Some thoughts on excursion and the infamous "Xmax"

[DevilDriver 28]

Excursion is a large contributor to high output in any driver on the market. Typically, larger excursion (seen visually as how far the cone moves in and out) means more sonic output. However, with excursion come challenges.

As the driver moves outward, the voice coil begins to move out of the gap. This causes a drop in the magnetic force (or BL) and results in a few negative effects, including a large theile/small parameter shift, lower efficiency (magnetic flux decreases), less control over the voice coil (transient response increases, making the driver sound sloppy), and introduces distortion into the output.

Knowing these effects, there was a need to design a parameter to describe the limit of excursion while still remaining

[DevilDriver 28]

To expand on an article that I wrote quite a while ago...

Defining Xmax is extremely challenging. If we are intending to define Xmax as the point at which distortion is audible (which seems to be the belief of many), then we are opening a very large can of worms. For one, we would need to get a better grasp of what distortion is acceptable, what distortion is most offensive, and how different types of distortion is perceived. While there has been great work contributed to this science of audibility (most notably by Drs. Geddes and Lee), it is still a field where more is discovered all the time.

From a purely theoretical standpoint, I look forward to many developments in the understanding of distortion. From a simplicity standpoint, it is best to stick with the 70% BL and 30% Cms targets. At the very least, a push for a true industry standard with confirmed measurements (ie. CEA compliance for subwoofer) would be a great step in the right direction.

Edited February 23, 2007 by DevilDriver

[Aaron Clinton 2,372]

Wow very nice info!!

[bdawson72 0]

Another good read

[filtor1 13]

I can honestly say that I had not thought about this topic very intensely. Great read. Thanks for the information!

[CBFryman 2]

Question concerning keeping CMS linear. Could motor designers not extend the pole to the point where the coil hits its maximum forward limit (due to suspension). Agian on the reverse, if shortening rings are used, the coil can not rock as much. Of course the forward extending part of the pole could be non magnetic as to keep the B-feild from straying. Essentiall an untapered phase plug with a different purpose. It's just an idea I though I would spit out there. I'm wondering why this has not been incorperated much except for in a few crazy motor designs and extreme excursion designs.

[helotaxi 46]

Question concerning keeping CMS linear. Could motor designers not extend the pole to the point where the coil hits its maximum forward limit (due to suspension). Agian on the reverse, if shortening rings are used, the coil can not rock as much. Of course the forward extending part of the pole could be non magnetic as to keep the B-feild from straying. Essentiall an untapered phase plug with a different purpose. It's just an idea I though I would spit out there. I'm wondering why this has not been incorperated much except for in a few crazy motor designs and extreme excursion designs.

I don't think that what he meant by linear and what you are thinking as linear are the same thing. You are talking about keeping the coil/cone centered in the basket and he is talking about the actual Cms value increasing in a geometric (therefore non linear) fashion as the cone moves away from the center point. An extended pole piece would not keep the cone centered in a fashion that would be desirable anyway. As soon as anything starts touching the moving coil, you have a serious and terminal problem.

[CBFryman 2]

Got it. My misconception of linear. Cant beleive I didnt catch that.