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
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
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
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
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
Cheers
Scott
Scottmoose said:
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.Joe
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
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
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?
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?
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
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
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
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
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.
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.
Scott,
In my opinion we spend too much time worrying about Xmax and we are very conservative in our design analysis assessing the capability of a speaker with a low Xmax. For example, I use Lowther drivers in my system, powered by a 200 watt SS amp, and have not experienced any problems due to the low Xmax value of 1 mm. Here is something I wrote up a few years ago on Xmax.
"Suppose I have a driver with an Xmax = 1 mm. I want to know what kind of input it can take at 20 Hz without exceeding this value. Lets assume that the standard Xmax plot, as a function of frequency from a T/S based computer simulation, shows that 1 watt of input will yield a displacement of 1 mm at 20 Hz. So 1 watt at 20 Hz corresponds to Xmax for a pure sine wave input signal.
Sinusoidal Input :
--------------------
If I excite this driver with a single frequency sine wave at 20 Hz, so that the maximum displacement is 1 mm, then the maximum input power is 1 watt as shown in the computer simulation plot. The driver is limited to 1 watt of input at 20 Hz before Xmax is exceeded.
Square Wave Input :
-------------------------
If I excite the driver with a 20 Hz square wave that produces a displacement magnitude of 1 mm, then the input power at 20 Hz can be greater than 1 watt before Xmax is exceeded. Hopefully, the following over simplified derivation will adequately demonstate my reasoning.
Definition of a square wave at 20 Hz :
Time = 0.000 to 0.025- sec the magnitude is +1 mm
Time = 0.025+ to 0.050 sec the magnitude is -1 mm
Using the Fourier Series to represent this wave shape yields the following sine waves that are added to reconstruct the required square wave. Please recognize that this is an infinite series and I am only showing the first 3 terms.
1st harmonic at 20 Hz = magnitude is 1.273 mm
2nd harmonic at 40 Hz = magnitude is 0.424 mm
3rd harmonic at 60 Hz = magnitude is 0.255 mm
.
.
.
.
Look at the magnitude of the first harmonic at 20 Hz, it exceeds Xmax. But the sum of all harmonics equals Xmax.
Conclusion :
--------------
If I have done my math correctly (maybe I screwed up and hopefully somebody can correct it NICELY!) then a 1 mm square wave would have a component that exceeds Xmax at 20 Hz.
Recorded music is much more complicated then a simple square wave, it is made up of many different sine waves all adding together to generate displacement of a driver as a function of time. I do look at the driver displacement plot, calculated in my T/S based MathCad models, and track the input power that will cause Xmax to be exceeded, but I do not consider it to be a rigid limitation in a TL (or bass reflex) design.
In my audio system, I listen primarily to acoustic jazz. I do not use my speakers in a HT set-up and I only play the 1812 cannon shots when demonstrating my speakers to uninitiated guests needing a big "WOW!" to understand why I take building speakers so seriously.
I probably do not exceed Xmax at the listening levels typically being played. I can play loud enough to simulate the live event in the room and still allow my family to go about normal activities, like sleeping.
I am sure that I have exceeded Xmax on occasions and I have even managed to bottom out my 10" Focal woofers against the stops a couple of times during 1812 cannon shots. Got a big "WOW!" on those occasions. HT, or other types of music, may force a designer to stick closer to the Xmax specified if signals with strong low frequency content are commonly being reproduced (cannon shots, walking dinosaurs, helicoptors, ....). Each application is unique so you can decide how rigid a limit Xmax really is in your design."
I wrote that a few years ago when I had a big Focal 3 way system so the 1812 overture was a standard demo CD. Now with the Lowthers I have retired the 1812 and use a few other jazz selections. I have never had the guts to try the 1812 cannon shots with the Lowthers. But I can listen to acoustic music at very loud levels and not have problems related to Xmax, my problems are complaints from my family members who do not appreciate jazz like I do.
