Hi Jamie,
The L=52" is for the F120a.
Since your first post asked about the FF125wk, I included comments/dims for that too. The FF125wk needs to be a bit smaller, so I shrank the dims Scott came up with for the FE127e, resulting in the L=48" set of dims for the FF125wk. The smaller box would not be so good for the F120A! (probably the magnet on the F120A would be too deep to fit the smaller box as well.)
Cheers, Jim
The L=52" is for the F120a.
Since your first post asked about the FF125wk, I included comments/dims for that too. The FF125wk needs to be a bit smaller, so I shrank the dims Scott came up with for the FE127e, resulting in the L=48" set of dims for the FF125wk. The smaller box would not be so good for the F120A! (probably the magnet on the F120A would be too deep to fit the smaller box as well.)
Cheers, Jim
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Ah, got it. I think I am going to look at your 52" L dimensions with a square cross section, keeping the area of S(0) and S(L) the same as your calc dims to best match the F(X)120(A) drivers. The square cross section may likely help me in construction and from what I understand should not depart too much from the spirit and function of the Met. Square would also slightly increase the depth at the Z(driver) point.
I'm toying with the idea of casting the Met enclosure in iron. For no good reason, I've wanted to cast an enclosure for a while, and own a foundry, and I've had my eye on the Met for this for a while. It's a bit large for casting, but aesthetically and functionally it would appear best suited to a process I have available. In order to keep the weight within some reasonable range the wall thickness needs to be thin as possible which makes magnet clearance more difficult. Or, we'll grow the wall thickness and go with aluminum to keep the weight reasonable. I will be able to provide very good back wave clearance for the small revealed spider on these drivers though since I'll have very stiff material for driver mounting that can be thin and shaped for back wave. As I've studied Dave's posts on cabinet construction, I'm less inclined to believe super rigid is much of an advantage and expect there is a tuning of sound that's happened in birch ply over 100's of builds, but I don't think it will hurt.
Not sure the project will do anything productive for the community at large, or necessarily put all the great engineering done in the forum to best use, but who knows maybe something interesting will happen.
Jamie
I'm toying with the idea of casting the Met enclosure in iron. For no good reason, I've wanted to cast an enclosure for a while, and own a foundry, and I've had my eye on the Met for this for a while. It's a bit large for casting, but aesthetically and functionally it would appear best suited to a process I have available. In order to keep the weight within some reasonable range the wall thickness needs to be thin as possible which makes magnet clearance more difficult. Or, we'll grow the wall thickness and go with aluminum to keep the weight reasonable. I will be able to provide very good back wave clearance for the small revealed spider on these drivers though since I'll have very stiff material for driver mounting that can be thin and shaped for back wave. As I've studied Dave's posts on cabinet construction, I'm less inclined to believe super rigid is much of an advantage and expect there is a tuning of sound that's happened in birch ply over 100's of builds, but I don't think it will hurt.
Not sure the project will do anything productive for the community at large, or necessarily put all the great engineering done in the forum to best use, but who knows maybe something interesting will happen.
Jamie
I'm not sure we'll need any additional bracing. I would cast the entire enclosure except for the port and bottom baffle panel as one piece and the Met designs tend to have inherently stiff geometry.
Both cast iron and cast aluminum have about the same modulus of elasticity at about 10 million psi. 18 mm birch ply is about 1.5 million psi. so both cast metals are roughly 7X more rigid. Figure they'd be about 1/2 as thick so maybe 3 to 4 times as rigid as built not factoring joinery, etc. (see previous debates about biscuit joints, oh boy) Neither birch ply, cast iron, or cast aluminum are highly dampening compared to many materials, but I believe that we must first put the material into motion with a force before worrying too much about resonance and stiffness (Young's Modulus, Modulus of Elasticity) should directly correlate as would mass as to the energy required to displace the material for resonance.
Cast iron is one of the most dampening metals though, approximately .005 to .010 damping factor, so it does provide some dampening and I assume most of this data is measure at the resonant frequency, not the rest of the band. I'm most curious about aluminum though at this point as it has the same modulus of elasticity as cast iron for 1/3 the weight. This is somewhat of a surprise to me. It also appears the modulus of elasticity for aluminum is about the same whether the material is 10,000 psi tensile or higher alloy heat treated 30,000 psi material. Our as cast architectural aluminum without heat treatment is relatively soft with course grain. Having straightened large flat panels with a hammer, my opinion is that it's not very resonant.
