Planar Control/Trampoline Effect

Discussion in 'Headphones' started by Thujone, Nov 1, 2015.

  1. Thujone

    Thujone Friend

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    This may be obvious to some but it has start to stump me a bit. Let's start with some measurements (HE-6 and HE-400S comparing angled velour (green) to focus A pad (purple)). These were taken under equal volume conditions between pads (not between headphones).

    HE-6
    [​IMG]

    HE-400S
    [​IMG]


    As you can see, the pads have an effect on both headphones, undoubtedly. The effect on the HE-400S is quite substantial in comparison (namely having a big difference in bass extension).

    Things like this have been seen before (the first thing that comes to mind is the jerg pads), where pads/mods are actually not created equal. I didn't think much about it though, at least until I saw the above measurements.

    blah blah blah



    Anyway, I'd like to start a discussion talking about the control of the planar soundwave (or the trampoline effect). At first, it seemed to me that equal dimensions and spacing (relatively) would produce equal changes in a FR, but lucky for me I have a friend who essentially slapped me in the face with the idea of diaphragm control. At this point, it would make more sense that the HE-6 is less fazed by a leaky pad than the HE-400s (dual vs. single magnet arrays). The HE-6 will quickly make a soundwave and (hopefully) not flex much when doing it. The HE-400S, however, would have less control, or at least that's how my brain logically breaks it down. The HE-400S would flex much more, like a trampoline, producing waves around in all directions as opposed to a planar wavefront. I was somewhat convinced it was as simple as that. Channel the HE-400S sound with Focus pads/electrical tape and you won't lose any bass. But then I remembered the jerg pads.

    Dig through some pages on changstar (HE-400 jerg and HE-500 jerg) and you'll find that the HE-500, much like the HE-6 above, doesn't care as much about the jerg pads as the HE-400 does. Okay, 500>400, so maybe, even with their dual magnet arrays, the 500 is just more controlled. Then what's up with the killer HE-400 bass distortion measurements (stock vs. stock)?

    It seems there is more going on here than simple driver control. There is info on this scattered around changstar but it was few and far between. And maybe distortion has nothing to do with any of this. Anyone have any ideas?
     
    Last edited: Nov 1, 2015
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  2. frenchbat

    frenchbat Almost "Made"

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    when it comes to the he6, Solderdude ahem, Astralstorm has been working on it and told me that it was basically linear in free-air conditions. So in this case it's most likely one of the two following factors (or probably a combo of both) :
    1. Pad reflections
    2. Spacing between driver and ear

    From what I gathered, he was working on some kind of k1k like contraption headband that would get the best result for the he6.
     
    Last edited: Nov 1, 2015
  3. Thujone

    Thujone Friend

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    Interesting, I didn't know he was experimenting there. Still, it seems something like the 400S would not compete in a free-air environment, but maybe it would. As the above would show, even when we control those two factors (pad reflections and ear spacing) the results may be very different simply based on driver differences. I should say I'm not positive on the ear spacing part for the 6 vs. 400S but I have to imagine the 400 and 500 are very very close.
     
  4. jerg

    jerg Friend

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    I know HE560 has diaphragm excursion control issues because of its single-sided magnets. In fact Bill-P took a HE560 apart just to show how exactly it was an issue (at low frequencies with open air, the diaphragm would actually hit the magnet-less side of the enclosure, making a rattling sound).

    Probably why HE1000 was a compromise in design, assymmetrical magnet arrays so excursion is kept under control, but the weight is kept down.
     
  5. Bill-P

    Bill-P Level 42 Mad Wizard

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    Yeah. Ever since my dissection photos, I think HiFiMAN improved on the design of their HE-560 slowly. First it was moving the baffle from plastic to metal, and then I think they ultimately caved and made the plastic mesh in front of the diaphragm further away. I no longer hear the bass rattling issue in the newer HE-560. At least... kudos to them for making that change.

    I think this is more a fundamental control issue between double-sided magnets versus single-sided. There's a reason the higher-end ortho models (HEK, HEX, LCD-4, etc...) that have come out recently still come with double-sided magnets.

    But... that is also to say, I don't think the lack of control is undesirable in all cases. Look at the Abyss. Due to its design, lack of control is sometimes preferable (breaking the seal causes a huge bass spike). I'm one of those who prefer the Abyss without a seal. With a seal and Marv's mod, it necessarily does have a pretty flat FR, but it sounds weak, anemic, and kinda uninvolving that way.

    Break the seal, and...

