I'm planning a voice coil for a 12'' subwoofer I'm going to build, and I already have the magnet and the frame.
I made a test coil to measure the force factor, and from measuring it's dimensions (plus assuming it uses some common materials) I created this model in FEMM:
From using those materials the force factor fitted right away.
You can see I've already added a voice coil to the model.
My doubt is the following: I planned the coil to be 27 mm tall so it could have a 10 mm Xmax. But, like that, when being fully "down" (like in the picture), the bottom of the coil itself would be 6 mm from the bottom (from hitting that curved part of the bottom).
Considering that I need a bit of former left below the coil (to avoid it from bottomming out when going "up"), this leaves me with very little window to anything. It could easily hit the bottom when going down.
So what you think is the wiser to do?
What you think it's a safe Xmax to avoid the chance of mechanical damage?
Note I'm planning to use this sub in a transmission line enclosure, and I'm planning to use it with a 150 W RMS class D amplifier. I'm not yet sure of how much excursion I'm going to need and how much control the enclosure will offer, but I'm following the parameters of known 12'' subs of similar power rating.
I made a test coil to measure the force factor, and from measuring it's dimensions (plus assuming it uses some common materials) I created this model in FEMM:
From using those materials the force factor fitted right away.
You can see I've already added a voice coil to the model.
My doubt is the following: I planned the coil to be 27 mm tall so it could have a 10 mm Xmax. But, like that, when being fully "down" (like in the picture), the bottom of the coil itself would be 6 mm from the bottom (from hitting that curved part of the bottom).
Considering that I need a bit of former left below the coil (to avoid it from bottomming out when going "up"), this leaves me with very little window to anything. It could easily hit the bottom when going down.
So what you think is the wiser to do?
What you think it's a safe Xmax to avoid the chance of mechanical damage?
Note I'm planning to use this sub in a transmission line enclosure, and I'm planning to use it with a 150 W RMS class D amplifier. I'm not yet sure of how much excursion I'm going to need and how much control the enclosure will offer, but I'm following the parameters of known 12'' subs of similar power rating.
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Is there a rule of thumb of how much Xmax you can get according to the magnet?
I have a 12'' subwoofer magnet and frame, and I'm building the subwoofer to be used in a transmission line enclosure.
The "depth" of the magnet is 33 mm, that's the distance from the top of the top plate to the bottom (in the inner side).
So I was wondering: Is there a rule of thumb for how much room I should leave "below" the coil at "negative Xmax" (when going down)?
I'm afraid trying to get a large Xmax (building a tall coil) will make it easily hit the bottom.
Thanks!
I have a 12'' subwoofer magnet and frame, and I'm building the subwoofer to be used in a transmission line enclosure.
The "depth" of the magnet is 33 mm, that's the distance from the top of the top plate to the bottom (in the inner side).
So I was wondering: Is there a rule of thumb for how much room I should leave "below" the coil at "negative Xmax" (when going down)?
I'm afraid trying to get a large Xmax (building a tall coil) will make it easily hit the bottom.
Thanks!
Given enough voltage, a coil can be driven completely out of the magnetic gap.
Your diagram looks like the coil would contact the back plate at about the excursion where the coil would leave the gap.
Speakers designed for large excursion and high power are designed so Xmech/Xlim (excursion mechanical limit) is about double Xmax. The suspension soft parts reach Xlim before the coil can bottom (or top) out.
What Art said.
However, the missing information is the application for this driver. Is it going to be used by a home-theatre person who must have 10Hz at 120dB?
Or is it going to be used by someone who wants to listen to chamber music in the background?
For the former, I'd absolutely worry about how the driver works past Xmax. For the latter, I wouldn't give it a second thought.
Chris
However, the missing information is the application for this driver. Is it going to be used by a home-theatre person who must have 10Hz at 120dB?
Or is it going to be used by someone who wants to listen to chamber music in the background?
For the former, I'd absolutely worry about how the driver works past Xmax. For the latter, I wouldn't give it a second thought.
Chris
Well, as I said I want to use it in a transmission line enclosure with a rated power of 150 W RMS. My pretension is that it's near flat at least between 40 and 100 Hz
But, it would be used in different ambiances for totally different styles of music, the rest is not well defined, so I'd should expect it to withstand playing rap and pop at highest power without destroying itself.
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You also need to pay attention to the electronic side of things. Plugging/ unplugging clicks. Power on/off thumps. If the amp is dedicated to this, as in a powered sub, you can control/limit the drive to the speaker fairly precisely.
Another thing worth considering is how catastrophic bottoming out is going to be. Is the former going to hit bare metal and deform/ shatter? Would it be worth sacrificing an mm or two of room to put a cushioning pad there?
Another thing worth considering is how catastrophic bottoming out is going to be. Is the former going to hit bare metal and deform/ shatter? Would it be worth sacrificing an mm or two of room to put a cushioning pad there?
There's a few ways that the former can be brought to a stop, two of which I think you should avoid in the design phase:
1. The suspension (spider, surround) allows the bottom of the former to hit the back plate.
2. The suspension (spider, surround) allows the joint between the spider and the former to hit the top of the gap
Both of those events can cause physical damage which will require replacing all of the "soft parts".
So, given that, I'd probably start with designing the driver so that the surround and/or spider does not allow the cone to go so far in that either of the above happens. And if the surround gets damaged, that's usually a lot easier to replace than the other the soft parts.
Secondly, if the height of the coil is short enough that the entire coil can leave the gap before the suspension brings it to a halt, then it might be a good idea to use a progressive, not linear, spider, to avoid "oil-canning".
Anyway, I'm not a subwoofer driver designer, so take all of the above with a grain of salt 🙂.
1. The suspension (spider, surround) allows the bottom of the former to hit the back plate.
2. The suspension (spider, surround) allows the joint between the spider and the former to hit the top of the gap
Both of those events can cause physical damage which will require replacing all of the "soft parts".
So, given that, I'd probably start with designing the driver so that the surround and/or spider does not allow the cone to go so far in that either of the above happens. And if the surround gets damaged, that's usually a lot easier to replace than the other the soft parts.
Secondly, if the height of the coil is short enough that the entire coil can leave the gap before the suspension brings it to a halt, then it might be a good idea to use a progressive, not linear, spider, to avoid "oil-canning".
Anyway, I'm not a subwoofer driver designer, so take all of the above with a grain of salt 🙂.