With both gray cast iron and cast aluminum it is very difficult to find data on the acoustic properties. In both cases most tables will show data for material with much higher tensile and hardness properties than what we would pour for architectural purposes. The data typically comes from automotive components for iron or rolled mill product for aluminum. I've seen natural frequencies for iron from 500 to 1000 hz.
The graphitic content of cast iron gives it the interesting dampening properties and we can manipulate this further by increasing carbon content. Consider that a somewhat normal range of tensile for cast iron is in the range of 20,000 to 40,000 psi, generic data for cast iron properties could be off by a factor of 100%! where the property correlates to hardness and tensile strength.
But, it's all theoretical till it's built, isn't it? I'm more concerned that I won't have a way to measure the result in a way that is useful to others. Some of the magic of the Planet10 designs is that they've been listen tested by multiple people and compared to dozens of different designs with the recommended materials. But, I've got a hammer, so everything looks like a nail to me.
Both cast iron and cast aluminum have about the same modulus of elasticity at about 10 million psi. 18 mm birch ply is about 1.5 million psi. so both cast metals are roughly 7X more rigid. Figure they'd be about 1/2 as thick so maybe 3 to 4 times as rigid as built not factoring joinery, etc. (see previous debates about biscuit joints, oh boy) Neither birch ply, cast iron, or cast aluminum are highly dampening compared to many materials, but I believe that we must first put the material into motion with a force before worrying too much about resonance and stiffness (Young's Modulus, Modulus of Elasticity) should directly correlate as would mass as to the energy required to displace the material for resonance.
Cast iron is one of the most dampening metals though, approximately .005 to .010 damping factor, so it does provide some dampening and I assume most of this data is measure at the resonant frequency, not the rest of the band. I'm most curious about aluminum though at this point as it has the same modulus of elasticity as cast iron for 1/3 the weight. This is somewhat of a surprise to me. It also appears the modulus of elasticity for aluminum is about the same whether the material is 10,000 psi tensile or higher alloy heat treated 30,000 psi material. Our as cast architectural aluminum without heat treatment is relatively soft with course grain. Having straightened large flat panels with a hammer, my opinion is that it's not very resonant.
With both gray cast iron and cast aluminum it is very difficult to find data on the acoustic properties. In both cases most tables will show data for material with much higher tensile and hardness properties than what we would pour for architectural purposes. The data typically comes from automotive components for iron or rolled mill product for aluminum. I've seen natural frequencies for iron from 500 to 1000 hz.
The graphitic content of cast iron gives it the interesting dampening properties and we can manipulate this further by increasing carbon content. Consider that a somewhat normal range of tensile for cast iron is in the range of 20,000 to 40,000 psi, generic data for cast iron properties could be off by a factor of 100%! where the property correlates to hardness and tensile strength.
But, it's all theoretical till it's built, isn't it? I'm more concerned that I won't have a way to measure the result in a way that is useful to others. Some of the magic of the Planet10 designs is that they've been listen tested by multiple people and compared to dozens of different designs with the recommended materials. But, I've got a hammer, so everything looks like a nail to me.
I have a couple of Dave's matched FE126's that I acquired last year. They've been broken in and matched, but not spotted. I've been thinking about what to build with them for some time. BLH's, featured in my original thinking, but I've always kept the idea of the Met in the back of my mind.
I have the wood, I have the drivers, I have the stuffing, need to order some binders, and the bits for the BSC.
So 2 questions:
1. I've looked for the 1.6 Ohm resistor indicated for the filter on the Metronome page, and have not been able to find an exact match, but PE has a 1.5 Ohm, my reading tells me that this is "close enough" and I should be OK. I will be driving with a KT88 based SET amp, so wonder from some recent comments if I will even need the BSC....
2. Looking at the page for the dimensions, and some of the plan drawings available, I'm thinking that the actual dimensions for the measurements of the side/front/back panels will differ from those shown on the drawings, as the measurements are given from the bottom, and not along the panels. I can compensate for this in my drawing program, or by "Old school" drafting ( I learned prior to the computer age.....) I do have to assume that the dimensions are to be measured from the base line or bottom, and not per panel, Is this right or am I missing something?
