seangoesbonk said:You need a ruler which has a precision of 1/10th of millimeter, as well as a very good eye.
EDIT: That was my 200th post... Too bad it was stupid...
Sean, that was unforgiveable. You are hereby sentenced to spend the next two weeks listening to nothing but an old $19.95 cassette boombox without a CD player!
Doug:
There is expensive measuring equipment out there, but it is generally not necessary for the home hobbyist.
There are two types of Xmax-linear and mechanical maximum. The linear type is the most quoted kind, and is smaller than the mechanical maximum. Within the linear range, the speaker operates relatively distortion free. Outside it's linear range, distortion rises sharply until the speaker hits it's mechanical limit-Xmech. Still, the speaker is able to operate and produce sound outside it's linear range.
Unless specified as Xmech or something similar, Xmax generally means linear Xmax.
If given the Thiele-Small parameters, you usually can calculate the linear Xmax for a speaker. You need the Voice Coil Length and the Height of the Magnetic Gap, (sometimes also called Depth of the Gap, or Air Gap). Just make sure the word "gap" is in there, LOL.
Then:
Xmax = (Voice Coil Length - Height of the Magnetic Gap) / 2.
If Voice Coil length is .75" and the height of the magnetic gap is .25", then Xmax = .25".
For Voice Coil Length, most hi fi speakers run from 3/4" up to 2" for some super-pumpers. The magnetic gap is generally about 1/4"-or 6 mm to 8mm.
If anything, the calculated response gives a stricter figure than the measured response. A calculated Xmax of .25" will often show up as .29" in a DuMax test, for instance. These differences are not important.
For an 8" or larger speaker that gives no specs, assume the Xmax to be .2 inches. If it gives the Voice Coil Length, assume the Height of the Magnetic Gap to be either 1/4" or 6 mm to 8mm. So a speaker with a .75" Voice Coil, you can assume it will have an Xmax of .25". If it has a one inch voice coil, you can assume it will have an Xmax of .36"-which is very good.
In hifi speakers, the Xmech is usually 1.5 to 2 times the linear Xmax. In PA speakers, the mechanical Xmax is frequently four times the linear Xmax-a PA speaker with a linear Xmax of .2 will have a mechanical Xmax of .8, usually.
NOOOOOOOOOOOOOOOOOOOOOOOO!!!!!!!!!!!!!!!!I should add that it is a good idea to check the voice coil length and magnetic gap height for another reason.
Most manufacturers give the Xmax as meaning the distance the cone moves from the center position all the way forward. Or, from the center position to all the way back.
However, some manufacturers, chiefly car manufacturers, give the disatnce the cone moves from all the way back to all the way front-double the distance from center to front.
That is why speaker manufacturers frequently give their Xmax figure as (+ or -).25 in, or something similar. A plus or minus figure means they are measuring from center to front, or center to back, the way they should. A plus or minus figure they are not measuring front to back, which gives an inflated figure.
However, when you know the voice coil length and the gap height, you can double check the Xmax figures, so you can know for sure.
Thought I would just give a graphic of the formula for Xmax. Voice Coil Length is sometimes called Voice Coil Height.
Most manufacturers give the Xmax as meaning the distance the cone moves from the center position all the way forward. Or, from the center position to all the way back.
However, some manufacturers, chiefly car manufacturers, give the disatnce the cone moves from all the way back to all the way front-double the distance from center to front.
That is why speaker manufacturers frequently give their Xmax figure as (+ or -).25 in, or something similar. A plus or minus figure means they are measuring from center to front, or center to back, the way they should. A plus or minus figure they are not measuring front to back, which gives an inflated figure.
However, when you know the voice coil length and the gap height, you can double check the Xmax figures, so you can know for sure.
Thought I would just give a graphic of the formula for Xmax. Voice Coil Length is sometimes called Voice Coil Height.
Attachments
Doug:
The wiz has it down on the money.
For a chiefly by the pants method you can try to shine a brite light onto the spider assembly and see if you can guess the length of the voice coil as you physically move the cone fully up then fully down. To put a real life touch on the magnetic gap height you should look at the thickness of the the top plate or the big washer on the top of the motor assembly that is closest to the cone of your woofer. Have fun
The wiz has it down on the money.
