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Gaps: What They Do For You

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1 Re: Gaps: What They Do For You on Fri Oct 22, 2010 3:54 am

Duran

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Split-Gap
This is a fairly (in)famous gap technology that many of you will already be aware of. Originally patented as XBL^2 by Daniel Wiggins and David Hyre (formally of Adire Audio/Extremis Audio, now of Acoustic Development Technologies), this is now being referred to (and trademarked) as Split-Gap, which is a much more accurate and understandable term. The term indicates exactly what you see pictured below:



As you can see, there is a notch of sorts that is machined into the top plate. If we forget about the notch for a moment, the speaker appears to be underhung (meaning the coil height is shorter than the gap height), though it is very close to being evenhung (meaning both the coil and the gap are of the same height). However, that little notch in the top plate makes the speaker behave in a different method than both underhung and evenhung. As the speaker moves, an equal amount of windings enter one region of the gap as the number of windings that exit the other gap. This gap treatment can be found in a large number of past and present speakers, including many products from Adire Audio, Creative Sound Solutions, Blueprint, Ascendant Audio, and many others. It is worth noting that of those 4 companies I listed, two no longer exist and one switched from using split-gap a few years ago. However, in no way does that represent the abilities of the technology.


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2 Re: Gaps: What They Do For You on Fri Oct 22, 2010 3:55 am

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LMT (Linear Motor Technology)
This gap treatment is growing in popularity. This treatment is a clever one designed and patented by Thilo Stompler of TC Sounds. If the name rings a bell, it’s because TC Sounds does OEM work for several companies. This technology is a few years old now and likely to be featured in future OEM and direct sales products from TC Sounds in the future. See the image below:



You’ll notice that the top plate is not machined in any way. TC Sounds manipulates BL through the speaker’s stroke by utilizing a variable density coil. In layman’s terms (and in case the image does not make this clear), the voice coil has varying layers of windings at different sections along the former. By adding more coil at the upper and lower limits, BL is higher than on a typical speaker when the coil approaches the outer limits of the gap. This gap treatment is featured on products from SoundSplinter (Rl-s), Eclipse (SW8200), and some direct offerings from TC Sounds (most notably, the LMS-5400).


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3 Gaps: What They Do For You on Fri Oct 22, 2010 3:57 am

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Split Coil
From what I can find via research, the concept of a split coil is roughly 3 decades old. Though there has been seemingly little interest in this gap treatment for many years, it is also increasing in popularity, due in part to a desire to achieve higher excursion levels while keeping BL linear, and in part to Scott Atwell’s utilization of the technology on a few very popular designs of late. If you haven’t already heard, Scott Atwell was the very talented engineer at Resonant Engineering (and the Destijl buildhouse), who has since moved on to Fi Car Audio (and its associated buildhouse). Of the three gap treatments listed here, Split Coil is the only one that is not covered via a patent, pending or otherwise. The concept, as you can see below, is pretty much the opposite of the Split-Gap concept.



Here we have no modifications to the top plate, nor do we have varying layers of windings on the voice coil. Instead, we have some of the voice coil located in the upper region of the gap with some of the voice coil located in the lower region of the gap, with a noticeable split between the two sections. As the speaker begins to move, an equal number of windings enter the gap as the number that exit. Pretty easy to grasp, I think. Perhaps the two best examples of speakers that utilize Split Coil would be the Resonant Engineering XXX (with an obscene 54mm of Xmax) and the Ascendant Audio Poly 6.5” (with an equally impressive 11mm of linear one-way excursion). I would expect to see more split coil designs in the not-too-distant future.

With the preliminary (and important) background information out of the way, let’s move on to the areas that I have identified as “relevant” to speaker design.


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4 Re: Gaps: What They Do For You on Fri Oct 22, 2010 4:09 am

Duran

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The term flux efficiency refers to how efficiently the flux in the motor is used. I know, weird. This concept is very simple: a given length of voice coil in the gap (L) is capable of integrating with a given magnetic field density or strength (B ). If L is too short, the flux capability of B will not be full utilized. This would be an example of low flux efficiency.

Efficiency of a speaker, on the other hand, is very different. A highly efficient speaker does not necessarily need the best flux efficiency. For example, a speaker with low B and small L while having high Mms (moving mass), will still be inefficient, even though it is possible that flux efficiency is very high. Hopefully that’s clear as mud.

So which gap treatment is the best? Well, let’s look at flux efficiency first. In case you were unaware, all of the B does not lie in the gap: there is some B in what’s called the “fringe field”, or more easily understood as the outer limits of the gap. If you wanted to achieve very high flux efficiency, you would ideally have a coil that is able to use all of the B in the gap AND all of the B outside of the gap. Of course, that’s near impossible to do. But which gap treatment does it the best? That’s likely Split-Gap. Split Coil does a great job of utilizing B in the fringe field, but sacrifices some of the B integration in the middle of the gap. LMT requires a larger gap than typical due to the variable layers of windings (to avoid scraping coil in the gap during its travel). So Split-Gap should be the most efficient design, correct? Nope.

