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Controlled Burn-In with Predictable Results for Headphones

Discussion in 'Sound Science' started by analogsurviver, Sep 3, 2012.
  1. analogsurviver
    This is Part 1 of the Controlled burn-in with predictable results for headsphones .
    Opinions on whether effects of burn in on various parts of the audio systems  are real or placebo are divided. There is no denying we get accustomed to a particular sound signature and then tend to do quite well at distingushing listening through that device for differences in either other gear or various recordings.
    The effects of out of the box vs burnt in headphones are not day and night differences as evidenced by quite a few objective measurements, that although they differ among themselves, were always the same for measuring the headphone »before« and »after« the burn in process. Variation of about 1 dB or so have been observed and although these are rather small, by any means they are not inaudible, particularly if the change occurs in the midrange where our hearing is the most sensitive.
    Judging from frequency response alone, one might conclude everything is in order and the performance of  that headphone would allow for accurate reproduction. There is a hidden defect that is very easy to overlook in the frequency response graph.  Fundamental resonance.  http://en.wikipedia.org/wiki/Resonance
    That resonance may well be very well damped or in technical terms, it has low factor of quality, (in further text) Q. However, there is always a 180 degree ( or close to that in real life ) phase shift occuring at the resonant frequency. With sharp ( high ) Q resonance that shows up clearly in the frequency response graph as pronounced peak, the range of frequencies affected by phase shift is relatively small – exactly opposite is true for low Q resonnces that are hardly visible in the frequency response, but the phase response shows anomally spread over several octaves above and below the resonance frequency. Both conditions lead to audible colouration and in practice a mean condition between the two extremes in Q  are most commonly used.
    The best way to deal with this fundamental resonance present in any headphone ( or any other electroacoustical transducer for that matter  ) is to place it above or below the frequency range of interest, in our case the commonly accepted range of hearing in humans of 16 Hz to 20000 Hz. However, only the costliest of principles and actual realization in designs commercially available meet or exceeed this criterion. Vast majority headphones exhibit its fundamental resonance within the audio band, generally between 16 and 150 Hz, with resonance nearing lower frequency limit of hearing with increasing price. Generally, you do get what you paid for.
    The fundamental resonant frequency of ( dynamic headphone) transducer is determined by the mass of the moving element and stiffness of its suspension.  The mass is fixed and we can not do anything about it – and we do not want lower the fundamental resonant frequency by increasing its mass, as this would have detrimental effect on high frequency response.  So the only course left open  is to decrease suspension stiffness  by enough to at least lower the fundamental resonant frequency within the audible range where it is less likely to get excited -  if not below and outside of audible range.
    How can we do that? By burn in – with precise specified signals that under close control allow to safely achieve our goal in the end. To use an analogy in sports – if you want to win a medal at the Olympics, you start to train/exercise at your current level, ever increasing its intensity/quality until, hopefully, your performance will surpass anyone else's – NOT trying to match top performance athletes' at your first training after it occured to you you would like to listen to your country's hymn with the medal around your neck .
    If you do not have it already, you will need to download signal generator SineGen .http://www.tucows.com/preview/502787  I also provide the Wikipedia links for sine wave http://en.wikipedia.org/wiki/Sine_wave, triangle wave http://en.wikipedia.org/wiki/Triangle_wave, square wave http://en.wikipedia.org/wiki/Square_wave, white noisehttp://en.wikipedia.org/wiki/White_noise and pink noise  http://en.wikipedia.org/wiki/Pink_noise. Please read all the Wikipedia links as I would like you to understand exactly what we will be doing with our headphones.
    I could have easily started advertising headphone burn in and charge for my service. That would give you a »fish« ( and me some coin ) - old chinese saying has it if you give the hungry man a fish, you would feed him for one day – if you teach him how to fish, you have fed him for life. I prefer to learn you how to "fish" for the good sound instead.
    I would like to raise the understanding of most head-fiers to another, higher level.  There are phrases commonly used on head-fi that make me frown each and every time I read them.  I understand where they are originating from – from the comment you are making it is clear that you can hear quite OK, yet the vocabulary used, and precision of that vocabulary in particular, leaves much to be desired.