Hope that helps,
In my opinion we spend too much time worrying about Xmax and we are very conservative in our design analysis assessing the capability of a speaker with a low Xmax. For example, I use Lowther drivers in my system, powered by a 200 watt SS amp, and have not experienced any problems due to the low Xmax value of 1 mm. Here is something I wrote up a few years ago on Xmax.
"Suppose I have a driver with an Xmax = 1 mm. I want to know what kind of input it can take at 20 Hz without exceeding this value. Lets assume that the standard Xmax plot, as a function of frequency from a T/S based computer simulation, shows that 1 watt of input will yield a displacement of 1 mm at 20 Hz. So 1 watt at 20 Hz corresponds to Xmax for a pure sine wave input signal.
Sinusoidal Input :
--------------------
If I excite this driver with a single frequency sine wave at 20 Hz, so that the maximum displacement is 1 mm, then the maximum input power is 1 watt as shown in the computer simulation plot. The driver is limited to 1 watt of input at 20 Hz before Xmax is exceeded.
Square Wave Input :
-------------------------
If I excite the driver with a 20 Hz square wave that produces a displacement magnitude of 1 mm, then the input power at 20 Hz can be greater than 1 watt before Xmax is exceeded. Hopefully, the following over simplified derivation will adequately demonstate my reasoning.
Definition of a square wave at 20 Hz :
Time = 0.000 to 0.025- sec the magnitude is +1 mm
Time = 0.025+ to 0.050 sec the magnitude is -1 mm
Using the Fourier Series to represent this wave shape yields the following sine waves that are added to reconstruct the required square wave. Please recognize that this is an infinite series and I am only showing the first 3 terms.
1st harmonic at 20 Hz = magnitude is 1.273 mm
2nd harmonic at 40 Hz = magnitude is 0.424 mm
3rd harmonic at 60 Hz = magnitude is 0.255 mm
.
.
.
.
Look at the magnitude of the first harmonic at 20 Hz, it exceeds Xmax. But the sum of all harmonics equals Xmax.
Conclusion :
--------------
If I have done my math correctly (maybe I screwed up and hopefully somebody can correct it NICELY!) then a 1 mm square wave would have a component that exceeds Xmax at 20 Hz.
Recorded music is much more complicated then a simple square wave, it is made up of many different sine waves all adding together to generate displacement of a driver as a function of time. I do look at the driver displacement plot, calculated in my T/S based MathCad models, and track the input power that will cause Xmax to be exceeded, but I do not consider it to be a rigid limitation in a TL (or bass reflex) design.
In my audio system, I listen primarily to acoustic jazz. I do not use my speakers in a HT set-up and I only play the 1812 cannon shots when demonstrating my speakers to uninitiated guests needing a big "WOW!" to understand why I take building speakers so seriously.
I probably do not exceed Xmax at the listening levels typically being played. I can play loud enough to simulate the live event in the room and still allow my family to go about normal activities, like sleeping.
I am sure that I have exceeded Xmax on occasions and I have even managed to bottom out my 10" Focal woofers against the stops a couple of times during 1812 cannon shots. Got a big "WOW!" on those occasions. HT, or other types of music, may force a designer to stick closer to the Xmax specified if signals with strong low frequency content are commonly being reproduced (cannon shots, walking dinosaurs, helicoptors, ....). Each application is unique so you can decide how rigid a limit Xmax really is in your design."
I wrote that a few years ago when I had a big Focal 3 way system so the 1812 overture was a standard demo CD. Now with the Lowthers I have retired the 1812 and use a few other jazz selections. I have never had the guts to try the 1812 cannon shots with the Lowthers. But I can listen to acoustic music at very loud levels and not have problems related to Xmax, my problems are complaints from my family members who do not appreciate jazz like I do.
Hope that helps,
Diplomacy is not my strength, I tend to go straight to the point without beating around the bush. I have no patience for reading between the lines and trying to figure put what somebody really wants but is not telling me. My wife learned that lesson a long time ago.
