My understanding was that Xmax was supposed to represent the excursion over which the number of coils in the gap was constant. Measured Xmax usually goes by some drop in measured BL... correct?
I notice that several drivers, tweeters in particular, list an Xlinear as well as Xmax. If Xmax was traditionally intended to represent the linear range of excursion, what does Xlinear tell you? Is this something specific to tweeter designs?
I notice that several drivers, tweeters in particular, list an Xlinear as well as Xmax. If Xmax was traditionally intended to represent the linear range of excursion, what does Xlinear tell you? Is this something specific to tweeter designs?
Xmax can mean one of two things. It can be
A) The linear excursion, approximately corresponding to the point where the voice coil leaves the air gap, or
B) The total excursion allowed before there is damage to the voice coil or speaker, which is a number much greater than the linear Xmax. this number is also sometimes given as Xmech, for maximum mechanical excursion.
For instance, an Eminence PA speaker has a linear excursion of ±9 mm, but an Xmech of ±25 mm.
Also of concern is the method of computing Xmax. The right way to do it is to compute it from the centerline. That is, if your linear excursion is 9 mm, then the speaker should be in the linear area when it is at rest, moves 9 mm forward, returns to the middle position, then moves 9 mm backward. This is known as ±9 mm.
Some speaker manufacturers give the Xmax = 9 mm, when actually the speaker moves from the centerline to 4.5 mm back, returns to the middle position, then moves 4.5 mm forward. They call this Xmax = 9 mm, when actually it really is Xmax = 4.5 mm. Sometimes this method of measurement is called "front-to-back" Xmax.
A) The linear excursion, approximately corresponding to the point where the voice coil leaves the air gap, or
B) The total excursion allowed before there is damage to the voice coil or speaker, which is a number much greater than the linear Xmax. this number is also sometimes given as Xmech, for maximum mechanical excursion.
For instance, an Eminence PA speaker has a linear excursion of ±9 mm, but an Xmech of ±25 mm.
Also of concern is the method of computing Xmax. The right way to do it is to compute it from the centerline. That is, if your linear excursion is 9 mm, then the speaker should be in the linear area when it is at rest, moves 9 mm forward, returns to the middle position, then moves 9 mm backward. This is known as ±9 mm.
Some speaker manufacturers give the Xmax = 9 mm, when actually the speaker moves from the centerline to 4.5 mm back, returns to the middle position, then moves 4.5 mm forward. They call this Xmax = 9 mm, when actually it really is Xmax = 4.5 mm. Sometimes this method of measurement is called "front-to-back" Xmax.
PS: Although the drop in Bl is the exact measurement, there is a very useful approximation that you can use. It is this.
Lv=Length of voice coil
Lg=Length of magnetic gap, (air gap)
Then Xmax = (Lv - Lg) ¸ 2
This an a useful approximation. Don't get too hung up on minor differences. 25% extra length in the voice coil only results in 3 dB extra output-so you have some leeway.
Lv=Length of voice coil
Lg=Length of magnetic gap, (air gap)
Then Xmax = (Lv - Lg) ¸ 2
This an a useful approximation. Don't get too hung up on minor differences. 25% extra length in the voice coil only results in 3 dB extra output-so you have some leeway.
I guess then that when I see Xlin and Xmax used, they are making an attempt to explicitly define the linear Xmax (what I always thought Xmax represented) and Xmech. Makes me wonder why they don't just use "linear Xmax" and "Xmech" as that would be less confusing.
I suppose the biggest thing to worry about is when you only see one number - Xmax, and it isn't immediately obvious whether this represents Xmech, Xmech p-p, Xmax, or Xmax p-p.
I suppose the biggest thing to worry about is when you only see one number - Xmax, and it isn't immediately obvious whether this represents Xmech, Xmech p-p, Xmax, or Xmax p-p.
Dumax measures how motor force factor (Bl) and suspension stiffness (Kms) vary with displacement of the cone from center.
Xmag = displacement where Bl drops to 71% of resting value
Xsus = displacement where Kms increases to 400% of resting value
Xmax = the smaller of Xmag or Xsus
Xmech isn't measured by Dumax but it's the displacement where the driver bottoms or causes other damage to itself.
Sample Dumax reports
Edit: as Keltic pointed out many unscrupulous vendors don't adhere to the Dumax definitions, if they even measure their drivers at all, so buyer beware.
Xmag = displacement where Bl drops to 71% of resting value
Xsus = displacement where Kms increases to 400% of resting value
Xmax = the smaller of Xmag or Xsus
Xmech isn't measured by Dumax but it's the displacement where the driver bottoms or causes other damage to itself.
Sample Dumax reports
Edit: as Keltic pointed out many unscrupulous vendors don't adhere to the Dumax definitions, if they even measure their drivers at all, so buyer beware.
Oops!
The correct figure is actually 50% extra length of excursion results in 3 dB extra output. Twice the excursion equals 6 dB rise in output at any given bass frequency.
As you can see, excursion is indeed important. However, it need not be the sole reason you select a subwoofer driver. If two drivers are reasonably close in excursion, I would choose on the basis of other factors, such as cost, sensitivity, etc.
kelticwizard said:
The correct figure is actually 50% extra length of excursion results in 3 dB extra output. Twice the excursion equals 6 dB rise in output at any given bass frequency.
As you can see, excursion is indeed important. However, it need not be the sole reason you select a subwoofer driver. If two drivers are reasonably close in excursion, I would choose on the basis of other factors, such as cost, sensitivity, etc.
catapult said:
So that's what those DuMax numbers mean! I always knew that Dumax was the strict standard by which excursion is measured, but I never actually knew how they arrived at the ratings until now.
Thanks, Catapult! 🙂
RHosch said:
You are absolutely correct. Welcome to the wonderful world of audio specifications. 🙂
That is why for comparison purposes, it is best to see if you can get the voice coil length and the gap length. Apply the formula in post #3, and you get a decent approximation of Xmax as it should be stated.
Another way of looking at it is that a normal woofer generally has a ¾", (17 mm), long voice coil, which usually results in an excursion of about ±¼", (6mm) linear excursion. That is, the magnetic gap is about ¼", (6 mm), long.
If you can find the voice coil length, and it is, say, 1", (25mm), long, then you probably have an excursion of ±9 mm. If the voice coil length, (VcL), is longer than 1", than you can judge your excursion length accordingly.
These are all approximations, but since excursion has to vary widely before you get any real difference in output, approximations are very useful in this application.
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