Headphones Frequency Response: Challenges & Solutions

Discussion in 'Measurement Techniques Discussion' started by samvafaei, Jun 19, 2017.

  1. ultrabike

    ultrabike Measurbator - Admin

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    What part of the pinna does a full size headphone bypass?

    The coupler may have an effect on the treble. Probably like a room does to a speaker. But I don't get as wild variations as you seem to get with full size headphones. Below are my coupler and free air measurements of the HD600s vs coupler (again):

    comparo.png

    IME the HD600 EQed to flat using coupler measurements does not sound very good. I got that super flat by the way. I think you've seen the thread.

    Above 5-7 kHz I see changes. See above measurements.

    I believe that a free air measurement is more accurate above 2 kHz. I could try to do a composite measurement using 2 kHz to couple the open and coupler measurements and equalize headphones that way. Then hear if the sound improves.

    What do you get for the HD600 on a free air measurement? No ear drum, or my random head, or whatever. Just mic pointing at the headphone.
     
  2. ultrabike

    ultrabike Measurbator - Admin

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    BTW, free air measurements are a pain in the ass if we want to also control SPL for distortion measurements. If free air-measurements are indeed more accurate, then I don't feel coupling the free-air measurements to the ones with coupling is the solution for that reason.

    Also, though I do respect your efforts, I don't fully trust your measurements. They vary too wildly and do not represent what I hear at all (http://superbestaudiofriends.org/in...easurements-by-serious.2518/page-3#post-93382).

    Your treble is all over the place and I don't think that's how I hear some of the cans you measured. I saw your HD800 measurements, it was modded, but judging by what you showed it seemed even less forward than an HD600. I have yet to hear that out of any modded HD800.

    My best guess is that you don't have full control about where you are putting the mic. @Hands recommended you to put it flush to the ear. Based on what I'm seeing so far, it doesn't seem like you are being consistent with that.

    You seem to jump into conclusions too fast and lack data. I do sometimes make that mistake. Like many. But as an example, your claim about finding out the cross over point and the type of filter just by looking at a 2-way speaker's phase response was very far fetched in my mind. I did follow your simple 360 degree comment there, and it's not w/o merit (though probably still useless in many situations). But you were basing your conclusion on a discontinuity that was fully due to phase wrapping. I'll be just as forward with you. That was fully incorrect, and shows severe lack of understanding. I'm not even sure you understand why that was incorrect even now.

    I'm not going to pretend I know everything. I don't. But I do own up to my mistakes if I see them. And try to learn from them. I also try not to cover up with a TL;DR deal. I expect the same from you if you want me to take you seriously at all.
     
    Last edited: Jun 25, 2017
  3. Serious

    Serious Inquisitive Frequency Response Plot

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    As I said, above 5-7kHz I prefer the coupler results. Below that the in-ear measurements. If you don't agree that's fine, but when EQing headphones to measure the same, the results are more similar based on the in-ear measurements than the coupler.*
    That'll depend on the coupler, headphone and other stuff.
    I think it's too early to jump to conclusions here. We could say it's more in line with what just the driver does. But the cups also add their own stuff. Otherwise even a stock HD800 would measure pretty well above 2kHz.

    What makes you think that? I don't think I have seen any in-ear measurements from you. The point is, it's not about what it seems, it's about what it is. Even if a headphone fully encloses the outer ear, without touching it (and as @samvafaei mentioned, very few actually do. The HD600 does touch my ears), the ear is still in the nearfield of the transducer, which makes the interactions between it and the ear very different than you would get with speakers. What I'm telling you is to look at my plots that compare the in-ear measurements with the coupler measurements, or compare Marv's measurements with Tyll's measurements. There are patterns in the differences. Differences that two different couplers don't have. In that case you can actually approximate a different coupler with a FR compensation. Orthos with their big drivers have a lot of ear-gain around 3-4kHz, on-ears have very little.


    Which measurements are you talking about here? I think you are mistaking my coupler-in-ear difference plots with actual FR measurements. Or you're talking about my in-ear measurements.

