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HD800 imaging and treble: Why and how it can be improved upon ...

Discussion in 'Headphones (full-size)' started by arnaud, Jan 24, 2010.
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  1. arnaud Contributor
    Introduction:

    Although I started posting about this in another thread, the ultimate goal is different so I thought this should deserve its own. The objective of this discussion is to highlight one potential area where the HD800 design can be improved upon, namely it's rendering of soundstage and tonal balance.

    The idea started after reading about the modification proposed by Jazz: http://www.head-fi.org/forums/f4/wan...sy-mod-464042/

    Actually, the first person who intimidated the thought that HD800 design, although extremely well executed, could be improved upon is iPodJ: http://www.head-fi.org/forums/f4/hd8...thread-446585/ However I was not convinced as I could not find much credibility in the idea: About iPodJ's mod . The mod was really low tech and cheap, which also contributed to it being dissed (such stark contrast to the high tech nature of the HD800 and a bit of an insult to years of R&D work that had been done).

    There is one interesting thing to note though: the head of Sennheiser's acoustic design team, Axel Grell admitted that they at least thought about voicing the headphone differently for various markets: What Hi-Fi? Sound and Vision - Blogs "He notes that different countries can prefer different sounds, too, but decided not to explore an idea to offer alternative versions of the HD800 'tuned' for specific markets around the world. ". To me, it means that the voicing of a headphone is always a compromise and one headphone, however how technically good it is, may not be the end of all to each and every one.

    My goal with the mod was originally to shift the tonal balance of the HD800 to a slightly warmer presentation as my not so stellar recording would not all be enjoyable to listen to through the HD800. I also tried to bring technical justifications as to why the modification is reasonable as you can read in the 3 posts here: Original foam mod

    Final version of the melamine foam mod:

    Since posting in Jazz's thread, I have digged a little further and Jazz has also been experimenting with my proposed modification. It turns out Jazz noticed bloating of the bass with the variable thickness foam pad which I proposed. I noticed it after Jazz brought it to my attention and decided to play further with the pad thickness. I settled down with a 5mm constant thickness pad of the same shape as originally proposed Original foam mod. It turns out the 5mm pad is the best compromise. There's no loss of detail from the stock headphone and, more importantly, no perceived degradation in the bass region. The highs are clearly more natural (particularly splash of cymbals and sibilant on voices) with slightly warmer tonal balance than the stock HD800.

    3D Finite Element Simulation:

    At the same time I was experimenting with the melamine pad thickness, I created a bit more involved simulation model. I performed 3D finite element simulations of the HD800 and foam mods to see if I could get some insight into why the mod is necessary. I wanted to share a few pictures here with those technically enclined to it (for those who could not care less, then just go directly to your drugstore, buy some melamine sponge and start experimenting! I want to hear your thoughts!)

    This is the Finite Element model of baseline headphone. It's a concept model with approximate geometry but probably fine enough for basic simulation:

    [​IMG]

    This is a plot showing the distribution of sound pressure level in the earcup in the baseline configuration, you can see reflection off the frame and observe that sound field is dissymmetrical. I am not sure about this but have a feeling this contributes to the HD800 "fuzziness". The soundstage is very wide and deep but placement is not so precise in stock configuration . I think reflections off the frame can be a cause for this:

    [​IMG]

    This is the model with variable thickness melamine pad and impact on SPL distribution, we can see clear improvement:

    [​IMG]

    Finally, this is the average SPL in the earcup as function of frequency. While the resonance observed at 3kHz may be an artifact of the model, the one at 6500Hz is very plausible and would explain why the headphone can sound tinny with some recordings. As you can see, the 5mm melamine pad clearly drops the peak, by almost 3dB. When listening to the headphone, we can feel it, it has become much more natural sounding in the highs:

    [​IMG]

    Conclusion:

    Considering the tonal characteristics of the HD800 stock headphone and stock cable, I believe Sennheiser was shooting for the most transparent sound possible, at the price of making it ruthlessly revealing of poor quality recordings and / or faulty upstream components.