For a chiefly by the pants method you can try to shine a brite light onto the spider assembly and see if you can guess the length of the voice coil as you physically move the cone fully up then fully down. To put a real life touch on the magnetic gap height you should look at the thickness of the the top plate or the big washer on the top of the motor assembly that is closest to the cone of your woofer. Have fun
Xmax
I looked on the Eminence website and if you download their catalog, go to page 6, they explain how they measure Xmax for their line of loudspeakers.
It seems that different driver manufacturers have their own way of measuring Xmax. I think it would be safe to say in general that as long as the voice coil remains within the magnetic gap the driver will remain reasonably linear ( BL ). Once the voice coil leaves the magnetic gap, the driver becomes non-linear rather quickly and the BL falls much faster.
Actually, if you look at the link I posted for DLC Designs, measurements with their DUMAX system shows that as soon as the driver starts moving from the zero ( at rest ) position, it starts to become non-linear already, the non-linearity speeding up once the VC leaves the magnetic gap. What do ya think guys?
DOUG
I looked on the Eminence website and if you download their catalog, go to page 6, they explain how they measure Xmax for their line of loudspeakers.
It seems that different driver manufacturers have their own way of measuring Xmax. I think it would be safe to say in general that as long as the voice coil remains within the magnetic gap the driver will remain reasonably linear ( BL ). Once the voice coil leaves the magnetic gap, the driver becomes non-linear rather quickly and the BL falls much faster.
Actually, if you look at the link I posted for DLC Designs, measurements with their DUMAX system shows that as soon as the driver starts moving from the zero ( at rest ) position, it starts to become non-linear already, the non-linearity speeding up once the VC leaves the magnetic gap. What do ya think guys?
DOUG
Xmax
Sorry, I forgot that I posted that web link for DLC on another forum. The link is:
http://www.dlcdesignaudio.com
Look at their reports in particular, they show graphs and parimeters generated with their DUMAX system.
It was the LinearX forum that I posted that web link on.
http://www.linearx.com Go to forums, LMS then Xmax and you will see my posting.
Any problems with the links, e-mail me directly and I'll send you the links.
DOUG
Sorry, I forgot that I posted that web link for DLC on another forum. The link is:
http://www.dlcdesignaudio.com
Look at their reports in particular, they show graphs and parimeters generated with their DUMAX system.
It was the LinearX forum that I posted that web link on.
http://www.linearx.com Go to forums, LMS then Xmax and you will see my posting.
Any problems with the links, e-mail me directly and I'll send you the links.
DOUG
Would the following procedure allow you to estimate Xmax?
Would the following procedure allow you to estimate xmax for a woofer suitable for sound reinforcement?
(1) Mount the speaker for which Xmax is to be measured so that it is pointed downward, and air movement is unrestricted above and below the speaker.
(2) Attach the speaker to an amplifier that has sufficient power to drive the speaker to distortion.
(3) The amplifier will be fed a 500 Hz sinusoidal signal. (500 Hz seems like a frequency any woofer should be able to handle at maximum excursion without running into frequency limitations)
(4) Make a tiny dot with white paint on the cone of the speaker for which xmax is to be measured.
(5) Attach one of the laser levels that you can buy in any hardware store to a platform that is otherwise rigid but can be moved up and down with a screw adjustment. The laser should be aimed horizontally so that it can point at the speaker cone near where it joins the spider.
[___Laser level________] ---------Beam-------> / Speaker \
(6) With the amplifier off, aim the laser exactly at the white dot (which should be about the size of the lazer's projected point of light). Measure the laser level's height relative to the speaker's mounting platform, and make sure that the level is really level.
(7) Now turn on the frequency generator and amplifier and slowly increase volume until the speaker just begins to distort.
(9) Move the laser level down until the laser beam is at the point where it barely touches the white dot when the speaker is at its maximum excursion, making sure that the laser level in fact remains level.