    Not really. I measured the crossover first and then looked at the phase data and saw the patterns. It works in this case. But you'll need time-aligned drivers. The thing is, drivers effectively behave as minimum-phase, so the difference from that is from the crossover and the time-alignement between drivers. The amount of phase shift will depend on the crossover. Depending on the topology sometimes the group delay will peak around the crossover frequency, sometimes it's smooth. These kind of things are visible in the measurements.
    Also keep in mind I was talking about simple analog crossovers. No allpass, no digital phase correction. Textbook analog crossovers. I know it doesn't quite work that way in the real world.
    You're right, I don't understand. So tell me why it doesn't work that way.***

    Yes, the modded HD800 sounds less forward than the HD600.** The HD600 is brighter. Note also how the unmodded HD800 measures much brighter than the HD600.

    *The treble peaks and dips from the in-ear method does tell us certain things about how the ear interacts with the cup and driver that the coupler doesn't tell us. When listening to sweeps with headphones the treble is never smooth, but with speakers it is. But with music I feel the brain starts to adjust and filter it out. Or at least it's harder to hear with music.
    **Well, it does and it doesn't, both at the same time. I do think the FR measurements are accurate, though. The HD600 has more dips that can balance out the peaks somewhat and the mid-treble region is more recessed on the HD600. And the HD800 driver has a very different presentation. It's sharper, more incisive. And without the same cable it's kinda pointless to compare them. Yes, cables make that much of a difference. Whatever, the modded HD800's treble is closer to the HD600 than the HD600 is to the HD650.
    ***The way I see it in a speaker there is the excess phase from the drivers and baffle response (should be 0, ignoring diffraction as that's a completely different issue), the excess phase from the driver alignement (which is not 0 depending on the microphone position) and the excess phase from the crossover and usage of the backwave (which is definitely not 0, unless the crossover is first order acoustically). The individual drivers driven from the crossover are still minimum-phase, but the speaker isn't, due to the crossover.
     
    Last edited: Jun 25, 2017
  4. samvafaei

    samvafaei New

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    So in that case I'll end the target curve conversation with this: If you guys think a feature that would allow you to upload your own compensation curves to our website, and/or providing you with our raw measurements would be beneficial, let me know.

    But it seems that you think there are issues even with my raw measurements. I'll try to clear some of that here:

    If my measurements are correct, then I showed that not only there are absolute differences between headphones in the treble range, but also relative differences (and of course in the bass range too). That's why I'm hesitant to comment on how a headphone should sound on me, based on measuring it on a different head/fixture. I think more data/research is needed here before I can make a judgment.

    1. Yes, but has a much smaller effect on open headphones. As I said before, a lot of the stuff in the video has been simplified to keep it short and accessible. However, regarding the SHP9500 that I mention in the video, the 120Hz bump wouldn't go away by breaking the seal. So I think it's just a matter of pressure. The wider the head, the more pressure on the pads, and therefore more bass.

    3. Two things. Take a look at the measurements below and compare them to Tyll's to see if you notice a big difference. Also, our new measurements are an average of 25 measurements that I've done on 5 humans, and in that pool of 5 we have one female and also a guy who wears glasses. Both of them consistently get less bass, even on open-backs to some degree (Todd Welti told me he noticed less bass on females too). So I would expect our final averaged-out measurements to be a bit lower in bass than what you'd expect. But not sure if this is what you are noticing.

    4. What matters to us is real life performance. We don't care if a headphone has perfectly matched drivers if the design is so bad that no one can get a proper and symmetrical fit. That's why measuring on humans is also helpful.

    7. In terms of measurements, what matters in the end is the in-situ performance of the headphones. And as you can see below, not only different headphones react differently to different people, but also under different test situations.

    I'll keep the commentary short, since you guys don't need any:

    ***CORRECTION: I think these may be HD 650 measurements and not HD 600. Sorry for the wrong label***

    [​IMG]

    These are raw HMS measurements along side human measurements (calibrated to match the response of the ear simulator). I'll still double-check the calibration of my earlier graphs when I get back to the office (even though the point to be taken there was the relative difference and not the absolute difference). These ones, I just pulled from our Dropbox storage. So these are actually the ones we are showing on the website. I would suggest you check my raw measurements against Tyll's to see if there's a big difference. The human measurements show the real variance among people here. Notice that I get more bass than Eric and HMS with the HD 600, but less bass than those two with the HD 800 S.