    Head-fi.org has become a very important organization with great influence on the global basis – with the uprising of decent sounding portable gear, this has become great force to be reckoned with in the music/audio market of today, rivalling, if not already exceeding the sales for the home speaker based systems in total value – not to mention the music/software part of the bussines.
    So, any education of new younger generations of music lovers and audiophilles alike we older guys are willing to share should reap a good crop in the future – educated choices made as a direct consequence of it should go a long way towards our common goal – perfect reproduction of a musical event. If you feel a bit intimidted by the slightly academic approach used – don't be.  No grades will be given – but the more you will understand, the more you will be able to enjoy your music in the end.  Not totally devoid of the budget involved, but always trying to get the most for your hard earned money.
    In Part 2, the actual process of controlled burn in will be presented. It will be posted in a couple of days – to give you time to download things required and digest the info in Wikipedia links for them to be able to sink in. For me, to try to find the online generator that most closely approaches  what I actually used in the end; if I fail, I will ask you to help, more heads know more and are also able to find more. To use sports analogy again, present SineGen will take you to the nationals, you would not be competitive at the Olympics level. Still, it would outperform any form of burn in by music of whatever genre.
    As always - enjoy your music !
  2. rickdohc
    Thanks Analog.. once the part2 is up i will use the write up to burn these new babys ( FX40 With Carbon Nanontubes)
  3. analogsurviver
    You welcome. FX 40 are known to take "forever" to burn in - I am interested how much less time it would take using my method.
    But please keep in mind it may take a couple more days than predicted for the Part 2 to appear - just got a possible project to record on Sept 5th and that may well mean zero head-fi activity until after this is over. But Part 2 will be up soon enough.
    As always, enjoy your music !
  4. rickdohc
    There is a huge diference between my 500+ hours burned red FX40 and the "Out of the Box" blue FX40.
    OTB,  they are way  to "trebel happy", bass is weak, that was by using them 5 minutes.  Once i switched to my Red Fx40,  its obviuos the sound change.(sound signature is the same)

    Very noticeable on this "Carbon Nano tubes" iems.
  5. xnor
    The opinions that are divided are the ones on whether burn-in causes audible differences and if people can remember the initial sound, burn their headphones in and finally recall the initial sound signature which is required to actually make a comparison. The problem is that you indeed get used to a certain sound, at which point a comparison with the initial sound signature becomes kinda impossible - still, reviews regularly contain very detailed before/after burn-in comparisons of one and the same headphone. This getting used to and (therefore) better sound after burn-in is what I call placebo.
    Playing loud noise for hours and hours through the headphones will heat up the voice coils so the question here is if those differences still exist if you let the headphones rest a few hours after burn-in.
    Tyll has measured a AKG Q701: "a headphone widely believed to change markedly with break-in - does not change much much over time." The frequency response after 100 hours is virtually the same, it looks like about 0.5 dB differences here and there. No big changes in the midrange.
    In the real world phase rarely exceeds 45 degrees. Anyway, this phase shift isn't audible in headphones.
    Such resonances only show up clearly in the FR if the headphone amp has a low damping factor. The only condition that leads to audible coloration is the changes in FR due to high output impedance.
    This is not a matter of price but of physics. Therefore, the best way to deal with this is to use a headphone amp with low output impedance.
    Ime, generally you do not get what you pay for with headphones. Resonances: HD800 = ~90 Hz, HD650 = ~80 Hz, HD212 = ~65 Hz, ... but it doesn't really matter.
  6. analogsurviver
    Willkommen Nachbar xnor, und DANKE fur dein crossfeed plugin fuer foobar2000 ! I am from the sunny side of the Alps...and enjoy your plugin immensely.
    I was just forwarding the link for your crossfeed plugin to a friend when notice about reply in the thread I started came in. Xnor... of all people !
    A bit busy and tired at the moment to answer it in earnest ( been making live recordings for like 3 days in a row...), I simply want to greet you. Looking forward to  "deal" with you in the future, as although I do not agree with some, if not all of your statements, any discussion/debate leading to our final goal, that is the perfect reproduction over headphones, is most welcome and encouraged. I hope you will like the fruit of our virtual chat in the end !
    Right now I am getting some well deserved sleep first. After I regain my composure , I promise to answer ASAP.