    I personally appreciate a slightly more relaxed presentation while I totally dig the extreme resolution and cleanliness of HD800 headphone. With a $5 melamine sponge you can address one potential shortcoming of HD800 design or at least tune its tonal balance to a warmer presentation.

    I have personally settled with the 5mm thick foam pad covering the area of the frame at the back of the earcup. I have tried to provide some technical justification for the modification, althouth ultimately what matters is the sound of the HD800. Basically, the effect of melamine foam is very noticeable so I encourage anyone who feels like it to start experimenting and report their thoughts!

    Happy Listening,

    Arnaud
     
  2. El_Doug Contributor
    FANTASTIC analysis! I cannot wait to try this out
     
  3. wower
    This will be an interesting thread. My only question is how? I had to jump around the links. You might want to include that info in an edit.
     
  4. arnaud Contributor
    Quote:

    Originally Posted by El_Doug /img/forum/go_quote.gif
    FANTASTIC analysis! I cannot wait to try this out



    Thank you! Yes, please try and report!

    Quote:

    Originally Posted by wower /img/forum/go_quote.gif
    This will be an interesting thread. My only question is how? I had to jump around the links. You might want to include that info in an edit.



    Thanks Wower, I tried to cleanup the links... I should have created a new thread from the first point...
     
  5. JaZZ Contributor
    Salut Arnaud

    I must confess that I don't fully understand your graphs. Also, in my book it would be essential to factor ear and head skin into the calculation – in terms of both acoustic properties and acoustic space. Moreover I think the steel mesh is far from being fully acoustically transparent, so should be taken into account as well. To me it is quite a dark horse, and I'd like to know more about its function – both intended and unintended. And finally the result should be somehow projected to the ear drum. That would be very hard (if not impossible) to do, since there are so many variables in play – similar to weather forecast. [​IMG]

    I'm still puzzled about the boominess introduced by the Melamine foam pad. How can a piece of damping material introduce a resonance? It would be more plausible if it was part of the sound-barrier on the steel mesh for reducing phase cancellation – which it definitely isn't.

    I have added two pictures with the latest state of my velours/velvet mod in my thread. But I will certainly give the Melamine some more chances when I have the time. And what I will do next is cover the metal ring surrounding the membrane with either Melamine foam or velvet. To make the damping even more effective and complete.
    .
     
  6. 3DCadman
    Wow! Awesome mad analysis skills. I dabble a bit at work with Solidworks Simulation, but I have nowhere near your abilities. Most impressive.

    I wish I owned a 800 to even think about playing with [​IMG]
     
  7. JaZZ Contributor
    Quote:

    Originally Posted by JaZZ /img/forum/go_quote.gif
    ...what I will do next is cover the metal ring surrounding the membrane with either Melamine foam or velvet. To make the damping even more effective and complete..



    Here it is:

    [​IMG] . . [​IMG]

    ...and on the right side an even better version with velvet ring.
    .
     
  8. IPodPJ
    How is the velvet and foam being held in place? If you have to use any tacky material, you are risking voiding your warranty.
     
  9. JaZZ Contributor
    Quote:

    Originally Posted by IPodPJ /img/forum/go_quote.gif
    How is the velvet and foam being held in place?



    The foam piece is kind of clamped between the velvet ring and the earpad/outer frame, and the velvet ring is attached to the driver ring by means of double-sided adhesive foil/tape. Easily removable.
    .
     
  10. arnaud Contributor
    Quote:

    Originally Posted by 3DCadman /img/forum/go_quote.gif
    Wow! Awesome mad analysis skills. I dabble a bit at work with Solidworks Simulation, but I have nowhere near your abilities. Most impressive.

    I wish I owned a 800 to even think about playing with [​IMG]




    I don't have such great skills or abilities, but the software is truly easy to use to make that kind of analysis and models!