    [​IMG]

    ^ that's an effect you can't achieve even with EQ, me thinks. The fact that it's all acoustic makes it even better.

    The Abyss does necessarily sound like it has a lot more impact than even speakers to me. That's something even my inner basshead can appreciate. It's too bad the design sux.

    Either way, though, I've been meaning to open a thread like this forever. It's time we dwelve deeper into this. So thanks, @Thujone!
     
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  6. BioniclePhile

    BioniclePhile The Terminal Man - Friend

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    I've always wondered about the newer single ended magnet arrays on HiFiMan's newer cans, though their FR seems almost a little better in the bass region with a tad more extension than the 500 and 400. Do they sound slower without the extra magnet pushing them back int place? I have no idea, all I've heard is the HE-500, but something just feels so much more complete with a double sided magnet array in my mind. Maybe it doesn't make that much of a difference, who knows, but here it looks like it can happen in the case of the Abyss. That's a pretty hilarious accentuation right there, I like it.
     
  7. Bill-P

    Bill-P Level 42 Mad Wizard

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    Yeah. In case it's not so obvious from the scale, that's a 12dB rise at 60Hz, which is the exact same thing that would happen to most speakers driven with a bass boost EQ, except this is purely acoustical.

    <60Hz is a wash IMO, since... in real life, if you measure speakers, you'll find that most of them drop off their response around 60 -80Hz, with the most extended ones only being able to sustain levels down to about 40Hz.

    Extension certainly doesn't hurt, but I'd maintain that dynamic range and control are more important in all cases, as long as FR is flat down to about 60 - 80Hz. Most music (and I suspect movies) don't even extend that low down anyway.
     
  8. BioniclePhile

    BioniclePhile The Terminal Man - Friend

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    The only thing I ever hear with bass down in the sub 40 Hz range is classical music with really sensitive mics picking up bass drum vibration in the room after a few seconds. The reverbration of the room is really cool to me, cause it just feels so much more real with it in there.
    Maybe some rap has bass down that deep tuned for massive car subwoofers, but not much else has it that deep. Electronic music, maybe. Oh, and organs.
     
  9. Thujone

    Thujone Friend

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    Can anyone confirm that the HE-5LE was the first asymmetrical magnet array? Tyll makes a comment about it in his HE-5, HE-5LE, HE-6 and HE-500 mega review but I haven't seen this information elsewhere. It would be interesting to know if Hifiman is leaning on their older strategies for their new flagships.

    Bottoming out the diaphragm is not comforting, I didn't realize that was a fault of the (original) 560. This would indicate more driver flex than the dual arrays, obviously, but I hadn't realized it was that significant.

    Single/dual magnet array differences set aside, the 400/500 difference is puzzling.
     
  10. knerian

    knerian Friend

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    I think you mean Astralstorm, right?
     
  11. briskly

    briskly Friend

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    Which came first, HE-5LE or HE-4?

    When I think magnetic control, I think of the variation of magnetic flux through the driver as it moves from rest position. That's what the double sided magnets are normally for, more constant magnetic field and reducing distortion from magnetic irregularities. There are many other sources of distortion, driver stiffness irregularity, material defects, but that doesn't seem to be what is going on here.

    If the bass is allowed to ring as such, using the Abyss as an example, that means the bass is being mechanically underdamped. Planar magnetics being what they are, they are electrically mostly resistors. So my ill-informed thought process leads me to wonder if they reduced mechanical damping for single-sided designs because they have less flux through the driver, and would be more power hungry and insensitive.
     
  12. frenchbat

    frenchbat Almost "Made"

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    My bad, corrected.
     
  13. Thujone

    Thujone Friend

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    I believe the 5LE followed the 5 and the 4 came after.

    The 4 is the only single ended HE series headphone that's difficult to drive, but it's still fairly close to the 5LE in terms of efficiency. If what Hifiman says is true, that the HE-4 is "a single ended HE-5LE," they may have simply removed the ear side magnets from the 5LE to create the 4. It's safe to say the 4 has higher distortion, likely due to the magnetic field breakdown as the driver flexes away (akin to all single ended magnet arrays).

    So what makes the 4 so much harder to drive than the 400S, 400i, or 560? A little off topic, I realize, but worth noting. Perhaps the diaphragm material has significantly improved in weight and elasticity over the years, allowing more driver flex/recovery at a lower current input. This way they can use a similar magnetic flux as the 4 while increasing efficiency overall.

    I need to do these same pad changes on the HE-4 and see how it's affected...
     

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