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Old 21st June 2008, 10:18 PM   #1
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Default Question about xmax

If the xmax of a sub is 20mm, how far will the 'surround' travel.

I am thinking half as far (10mm) since it is right in the middle of the speaker frame which doesn't move at all, and the cone which moves the entire 20mm. Is this correct?
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Old 21st June 2008, 10:29 PM   #2
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Default Re: Question about xmax

Quote:
Originally posted by mitchyz250f
If the xmax of a sub is 20mm, how far will the 'surround' travel.

I am thinking half as far (10mm) since it is right in the middle of the speaker frame which doesn't move at all, and the cone which moves the entire 20mm. Is this correct?

It depends how many folds the surround has.

It will be xmax/number of folds.
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Old 21st June 2008, 10:46 PM   #3
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This is a rubber surround. Let's say the high point of the surround is the middle of the surround, half way between the cone and the frame.
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Old 21st June 2008, 10:48 PM   #4
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Quote:
Originally posted by mitchyz250f
This is a rubber surround. Let's say the high point of the surround is the middle of the surround, half way between the cone and the frame.

So it would be 1/2 xmax
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Old 21st June 2008, 11:17 PM   #5
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That's is what I was thinking but I needed to verify my thinking with someone else or my surround might hit it's grill 40 times a second.

Thanks.
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Old 21st June 2008, 11:22 PM   #6
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Quote:
Originally posted by mitchyz250f
That's is what I was thinking but I needed to verify my thinking with someone else or my surround might hit it's grill 40 times a second.

Thanks.

Well that would add some interesting harmonics !

In the 60's some guitarists used to put rivets through the speaker to vibrate.
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Old 22nd June 2008, 05:06 AM   #7
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Quote:
Originally posted by mitchyz250f
This is a rubber surround. Let's say the high point of the surround is the middle of the surround, half way between the cone and the frame.
The point with the farthest travel will be where cone and surround meet, as it is the cone (center) that is driven - not the surround.
In other words: The surround gets bent, not stretched!

But for mounting a grille, a safety margin has to be factored in, as the manufacturer's X(max) is not the farthest travel possible! Look for an X(damage) in the datasheet, this is the actual travel way the suspension allows for (as a nonlinear drive, which is possible, could otherwise still slam the surround onto the grille).

If no reliable information is available, a safe bet could be spacing the grille at a distance (to the baffle) equivalent to the surround straightened width, as the cone can't pull the surround any farther than bending it straight.

In this case, the surround would form an imaginative straight line between basket and cone with an angle depending on the surround's actual form (i.e. half circle, half ellipse, etc.). With a true half circular surround this would be half of the circle's circumference.

The safe distance then is a triangular function of straightened surround length and creepage between cone and basket.
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Old 23rd June 2008, 03:52 AM   #8
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Whut he sez.  The attachment of the surround to the cone will move exactly as much as the cone.  The attachment of the surround to the frame will move exactly as much as the frame, approximately zero.  A point in the geometric center of the surround will move approximately the mean of these, depending on surround configuration/geometry.  If any part of the surround hits the grille, that will be because the edge of the cone is doing so.

Why do you care?  This is a nonparameter.  Let the guys who design the driver sweat this stuff, and think about the mechanical and T/S parameters that are your job.

In answer to your implied question about grill clearance, keep the grill a little more than Xlim away from the edge of the cone-at-rest.  Xlim is a measurement parameter, and is defined as the distance the cone can move in either direction before it hits something.  This will usually be the back plate of the motor, but it does define how far the cone will also be moving forward before very bad things start happening.

Aloha,

Poinz
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Old 26th June 2008, 02:14 AM   #9
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I believe that xmax is measured one way, so the cone would move out 20 mm linearly, plus another 5-10 mm nonlinearly past xmax, until the suspension bottoms out (surround is straightened out).
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Old 26th June 2008, 04:31 PM   #10
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Default Re: Question about xmax

Quote:
Originally posted by mitchyz250f
If the xmax of a sub is 20mm, how far will the 'surround' travel.

I am thinking half as far (10mm) since it is right in the middle of the speaker frame which doesn't move at all, and the cone which moves the entire 20mm. Is this correct?
As stated previously, there are various ways of measuring Xmax.

Some manufacturers measure the linear extension in one direction, going all the way to the linear extension in the other direction. So a speaker which travels linearly 20 mm inward, then returns to it's midpoint and travels another 20 mm outward will be rated as 40 mm. Most of the compainies which do this make equipment for cars.

The more respected way is measure the distance the cone linearly travels in one direction. So the speaker above would be rated as 20 mm Xmax, not 40.

For those who want to be sure, check the voice coil length, (otherwise known as voice coil winding length) and the length of the air gap, (also called gap height).

Then go by the following formula:

Linear Xmax = (Voice Coil Length - Air Gap Height) / 2

Example: A typical 10 inch speaker has a voice coil winding of 18 mm. The air gap is 6 mm.

Xmax = (18mm - 6mm) / 2 .

The linear Xmax = 6 mm.


This is the old method of measuring linear X max. I have been informed that some manufacturers have begun to add an additional 25% onto this, but it still stands as a decent guide regardless of whether these manufacturers do that or not.
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