    [​IMG]

    I have an extra HMS ear with the pinna cut-off (so it only has the ear canal). It's part of my Soundstage test bench, which I'll discuss more after I have done all the necessary research on it (called PRTF on our website). Regardless of how you align the with-pinna measurement against the no-pinna measurements, you'll see that the HD 600 and HD 800 S don't react the same to the lack of pinna.

    [​IMG]

    I decouple headphones as part of a bass quality test that I'm developing (more on that soon). The reason they don't look quite flat is that they are measured with the ear simulator of the HMS (which doesn't have a flat response) and also, because the ear simulator mic is not at 0 distance (I basically remove the ear and replace it with an acoustically transparent grid. So there is a cavity between the headphones and the ear simulator mic, where the ear coupler used to be). But again, the point to be taken here is that each headphone reacts differently to being decoupled.

    My point in posting these is to show that although different tests tell us different things about headphones, the one that really matters is the in-situ performance, on a specific individual, with a compensation curve made specifically for that individual. Hence, the need for more self-calibrating headphones. and more averaging in headphones measurements.
     
    Last edited: Jun 25, 2017
  5. samvafaei

    samvafaei New

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    ***CORRECTION: I think these may be HD 650 measurements and not HD 600. Sorry for the wrong label***
     
  6. Serious

    Serious Inquisitive Frequency Response Plot

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    Would be pretty much the same as yours. I have free air measurements here, but those are at 1ft distance, which isolates the driver even more, but also gives you more weirdness. And obviously the bass cancels due to the lack of seal and OB cancellation. Those were very rough measurements that I mainly made to compare the driver efficiency.

    @samvafaei I generally agree a lot with your comments on measurements, it's just that I don't think the Harman target is a very good target to use for your measurements. The bass emphasis built into the target makes no sense at all and the rest of it could also need some work. I'll maybe to come up with my own compensation for your plots.
    See @ultrabike. This is the kind of stuff I was talking about. In this case the HD600 seems to almost completely bypass the pinna, while the HD800 relies on it to meet its tonal target (which you can like or not, but the voicing was definitely intentional). The 4kHz dip is there on these measurements too and is gone with the pinna. These differences do show us a few things about the soundstaging of cans, but I don't think they show us everything as a lot of Orthos would probably get similar effects, even if they don't have great staging.
    I don't think 100% accuracy in the FR is needed for it to sound relatively neutral. And as @ultrabike mentioned, some of these differences in the measurements are just differences in how we perceive sound altogether, so the brain filters it out. But contrary to ultrabike I think this mainly affects the ear canal, less so the pinna and concha since these will have greater variance with different headphones. But then again my blocked ear canal measurements for the 4 people I measured were quite similar up to 5kHz, unlike yours. These measurements are in a dropbox folder here. And even then they were relatively consistent. So I do think we generally hear very much the same thing when listening to the same headphone, at least when ignoring seal issues. Especially when considering that we all tend to agree on how headphones sound.
    And, as I said previously, I feel that peaks or dips in the treble region aren't all that important as long as the headphone itself doesn't cause them.
     
  7. samvafaei

    samvafaei New

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    Agreed. As I said in the video, research has shown that we do have an inverse filter of our hardware (HRTF) that we compensate for. But I think that develops in real life where our head is in a "free/diffuse field" for lack of a better term (and not coupled to a headphone). The relative differences in headphones, although caused by HRTF, is not something that human brains can predict. So I don't think we have an inverse filter of how different headphones react on our heads. How would our brain learn that? For example, if my brain learns how the HD 600 reacts to my pinna, and develops an inverse filter to adapt, that inverse filter is not going to work on the HD 800 S.
     
    Last edited: Jun 25, 2017
  8. Serious

    Serious Inquisitive Frequency Response Plot

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    Maybe part of the fun of getting headphones to sound less like small speakers strapped to our heads is in trying to recreate the most speaker-like interaction between headphone and ears. But I think that's overthinking the issue. Better to just listen. The best stuff in Hi-Fi is made by people who do a lot of listening and often times very few measurements.
     