    As always, enjoy your music !
  7. xnor
    I'm glad you like it and I'm also fine with any kind of disagreements. I can be wrong too since I'm only human, at least I hope so. :)
  8. analogsurviver
    I agree it is difficult to remember the differences in sound in the long run. That is why I always use reference for direct comparisons. Reference means just that - reference, not necessarily something with the "best" in everything, but constant well known quality you can always compare to the Device Under Test - DUT in further text. This holds true for the more subtle differences; gross ones, like the bass resonance in new out of the box JVC HA-S500 Carbon Nanotube headphones, are so large that no reference is required if you know the recording played well. Differences that large are not placebo.
    The purpose of burn in should have nothing to do with change of impedance of the coil in electrical terms of resistance, inductance, etc. The coil would normally heat up and increase its resistance with large(er) peaks, only to rapidly cool down and return to the same resistance as in no signal/stand by condition. Any burn in that leaves permanent change in electrical parameters is excessive and leaves permanent demage. A definite no go.
    Regarding AKG K701 ; after quite a few reviews of it claiming it is superiour to the K 1000, I went to the store and tried to listen to it. With my own recordings I am intimately familiar with. I will remain polite, stating only I will never have back about one hour of my life wasted listening to totally new 
    ( I almost had to agree to purchase them before they were willing to open the box ) pair that sounded dreadful. The front end used at the store was inferior to mine, but should allow for decent demo. Upon returning home, I panicked and imediately played the same material on K 1000 - blessed relief. 701s are not for me, new, burnt in - or burned down to the ground. Could not posibly care less for the sound this pair was producing. Should the opportunity to listen to a burnt in pair under better conditions present itself, I am willing to give it another try. But possibility that K701 might eventually turn out to be my cup of tea are minimal indeed - but I do not dismiss it totally.
    If there is anyplace phase is clearly audible, it should be headphones. No room acoustics problems that pleague speakers by masking speaker defects, whatever they are, by even larger defects due to room resonances etc. There is one condition you absolutely have to adhere to hear phase related, good or bad, effects : good recordings, as simple mic count and techniques as possible; multimiking and overmastering will always give you goulasch instead of Wiener Schnitzel ! Compared to those gross phase distorted recordings phase effects in headphones are minimal, generally inaudible and of no consequence - but tables reverse once well made 2 mike recordings are used. I will post the links for some of recordings that are well made regarding phase ( and everything else...) in my future posts; stay tuned.
    I agree damping factor of an amplifier plays a significant role of suppressing changes in FR due to impedance changes headphones usually exhibit at their resonance(s); but damping factor should approach infinite in order to be able to supress these variations completely, which is practical impossibility. So, some, admittedly small, residue of the problem will always remain in real world conditions. BA IEMs are especially vulnerable to FR differences due to output impedance of the amplifier driving them due to their wild impedance fluctuations within audio band. A headphone that needs not to rely on damping factor of the amplifier will always have the advantage.
    Certainly is the fundamental resonant frequency of the headphone governed by physics and not the price; I merely wanted to emphasize that if wider frequency response is desiderata, manufacturers tend to stick higher price tags to those phones that go higher and lower in FR than to those with more limited response. So, the two do go hand in hand in real life.
    The resonant frequencies of the very headphones mentioned kept me at bay from them - it is simply too high. I come from electrostatics, from what head-fiers should be familiar with, the Stax Lambda Pro. The Pro version was first commisioned and used by Mercedes Benz, world known automobile manufacturer. They were conducting series of test recordings using dummy head/Kunstkopf binaural recordings in order to make the inside of the car as pleasent as possible for both passengers and driver regarding sound conditions. They found that normal Lambda could not keep up in the lower registers, either in frequency extension or ultimate loudness capability at both audible and, in this case really meaningful, sub bass frequencies. So they specified the driver with larger electrode spacing/higher bias voltage AND lower fundamental resonant frequency - that audiphiles got hold of it is kind of collateral demage and natural desire for the manufacturer to take advantages of it to increase sales to broader market - I can not imagine Mercedes Benz actually buying more than say 100 "automobile testing" Lambdas, actual number might well be much lower than that; audiophiles eventually bought it in far greater numbers.