    I have now refined the mod slightly following Jazz's work. Here's the latest geometry of the melamine foam:

    [​IMG]

    Compared to previous mod, there's now a circular wedge (height varies from 0 to 5mm) covering the ring part of the driver frame (magnet). I used hot glue to attach the foam pieces together, it works well. I am quite happy with the results: nice taming of excess upper-mids / highs for some rock CDs and large improvement of the soundstage. However, I still feel there's no free lunch and that the mod brings a slight bloom in the mid-bass region, similar to previous Sennheiser headphones actually but nowhere near as obvious. It's only visible on some recordings. I'd be happy to hear comments from others who may try the mod.

    Jazz, I am not using the velvet because it's peeling too much, I am concerned some fibers will get through the dust screen and get on the transducer. I may try to use some hard felt you place under the chair (at least for the ring part of the driver frame).

    arnaud
     
  11. arnaud Contributor
    Jazz, these are very good points you're bringing...


    Quote:

    Originally Posted by JaZZ /img/forum/go_quote.gif
    Salut Arnaud
    Also, in my book it would be essential to factor ear and head skin into the calculation – in terms of both acoustic properties and acoustic space.




    I agree with you that the current simulation is missing some physics. That's why I am not paying too much attention to absolute levels. It's more like a comparison with / without treatment so should still be not too bad for that.

    Quote:

    Originally Posted by JaZZ /img/forum/go_quote.gif
    Moreover I think the steel mesh is far from being fully acoustically transparent, so should be taken into account as well. To me it is quite a dark horse, and I'd like to know more about its function – both intended and unintended.



    Yes, indeed, the mesh properties were likely tuned to provide just the right amount of damping (you get significant viscous damping when sound is squeezed through tiny apertures) and isolation between front and rear radiation of the driver. Indeed, it's not fully transparent acoustically, but not too far from it though, esp. toward higher frequencies.

    Now, simulating this stuff is possible, but I haven't bothered as it's not as straightforward. Also as I expect the screen to be transparent at higher frequencies (where I wanted to evaluate the impact of the foam).

    Quote:

    Originally Posted by JaZZ /img/forum/go_quote.gif
    And finally the result should be somehow projected to the ear drum. That would be very hard (if not impossible) to do, since there are so many variables in play – similar to weather forecast. [​IMG]



    Yes, if there are a model of the pinna + skin impedance, I'd want to look at the result at the entrance of the ear canal. But for this simplistic model, I thought looking at overall level in the earcup was sufficient as it gives you an idea of the added damping from the foam.
    And yes, you are a tough one to please. I will rather enjoy the mod for now than spending so much more time simulating it [​IMG].

    Quote:

    Originally Posted by JaZZ /img/forum/go_quote.gif
    I'm still puzzled about the boominess introduced by the Melamine foam pad. How can a piece of damping material introduce a resonance? It would be more plausible if it was part of the sound-barrier on the steel mesh for reducing phase cancellation – which it definitely isn't.



    I am not too sure about this one either. I have two ideas in mind though:

    1. Alteration of mid/highs which affect your perception of lower notes. In particular, the foam / velvet reduces higher frequencies so impacts / transients are less pronounced. Subjectively, that may be interpreted as bass bloom.

    2. Foam can really affect the acoustic properties of the earcup. In particular, the effective acoustic volume is larger when foam is added (just like the effective volume of charge in a closed or bass reflex enclosure needs to be adjusted when you fill it with some absorber material). I thought that does not matter for open design like the HD800 but maybe it does after all? Ahhh, if only some people at Sennheiser could contribute to this and enlight us [​IMG]

    arnaud
     
  12. complin
    What is melamine foam?
    Is it the stuff used in packaging electrical equipment?
    How about polystyrene foam?
     
  13. wualta Contributor
    BASF calls it Basotect. It was developed as a soundproofing and insulating foam, but it has gotten out and found uses far beyond its original design intent. You might find it in the UK as "JML Magic Eraser", a household cleaning sponge. You might also find it in the cushion of your airline seat.
     
  14. Tyll Hertsens Contributor
    Wow.  
     
    I do a bunch of work on this subject, only to find this just before I publish.  Friggen outstanding work, Arnaud, OUTSTANDING!
     
     
     
  15. socialscene
    I can not understand a bit of this, but have to dig it. 
     
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