Very interesting article by Tyll with regards to headphone burn in - Page 4

Quote:

Originally Posted by upstateguy 

Did Tyll make frequency plots of the headphones before starting?  If so, how similar were they?

I don't think he ever measured the brand new white Q701 used in the test. It's something that could be done, and compared to measurements of the green Q701 when it was new and burned in. Inner Fidelity isn't loading for me right now, but search around for "break-in" or "burn-in" or something and you should find the tests done on the green Q701 that measured burn-in. From memory, they weren't very large, but potentially audible.

Quote:

Originally Posted by Head Injury 

I don't think he ever measured the brand new white Q701 used in the test. It's something that could be done, and compared to measurements of the green Q701 when it was new and burned in. Inner Fidelity isn't loading for me right now, but search around for "break-in" or "burn-in" or something and you should find the tests done on the green Q701 that measured burn-in. From memory, they weren't very large, but potentially audible.

Are you referring to pre-burn in plots of the two headphones in question?

Quote:

Originally Posted by upstateguy 

Are you referring to pre-burn in plots of the two headphones in question?

No. Read my post, specifically the first sentence.

Quote:

Originally Posted by Head Injury 

No. Read my post, specifically the first sentence.

I'll have to reread Tylls test again. 

I think you guys are over-analysing this. I think he explained and qualified the limits of his experiment quite well.

Yep, too much talk.

I've re-read the test and this is what I think.

Since the 2 headphones were never compared (or measured) before break in was done to the green one, I fail to see the relevance of this test. 

Tyll starts off by saying:

"The green Q701 that was used in the first test has been on my bench playing pink noise at about 90dB for well beyond 1000 hours at this point. The white Q701 remains sealed, brand spanking new in its box."

He finishes by saying:
 

" All I've proven is that I could tell one headphone from another."

The set up looked like this:

"I did notice that the music playing computer in front of me did provide some reflections in which I could potentially see the headphones on my head, so the screen was tilted way back to prevent seeing any reflection."

And let's not forget that in Tyll's prior "measurement test" of the green headphone, he  wrote:

"Summary:  Did I show break-in exists? No."

Good summation upstateguy.

Post-test only designs aren't that uncommon - but depend on adequate sample size to achieve the desired statistical and study power, and other procedures to achieve good experimental control. Obviously this (sample size) is not remotely adequate for post-test only.

Simply, this was not a scientific test.

I see it instead as a pilot for a serious study. Tyll tested the water. Had he found no difference, it would seem to not be worth going further. He did find a difference, so it might be worth pursuing. However, pilots are by definition inadequate as studies, hence decisions to proceed or not on that basis alone aren't justified.

Problem is, I don't know head-fi is the sort of forum to get us there. It would take a lot of time and analysis to construct a validated measurement instrument, and then there's up-front cost getting various samples of headphones etc etc. Altogether, an expensive exercise.

At this point, I'm going to take my own advice and move on. Over and out.

I cannot think of one thing made of physical materials that does change as it ages.  Believing that speakers or headphone diaphragms do not change with time is ignoring the fact that all things change over time.  They flex thousands of times and just like a drum head they are going to move, warp, wobble, dent, and eventually wear out.

I am all for the measurements as this tends to cross off the counter-hypotheses.

I guess taking this to the extreme case I ask anyone out there if they have had a driver physically fail?  This would be a boundary condition for sure as the sound has certainly burned in to an extreme at that point.  The simple fact that overdriving a speaker can damage it also provides evidence into permanent burn in be it the extreme case or not.  I doubt any manufacturer tests their speakers / headphones over years which is how long many of us use headphones for.

My hypothesis is that burn in exists.

My assumptions are:

1.)  All things made of physical materials age with time

2.)  Measurements can plot the change as a function of frequency spectrum or total harmonic distortion vs time

Evidence:

1.)  Tyll's initial measurements show some changes over time

2.)  The diaphragm of a headphone is made of physical materials which have to change with time ( age )

3.)  Many people claim to hear it and by many I mean thousands not just a few isolated cases

The question is how audible the result is, and whether it changes significantly in 10, 20, or 200 hours, and whether that change is as immediate and pronounced as burn-in believers tend to suggest.

Driver failure isn't necessarily burn-in, by the way. More often it's a problem with the driver. There are decades-old headphones that have had plenty of use and never failed.

It's extremely important to differentiate between change in properties through wear, and change in properties through short-term destructive use, against change in properties through aging - they are most definitely not the same.

Take, for example, a steel cantilever beam put through repetitive loading/unloading cycles at a high frequency and with a constant (but different in my different examples) load.

1. If the hypothetical load on the beam results in a maximum stress below the fatigue endurance limit on the steel, the beam can undergo (essentially, this is an area of science still under investigation) infinite loading cycles without change.

2. If you increase the load so that the maximum stress is above the endurance limit but not so high that the beam will bend, in a finite number of cycles the beam will eventually break from fatigue failure.  This happens at an increasing rate, in that microscopic cracks in the material expand as a result of the high stress concentrations at them, and as the area of the crack increases the stress concentrations increase even more - until a critical point is reached and catastrophic failure occurs.

