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Old 8th November 2005, 01:22 PM   #1
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Default Xmax -views on importance?

I both ask for and invite your comments and ideas guys, as there are so many different views. There's been some discussion about this point in recent threads, so I thought I'd start one up devoted to this topic that might help people when making driver choices. I know we've covered aspectd before, but I thought it might be an idea to have the knowledge combined into one easily accessable and found thread. I don't pretend to be particularly up on the subject, so hopefully I should learn something myself.

Comments on the nature of Xmax, the value of it (a worthwhile guide, or to be taken with a pinch of salt et al), driver suitability for general enclosure types etc with examples will be probably the most useful.

Cheers for now
Scott
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Old 8th November 2005, 01:50 PM   #2
Mark Kravchenko --- www.kravchenko-audio.com
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Default Maybe a little smaller scope to the questions would help !

Hi Scott

Are you refering to X-max in full range drivers or X-max in drivers in general ?

To be sure the figures claimed can be suspect. But more often than not they are accurate.

When you are looking at fullrange drivers there can be some problems. Generally they want the lightest stiffest cone to be able to get the frequency response as high as possible. The overall mass is also very important. SO making a voice coil with a long length to gain a longer excursion not only adds unwanted mass but also unwanted inductance which will limit the high frequency responce.

The only other way to get larger X-max is to use an underhung coil and a thick top plate. But that makes for a massive magnet structure when using conventional magnets or an expensive slug of neodymium.

The other thing to note that strictly speaking using the T/S guidelines is that X-max = voice coil height minus top plate thickness / 2. An underhung motor structure must be understood to not really work with that little explanation. It's almost the same idea just a rearrangement of the top plate versus the coil height.

Hope this helps a bit!

MArk
one time designer of a lowther clone
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Old 8th November 2005, 02:33 PM   #3
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Indeed it does. Many thanks for a very clear explanation, and for the comments on the underhung voice coils too. (One time designer of a Lowther clone? Excellent, always nice when someone involved in design can step forward). In answer to your question, yes, I was refering to xmax related to full-range drivers, which will need to be considerably lower than a long-throw subwoofer intended for dipolar operation! One idea to thow into the arena: xmax and maximum safe cone excursion are not necessarily the same thing, correct? As I understand it, when a driver goes beyond this limit, distortion increases, but providing the suspension limits it and prevents it fouling the voice-coil windings, things remain safe?

Cheers
Scott
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Old 8th November 2005, 03:41 PM   #4
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Quote:
Originally posted by Scottmoose
One idea to thow into the arena: xmax and maximum safe cone excursion are not necessarily the same thing, correct? As I understand it, when a driver goes beyond this limit, distortion increases, but providing the suspension limits it and prevents it fouling the voice-coil windings, things remain safe?

Cheers
Scott
That's pretty well how I understand it as well. For example before I learned the importance of x-max figures I was running my fostex 108's open in a sealed enclosure on top of a pair of subwoofers. Quite compromised in retrospect... anyways I took them to the AlantaDIY event and Greg Monfort (GM) cranked them up much louder than I had ever listened to them before (trying to teach me the wrongs of my ways, not melt my drivers, I assume) . The little guys were jumping all over the place well past their .28mm x-max. The louder they got the worse they sounded, however they still held together, didn't self destruct or anything. A definate learning experience for me about the importance of staying within the the excursion limits. I believe Vance Dickason states +15% x-max is reaching 3% 3rd order harmonic distortion. Which he seems to think is a safe limit for listening, I'll defer to the experts on this one.
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Old 8th November 2005, 03:46 PM   #5
Mark Kravchenko --- www.kravchenko-audio.com
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Default Yep

Generally under normal drive conditions the suspension will protect a driver.

Yes as X-max is exceeded the distorsion rises as the cone is not in the optimum magnetic field density set up by the motor. ( The magnet/ pole piece and top plate are the motor ) As it leaves it's happy spot it misbehaves!

