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Comparing LF drivers
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<blockquote data-quote="Bennett Prescott" data-source="post: 106447" data-attributes="member: 4"><p>Re: Comparing LF drivers</p><p></p><p></p><p></p><p>Michael,</p><p></p><p>There are a couple of problems with scaling listed 1w sensitivity by rated power, but in this context we'll ignore power compression and sound quality and focus on what makes subwoofers different.</p><p></p><p>The listed sensitivity is usually measured over a frequency range like Fs -> 10Fs, so maybe 35 - 350Hz. In practice this number is dominated by the midband sensitivity, which in fact doesn't matter to us very much at all. This kind of statistic is very useful for midrange and HF drivers, which are often used for several decades and so a free air measurement of their midband sensitivity corresponds well to real world performance. Subwoofers are somewhat unique in that operation for even two octaves is relatively rare, so it is better to look elsewhere for performance comparisons.</p><p></p><p>It is very important to look at subwoofers as more than a discrete driver, since every application marries a subwoofer with a box, and the two form an integral system. Modern sub woofers are over-damped, with a Qts in the range of 0.2 - 0.4, which ends up working well with under-damped box and port designs - not to mention the atmosphere. This makes life a lot easier, actually, since given a woofer designed with this in mind you can pretty much tune the box to the woofer's Fs and get a good magnitude response. That said, apparent output in the subwoofer range is mostly a function of Qe, as more damping equates to reduced LF. Therefore, you can build a woofer with a light cone (which will give good sensitivity), and put a loose suspension on it (which will get the Fs back to the expected range), and put a weaker motor on it (which reduces Bl and improves apparent LF), and have a woofer that looks great on paper with significantly increased LF sensitivity but reduced cone control. The real tradeoff is in Vas, or how big a box the woofer needs. If you look at the JBL 2242H you reference, its Vas is almost twice our 18SW115. What we've done is put on a massive motor, a very stiff suspension, and a heavier cone. In our experience this results in better reliability and sonics, not to mention the ability to get similar performance in nearly half the box size. Certainly there are many applications where a high Vas is no big deal, but most of our customers build products that have to travel regularly by truck and so this advantage in output from a relatively small box is A Big Deal™. Look at our 21SW152, for example - significantly higher Bl (with a lower Re!) than the 18SW115, more cone area, more excursion, more power handling, while maintaining a Vas of about 7ft^3!</p><p></p><p>Many of these tradeoffs result in off-spec-sheet differences that can end up being really significant. Careful attention to Klippel data will reveal more of the magic here, all our woofers exhibit extremely low DC offset, excellent symmetry, and relatively flat Bl. The end result is a woofer that sounds better, especially when driven to rated power. Our voice coil, former, and motor cooling technologies are a big part of the jump in power levels you have observed as well, but don't think of them as spurious. We really can build woofers now that have more than twice the power handling of the best of the previous era's, while having better usable LF sensitivity, and that behave much better at max output - letting you actually utilize these hard won advantages.</p></blockquote><p></p>
[QUOTE="Bennett Prescott, post: 106447, member: 4"] Re: Comparing LF drivers Michael, There are a couple of problems with scaling listed 1w sensitivity by rated power, but in this context we'll ignore power compression and sound quality and focus on what makes subwoofers different. The listed sensitivity is usually measured over a frequency range like Fs -> 10Fs, so maybe 35 - 350Hz. In practice this number is dominated by the midband sensitivity, which in fact doesn't matter to us very much at all. This kind of statistic is very useful for midrange and HF drivers, which are often used for several decades and so a free air measurement of their midband sensitivity corresponds well to real world performance. Subwoofers are somewhat unique in that operation for even two octaves is relatively rare, so it is better to look elsewhere for performance comparisons. It is very important to look at subwoofers as more than a discrete driver, since every application marries a subwoofer with a box, and the two form an integral system. Modern sub woofers are over-damped, with a Qts in the range of 0.2 - 0.4, which ends up working well with under-damped box and port designs - not to mention the atmosphere. This makes life a lot easier, actually, since given a woofer designed with this in mind you can pretty much tune the box to the woofer's Fs and get a good magnitude response. That said, apparent output in the subwoofer range is mostly a function of Qe, as more damping equates to reduced LF. Therefore, you can build a woofer with a light cone (which will give good sensitivity), and put a loose suspension on it (which will get the Fs back to the expected range), and put a weaker motor on it (which reduces Bl and improves apparent LF), and have a woofer that looks great on paper with significantly increased LF sensitivity but reduced cone control. The real tradeoff is in Vas, or how big a box the woofer needs. If you look at the JBL 2242H you reference, its Vas is almost twice our 18SW115. What we've done is put on a massive motor, a very stiff suspension, and a heavier cone. In our experience this results in better reliability and sonics, not to mention the ability to get similar performance in nearly half the box size. Certainly there are many applications where a high Vas is no big deal, but most of our customers build products that have to travel regularly by truck and so this advantage in output from a relatively small box is A Big Deal™. Look at our 21SW152, for example - significantly higher Bl (with a lower Re!) than the 18SW115, more cone area, more excursion, more power handling, while maintaining a Vas of about 7ft^3! Many of these tradeoffs result in off-spec-sheet differences that can end up being really significant. Careful attention to Klippel data will reveal more of the magic here, all our woofers exhibit extremely low DC offset, excellent symmetry, and relatively flat Bl. The end result is a woofer that sounds better, especially when driven to rated power. Our voice coil, former, and motor cooling technologies are a big part of the jump in power levels you have observed as well, but don't think of them as spurious. We really can build woofers now that have more than twice the power handling of the best of the previous era's, while having better usable LF sensitivity, and that behave much better at max output - letting you actually utilize these hard won advantages. [/QUOTE]
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