  9. purr1n

    purr1n Desire for betterer is endless.

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    I believe Olive did provide a curve where the speakers were calibrated to be flat when measured by the microphone. It wouldn't be that hard to derive from this a 1db/octave or B&K equivalent type curve.

    But as far I am concerned, Olive-Welti is dangerous to the hobby for use a a reference. We've already seen how so many wannabe measurers and armchair quarterbacks have taken it as gospel, the word of God, without understanding the nuances of it.

    Olive-Welti is a good consumer target. However, I firmly believe that a reference target curve should be derived from neutral speakers in a decent room. Not a panel of random porkers with a few other doods who took a few training ear courses.

    I don't want reference headphones to start sounding more like the MDR-Z1Rs than the HD600/HD650s, even though I do have a secret desire for bass myself.
     
    Last edited: Jun 25, 2017
  10. ultrabike

    ultrabike Measurbator - Admin

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    Because we were looking at the overall phase response of a 2-way speaker at a distance, where the individual responses combine, and it's difficult to separate their individual contributions + the crossover. I was unable to tell were the crossover point was on your results and on mine. I took a close mic measurement to get an idea of that with my speakers.

    And because I know phase wrapping is not necessarily due to discontinuities of a crossover or driver exclusively. Phase has a range of 360 degrees (+/- 180 depending on how you define it). Multiple cycles wrap around. If you took measurements of the same 2-way speaker at different distances, under anechoic conditions, your excess phase will be different and the phase will wrap around differently. If you were to base your crossover conclusion purely on where you see jumps in phase due to wrap around, then different measurements of the same 2-way speaker would yield several different crossover results as a function of measurement distance.

    It is possible that the bass response you are getting is due to (less) pressure. Many coupler measurements stretch the cans and push them harder against the coupler than perhaps when a mannequin or real head is used. With Stax headphones, even flat coupler measurements need extra pressure to get bass out of them.

    I also understand your point about that different headphones might interact differently with different couplers. I believe that is @Serious point as well. I will keep an eye on that.
     
    Last edited: Jun 25, 2017
  11. ultrabike

    ultrabike Measurbator - Admin

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    And yes. The Olive-Welti makes little sense to me.
     
  12. Serious

    Serious Inquisitive Frequency Response Plot

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    @ultrabike I think we're on the same page now. I just wanted to show that you can get rough estimates of the overall crossover design by just looking at the phase or step response, not precise readings of where one driver "crosses over" to the next. And I don't like to think of it as a crossover point, where suddenly one driver is louder than the other, but as a crossover region where both drivers add together.

    Yes. Because when both drivers aren't at the same location (and they almost never are) then there will be only one axis where crossover integration is proper, which could be pointing down somewhat, or up (less likely) and will depend on the speaker. This axis is where one should measure to get proper phase readings.
    Distance probably less so, because for smaller speakers you're almost in the far field, even at 1m. For bigger three way speakers you can often see changes in FR and phase with different distances and as an effect not only the crossover integration, but also the crossover frequency will shift.

    The wrapping around part was mainly because with an ideal 4th order crossover I think that's exactly what happens, at least as long as drivers are time aligned, etc.

    The thing is it's still weird to see more bass from the HD600 on the wider head, while also seeing less bass with the HD800. That even surprised me. TBH I didn't pay much attention to that part with the measurements on the first page. But then again more factors play a role here and it could be that the large earcups of the HD800 make it harder for some head shapes to get a perfect seal.
     
  13. ultrabike

    ultrabike Measurbator - Admin

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    LOL! Man you are stubborn.

    I don't think it is weird that the HD600 has more bass on the wider head. I think that means the pads are pushed a little more in, and effectively the headphone is driving a smaller volume, which in return increases the bass.

    If the HD800 produces less bass when pushed harder against the coupler, that would be surprising. But I would actually expect less of a change because the HD800s pads are pretty skinny, and perhaps this may be a measurement variation problem.