Looking forward to butchering a bit of timber.
John
I have the wood, I have the drivers, I have the stuffing, need to order some binders, and the bits for the BSC.
So 2 questions:
1. I've looked for the 1.6 Ohm resistor indicated for the filter on the Metronome page, and have not been able to find an exact match, but PE has a 1.5 Ohm, my reading tells me that this is "close enough" and I should be OK. I will be driving with a KT88 based SET amp, so wonder from some recent comments if I will even need the BSC....
2. Looking at the page for the dimensions, and some of the plan drawings available, I'm thinking that the actual dimensions for the measurements of the side/front/back panels will differ from those shown on the drawings, as the measurements are given from the bottom, and not along the panels. I can compensate for this in my drawing program, or by "Old school" drafting ( I learned prior to the computer age.....) I do have to assume that the dimensions are to be measured from the base line or bottom, and not per panel, Is this right or am I missing something?
Looking forward to butchering a bit of timber.
John
Something I would consider is that the surface absorption on both metals is way lower than that of wood, and this implies more HF internal reflections so some internal surface treatment (bitumen or the like) besides the mass loading would be in order to avoid HF back waves passing through the driver's cone.
Gaston
Edit: Added the "internal reflections" clarification to the surface absorption remark.
Gaston
Edit: Added the "internal reflections" clarification to the surface absorption remark.
Resonant frequency would be dependent on shape and panel length one would think.
To simplify things for the sake of explanation, a 30cm metal rod will not have the same resonant frequency as a 100cm one. The latter being lower if course.
As to bracing, the idea is to push the resonant frequency of the enclosure higher than the audible range (typically 20khz) and/or dampen anything below that enough so as it does not cause any audible artifact. Bitumen sheets come to mind, as they have the dampening and the mass required.
And in general the stiffer a metal emclosure is the more likely it is to resonate.
I might be wrong as i'm relying on the info i remember from my mechanics of materials class some 15 years ago...
Please do build these
1. Yes, 1.5 ohms is close enough, but your SET amp will probably have >1.6 ohms of output impedance, maybe enough more to make the bass too under-damped [bloated] unless at least some of it is accounted for in the cab design, i.e. raise Qes_Qts somewhat in the sim to calculate a larger net Vb, though ideally tuning will be at the driver's actual Fs regardless.
This requires knowing the amp's output impedance or at least its damping factor, then divide the tap rating by it; otherwise it has to either be measured or calculated from its design parameters or assume a probably too high an amount and if too much, then add some series resistance to get the desired tonal balance.
2. Not sure what you’re driving at, so I’ll leave this one for the builders.
GM
It only needs to be above the cab's pass-band, i.e. > 2xFs/Qts, which is well below a 3/8" thick cast iron cab's. This is moot WRT 'FR' drivers though since they can't generate enough acoustic power down low to flex it.
It will be highly reflective to a 'FR' drivers mids/HF output though, but factor in that the tapered walls will reduce any reflection's intensity and minor stuffing or just lining portions of the walls should quell these short WLs.
GM
Interesting. The recommended dampening for most Mets is some acousti stuff up in the narrow end and 1" or so adsorbing material just on the back wall behind the driver. I think the idea is to not make it too "dead". The metal enclosure might need more than just the back wall, potentially sides as well to avoid these reflections back. One person has written that these bad reflections back tend to be in the 800 hz range, ironically about the natural frequency of iron?.
Or, I had thought about potentially adding some diffraction texture to the inside of the casting. But, unless the diffraction pattern is randomized, etc. it has a comb effect, AFAIK. B&W has done this, not sure if it's a gimmick unless properly done.
I would think the reflection should be easily solved by the absorbing lining. My acoustics manual suggests that higher frequencies can be effectively absorbed with even rather thin materials.
There is an added difficulty with the F120A driver in that the area behind the driver gets tight to the back wall. I haven't modeled it yet. I have no idea what that does acoustically, but it's an area of design I'm curious about. One of the benefits I was expecting of the metal enclosure was that with 3/8" wall thickness nominal and contouring the driver baffle opening I could prevent some congestion around the spider for the back wave that might occur with the large magnet drivers and thick front panels.