    The fundamental resonance of Lambda Pro is about 5 (in a word - five ) Hz, give or take a Hz due to sample/temperature variations etc that occur with real life electrostatics. That is why its bass is so uncoloured and may well not appeal at first to the  listeners coming from other less accomplished phones that stamp their editorial comment on any bass they try to pass on unaltered. Lambda Pro has its share of problems higher up in range, all known and possible to take care of, but you can not fault it in bass. It is free in the audible band from resonance/phase/damping factor(not really applicable with electrostatics in same manner as dynamic drivers)/FR problems of the above mentioned headphones.
    As generous and pure as the low frequency performance of Lambda Pro is, it is outclassed by orthodynamic types, such as Audeze or Hifiman models - they can play louder, much louder in the bass. But other than that, I see Lambda Pro as "good enough" and look for problems elsewhere in the system.
    The purpose of my burn in method is basically trying to lower the fundamental resonance as much as possible, so that the effects it has on reproduction are minimized. One noted exception that can not and should not be attempted to improve with this method is AKG K 1000, due to its inherent design requiring resonance that is placed at around 50 Hz in order to counter the effects of acoustical short circuit as much as possible. It could lead to de-tuning of the system and should not be attempted.
    Part 2 describing this method will be online within hours.
  9. lovleylady
    Interesting... Waiting with great anticipation.

  10. xnor
    Do you mean a references as in a different (already burned-in I guess) headphone, or another headphone of the same model but not burned-in (at least not as much)?
    I think now we're talking about different things. There's the driver resonance and then there's also ear cup resonances. Cheap headphones typically have no damping material, there's just a driver screwed on to a piece of plastic. A small change in the drivers resonant frequency, or a higher output impedance of the amp could change the FR in a way so that (audible) ear cup resonances will be induced. Of course it could be the other way around too. Anyway, IMO it's best to mod such headphones and add some damping material or just get "proper" headphones instead. :S
    Yes, I agree. But what I meant was that if you take measurements, make sure that you give the headphones some time to rest. I'm not sure if a few seconds are enough for a voice coil to return to the initial temperature after a couple of hours of burn-in.
    Since burn-in changes are small such measurements are incredibly sensitive. For example, in Tyll's measurements there are ups and downs in the FR due to the changes in the room temperature (night/day), external noise and vibrations etc.
    But it doesn't matter whether you care or not, it's entirely irrelevant. I also don't like the 701s but that doesn't change what I said before: it's "a headphone widely believed to change markedly with break-in - does not change much over time."
    You know head-fi, people report day/night differences. Multiple measurements showed the opposite seems to be the case. Placebo anyone?
    But it is not under normal circumstances. This is backup up by studies. It's only audible if you use very specific test signals (e.g. sawtooth waves) at high SPLs at specific frequency ranges and using all pass filters (which are not minimum phase, but headphones mostly are!) with -180° and -360° phase shifts.
    Headphones usually consist of a single driver, there's no crossover networks. Therefore they are typically minimum phase. There's also no crosstalk between left and right like with speakers. Sure, there are no room resonances, but ear-canal and ear-cup resonances. So headphone sound is a lot "drier" and details are easier to pick out, but "on normal music or speech signals distortion appears not to be generally audible".
    It doesn't have to be infinite. Once a certain damping factor is reached there are not going to be audible changes anymore. Many competent amps have an output impedance < 0.5 ohms, which will cause FR deviations on the order of a few hundreths of a dB with an average 32 ohm dynamic headphone.
    Almost every dynamic headphone relies on the damping factor of the amplifier.
    Yeah but a couple of expensive headphones have bass roll-off, weak treble response, high distortortion etc. so I wouldn't say price = sound quality. When Tyll was asked if he thinks there's a correlation between a headphones price and its sound quality he said no.
    Yeah, electrostatics do not compare to dynamic headphones. But may I ask, where do you know from that the resonance sits at 5 Hz?
  11. rickdohc
    I am no expert and dont have high end stuff, but from all of my headphones and Iem`s, the worst sounding phone right out of the box has been the FX40,   funny thing is that they where unused for weeks even a month. But after almost 200 hundreds of hours of "burn in" in my personal opinion and experience, these are the better iem`s i have.  Why?
