otherwise system may be marred by phase distortion. If you design units off centre then make sure that for stereo the other speaker is a mirror image of the one you have just made.
If one is building a tower speaker (slim narrow enclosure) place driver 1/5 from end, which supresses 5th harmonic, (tube type resonance effects).
Two drive units in an enclosure sound far better than singles and one can relax a bit on the double up volume requirements without a detriment in sound.
Moto.... keep it simple and you will be more than satisfied.
rich
>i.e. MDF is uniform. Real wood is not so much due to knots making it stronger/weaker in places.
====
Ah, OK, you're right, when using other types of wood it needs to be knot free, etc., i.e. no-void grade. I used marine grade for larger cabs for this reason, but they don't guarantee it being void-free anymore, so BB ply and Apple ply are the only plys left AFAIK.
There are exceptions though, such as very old, weather dried wood. I built smaller cabs out of free >150 yr old barn planking that has to planed down from 7/8"-1"+ to ~1/2"-5/8" to get it flat, and it's still stiff enough to not need any bracing, yet easily damped. Tough on blades, etc., though. Knots, minor cracks that didn't go all the way through were sealed with JB weld.
====
>I was looking for something that would tell me whether a particular material provides good damping.
I'm interested in some densified laminated wood which has a whole bunch of technical properties (on a spec sheet) that I don't understand. I was wondering whether any of them would give me a clue to its damping properties. Its MOE is greater than the Baltic Birch you mentioned, so if I could also be sure of its damping properties, then I'd pay the premium and give it a go...
====
Material density determines its damping factor, so lead or sand filled panel cabs is the ideal for folks who believe that anything worth doing is worth doing to excess.
Again though, the stiffer (MOE) the panel, the higher its Fs, and since the acoustic energy (amplitude) exciting it falls at the square of increasing frequency, the stiffer a panel is for a given thickness, the more DF it has, ergo the stiffest/densest material (or combination of, in a constrained layer design) at a manageable workability/weight is the practical ideal.
Recap:
Stiffness increases at the cube of thickness.
Damping increases at the square of stiffness.
As a generality, a panel's deflection is a function of the square of its length, so dividing it into two sections via bracing increases its stiffness 4x, in thirds, 8x.
From this we see that starting with a stiff material, then bracing it yields the best stiffness:damping:weight ratio.
Where weight is less critical (or even desired), increasing stiffness/damping through increased thickness (2x as thick, 8x as stiff) and/or combining different material properties is preferred.
Of course you're not likely to get the cab stiff/damped enough to quell all resonance back through the diaphragm(s) in some cases, so spot damping with dynamat (or similar), or just lining one wall, back, and end opposite the vent (if used) should suffice if you got it stiff/damped enough.
I can balance a quarter on edge on either of my 20ft^3 cabs while four 15" are pounding out pipe organ music, so I'm satisfied that theory translates to reality pretty well in this case.
GM
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Ah, OK, you're right, when using other types of wood it needs to be knot free, etc., i.e. no-void grade. I used marine grade for larger cabs for this reason, but they don't guarantee it being void-free anymore, so BB ply and Apple ply are the only plys left AFAIK.
There are exceptions though, such as very old, weather dried wood. I built smaller cabs out of free >150 yr old barn planking that has to planed down from 7/8"-1"+ to ~1/2"-5/8" to get it flat, and it's still stiff enough to not need any bracing, yet easily damped. Tough on blades, etc., though. Knots, minor cracks that didn't go all the way through were sealed with JB weld.
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>I was looking for something that would tell me whether a particular material provides good damping.
I'm interested in some densified laminated wood which has a whole bunch of technical properties (on a spec sheet) that I don't understand. I was wondering whether any of them would give me a clue to its damping properties. Its MOE is greater than the Baltic Birch you mentioned, so if I could also be sure of its damping properties, then I'd pay the premium and give it a go...
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Material density determines its damping factor, so lead or sand filled panel cabs is the ideal for folks who believe that anything worth doing is worth doing to excess.
Again though, the stiffer (MOE) the panel, the higher its Fs, and since the acoustic energy (amplitude) exciting it falls at the square of increasing frequency, the stiffer a panel is for a given thickness, the more DF it has, ergo the stiffest/densest material (or combination of, in a constrained layer design) at a manageable workability/weight is the practical ideal.
Recap:
Stiffness increases at the cube of thickness.
Damping increases at the square of stiffness.
As a generality, a panel's deflection is a function of the square of its length, so dividing it into two sections via bracing increases its stiffness 4x, in thirds, 8x.
From this we see that starting with a stiff material, then bracing it yields the best stiffness:damping:weight ratio.
Where weight is less critical (or even desired), increasing stiffness/damping through increased thickness (2x as thick, 8x as stiff) and/or combining different material properties is preferred.
Of course you're not likely to get the cab stiff/damped enough to quell all resonance back through the diaphragm(s) in some cases, so spot damping with dynamat (or similar), or just lining one wall, back, and end opposite the vent (if used) should suffice if you got it stiff/damped enough.
I can balance a quarter on edge on either of my 20ft^3 cabs while four 15" are pounding out pipe organ music, so I'm satisfied that theory translates to reality pretty well in this case.
GM
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