Steven Kephart wrote:
| Well how would you describe the excursion of an underhung motor using that technique? Does it have negative excursion since the coil is much shorter than the gap? Also, how would you describe our motor? Our coil is shorter than the top plate height, so would it have negative excursion? Or would you use just one gap, so it would be a very short overhung. If that were true then the Brahma would have an excursion around 7 mm. However DUMAX put it at around 29mm for the Brahma 10 (BL 70% down). In theory the above works only for overhung motors. However in practice, it doesn't provide an accurate number because it doesn't take into account the fringe field. Also, the optimization within the motor can make a difference like whether it is an LGLC (long gap long coil) or SGLC (short gap long coil). And now for your follow-up questions. 1) Not necessarily. You can decrease your gap height and gain excursion as well. And of course this is only true of Overhung motors. 2) Due to the added mass, yes it would. 3) The only negative I can see is reduced heat dissipation. However there is more to it than that as you will see below. That actually get's into the rest of what I was going to add to my previous post. The probablem is that this topic is such a HUGE one. It is difficult to cover without writing a novel about it. The main problem here is to fully understand where excursion comes from. Here's a quote by Dan Wiggins: "Xmax isn't really a physical parameter of the driver - it's a "side effect" of a given driver design. Note that you can have the same 6 physical parameters, but Xmax values all over the place. And Xmax does not affect any of the derived T/S parameters." You can't just increase the coil length to gain excursion. This is because you change everything else in the driver as well. Engineering is a practice of compromises. You try to make the best compromises to obtain your goals. |
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Cool. Thanks for the continued education. A couple points / questions:
I was under the impression that xmax can be determined using the coil / gap height method for underhung motors as well. I'm talking about the absolute difference. So, take the absolute value of the "distance over which the coil can travel in one direction and maintain a constant number of [coil] turns in the gap." See Vance Dickason, The Loudspeaker Design Cookbook, p 4-5. Since BL decreases substantially once the coil leaves the gap, aren't we talking about the same thing here? Don't these two methods of measurement produce similar results?
Anyway, the method of measurement is neither here nor there, but I assert, based on my limited reading and some amount of common sense, that a decent approximation at xmax can be attained via a measurement of distance relating to the voice coil and the gap, regardless of whether the design is overhung or underhung. I.e. while perhaps the best way, the BL method is not the only way to measure underhung xmax.
But that's just describing a normal underhung motor. I have no idea how this would work, if at all, for multiple gaps, like in your XBL^2 motor. :-)
Anyway, the above was just for my understanding more than anything.
I was pushing because my hypothesis was this - hypothesis: It is valid to state that generally high excursion drivers are less efficient than similarly sized and built drivers with lower excursion capabilities. This hypothesis isn't based on anything scientific, only an observation that, almost without exception, high efficiency drivers I see are also inefficient. I was having a hard time believing this is _just because_ the manufacturers don't need to develop for efficiency anymore. Intuition was telling me that _something_ about increased xmax negatively affects efficiency.
A closer examination of all the efficiency calculations that I've been able to get my paws on support your conclusion and shoot my hypothesis to death. Thank you for the education. Some of the posts on that board helped me put some of the my other findings together. Page 31 of the loudspeaker cookbook also helped.
So, while I've learned the long way what you tried telling me from the beginning: there is no direct relationship between xmax and efficiency. I am left with the question...
Why do all the higher excursion drivers seem to have relatively low efficiency numbers? If I read your previous post correctly it is because modern subwoofers will generally be designed with lower fs and lower vas. Am I reading your post correctly?
New Project: 2003 Pathfinder
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