Remember that notch that Split-Gap has? Guess what: it actually sacrifices some B, similar to how the wider gap required for LMT sacrifices some B. The further you move the top plate from the coil, the less B is utilized. So when you machine a notch in the gap, you are giving up B. This will actually drop your speaker’s efficiency, even though you have likely increased flux efficiency. Split Coil, though it is less flux efficient, does not sacrifice any B from the top plate at all. Likewise, LMT does not do so either. Split-Gap is likely the least efficient design from this standpoint. These issues can be overcome by adding more to the magnet structure, though this is pointless once you have saturated the top plate. You could make a taller top plate if you’d like, but this grows increasingly expensive and it is harder to achieve a consistent magnetic field through a tall top plate. Split Coil drops efficiency because it intentionally sacrifices the most dense flux (in the middle of the gap) for the least dense flux (in the fringe field). LMT sacrifices efficiency because the gap needs to be wider to accommodate the thicker layers of windings at various points in the coil.

The last part of the speaker efficiency equation is the moving mass of the speaker. Forget about the other soft parts for a moment and think only of the voice coil’s role here. Split-Gap uses the smallest voice coil, so it’s moving mass is the lowest. Split coil generally uses a smaller voice coil than LMT (though this depends on the number of windings on the LMT coil) and thus has the second lowest moving mass. LMT usually has the highest moving mass. However, minor differences in the moving mass pale in comparison to large changes in the BL of a speaker.

In conclusion, Split-Gap is the most flux efficient design. However, Split Coil will likely result in the most efficient speaker, followed by LMT, and, lastly, Split-Gap.


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5 Re: Gaps: What They Do For You on Fri Oct 22, 2010 4:09 am

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All three gap treatments very clearly help to linearize BL over a speaker’s stroke; there is no arguing that. But are all the gap treatments equally linear over long distances? Which gap treatment is easy to achieve ridiculous levels of linear BL? The answer here is very simple.

If you’re familiar with underhung vs. overhung arguments, you’re likely aware that, while underhung designs are capable of more linear BL, they are very hard to achieve linear BL over a very large stroke at an affordable price. Remember how I mentioned that tall top plates can get pricey with inconsistent magnetic field strength? These disadvantages come directly into play with an underhung speaker, and sadly, making taller and taller top plates is the only way to achieve more stroke with an underhung design. This same argument is very applicable when comparing the three gap treatments we’re looking at.

Split-Gap uses a coil that is smaller in height than the height of the gap and suffers from the same effects outlined above for underhung speakers. If you want really huge stroke from a Split-Gap speaker, be prepared to fork over some cash for your top plate. Oh, and make sure that you have a good magnet structure to keep that top plate as saturated as possible. Another consideration: by machining a notch in the top plate, Split-Gap sacrifices some BL at rest. This can be controlled by working with the depth of the notch in your top plate, but is clearly visible in many Klippel and Dumax results for Split-Gap speakers, where there is a plateau of sorts at rest and then BL product rises once the coil begins to move in the forward and rearward directions. LMT and Split Coil, on the other hand, fall more within the realm of overhung speakers. If you want to add more stroke, add more coil height. As the coil gets taller and taller, the backplate needs to be moved further and further back on the speaker. Here, the use of double or triple stack magnet structures proves much more affordable than machining a very tall top plate. It’s easy to get the stroke and gain the clearance with Split Coil and LMT. Now it’s worth noting that because LMT allows the manufacturer to customize the layers of windings at various points in the coil, the voice coil can theoretically be designed for an absolutely obscene amount of stroke AND LMT will allow the speaker engineer to carefully guarantee ruler flat BL until the entire coil has left the gap. Split Coil, on the other hand, will see BL begin to drop slowly as it reaches its outer excursion limits. Still, it is very capable of huge excursion without a very tall top plate.

All things considered, the LMT is probably the best for huge stroke with perfectly flat BL. Split Coil is next, with Split-Gap usually bringing up the rear.


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6 Re: Gaps: What They Do For You on Fri Oct 22, 2010 4:10 am

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This is a key factor in many speaker designs. Inductance (Le as a thiele/small parameter) is a key limiter in the bandwidth of a speaker. If a speaker begins to roll-off at a given frequency, it can usually be attributed to inductance. If you can keep inductance low, you will have a wider bandwidth in which the speaker will play accurately.

NOTE: Le is a tricky parameter to compare. First, it varies with a number of factors, including frequency and power. When comparing various speakers using their published Le specification, be sure to divide the value of Le by the value of Re (the DC resistance of the voice coil). A speaker is a reactant load and an increase in Re will result in an increase in Le, though the corner frequency at which roll-off begins remains the same. In short, a speaker with a DC resistance of 4.0 ohms and an inductance of 2.0mH is actually better than a speaker with a DC resistance of 2.0 ohms and an inductance of 1.5mH.

So what is the key factor in the inductance equation? It’s almost exclusively the length of the voice coil. A longer voice coil will have higher inductance. A smaller voice coil will have lower inductance. Though inductance can be decreased through use of shorting rings or copper sleeves, the voice coil length is the primary variable when considering inductance.

Based on what we know so far, Split-Gap usually has the smallest coil, followed by Split Coil, followed by LMT. Let’s also consider the usability of shorting rings. The notch in the top plate of a Split-Gap design is very easy to fill with copper and help lower inductance. If you want to add copper to the gap in a Split Coil or LMT motor, you need to widen the gap, which drops your B, which drops your efficiency.

In terms of inductance, Split-Gap is the clear winner, followed by Split Coil, with LMT taking the last position.


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7 Re: Gaps: What They Do For You on Fri Oct 22, 2010 4:11 am

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Article written by Neil, over on SSA.


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8 Re: Gaps: What They Do For You on Fri Oct 22, 2010 4:14 am

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A link to JBL's white papers on Dual Gap Technology http://www.jblpro.com/catalog/support/getfile.aspx?docid=1253&doctype=3


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