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<blockquote data-quote="DanWiggins" data-source="post: 7133695" data-attributes="member: 544847"><p>Except it doesn't work nearly as well. Look at the reluctance of steel relative to magnetic material - you have considerable losses with a deep-draw back, and then of course - the costs of steel (and weight) are considerably higher than most ceramic magnets meaning an even more expensive build to get where you want to go. You thus need even more magnet to make up for the extra losses incurred.</p><p></p><p>Forcing the same magnet dimensions is much like deciding to do an automotive test between different cars, but mandating all shall have the same tires and wheel diameters, or all will use the exact same displacement engine, when in fact choice of tires, wheels, and engine displacement is key to overall performance of the speaker.</p><p></p><p>XBL allows for a much more efficient use of the volume of magnet utilized, which means lower cost, higher performance, or a mix of both. That is perhaps the primary advantage from a physical build - the shorter, wider magnets - and should not be overlooked as it is crucial in so many applications. And no one ever turns down the lower cost thanks to less magnet - and shallower steel/no bumped backplates - required!</p><p></p><p></p><p>OK, assume everything in FRONT of the motor is the same; we're talking motor height only. Let's assume the same thickness of backplate, so the reluctance from magnet stack to pole is the same. We're looking at top plate height and magnet stack height.</p><p></p><p>In 36mm of top plate and magnet stack, I can get you 18mm of one way (rearward) voice coil clearance. And that's for a speaker with 14mm one way linear stroke. So a 1:2 ratio of Xmech:height, and a 1:2.5 ratio of Xmax:height.</p><p></p><p>I'd be interested to see how much mechanical stroke you can get with an overhung, with 14mm of linear Xmax, and top plate plus magnet stack of 36mm.</p><p></p><p></p><p>However, those are static analyses you're most likely doing, correct? Dynamic inductance measurements will show a smoother balance of inductance over current and frequency versus position with the shorting ring placed at the average position of the voice coil - which just so happens to be in the middle of the gap (note this not show up in Klippel as Klippel cannot measure inductance versus frequency).</p><p></p><p>Take a look at the papers from Button, Skaaning, and Vercelli for information about where best to counter and linearize inductance. There's a reason JBL uses laminations of steel and copper, and all of Ejvind's companies have used sleeves. A ring under the gap helps, but is nowhere near as effective as IN the gap, especially when linearity versus position in taken into account. In a static/small signal analysis it looks great but as you move away from that ring it all goes away.</p><p></p><p>Additionally, undercutting the pole significantly chokes off flux in the motor, meaning you need a larger diameter voice coil/pole to maintain the same flux level (or much larger magnets). Of course, that increases the native inductance of the voice coil. The rebate in an XBL motor - at most - cuts of half the amount of flux an undercut would, because it's only the upper gap that is affected, thus meaning it's not nearly as limiting as a full undercut.</p><p></p><p></p><p>How many layers are required for LMS, relative to XBL? For an average 4 layer LMS, the outer areas are 8 layers, and the inner are 2. Thus you end up with an even larger gap. And we'll not talk about the higher Mms that comes along. Often that Mms is desired - and needed! But many times it is not, and that becomes a limiting factor of application of that approach.</p><p></p><p>In general, my experience building a few thousand versions of underhungs, overhungs, and XBL (as well as split-coil, as it was covered in the original '72 patent from Babb) is that the gap can be considerably tighter overall with an XBL motor. Adding two more layers (going from a 2 layer overhung to a 4 layer XBL) does not double the width of the voice coil because of stacking of the wires; you see about a 60% increase in width for doubling the layers. Thus one reaches a doubling of L without a corresponding increase in gap width and the corresponding loss of flux.</p><p></p><p></p><p>Take a look at inductance over current and frequency versus position (of course, over frequency versus position is difficult to do, since Klippel cannot measure it, you need to either analyze it or measure with different tools). Dynamic stability is increased with the bulk of the inductance countering being right in the middle of the gap.</p><p></p><p>If analyzing, remember to set the current negative to position, too; most people don't realize that when the driver is moving forward, the current is actually NEGATIVE; current is negative the position of the coil, and that is why so many get inductance compensation wrong - they place too much copper or aluminum at the wrong position (and, more often seen, too much at that wrong position as well).</p><p></p><p></p><p>I guess my point is that you are holding "fundamental constraints" arbitrarily that negate the advantages of XBL.</p><p></p><p>XBL will yield shorter motors for a given level of mechanical and linear stroke (meaning lower costs, or wider - and thus higher flux - motors). That's a straight geometric proof; it's hard to argue.</p><p></p><p>XBL will give you greater inductance stability over stroke because of the placement of the shorting rings in the middle of the gap (and does not incur the flux loss issue when you sleeve the pole). Pure research papers from others in the field, as well as measurements and FEA simulations will confirm this result.</p><p></p><p>XBL provides for greater B field stability over power because of the breaking up of the gap into multiple parts, so skewed fields are not as much of an issue (which is usually addressed by assuring another point in the magnetic path - usually the backplate-to-pole junction - is saturated at all times, meaning a loss in total flux in the motor). Again, measurements, theory, and FEA all coincide to show this to be the case.</p><p></p><p>I'm not anti-overhung; I use it quite a bit for many things. Likewise underhung as well. However, there are several distinct advantages of XBL motors that I feel your analysis simply ignored; I'm just providing a little more insight into the situation and hopefully providing some input into why holding parameters arbitrarily constant is not a good engineering exercise as it does not provide a real-world look at the issues at hand.</p></blockquote><p></p>