(10) Turn the amplifier off (to minimize time at high level output and to avoid unecessary stress on the speaker).
(11) Again measure the level's height relative to the speaker's mounting platform. The difference in height between this measurement and the one made in step 9 should at least approximate Xmax shouldn't it?
I guess that the basis for this whole idea is that distortion would occur under the circumstances outlined above (response to relatively low frequency sinusoidal output) primarily from increasing nonlinearity that would result from excursion approaching xmax. Does this assumption make sense? I'm a novice at this, so if I've got this badly wrong, put me down easy, OK?
I also assume that there would be some risk to the speaker from this procedure, but if the time spent at high output is kept to a minimum, and the speaker is pushed just to the edge of distortion, the risk should be minimal.
Would the following procedure allow you to estimate xmax for a woofer suitable for sound reinforcement?
(1) Mount the speaker for which Xmax is to be measured so that it is pointed downward, and air movement is unrestricted above and below the speaker.
(2) Attach the speaker to an amplifier that has sufficient power to drive the speaker to distortion.
(3) The amplifier will be fed a 500 Hz sinusoidal signal. (500 Hz seems like a frequency any woofer should be able to handle at maximum excursion without running into frequency limitations)
(4) Make a tiny dot with white paint on the cone of the speaker for which xmax is to be measured.
(5) Attach one of the laser levels that you can buy in any hardware store to a platform that is otherwise rigid but can be moved up and down with a screw adjustment. The laser should be aimed horizontally so that it can point at the speaker cone near where it joins the spider.
[___Laser level________] ---------Beam-------> / Speaker \
(6) With the amplifier off, aim the laser exactly at the white dot (which should be about the size of the lazer's projected point of light). Measure the laser level's height relative to the speaker's mounting platform, and make sure that the level is really level.
(7) Now turn on the frequency generator and amplifier and slowly increase volume until the speaker just begins to distort.
(9) Move the laser level down until the laser beam is at the point where it barely touches the white dot when the speaker is at its maximum excursion, making sure that the laser level in fact remains level.
(10) Turn the amplifier off (to minimize time at high level output and to avoid unecessary stress on the speaker).
(11) Again measure the level's height relative to the speaker's mounting platform. The difference in height between this measurement and the one made in step 9 should at least approximate Xmax shouldn't it?
I guess that the basis for this whole idea is that distortion would occur under the circumstances outlined above (response to relatively low frequency sinusoidal output) primarily from increasing nonlinearity that would result from excursion approaching xmax. Does this assumption make sense? I'm a novice at this, so if I've got this badly wrong, put me down easy, OK?
I also assume that there would be some risk to the speaker from this procedure, but if the time spent at high output is kept to a minimum, and the speaker is pushed just to the edge of distortion, the risk should be minimal.
Adding to what KelticWizard has said:
For underhung drivers, you will often find that the Coil height is ~3 times the gap height - and the gap height is usually the same as the top plate thickness.
This means that the top plate thickness is approximately Xmax. It's the best estimate you can get unless you can see the coil through the spider. It is very likely that the real Xmax will be somewhere within 0.7 to 1.5 times the top plate thickness.....
For underhung drivers, you will often find that the Coil height is ~3 times the gap height - and the gap height is usually the same as the top plate thickness.
This means that the top plate thickness is approximately Xmax. It's the best estimate you can get unless you can see the coil through the spider. It is very likely that the real Xmax will be somewhere within 0.7 to 1.5 times the top plate thickness.....
Xmax
Here is the proper link to the thread on X-max on the LinearX forum, the folks who built the software for Leap-5 and Lms systems.
http://www.linearx.com/forums/showthread.php?threadid=119&highlight=Xmax
Doug
Here is the proper link to the thread on X-max on the LinearX forum, the folks who built the software for Leap-5 and Lms systems.
http://www.linearx.com/forums/showthread.php?threadid=119&highlight=Xmax
Doug
Hi Forum,
my English is not that good and I would like to know cone travel relative to the voltage of a speaker, a graph depicting volts vs excursion basically.