Another example, 20 years ago my boss's boss was debating if we should spend the money and buy a color printer for the computer network. He really liked those color engineering plots that some young guy in another group was wowing the Genral Manager with during meetings. He asked me what I thought. I told him point blank that I could BS him and the GM in black and white just as well as I could in color and recommended we spend the money on better software. He bought the color printer, I only used it to make black and white plots for all my presentation. He really liked me after that, my co-workers thought it was very funny. The young guy they eventually canned, he was more fluff then substance. I quit a couple of years later.
Another example, 20 years ago my boss's boss was debating if we should spend the money and buy a color printer for the computer network. He really liked those color engineering plots that some young guy in another group was wowing the Genral Manager with during meetings. He asked me what I thought. I told him point blank that I could BS him and the GM in black and white just as well as I could in color and recommended we spend the money on better software. He bought the color printer, I only used it to make black and white plots for all my presentation. He really liked me after that, my co-workers thought it was very funny. The young guy they eventually canned, he was more fluff then substance. I quit a couple of years later.
Rather than just look at Xmax, it's really necessary to look at total displacement volume. Taken FR extends to the same frequency, no significant cone resonances close to XO point, same efficiency and displacement volume, etc. I think the larger cone with less Xmax would be better.
Of course there are many other design issues involved, and this could really end up in a discussion as hot as wires.
Of course there are many other design issues involved, and this could really end up in a discussion as hot as wires.
Thanks soongsc,
My coworkers are always amused when I cut to the significant facts in a meeting and ridicule the self inflating double talkers and beaurocrats. Why just today I stuck some human resource fool in a group meeting and I don't think the guy was even smart enough to realize. The guy next to me almost fell out of his chair laughing. I have perfected that totally innocent look.
My bosses have usually appreciated my candor and have treated me well. But I doubt I will ever be promoted to a position where I could make a difference because I won't play the games. I am not sure I could stand being in a position of responsibility and being handcuffed by politics, company policies, and management infighting. Doing the real work is more fun and interesting.
OK back to Xmax, the real question.
My coworkers are always amused when I cut to the significant facts in a meeting and ridicule the self inflating double talkers and beaurocrats. Why just today I stuck some human resource fool in a group meeting and I don't think the guy was even smart enough to realize. The guy next to me almost fell out of his chair laughing. I have perfected that totally innocent look.
My bosses have usually appreciated my candor and have treated me well. But I doubt I will ever be promoted to a position where I could make a difference because I won't play the games. I am not sure I could stand being in a position of responsibility and being handcuffed by politics, company policies, and management infighting. Doing the real work is more fun and interesting.
OK back to Xmax, the real question.
Ha! We're getting some excellent views and answers from everyone here! Martin -that's very interesting, it seems to prove a recent experience I had with a very low xmax driver that I assumed flat wouldn't work in a hefty load, yet seemed to do just fine (so long as it wasn't asked to replicate live Motorhead decibels, which I decided might be unhealthy).
Cheers for everything so far guys
Scott
Cheers for everything so far guys
Scott
... anybody remember the Elemental Designs eD12A ? It had a large xMax, around 18mm. It's xMag was greater than it's xSus (I think I got the terminology right) and when it was overpowered it tore it's surround and spider. The Phoenix Gold Ti10 in my car had a similar problem that needed warranty work. The tinsel leads were too short for it's 18mm xMax.
I used to thind that the driver with the most SD and most xMax for a given size was the way to go. Now I want the driver with a cone that can most closly follow the input signal.
I used to thind that the driver with the most SD and most xMax for a given size was the way to go. Now I want the driver with a cone that can most closly follow the input signal.
I would like to suggest that different manufacturers choose the Xmax they specify differently, so it is best taken as a useful guide rather than a "engineering" specification.
Picking up on the pont made by Squalor, I have seen one cheap driver specified with a large Xmax, (and probably justifed by its magnetic circuit), that failed entirely to transfer the energy to the cone at large excursions. It sounded good at low volumes, You can guess what it sounded like at high!
Picking up on the pont made by Squalor, I have seen one cheap driver specified with a large Xmax, (and probably justifed by its magnetic circuit), that failed entirely to transfer the energy to the cone at large excursions. It sounded good at low volumes, You can guess what it sounded like at high!
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