Mark
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Old 8th November 2005, 04:21 PM   #6
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A series of articles called "Speakers Corner by John Watkinson" in Electronics World beginning from mid 1998 till end of 2001 or so is very instructive and addresses many issues related to x-max. A must read for everyone.

Even if someone who has these back issues is able to scan the articles, they cannot be posted on the forum for public view due to copyright issues, but sending the scanned images zipped to individual email ids....... is someone willing to take the effort?
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Old 8th November 2005, 09:03 PM   #7
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I was wondering if someone could explain what cone slap is to me. I am also interested in "turning up the bass" without turning up the volume and how that effects the diver. I've knoticed with the wr125 and fe127 that they can't handle turning up the bass, but the pioneers can, even though the pioneers only have a 3mm xmax.

This may be a little off topic, but if a drivers fs is 50hz and has 10mm xmax how does it play a 40hz note differently from a driver with an fs of 50hz and xmax of 5mm, to add to that, how does that differ from a driver with an fs of 45hz and and an xmax of 5mm.

And how does power handling play into xmax and what a makes a diver have a greater power handling from another driver? ex why can fostex driver not handle much power?

Thanks,

Josh
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Old 8th November 2005, 11:38 PM   #8
Mark Kravchenko --- www.kravchenko-audio.com
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Default Questions and hopefully some answers

Some of you guys should do some searching on the forum.

I'll try to help out with a couple of answers.

Cone slap is when a drivers is being driven beyond it's safe mechanical X-max. It is not good. Another common problem has been called oil canning where the cone goes into flexture due to the combination of high excursion and not so strong cone material and it makes a popping noise like the old oil cans.

Driver power handling is a function of a number of components. The diameter of wire in the voice coil, the diameter of the voice coil and the mechanical clearances and cooling paths.

The larger the wire the more current it is capable of carrying. The larger the wire the more room it takes up and you get a smaller BL factor or a driver that is not as efficient. Also a larger wire is greater in mass and consumes more magnetic flux per area taken up on a voice coil stack. It means a loss in efficiency that will mean more power is required to reach a certain loudness.

With a larger voice coil diameter you can spread the heat over a larger area and facilitate cooling. But a larger coil is heavier and requires a much larger motor to get it moving with the same level of efficiency as a smaller lighter coil.

Holes in the pole piece and the surround are used to pump air into and out of the motor to cool the coil. Pro drivers even have heatsinks cast into the frames and the back plates.

As for the sound of different drivers playing the same note at the same SPL. OH boy! Big one here. Different cone materials, different suspensions, different baskets etc. Drivers are not suposed to add their own sound but we all know that they do. The trick is in finding the ones that do it the least.

Just a side note . There are some very talented engineers on this forum that are currently designing drivers. I'm admittedly rusty on some of the proper terminology. BL is something that I wanted to explain more clearly but my brain is on strike. They can and have answered all of these questions before. If you want a quick overview of how drvers work there are two threads that I cn think of off the top of my head. Vikash did a DIY driver and there is the DIY Parthenon thread.

Mark
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Old 9th November 2005, 07:33 AM   #9
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Default linear xmax

While xmax is important as you want to know how far a driver can be pushed before damage will occur linear xmax is of more interest. This tells you how much travel you have without distortion. In most cases the linear xmax is a little less than one third of the top plates thickness. As a general rule of thumb 1/3 thickness is an ok guide. While drivers can be designed to have lots of travel the further you go beyond 1/3 top plate peak to peak the more non linear the output becomes. Cabinet designs that do a better job of loading the driver and which provide a better impedance match between the driver and the air in the room will yield cleaner output. The less your driver has to travel the lower the overall distortion. No free lunch as the saying goes. Regards Moray James.
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Old 9th November 2005, 07:21 PM   #10
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I thought x-max was the amount of excursion a driver can handle without distortion, there is a different paramater for the physical limit a driver can handle (x-mech I think...). I could be wrong though.
Joe
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