hey all, i posted this up in response to a question on e1 burn in but it was an older thread and i wanted to get some feedback as to wether it sounds reasonable or not to all you experienced heads..

of course it's possible, people love to argue about it but as i figgure it, the majority of headphone diaphrams are made out of polymers, not having true surrounds as a conventional speaker has, which means that the diaphram is expected to act as both the suspension (allowing movement back and forth) and the transductional element (the rigid cone of the conventional speaker).

Polymers consist of small molecules linked together to form long molecules which are then linked to each other by intermolecular bonds. When a polymer sheet is distorted some of these bonds will come under greater strain than others because they are either stretched or compressed by the bend. Some bonds will break immediately because the relatively weak bond will be overcome by the powerful electromagnetic force of the voice coil (or however headphones do it) whilst others will not be in opposition to the force and will be able to distort without braking. Inbetween those which break imediately and those which will last for the life of the headphone are billions of bonds which are able to distort enough to allow the movement most of the time but have a chance of breaking with each stretch/compression. Over a time period more and more of these bonds wil break, allowing the diaphram to oscillate further with the same amount of power, thus producing the longer wavelengths of the lower frequencies more efficiently, bringing bass where before there was none (or very little)

well thats what i reckon. if there is a fundamental flaw in my reasoning im sure you all will point it out.

Seems straightforward enough for me.

I read some arguement a long time ago that pointed to the charge characteristics of the dielectric sheath on the wiring, and how it will change over time when there is current being passed through it.

I think that would apply more to speaker cables though, there's only a whisper of current going through headphone cables.

It seems to be a sound theory. It has been brought up a few times before (although usually not as well written and in detail). However, my question is--would the voice coil be powerful enough to break the bonds between the polymers or stretch them? This is similar to the question of stretching the diaphram of the headphones passed their elastic limit (point where the material will actually stretch--below that it will not--it will just spring back to it's original shape) which one head-fier stated is likely not possible through break-in because the voice coil would not be able to generate nearly enough force to do such a thing no matter what the decibel level.

I think it would depend on how brittle that particular molecule is and how much strain it's in already.

yeah, the voice coil will have the power to break individual bonds in the polymer sheet, compared to the amount of power a headphone driver has, individual bonds are miniscule... i know this can be answered by pointing out that the force of the driver is spread amongst countless bonds but if some of these are taking more of the strain (because they're in greater opposition to the distortion) then they are going to be broken eventually. You can't stretch these bonds really or they wont stay stretched unless you supply a steady permanant force- otherwise they will either just spring back as they were or brake entirely.

What you are describing with polymers is plastic deformation, and this really only occurs when you stretch an elastomer to it's limit.

One thing that may be at play here, is the well known 'stress relaxation', or viscoelastic behavior. They strain progressively over time when load is applied.



This could also explain why some drivers need to be 're-broken in' when you shelve them for a while, the strain returns to their 'rest' state:

(Of course, this curve shows the response under CONSTANT load, so the relaxation effect would be the same, MUCH slower for the drivers as they under go very intermittent loading).

The other thing that just occured to me, is that if this theory is correct, why wouldn't bass become more flabby and bloated as you break in?

Yet, it many cases, it becomes tighter and more defined!

When you improve the "elasticity" of the material by break in, what you are doing is improving its tendency to return to its initial unstressed condition once the stress is removed.

As a result of this - the response becomes tighter and more well defined.

When the elasticity is poor you end up having bloated, lagging sound and Bass extension.

I hope thats clear enough

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....and why do you believe it will not continue to do so, and stop just when it's perfect? What mechanism governs this?

This has to be the first Mike post I've peeked at for a long while that has a valid point/question... although I know it doesn't come from knowledge or experience but merely a different tack to attack these theories about burn in.


Burn-in, if you adhere to the principles laid down so far, doesn't stop during the life of the phone. But I believe there's an initial 'burst' of excessive burn-in on some phone drivers caused by their manufacturing process. Be this the evaporation or destruction of coatings used during the manufacturing process, or just a particularly 'stiff' driver, I can't tell you what this is... But I do agree with some of the theories advanced above and yet I hear no 'burst' burn-in occuring with some phones. I think it depends on how phones are made and initially tested. Thereafter, yes I'm sure all phones experience continuous 'burn-in'... but those changes tend to be much more subtle.

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There are arguments that could be made about it, but no-one has advanced any of them. I present two here:

1. The manufacturer makes the item with an 'aim point' requiring several hours of use to reach optimum, because the product otherwise would be inferior and unstable.

Against that, one could speculate:

2. The manufacturer performs all necessary 'burn-in' at the factory, before they are delivered to the consumer.

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Yes, but how does the elasticity improve, or increase during break-in. The theories we were just talking about explain how elasticity might DECREASE:

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So what could lead to the material stiffening during usage?

Elastomers age. Cross-linking of the molecular chains will make them stiffer. A number of things can cause this, UV light, exposure to ozone (trace amounts are in the air), but I'm not sure how playing the speakers/headphones would promote crosslinking, and I really doubt the plastic deformation would occur to a noticable degree by the excursions of the driver during playback.

I don't think anyone has quantitatively documented break-in phenomenon, but it's fun taking shots in the dark here...

plastic deformation is all very well but i think i agree that it is unlikely to come into play here, its a whole different thing to what im talking about. if you take polymer sheet, and stretch it heaps, measing the force as you go then measure it whilst it returns to the the state it was in prior to the application of the force you may get a very broad overall view of what happens when you stretch them to the limit but it won't tell you much about whats really going on on the molecular level in lower stress applications.

Im sure people will agree that as bangraman said the change here is in the subtlties.

Im talking about polymers, not elastomers chillysalsa. By breaking those bonds which overly restrict the distortion of the sheet i think you may well improve its elasticity, not decrease it. the theory above might explain why rigdity will decrease, but maybe not its elasticity.

This isn't put across as a be all and end all for burning in, just maybe one of the contributing factors.

But, all elastomers=polymers.

I was also assuming we're talking about the material used in the surround of the driver. I'm not sure wich elastomer is mostly commonly used in headphone drivers, but butyl is used a lot in speaker designs. Of course, you were talking specifically about the cone of the speaker.

I was focusing on what happens to the surround, because I don't think the cones will distort that much in a headphone design. B&W have done interesting modeling and research on their kevlar cones to find out what it takes to distort them, interesting stuff is on their website: B&W Kevlar Cones, it turns out the 6" cones require kevlar to keep from flexing and distorting at high volume + high frequencies.

But I digress... My point is that I'm very skeptical that any physical changes are happening to the cone to cause a difference during burn-in. I believe it has more to do with the surround. My experience with new speakers has lead me to this bias. When I first got them, I played some bass test tones, and the cones would hardly move. Two months later, playing the same tones made the woofers look like they were going to fall out. Might the butyl surround must have loosened up due to stress relaxation? I wonder...