This is the speaker Eminence em 300 - 15 15880
It is not a Subwoofer
300/600 Watts
3 Inch coil
XMAX MM 1.60
BL TM 17.36
RE OHMS 5.19
FS HZ 29.45
LE MH 1.07
MMS GMS 89.80
QM 6.06
CMS mm/N .3253
QE .290
RMS NS/M 2.7393
QT .270
VAS LTRS 335.06
SD SCM 856.34
EBP 102.9
EFF % 2.88 20-
SPL dB 96.6
The xmax is stated 1.6mm, which might be voice coil overhang,( I looked at the coil with flashlight) . Some speaker manufacturers calculate xmax= overhang+ 1/4 plate thickness, which would mean 3,6mm (8 mm plate) xmax (with a little diminished BL) - and that would be ok for me. But the mechanical properties ofcourse also play a part, surround, spider limiting forces, ect.
I would basically like to attach a power supply and measure the (static) excursion at different voltage levels? Would that be risky with this speaker. How long could I hold the voltage, Im guessing I would only need 1 sec per voltage step?
If i statically measure the excursion vs voltage, does that mean dynamically with with music the response will reflect these measurements?
Mechanically this speaker seems very robust.
If that is a bad idea, could i conclude that if I have linear relationship cone travel vs "resisting forces" of spider and surround , that I might have somewhat linear xmax +- 3,6mm, as calculated above? I could measure the resisting forces vs excursion.
my English is not that good and I would like to know cone travel relative to the voltage of a speaker, a graph depicting volts vs excursion basically.
This is the speaker Eminence em 300 - 15 15880
It is not a Subwoofer
300/600 Watts
3 Inch coil
XMAX MM 1.60
BL TM 17.36
RE OHMS 5.19
FS HZ 29.45
LE MH 1.07
MMS GMS 89.80
QM 6.06
CMS mm/N .3253
QE .290
RMS NS/M 2.7393
QT .270
VAS LTRS 335.06
SD SCM 856.34
EBP 102.9
EFF % 2.88 20-
SPL dB 96.6
The xmax is stated 1.6mm, which might be voice coil overhang,( I looked at the coil with flashlight) . Some speaker manufacturers calculate xmax= overhang+ 1/4 plate thickness, which would mean 3,6mm (8 mm plate) xmax (with a little diminished BL) - and that would be ok for me. But the mechanical properties ofcourse also play a part, surround, spider limiting forces, ect.
I would basically like to attach a power supply and measure the (static) excursion at different voltage levels? Would that be risky with this speaker. How long could I hold the voltage, Im guessing I would only need 1 sec per voltage step?
If i statically measure the excursion vs voltage, does that mean dynamically with with music the response will reflect these measurements?
Mechanically this speaker seems very robust.
If that is a bad idea, could i conclude that if I have linear relationship cone travel vs "resisting forces" of spider and surround , that I might have somewhat linear xmax +- 3,6mm, as calculated above? I could measure the resisting forces vs excursion.
Hope my very old stats from above mean (Xmax: Voice coil height minus top plate thickness, divided by 2) and not 10% THD
Eminence used physical overhang a lot in their factory spec sheets.
I believe 12LTA still has an 0.08 inch overhang.
Lc= coil height, Hg = gap height. I have an Eminence 12cx with 1.6mm xmax figure and by the 70% BL factor mark formula below, would be rated `"4mm"
from DJK "The coil is 0.5" long, the top plate 0.375" thick" on my Eminence 12cx.
While the coil overhang is about 1.6mm, the 70% Bl point is about 4mm.
(Lc - Hg)/2 + 1/4(Hg) = x-max (70% Bl).\
Google Translate
I believe 12LTA still has an 0.08 inch overhang.
Lc= coil height, Hg = gap height. I have an Eminence 12cx with 1.6mm xmax figure and by the 70% BL factor mark formula below, would be rated `"4mm"
from DJK "The coil is 0.5" long, the top plate 0.375" thick" on my Eminence 12cx.
While the coil overhang is about 1.6mm, the 70% Bl point is about 4mm.
(Lc - Hg)/2 + 1/4(Hg) = x-max (70% Bl).\
Google Translate
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