    To @Serious, @samvafaei and @Marvey , here are a couple of ideas:
    1. Bass: Could we say that the increase of bass response from head to head, or even coupler is in large part due to the pads being pushed into the coupler making the headphone indeed drive a smaller volumen and increase bass response? I know in IEMs ear canal volume may play a significant roll on perceived bass amount.
    2. Upper-Mids and Treble: Could we say that if the headphone pushes against the ear and deforms the ear differently from headphone to headphone, then that could partially account for the differences?
    3. Upper-Mids and Treble: An antenna and an ear has an associated manifold (phase and magnitude response vs. angle). The mic will experience different responses as a function of sound wave angle of arrival. Could we say that different headphones have their drivers oriented slightly different relative to the measurement pick up head and that this may partially account for the differences?
    4. Upper-Mids and Treble: Could we say that a flat coupler that has a more uniform manifold vs an ear may exhibits less variation as a function of angle of arrival?
    I agree that different couplers and people may yield different results. But if the headphone measures somewhat flat on a flat coupler, or any coupler that is less angle of arrival dependent, wouldn't that sound flat but coming from a different direction on actual application? (Ignoring cross channel effects)
     
    Last edited: Jun 25, 2017
  14. spoony

    spoony Spooky

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    I agree. This happens even with 'smaller-drivered' planars like the T50RP. See the Paradox and PMx2 graphs (often touted as very neutral), they both show a dip @ ~3 KHz. I bet that dip disappears when measuring using in-ear mics.
     
  15. purr1n

    purr1n Desire for betterer is endless.

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    For bass: it's easy to figure out what is going on. Look at the uncompensated measurements and compensated ones. If there is a big difference, then it is the result of Olive-Welti fuckery. Otherwise if the bass still seems too low, particularly below 60-80Hz then it's seal.

    For phase on multi-driver systems: This is just a huge clusterfuck in reality. Keep in mind that we also need to consider time delay fuckery from xover parts. And you guys already mentioned the other issues, positional changes, bounces from boundaries. Heck, add cabinet refraction too. This stuff does make a small difference, but not enough to worry about. Large venue sound reinforcement, film theater sound, and studio monitoring systems will work quite well, even with LR4 xovers.

    I dislike LR4 for home audiophile applications, but it's because of the more immediate transition between drivers with different sonic characteristics.
     
    Last edited: Jun 25, 2017
  16. Bill-P

    Bill-P Level 42 Mad Wizard

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    I believe it's a function of reflections and other voodoo stuffs that we have yet been able to measure. My guess is that some of it is buried in the mess that is distortion graph and CSD/spectrogram. If we can just "isolate" it somehow, I think we'll see some very interesting shit.

    --------------------------------

    I call that "proper engineering" and "best headphone evar" and "HD800 is actually trash." :p

    Okay, kidding aside, this is my guess: HD600 has very little opening inside for any reflection to even happen, as the driver is so close (for some, it's literally resting on their ears), and thus you don't get weird treble gain shit. HD800 is a different case: the inner chamber is so massive that any small thing will have an effect. It's a design optimized for soundstage by trying to "simulate a small room" per se, and I'd say they're fairly successful. But the problem is not everyone reacts the same way to that design.

    Ya know what's betterer? When I mod Andromeda, everybody that listens to them tell me the exact same thing about how they sound. And I'm guessing that's because with Andromeda, essentially everything is bypassed so however I hear it will be exactly what someone else will.

    Size does matter. :) (in before someone quotes me and writes "that's what she said")
     
  17. Serious

    Serious Inquisitive Frequency Response Plot

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    Actually this doesn't seem to be the case all the time. With the HD600 and its relatively open pads, yes. With the HD800 getting the driver closer to the ears actually decreases the bass relatively, or rather the opposite: It increases the midrange.
    Not to say that it doesn't, but with your original in-ear measurements it seems like there was a leak somewhere. I know you tried everything to get rid of possible leaks, but I had the same problem. In that case it turned out that it was just completely impossible to get an airtight seal with the in-ear tube because of the omni microphone design, even if it gave me good bass readings on open headphones. The mic body itself was leaky. The WM61A doesn't have such issues, because there is no extra body around the mic. Even with a very long tube I don't get subbass rolloff like you did. But yes, generally larger, longer ear canals with more shallow insertion will give less bass.