    Bass is tighter, highs are not harsh, sounds way better.
    It could be those "nanotube" drivers.

    I have 4 of these,  all i can say is that these can be easly used as a reference of how a burn in process afects the sound of this iems,  i  even have made "blind tests" with co-workers and 5 out of 5 times they can hear and pick the one that sounds the best,. its the red Fx40 with almost 500 hours of use and burn in. the diference is way obviuos, same thing happens with the other 2 FX40 with little or no burn in.

    I dont know if the burn in process is placebo or a myth on other iems, but i am convinced that on this FX40 the burn in process is not a placebo or a "adjusting" thing.
  12. analogsurviver
    Very quick reply - would like to finish Part 2.
    For reference I meant "ANY HEADPHONE WITH KIND OF DECENT PERFORMANCE" that is there forever, you are familiar with and has proven to have SQ relatively unaffected by temperature, humidity etc. The higher quality the better, but your familiarity with it is more important than the quality of the can per se. I use Stax Lambda Pro for reference.
    By driver resonance I meant exactly that. Not ear cup resonance that could be ameloirated by means you mention.
    A propos "proper" headphones -
    I guess no commercially available phones are really finished article beyond which no improvement is possible by atention to ever minutest details that combined mean make or break of that particular phone in the end. Every commercially available phone is made to a price point.
    It is normal to give the headphones rest time beetween mesurements "before and after "  It is not electrical parameters that change, but mechanical. Suspension can be made a bit softer by using appropriate signals, rendering lower resonant frequency that would ideally fall below audible range, lower than 16 Hz. What I am concerened with if there is possibility for the matereial to "stiffen back" and return to or close to its original resonant frequency after long(er) period of inactivity.
    Regarding 701 - I believe the measured differences are small indeed, like 0,5 dB or so. I might have overreacted regarding 701s, but that review stating 701 > 1000 made me mad for like years now, and I am normally not that temperamental. I am inclined to discuss any other phone, but mentioning of 701 is to me the same thing as red flag is to a bull. I prefer to close the 701 subject here.
    I come really from phono cartridges. These are VERY temperature sensitive, far more so than headphones. They may well exhibit totally different performance, not just FR, for the difference say 4 degrees Centigrade in temperature. Yet this is entirely different and completely separated from their burn in. To make matters worse, audible effects are in some cases similar if not entirely the same.
    I agree with what you say regarding speakers and headphones - in general and in mostly what is available.. However, there are important exceptions. In speakers that would be full range models (both dynamic and electrostatic , minimum phase ), in headphones there are two or more ways ( JH has 8 drivers and ? way x-over packed in an IEM ...), headphones do provide for natural crossfeed to occur ( AKG  K 1000, Jecklin Float, Sony PRF ), etc.
    Yes, the difference in FR in described amp and phones would be about what you mention.
    Yes, almost any dynamic headphone relies on the damping factor of the amplifier. Still, more predictable response will have the phones that exhibit the least frequency dependant impedance variations.
    I agree with you and Tyll on that one ( Qualia ? ). But in majority of cases quality does go up with increasing price. Today competition is so fierce in every price segment of the market that underachievers/ovedrpriced ones get out of the picture pretty fast. Threre are and will always be exceptions.
    I agree electrostatics are completely different animals. The resonant frequency of Stax Lambda Pro is about what I posted - later incarnations used even thinner material for the diaphragm with even lower resonant frequency - IIRC, this ended around 3 Hz for the thinnest/most compliant film diaphragm used in Lambda ??? - recent x0x series is a bit stiffer, but still in the general ballpark. In any case, below 10 Hz. You can confirm that both by listening to sine wave "sweep" from zero to say 20 Hz at fairly high level; at the resonant frequency the amplitude would of course increase and the diaphragm would start touching stators (equivalent to bottoming out in dynamic driver) and/or observing the reflection of a light bulb in the headphone diaphragm - there would be much more activity at the resonant frequency. All models of Lambdas with pro bias will be fairly close in resonant frequency, and yet even these ultra low resonant frequencies do affect the soundstage etc - the ones that resonate lower are always described with
    better, more extended bass and more defined soundstage. Couple Hz difference(s) in resonance frequency well below audible range - go figure then what say 60 Hz resonance in dynamic driver does with music...