Or, I had thought about potentially adding some diffraction texture to the inside of the casting. But, unless the diffraction pattern is randomized, etc. it has a comb effect, AFAIK. B&W has done this, not sure if it's a gimmick unless properly done.
I would think the reflection should be easily solved by the absorbing lining. My acoustics manual suggests that higher frequencies can be effectively absorbed with even rather thin materials.
There is an added difficulty with the F120A driver in that the area behind the driver gets tight to the back wall. I haven't modeled it yet. I have no idea what that does acoustically, but it's an area of design I'm curious about. One of the benefits I was expecting of the metal enclosure was that with 3/8" wall thickness nominal and contouring the driver baffle opening I could prevent some congestion around the spider for the back wave that might occur with the large magnet drivers and thick front panels.
Oh, and I had thought that possibly the Met helps solve some of these reflection issues due to it's geometry. Tapered panels, no parallel surfaces. etc. For what I want to do it has quite a few inherently robust design features that help avoid errors in execution or maybe in this case materials. This same geometry will make a very stiff enclosure, and very might be an understatement compared to typical box construction.
I've earlier posted the sizes for both the FF105WK and the FF125WK. in both my metronomes I have the driver more towards half height than towards the narrow end. I much prefer the FF105WK, it does not give a lot away to the FF125 which admittedly goes a tad lower. In my office the FF105WK is sitting on the desk and it is a far less obstructive size while bass is still very satisfactory. I've earlier on mentioned that the FF125WK has displaced the Monitor Audio Studio 12 that we have. Even the FF105WK has a nicer and deeper bass than the MA. The metronome will not go as loud but still with a 2 x 3 watt tube amp I have to walk out of the office so it is plenty enough for me. But it will not shake the plasterboards from the wall what some people are doing with HT and subs.
The weakness here is not the enclosure or drivers, no the weakness is that it needs a good tube amplifier (power output is not that important). Unfortunately this will require you to either build one yourself or otherwise spent a small fortune. The whole secret in the tube amplifier is in the output transformer and these days it is no different from 60 years ago: Less than a handful of transformer makers give you honest performance data / test results. And price and/or size on their own are no guarantee. You have to know exactly the specifications before you can make an informed decision. A 2A3 comes to mind and I am fond of SE since PP has harsher clipping.
If you want to improve the metronome then you take an octagon and flatten the front so that the shape basically becomes a "D", attach the base to the speaker and then put it on spikes if you are so inclined.
The weakness here is not the enclosure or drivers, no the weakness is that it needs a good tube amplifier (power output is not that important). Unfortunately this will require you to either build one yourself or otherwise spent a small fortune. The whole secret in the tube amplifier is in the output transformer and these days it is no different from 60 years ago: Less than a handful of transformer makers give you honest performance data / test results. And price and/or size on their own are no guarantee. You have to know exactly the specifications before you can make an informed decision. A 2A3 comes to mind and I am fond of SE since PP has harsher clipping.
If you want to improve the metronome then you take an octagon and flatten the front so that the shape basically becomes a "D", attach the base to the speaker and then put it on spikes if you are so inclined.
+1. This applies to most 'FR' and all compression horn drivers IME when best overall SQ is the primary performance goal.
GM
The resonance i'm talking about is the one that would be transmitted by the driver frame to the enclosure, not the enclosure's passband.
In a metal enclosure, the former is something to be adressed with more care than a plywood one.
So of course there is the typical damping scheme of the Metronome as mentioned in many posts, but added to that one needs to also make sure the driver itself doesn't make the metal enclosure resonate with the energy transmitted by the driver to the driver.
No 'FR' driver ever made will excurse enough to excite cast iron in its pass-band and anyway, that's what the driver's gasket is theoretically for. Ditto 10-12 ga stainless and hot or cold rolled steel for that matter. Been there, done that using old electrical equipment boxes.
What I do recommend regardless of cab material/construction though is to mass load the driver to it, so this addresses your concern as well as damping the driver.
GM
What I do recommend regardless of cab material/construction though is to mass load the driver to it, so this addresses your concern as well as damping the driver.
GM