[QUOTE="DanWiggins, post: 7133695, member: 544847"] Except it doesn't work nearly as well. Look at the reluctance of steel relative to magnetic material - you have considerable losses with a deep-draw back, and then of course - the costs of steel (and weight) are considerably higher than most ceramic magnets meaning an even more expensive build to get where you want to go. You thus need even more magnet to make up for the extra losses incurred. Forcing the same magnet dimensions is much like deciding to do an automotive test between different cars, but mandating all shall have the same tires and wheel diameters, or all will use the exact same displacement engine, when in fact choice of tires, wheels, and engine displacement is key to overall performance of the speaker. XBL allows for a much more efficient use of the volume of magnet utilized, which means lower cost, higher performance, or a mix of both. That is perhaps the primary advantage from a physical build - the shorter, wider magnets - and should not be overlooked as it is crucial in so many applications. And no one ever turns down the lower cost thanks to less magnet - and shallower steel/no bumped backplates - required! OK, assume everything in FRONT of the motor is the same; we're talking motor height only. Let's assume the same thickness of backplate, so the reluctance from magnet stack to pole is the same. We're looking at top plate height and magnet stack height. In 36mm of top plate and magnet stack, I can get you 18mm of one way (rearward) voice coil clearance. And that's for a speaker with 14mm one way linear stroke. So a 1:2 ratio of Xmech:height, and a 1:2.5 ratio of Xmax:height. I'd be interested to see how much mechanical stroke you can get with an overhung, with 14mm of linear Xmax, and top plate plus magnet stack of 36mm. However, those are static analyses you're most likely doing, correct? Dynamic inductance measurements will show a smoother balance of inductance over current and frequency versus position with the shorting ring placed at the average position of the voice coil - which just so happens to be in the middle of the gap (note this not show up in Klippel as Klippel cannot measure inductance versus frequency). Take a look at the papers from Button, Skaaning, and Vercelli for information about where best to counter and linearize inductance. There's a reason JBL uses laminations of steel and copper, and all of Ejvind's companies have used sleeves. A ring under the gap helps, but is nowhere near as effective as IN the gap, especially when linearity versus position in taken into account. In a static/small signal analysis it looks great but as you move away from that ring it all goes away. Additionally, undercutting the pole significantly chokes off flux in the motor, meaning you need a larger diameter voice coil/pole to maintain the same flux level (or much larger magnets). Of course, that increases the native inductance of the voice coil. The rebate in an XBL motor - at most - cuts of half the amount of flux an undercut would, because it's only the upper gap that is affected, thus meaning it's not nearly as limiting as a full undercut. How many layers are required for LMS, relative to XBL? For an average 4 layer LMS, the outer areas are 8 layers, and the inner are 2. Thus you end up with an even larger gap. And we'll not talk about the higher Mms that comes along. Often that Mms is desired - and needed! But many times it is not, and that becomes a limiting factor of application of that approach. In general, my experience building a few thousand versions of underhungs, overhungs, and XBL (as well as split-coil, as it was covered in the original '72 patent from Babb) is that the gap can be considerably tighter overall with an XBL motor. Adding two more layers (going from a 2 layer overhung to a 4 layer XBL) does not double the width of the voice coil because of stacking of the wires; you see about a 60% increase in width for doubling the layers. Thus one reaches a doubling of L without a corresponding increase in gap width and the corresponding loss of flux. Take a look at inductance over current and frequency versus position (of course, over frequency versus position is difficult to do, since Klippel cannot measure it, you need to either analyze it or measure with different tools). Dynamic stability is increased with the bulk of the inductance countering being right in the middle of the gap. If analyzing, remember to set the current negative to position, too; most people don't realize that when the driver is moving forward, the current is actually NEGATIVE; current is negative the position of the coil, and that is why so many get inductance compensation wrong - they place too much copper or aluminum at the wrong position (and, more often seen, too much at that wrong position as well). I guess my point is that you are holding "fundamental constraints" arbitrarily that negate the advantages of XBL. XBL will yield shorter motors for a given level of mechanical and linear stroke (meaning lower costs, or wider - and thus higher flux - motors). That's a straight geometric proof; it's hard to argue. XBL will give you greater inductance stability over stroke because of the placement of the shorting rings in the middle of the gap (and does not incur the flux loss issue when you sleeve the pole). Pure research papers from others in the field, as well as measurements and FEA simulations will confirm this result. XBL provides for greater B field stability over power because of the breaking up of the gap into multiple parts, so skewed fields are not as much of an issue (which is usually addressed by assuring another point in the magnetic path - usually the backplate-to-pole junction - is saturated at all times, meaning a loss in total flux in the motor). Again, measurements, theory, and FEA all coincide to show this to be the case. I'm not anti-overhung; I use it quite a bit for many things. Likewise underhung as well. However, there are several distinct advantages of XBL motors that I feel your analysis simply ignored; I'm just providing a little more insight into the situation and hopefully providing some input into why holding parameters arbitrarily constant is not a good engineering exercise as it does not provide a real-world look at the issues at hand. [/QUOTE]
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