    It is this I am after - to push the resonant frequency as low as it would go in order to minimize each  and every possible audible colouration(s) arising from it either due to FR anomaly or phase shift within audio range.
  13. xnor
    Ok, just a few things:
    The HA-S500 you mentioned has boosted bass, it's not just a "narrow" resonance and this boost doesn't go away with high damping factor either => it's the cup construction that creates that bass boost.
    As long as the diaphragm doesn't lose its rigidity no harm is done. That's what I'm concerned about but I have yet to see what you're posting in part 2.
    I'm not so sure. I've seen a lot of people dislike many "high-end" headphones. I'd argue that those are not exceptions. You and I for example don't like the K701, others don't like the t50p, fidelio l1, or the new bassy Denons, some think the HD650 is too dark and veiled, the HA-MX10 is not cheap but the FR is everything but flat, what about the entire Ultrasone lineup? The expensive Grados?
    => Sound quality does not correlate with price, at least not above a certain price tag. (This doesn't mean that an expensive headphone cannot be of high quality and sound excellent.)
    You cannot determine the resonant frequency of the driver by listening to a sweep. If you sweep from 1 - 200 Hz you'll bottom out most dynamic drivers long before you reach the actual resonant frequency. What you need to do is to measure the impedance and calculate the phase, the resonant frequency is where the phase angle is 0.
    Also what I don't understand is you said that "later incarnations used even thinner material for the diaphragm with even lower resonant frequency" but doesn't thinner material make it lighter and therefore raise the resonant frequency? I'm no electrostatics guy but this seems backwards.
    Not much with a nice amp with low output impedance.
    So what changes if you push the resonant frequency from lets say 80 to 75 Hz? Not much either I guess.
    Electrostatic loudspeakers have resonant frequencies as high as 800 or even 1200 Hz ... and according to the reviews still sound excellent.
  14. analogsurviver
    I will limit myself to answering the electrostatic questions this time and will return to the rest when time will be available. You obviously posses great knowledge on dynamic headphones, therefore is your noob status regarding ES all the more glaringly appearent. Please note that I think VERY highly of you and would be glad to meet you in person - who knows, perhaps we would make a great team ? 
    Let's first get the fundamental resonance in ES straight and separate it from the structural resonances that are inavoidable in any phisical structure.
    Structural resonances can be even much higher than what you posted, Jecklin Float Electrostat has it about at 11 kHz, for example.
    Fundamental resonant frequency in ES is ultimately governed by the tension and elastic properties of its diaphragm, which is a stretched sheet of mylar in practice. NO MASS is involved, mass of the ES is only relevant to the high frequency limit - and at that, in any competently designed ES, the mass of the air between the two spacers and mass of the air in openings (holes) in the stator electrodes that has to be regarded as moving mass of the complete driver outstrips that of the mass of the diaphragm itself by quite a margin. The thickness of the diaphragm used in Stax Lambda series
    is from 1,5 micrometers to 1 micrometer (and rumor has it, there were unofficially even thinner prototypes). IIRC, spacer thickness is 0,5 milimeter, so there are 2 of those bringing the air layer thickness to 1 milimeter ( plus air in stator holes) - do the math, mass of the diaphragm itself is next to negligible.
    That fundamental resonance is a tradeoff among efficiency, ultimate SPL capability and fundamental resonant frequency. Fundamental resonant frequency can be as high or higher as you posted in multi way ES speakers, but they are crossed over to the bass panel at frequency at least double its fundamental resonance ( ie a speaker with 1000 Hz fundamental resonance should not be crossed over below 2000 Hz, preferably higher, if phase response is to remain "good" ). This multy way bussines results in higher sensitivity/ultimately higher SPL capability - at the cost of the coherence full range drivers have by default. Sennheiser used two way approach in order to get higher SPLs in Unipolar 2002 headphones than were achievable in a full range Unipolar 2000, both electret types that differ from real electrostatic only by fact they do nor require polarizing voltage supply as they are energized from permanent electret. ( which was unfortunately anything but permanent in the said example ).