3. Increase the load even more so that the beam will bend but not break and you will have exceeded its yield strength, strain hardening the (now bent) beam and quickly and permanently changing its properties.  It will still spring back somewhat towards its neutral position, but not all the way.  After strain hardening, it is now stiffer (at least at the strain hardened area) and will not bend as much under a given load.

4.  Lastly, you can increase the load so high so that the beam just breaks (after bending as the load is applied); exceeding the ultimate tensile strength (UTS).

5.  Note that all of these are different than just "aging" as a process only dependent on time and the environmental conditions.  A steel cantilever cycled below its fatigue limit would last an infinite number of cycles, but if it rusts and fails from that, it's an entirely different process causing failure.  Same goes for plastics out-gassing, etc.

Compare to transducers - as far as burn-in is considered, we are not worried about 1, 2, 4, or 5.  1 results in no change, 2 and 5 result in changes that are constant throughout the use of the driver, and 4 results in failure of the driver.  So that leaves us with 3 (plastic deformation, from exceeding yield but remaining under UTS) as the only plausible way a driver changes (mechanically) in a manner similar to that of "burn-in".

Now, the materials used in drivers and suspensions are not perfectly uniform - there are some small-scale variations where some sections are under local pre-tension while others remain untensioned or in compression - so even if the driver/suspension as a whole doesn't exceed the material's yield limit, you may see small-scale plastic deformation.  Larger-scale plastic deformation, on the order of the entire driver, would be much larger in magnitude and thus much easier to measure.

The problem I see with burn-in is that it is described as occurring at a large initial rate, slowing down over time until it levels out and is progressing very slowly, approaching "no change" over time.

My understanding of plastic deformation doesn't explain if (or how) plastic deformation continues to happen with repeated loads at the same level - as far as I understand it, no additional plastic deformation would happen without an increase in load.  This would support some manufacturers' claims that burn-in is instantaneous with the first time a driver is run-in.  But I'm not an expert on plastic deformation, nor am I an expert on polymers and their unique properties.

Anyway, my point is that if any change in burn-in happens, you would be able to measure it in driver compliance and displacement for a given voltage.  This makes sense - duh - because all the sound produced by driver is a result of displacement of the driver.  I know another member here had begun preliminary testing of drivers for this.

So beyond all this, as others have said on top of (A) determining the measurable degree to which burn-in happens and (B) the mechanism that causes it, we need to explore (C) the actual audible impact.  Tyll has done pilot studies on (A) and (C) of these; further investigation is definitely warranted based on his results.

Hmmm.... I see potential for another master's thesis...

Awesome post BlackbeardBen 

Plastic deformation would only apply to dynamic drivers, right? Planars and electrostatics don't bend and stretch in the same way.

Quote:

Originally Posted by BlackbeardBen 

It's extremely important to differentiate between change in properties through wear, and change in properties through short-term destructive use, against change in properties through aging - they are most definitely not the same.

 

 

I agree.   Anything mechanical is going to wear and eventually wear out.  So headphones will slowly change over time.  But that is not this short term thing called "burn in".

Quote:

Originally Posted by Head Injury 

Awesome post BlackbeardBen 

 

Plastic deformation would only apply to dynamic drivers, right? Planars and electrostatics don't bend and stretch in the same way.

No, the same rules applies to them as well - planar drivers, whether magnetic or electrostatic fields are moving them, are tensioned in place and deform during playback.  In fact, they have very interesting modal resonances; I think the when someone did an FEA analysis here of a planar driver they touched on that.

They are also damped by foam around the edges, so the compliance (and any plastic deformation) of the foam will also come into play.

Like I was saying, the key thing to investigate from a broader perspective than just audio transducers is if plastic deformation can continue to happen from a repeated load of the same magnitude, particularly on the microscopic scale.  It may be the case that macroscopically plastic deformation does not happen with repeated similar loads, but microscopically it does, or that it happens in some materials (polymers) and not others.  A bit of Google-fu may turn up an answer, but it may take more investigation or it may even be a poorly-researched subject.

Quote:

Originally Posted by BlackbeardBen 

No, the same rules applies to them as well - planar drivers, whether magnetic or electrostatic fields are moving them, are tensioned in place and deform during playback.  In fact, they have very interesting modal resonances; I think the when someone did an FEA analysis here of a planar driver they touched on that.

 

They are also damped by foam around the edges, so the compliance (and any plastic deformation) of the foam will also come into play.

 

 

Like I was saying, the key thing to investigate from a broader perspective than just audio transducers is if plastic deformation can continue to happen from a repeated load of the same magnitude, particularly on the microscopic scale.  It may be the case that macroscopically plastic deformation does not happen with repeated similar loads, but microscopically it does, or that it happens in some materials (polymers) and not others.  A bit of Google-fu may turn up an answer, but it may take more investigation or it may even be a poorly-researched subject.

Another thing to consider with plastic is that the plasticizer continues to dissipate and the plastic becomes brittle over time.