mitchyz250f said:
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.
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
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
mitchyz250f said:
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.
The above applied to linear Xmax. That's the distance the cone travels before distortion levels get high-generally 5% or 10% is the standard.
But others have pointed out that there is another type of Xmax-the distance a cone travels before the speaker becomes damaged. This is somewhat greater than the linear Xmax. This stat is variously given as X damage, or Xsus, (for suspension limit), or Xmech, (for mechanical limit).
In normal woofers, Xmech is normally about 50% longer than the linear Xmax. So a home woofer with a linear Xmax of 6mm will likely have an Xmech of 9mm.
In PA speakers, the Xmech is normally much, much greater than linear Xmax. If the linear Xmax of a 15 inch PA speaker is 5mm, you can count on the Xmech being at least 12mm, and very often it will a full inch, (25mm). Happily, most PA manufacturers give you the Xmech in the stats, so you know what the limits are when you buy the speaker. They may call it X damage or whatever, but the mm at which damage occurs usually is listed.
But others have pointed out that there is another type of Xmax-the distance a cone travels before the speaker becomes damaged. This is somewhat greater than the linear Xmax. This stat is variously given as X damage, or Xsus, (for suspension limit), or Xmech, (for mechanical limit).
In normal woofers, Xmech is normally about 50% longer than the linear Xmax. So a home woofer with a linear Xmax of 6mm will likely have an Xmech of 9mm.
In PA speakers, the Xmech is normally much, much greater than linear Xmax. If the linear Xmax of a 15 inch PA speaker is 5mm, you can count on the Xmech being at least 12mm, and very often it will a full inch, (25mm). Happily, most PA manufacturers give you the Xmech in the stats, so you know what the limits are when you buy the speaker. They may call it X damage or whatever, but the mm at which damage occurs usually is listed.
Xmax
... the geometric calculation is sometimes a little optimistic
Have a look on this:
http://www.fesb.hr/~mateljan/arta/AppNotes/AP7-EstimationOfLineardisplacement-IEC62458-EngRev01.pdf
Regards
Heinrich
... the geometric calculation is sometimes a little optimistic
Have a look on this:
http://www.fesb.hr/~mateljan/arta/AppNotes/AP7-EstimationOfLineardisplacement-IEC62458-EngRev01.pdf
Regards
Heinrich
Their is only one professionally accepted definition of Xmax. Period. Other standards have been proposed, but none have gained any wide spread acceptance.
It is, as quoted above, Xmax=(top plate thickness-VC winding height)/2
If you find a driver manufacturer using double this value. Run. Don't touch their stuff. They are probably making up other specs and/or fabricating measurements also.
Regarding grille cloth to surround clearance, there are some other things you need to consider. When the driver starts moving at high excursion, the air movement will start to move the grille cloth also. If their is anything underneath the grille cloth (usually the frame) that the cloth can hit, it will make twacking noises bouncing against it.
If the grille cloth is not tensioned enough, its motion can become out of phase with the woofer motion and hit the surround also.
It is, as quoted above, Xmax=(top plate thickness-VC winding height)/2
If you find a driver manufacturer using double this value. Run. Don't touch their stuff. They are probably making up other specs and/or fabricating measurements also.
Regarding grille cloth to surround clearance, there are some other things you need to consider. When the driver starts moving at high excursion, the air movement will start to move the grille cloth also. If their is anything underneath the grille cloth (usually the frame) that the cloth can hit, it will make twacking noises bouncing against it.
If the grille cloth is not tensioned enough, its motion can become out of phase with the woofer motion and hit the surround also.
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