    The ultimate in ES is the full range driver. You can achieve extremely low fundamental resonant frequencies - below 1 (in a word: one ) Hz - at great penalty in efficiency and ultimately achievable SPL. Stax Lambda series is about the most sucessful compromise/tradeoff of everything that applies to headphones - no wonder it is still with us, more than 30 years after its introduction. I am no advocate of Stax amps, but reliability of their electrostatic drivers is second to none - if you do not use it as a puck for hockey or throw it in liquid, it should be fine even after 30 years of normal (ab)use.
    I could bore you ( and other head-fiers) to tears with math formulas governing ES - we can pursue the matter further in PMs, if you wish. 
    As always, enjoy your music !
  15. analogsurviver
    Part 2
    Today I will describe the actual burn in process that under controlled conditions should result in properly burnt in headphone drivers squeezing the last possible performance out of any given design.
    Please first set the volume levels on your computer/soundcard the same way you listen to your musical files. Then please open SineGenyou were instructed to download and install in Part 1.
    Go to the Work function and set sine.  Set the Level at -19,66 dB (closest value to -20 dB I can set using slider) for  start.  Set the frequency with the slider to approximate 150 Hz. Then select the D#3 note that will adjust the frequency exactly to the 155.56 Hz. You will see SineGen is the most musical oriented generator, each note has its PRECISE value in frequency in Hz. In beetween note adjustments are still available through slider.
    150 or so Hz is chosen because it is unlikely FUNDAMENTAL RESONANT FREQUENCY OF THE DRIVER will be higher up in any decent headphone. Please note the -20 dB  or so level is quite high; depending on sensitivity of your headphones and amp used, actual sound pressure level(s) can reach anything from 80 to 100 dB constant pressure  levels which should be listened to the least amount of time possible to determine the correct  procedure of burn in. Reduce the level if you feel uncomfortable or headphones start to show their limitations.
    WARNING !!!!                 HEARING LOSS IS NOT REVERSIBLE !!!
    Now  you can reduce the frequency with the slider all the way down – lower limit of the SineGen is 16.00 Hz . Listen carefully while reducing the frequency; at first, close to 150 Hz, everything should be OK, the sound should be equally loud in both earpieces and should present the dead center image in your head.  Lower the frequency further , and if everything is stil sounding OK, go lower and lower. Sooner or later, with the exception of trully exceptional headphones to begin with, you should reach a point sound will no longer appear to be dead centered within your head, but will »spread« to both the left and right hand side, leving some hole in the middle and sounding a bit fuzzy in overall character. This is approximately the fundamental resonant frequency of the headphone driver(s). If you lower the frequency further still, you should get even more »spreading« and overall sound should be getting weaker. The lower you go, the louder will be sounds emanating from both extreme left and right ( yes, this is something that should not have been there in the first place -  distortion, but this is real world ! ) and weaker to inadible the sound from the dead center, which is the desired sound output.
    Note the frequencies where these changes occur; the most important one is of course the fundamental resonant frequency. Of interest is of course also frequency where everything is still »perfect« and the lowest frequency you still have useful output of the bass in the mid dead center position.
    That is your starting point. Sporting analogy – this is your current level/form from which you should increase your practice both in quality as well as quantity if you are serious about bringing that coveted medal back home.
    In headphones, you want that fundamental  resonant frequency as low as it would go, preferably below the audible range, that is to say below 16 Hz. To be brutally honest, getting that resonance below 16 Hz would be quite an achievement, usually possible only for the finest/best/usually but not always costliest designs. What is realistically achievable is getting 16 Hz at least as still audibly usable signal – even that much is drastic improvement of what majority will experience after the above mentioned procedure used to establish the starting point to begin the burn in with.
    Sporting analogy again – you start your practice at the level you are still comfortable with and up it by a notch, just to the onset of »pain«, then easying it perhaps a bit between the two. That would still lead you higher – more pain, more gain, but you should not overcook it, as too much or inapropriate exercise does more demage than lying around.
    Assume the fundamental resonant frequency is 40 Hz. Everything OK would be about 50 Hz and the last frequency with useable output is 30 Hz. Below that – that flapping distortion only. This is quite realistic scenario as far as headphones go.  Instead of 30/40/50 Hz scenario in text please use the frequencies you have established with your real headphones.
