General modding theory questions

Discussion in 'Modifications and Tweaks' started by redrich2000, Dec 7, 2018.

  1. redrich2000

    redrich2000 Rando

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    I want to understand more about how certain mods achieve sound changes. I'd like to have a better understanding of how these things work so I can tinker a bit with headphones that are not commonly modded, as well as understand how common mods work better.

    Firstly, the application of damping material in the HD650 mods... my understanding is that is adding weight to the driver, preventing unwanted vibrations. Is that correct? If that is correct, what then does less unwanted vibration do? Does it just allow the driver to reproduce more accurately?

    Next, what exactly does non-slip matting over the driver do? Is it absorbing reflections? If so, what does absorbing reflections do exactly?

    Lastly, when I was trying to get more bass in my Koss A250s, I was recommended to add some venting holes in the fabric covers of the back driver vents. How exactly does that increase bass?
     
  2. Lyer25

    Lyer25 Too sensitive for SBAF

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    1. Unwanted vibrations generally mean more distortion— they add things to the final sound that a good reproduction (i.e. input is replicated with nothing taken or added) of the signal wouldn't have. Mass dampening is pretty easy to grasp in theory, basically more massive objects have more inertia, and thus require more force to move and by extension you need to crank e.g. headphones up much louder to get the same amount of distortion.

    Also, you aren't really applying the dampening material to the driver itself, that'd be messy and could possibly ruin your driver permanently. The Creatology+Dynamat Xtreme sandwich goes on the plastic frame surrounding the actual driver assembly; unless I'm very much mistaken (possible) counteracting plastic's inherent resonant frequency is the goal.

    I'm a social sciences guy. I did okay in high school physics but that's mostly because I fancied my teacher then :p

    2. You know how you get a different sound singing in the shower than you do in, say, a room with loads of soft, plushy curtains and divans and whatnot? Yeah all that hard tile is great at reflecting sound waves and basically messing with signal. It sounds pleasing, at least when I'm trying to sing along with Hayley Williams, but it's still degradation of the original signal in the sense that it's introducing sonic artefacts/things that make you think you're a much better singer than you actually are ;)

    I did a quick search and came across this. Promptly laughed my butt off. Useful.
    https://mic.com/articles/109012/sci...y-do-sound-better-when-you-sing-in-the-shower

    3. Bass porting seems counter intuitive, or at least it did to me at first, but basically having more air at hand to move the diaphragm/driver allows for greater excursion/movement of the same. I won't be able to explain this well at all, but because physics drivers require significantly more excursion to reproduce signals than stuff higher up like in the midrange; it takes twice as much actual excursion to reproduce 50Hz than it does 100Hz, for example*. For an example I've played with myself, taking the cups off the Fostex X00 makes for BIG bass, but stuffing material into them helps pat down the lower frequencies. You can't just shove a bunch of cotton into a closed-back and call it a day though, there's a lot more nuance to it than that, as Serious demonstrates: https://www.superbestaudiofriends.o...ts-and-succinct-review.389/page-2#post-233602

    You can also read up a bit on how bass ports affect sound in james444's thread about modding the Blitzwolf ES1s here: https://www.superbestaudiofriends.o...al-1-mod-yourself-a-pair-of-killer-iems.4019/. TL;DR is that reducing the amount of air available behind the driver tamps down the bass, while increasing the amount of air escaping in front actually reduces bass presence; I do something similar when I feel like making the Klipsches less dark :))

    *I actually just (re)learned this specific thing now: https://barefacedbass.com/technical-information/the-mysteries-of-ports.htm
     
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  3. AstralStorm

    AstralStorm Friend

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    Then there's also material science such as absorption coefficients of various materials (and foams) vs frequency.
    Generally, you want smooth and even absorption along all of the spectrum.

    Usually the best is rock wool also known as mineral wool, but it's pretty hard to apply in tiny amounts as needed for headphones and it needs to be covered with something to not make a real mess.

    For minimal absorption, cotton felt is good. It is most active around 9 kHz but has smooth, rolled off absorption spectrum. Good at reflection damping due to rough surface.

    Next is probably cotton wool, natural wool, and kapok, these are stronger absorbing and most active at 6 kHz. Mundorf makes preparations of the latter two as "angel hair" and "unicorn tail" respectively - overly expensive if you ask me, get them elsewhere.
    Here, the main material property is the "twist coefficient" which depends mostly on the fiber length and density. The latter is tweakable by pulling the strands apart, fluffing it up.
    These are also good

    Foams - the specific absorption pattern depends on cell size and what amount of cells are closed (unlinked) or open (linked to other cells). There's no hard and fast rule as the whole combination is critical.

    Open cell foams are typically most active around 4 kHz and again at high frequencies, more peaky absorption. This kind of foam can be left with the rough surface which makes for superior reflection results.

    Next, closed cell foams. These usually have a smooth surface which makes for worse reflection damping.

    As for materials for foams, you can find PET, PU, melamine, butyl rubber and silicone rubber. These are ordered in terms of absorption coefficient. PET and melamine foams are typically stiff, while others are either soft and even stretchy.

    For bass, you will find that mass and vibration damping properties play the main role as opposed to how rough the surface is. Here, the common materials are bitumen, butyl rubber and urethane rubber. Sometimes natural rubber and latex but these tend to be inferior.
    These materials are used in subwoofers, car damping and mechanical devices such as record players, but they can find some use in headphones if you want to affect just bass.

    Since this is all highly nonlinear physics, the best solution is to measure. Stuff the material between the speaker and a microphone in a tube via standing waves. That's how you measure absorption coefficient. (Sometimes seen as alpha in papers. The unit is called sabin.)
    Next, put same material in desired shape on a plane or in a box made of highly reflective material - usually thick metal or smooth stone. Put the emitter and microphone in at a specified angle with goniometer against the surface. This will measure sound dispersion coefficient which is related more to surface properties and shape. (Sometimes seen as R or d in papers. The unit is still sabin but you get a polar characteristic.)
     
    Last edited: Dec 8, 2018 at 6:01 PM
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  4. redrich2000

    redrich2000 Rando

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    This is great info, thanks.

    On reflections... so it tends to be high frequencies that reflect more than low?

    I'm working on my B&O H6 I use while commuting... they can at times be a bit hot in the treble. Based on the above I just switched from the non-slip liner I had tried to some cotton make removal pads my partner had (that were the perfect size!). I suspect they're somewhere between cotton felt and cotton wool. They have definitely reduced the treble more than the non-slip closed cell foam. Can't decide yet whether it's too much. I'll keep playing.
     

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