    I agree with your other three points.
    It's not just the angle, but that plays a role too. Driver design, earcup design, earpad design, ... it all plays a role and a flat plate coupler isn't a natural environment for the headphones. I guess you could say it's kind of like how some speakers measure worse in an anechoic room than in an actual listening room and some measure better in the anechoic room. Ok, that comparison is flawed because with headphones the ear is so close to the drivers - it's effectively part of the earcup design.

    Actually the PM2 I measured behaved much more like a HD600. I think it has more to do with driver size and distance to the ears. It's probably mostly that the FR is super smooth already coupled with a nicely done tonality.
    Comparing the innerfidelity graphs for the LFF Paradox and Hands' Slants measurements to Marv's V1 and V2 coupler measurements does make it seem like the T50RP uses more ear gain than the HD600, but nothing really extraordinary.

    @Bill-P Actually I saw no significant differences between how the people I measured reacted with the HD600 and the HD800. The differences were the same. But at 4 heads and 2 headphones each it's too early to jump to conclusions. One could argue that because less is bypassed it's a more realistic representation. "Proper engineering" is whatever gives the best sound. This could even be an 8" widebander with whizzer cone.
    I doubt it's so much reflections and more a completely different directionality from very different driver and housing design. The HD800 driver was made to emit close to a plane wavefront, which is probably why it's more like planars in how it intereacts with the ears, but also pistonic in its operating range.
    I do think diffraction effects inside the cup are an issue, but not so much reflections. I use mods to control diffraction effects at spots where I thought it made a difference. And the liner more or less decreases the earcup size making the HD800 more like a normal headphone design - but still an HD800.
     
    Last edited: Jun 25, 2017
  18. Bill-P

    Bill-P Level 42 Mad Wizard

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    Regardless of whether or not the wavefront is more planar, I think the reflection issue still stands, otherwise mods like the InnerFidelity mod that Tyll introduced a while ago wouldn't make any sense since nothing was obstructing the driver directly. One could even say that internal reflection in this case is directional, which I think I have observed during measurement at one point (but now my mind is too senile to remember).

    The thing is... talking about the HD800, trying to use it as a reference, and etc... is a bit harder than trying to use something like HD600 IMO. Not only is HD600 less affected by the quality of the source, it's also less affected by people's different head shapes and ear shapes. I have asked at least 8 different people how a modded pair of HD800 sounds like and they all give different answers. Even the more similar impressions are more spread out when zoomed in to certain frequencies, like say some folks hear the 6KHz peak while some others don't. Same track and all, of course.

    One thing I've always done at meets is that I get people to listen to my gears and then ask them for impressions. It helps me understand what I'm missing in my mods and tunings, as well as help me understand more the target response. I'm sure those who have had the chance to... interact with me recognize this behavior.

    And I have done that to multiple headphones over the years, on multiple different heads, and hell, in different countries and climates as well. It's interesting to note that if it's hot out, people can also hear less treble, by the way (observed in Vietnam). :p
     
  19. TomHP

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    1. Yes and no. If there is a leak (in practice there is always some degree of leakage), the bass level will actually be higher relatively for larger volumes.
    2. What is more important I think is the cavity resonances and how compressing the ear and changing the volume influence those. From the frequency point where the wavelength becomes comparable to the housing dimensions, a distributed pressure field will exist in the cavity. The frequency response measured will depend on where in that distributed pressure field the measurement is done (hence the wide variation seen in higher frequencies in moving the headphone around). Low frequencies are much less affected by movement (if the same leakage condition is maintained) as the whole volume will be acting as a pressure chamber and all parts in phase. The ear inside the cavity will change the distributed pressure field inside the cup and I can imagine that compressing it differently will surely change the FR. Harman has done some interesting work on multi-mic measurements inside the cup for identifying the distributed field at higher frequencies and adding damping to counteract.
    3. See point 2. For very high frequencies, directionality of the driver will come into play. This is still somewhat uncharted territory and something I'm sure a lot of researchers are working on as we speak.
    4. See point 2. Certainly not. I've witnessed a case where a very strong standing wave in the cup was basically "invisible" by incorrect placement on a flat coupler.
     
  20. purr1n

    purr1n Desire for betterer is endless.

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