    So far, you have been using sine wave only. Leave the frequency at the lowest useable frequency, 30 Hz in our hypothetical case, and switch from sine wave to triangle wave in SineGen. Listen a bit, then go to the everything OK frequency, 50 Hz in our case. Hear the difference? Go to the resonance, 40 Hz in our case. Hear the difference? Go anywhere higher than everything OK and listen to the consistency of the triangle wave – it changes in pitch, but not in character as it does once you reach lower fundamental resonant frequency or even worse, last usable audible frequency.
    Repeat the exercise with square wave – this should make it even clearer something fishy is going on around and below fundamental resonant point.
    Now – what ? You could go to usuaL METHOD OF BURN IN WITH MUSIC, ANYTHING WITH GOOD BASS, LIKE DRUM & BASS. Trouble is, most musical recordings contain very little, if anything,below about 40 Hz. If you want to push the fundamental resonance below our hypothetical 40 Hz, you need to »exercise« it with frequency that is lower than 40 Hz.
    What waveform? To cut the long story short – square wave . What frequency ? About 20 – 30 percent lower than the resonant frequency at any given time. You should find the fundamental resonant frequency will keep getting lower. What duration in time ? About 3-4 hours  per each ever decreasing frequency. What level ? Keep it conservative for starters : remain at -20 dB or so for one day/24 hours, while decreasing the frequency every 3-4 hours.  You should check for and write down the three frequencies at each change/lowering of the frequency of the burn in square wave signal.
    If you wish, you can open sineGen in an additional window.  Please select white noise. Set the level at about – 10 dB this time. You can run both sineGen generators in paralel.
    Although square wave for the fundamental resonance lowering does contain high frequencies as per formula given in the Wikiopedia link, the amplitude of highs is getting ever lower with increasing frequency and it is not sufficient to burn in the drivers in the upper part of their frequency  response.
    Adding white noise so helps shortening the time required for complete burn in. To be honest, I have not yet been able to find a signal that would properly burn in drivers in the high frequency range, if you know of an internet radio that analogous to drum  & bass music plays »cymbals & triangle« music 24/7, you are one step ahead of me. By the high frequency burn in I mean the tendency of some headphones to introduce sibilance and grain where there is none in the recording . Such sibilance is almost always present on open box and does get better with playing time. One noted example are JVC Carbon Nanotube driver equiped range of IEMs and on-ears that are notorious for requiring extremely long time of burning in by music to settle down to their final signature. I hope my method will at least speed up the process, but as noted above, high frequency burn in is still best achieved by (trebly) music.
    Taken together, this should have quite marked improvement in SQ. On purpose I will not provide any pointers what to listen for – other than that you should use highest quality recordings you can possibly get.  If you have 2 pairs of the same type of headphones that are either completely new unplayed or with about the same (low) count of hours on both, please consider subjecting one pair to burn in process described and then compare the SQ between the two pairs
    You will find that this burn in is quite loud – even IEMs can be heard across the room and it would make sleep in the same room as your headphones burn in impossible . Larger on and over ears are much louder still. Muffling IEMs is easy – put them in a drawer and some clothes over them – that should silence them to bearable level. For larger closed cans the best solution that anyone should have at hand is to use a 10 CD/DVD Jewel Case ( Box or Wrap ) and place the earpads along the longest dimension of the box. As this is about the sized of a human head, headphone should clamp the drivers to the flat sides sealing the leakage of the sound to the surrounding enviroment. If that is still too loud for you, place the contraption in a cupboard or similar closed place. For open headphones cupboard solution is the only solution. Or distant room. Or you get an excellent excuse to leave home for some exercise to your butt outdoors while you are still spiritually connected to your beloved headphones…!
    However,  all of the above is – kindengarten level. Some of you would probably be able to reach 16 Hz as signal frequency – the limit of present SineGen.
    Once you familiarize yourselves and get comfortable with Part 2, for which I reckon a week should be enough to convince Doubting Thomases and other naysayers in the existance of burn in and its positive effects on SQ of headphones, I will publish Part 3  - the adult, XXX version if you prefer, of the real deal. Very few of you will be able to exploit it to the fullest due to the limitation of most gear that is out there. I will work with you so that you would be able to safely squeeze the most out of your available equipment.
    As always – enjoy your music !

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