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My DIY electrostatic headphones - Page 40

post #586 of 1477
Quote:
Originally Posted by chinsettawong View Post

For Stax 007, the sleeve on the pad simply goes into a small opening on the side of the cup.  As for my design, I use the same mounting mechanism as HE-5.  It's rather difficult to explain.  I can take a few photos to show if you want to.

 

Wachara C.

If I understand correctly, the cross section would look something like this? That could actually work quite well for my design. Do you know what D measures for a good tight fit? Don't know if you actually have a pair of 007's to measure.

 

If you don't mind taking pictures, I'd definitely be curious to see other mounting methods that have worked for you.

 

post #587 of 1477
Thread Starter 

Hi Dude_500,

 

What you understand is correct.  I'll take a few pictures later on today and will post to show you how mine looks like.

 

Wachara C.

 

P.S.  D is quite thin.  It's about 1 -1.5 mm.  However, the sleeve can go in deep.  The inner diameter of the sleeve is around 90.  I'll check it for you later just to make sure.


Edited by chinsettawong - 9/24/12 at 10:34pm
post #588 of 1477
Thread Starter 

Here is a simple drawing of how my pad is put on.  I hope you can understand. :)

 

Here is a picture of my cups before putting the pad on.

 

 

 

After putting the pads on, they look like this:

 

 

post #589 of 1477
Thread Starter 

Here are some more photos:

 

 

 

 

 

 

 

Wachara C.


Edited by chinsettawong - 9/27/12 at 8:42pm
post #590 of 1477

I have been looking further into the 1kHz resonance issue of dude500's previous iteration. I will show the headphone simulation results later on, but basically, the earpad cavity is not responsible for the 1kHz resonance, and it indeed comes from the pad / leather arrangement. For now, have a look at this simulation of the earpad absorption (the first peak is the foam/leather fundamental resonance, like a mass/spring system if you will).

 

These results are for an infinite size sheet (1mm leather - 10mm foam - 1mm leather) so it's not quite was is happening with the actual earpad geometry but this highlights the potential resonance when using soft memory foam:

 

 

The simulations I've run so far where for melamine foam / no leather, I will try with "soft PUF + leather" next. The one strange point about this is that, considering only the absorption effect of the pad, you should get a dip at 1kHz, not an extra resonance. Hopefully, the actual simulation including 3D earpad model will tell... 

post #591 of 1477

Finally, here are the simulation results of dude500's previous phones (large earcup, thin pads). One important thing is that I realized how critical the pads construction / configuration / size affects the results, in particular the fundamental resonance of the diaphragm (it is heavily influenced by the effective mass and stiffness of the earpad cavity which in turns depends on the earpad to some extent).

 

Since I only had one dimension with work with (old earpad inner surface area), everything else is approximate base of the pics posted by dude500 and of coarse I did not have any of the materials properties so I don't expect things to line up with tests. However, it was very good learning experience to find out about the importance of the pads.

 

First, a simulation of the response of the headphone in free air for mics along the vertical axis between the diaphragm and base of the earcup. A couple of notes: this type of simulation is based on "Boundary Element Method" in which you only mesh the boundaries of the domain, compute the pressure and particle velocity at every nodes of the surface(s) and can then post-calculate the SPL at any point in the domain. The nice thing is that this enables calculation in free field / over large volumes:

 

Below is a contour plot of the diaphragm velocity and SPL around the headphone at a single frequency (~100Hz, the free air resonance of the diaphragm):

 

And here is the 3rd resonance of the diaphragm (+/-/+ across the length) at about 300Hz. 

 

Note that the 2nd resonance (+/- across the length) isn't visible in the SPL response because this "mode" is a very poor "radiator" (+/- cancellation over the surface).

 

Next step consisted in comparison the results from "BEM" simulation above to those of more traditional "Finite Element Method" where the whole acoustic volume is meshed. In that case, "rigid walled" acoustic modes are first computed and combined to "in-vaccuo" structural modes of the diaphragm during a coupled structural/acoustic solution. The main limitation is that you cannot resolve the response outside the meshed domain and computations get expensive with increasing domain size. Special care needs to be taken to make sure you don't get reflections from the boundary of the exterior domain, it then returns reasonably close results to the BEM approach (red vs. black curve). Note the drastic shift in the diaphragm resonance frequency compared to free air response:

 

Contour plot around 3.5kHz where the SPL dips at the center of the earcup (this is an acoustic resonance in the earcup). First the BEM model, next the FEM model:

 

Then comes the question why bother with FEM instead of using BEM? The reason is to be able to include more complicated earpad dynamics. In particular, in the BEM and FEM simulations above, the earpad effect was modeled as just a surface impedance (using infinite size trim model discussed in previous post) with every node acting independently of the other. It turns out this simplified earpad model totally misses the physics at low frequency, in particular because the earpad has such large influence on the first diaphragm resonance frequency. Here's a comparison with a "FEM/PEM" coupled model with the earpad (PU foam + leather) modeled in 3D using "Porous Finite Elements". The pad is assumed bounded to on the diaphragm side and "sliding" on the base (the surface in contact with the skin). You can see the diaphragm resonance (above 25Hz) shifted back to about 100Hz and you barely see the blip (I now understand why Stax headphones measure like they do below 100Hz: this is actually a coupled diaphragm / earcup cavity / earpad resonance):

FEM model (rigid earpad) at ~25Hz (note the SPL response inside / outside is quite boring, this is because we're at very low frequency):

FEM/PEM (3D earpad visible in grey color below) at ~100Hz:

 

 

 

So, this brought me to the next stage. Now, it was clear I couldn't cut corners and had to include a full 3D model of the earpad. I did the rest of the simulations using this coupled FEM/PEM approach and investigated the sensitivity of the response to the earpad. I first changed the foam (PU foam) stiffness drastically. The disappointing thing is that I could not reproduce the huge 1kHz peak in dude500's response (it's rather easy to imagine though since I probably used the wrong material properties, foam dimensions, leather properties/dimensions and especially mounting configuration...). But the interesting thing was how sensitive the low frequency response was to the foam used. Below I am showing the SPL in the middle of the earcup, diaphragm velocity response and contour plot at 60Hz (soft foam only):

 

 

 

 

 

At last, I investigated the effect of removing the leather and having a porous ("unsealed") or smooth foam surface ("sealed", impervious...). The motivation was that I did not know the actual leather properties / thickness and noticed that dude500 was only partly covering the foam with leather. Below are the SPL and diaphragm velocity, effect is pretty drastic:

 

 

 

Conclusion: well, it's no surprise to some of you maybe but indeed the earpad matters a lot more than I thought! This actually has taught me a thing or two and I will be able to revisit my 009 model next...

post #592 of 1477
Thread Starter 

Hi Arnaud,

 

I'm impressed of your simulations.  You really have spent a lot of time and effort into this.  Thanks a lot for sharing the info with us.

 

As I have no knowledge about this subject and I cant't really understand the graphs (I'm sorry), could you kindly summarize the simulation results?  Can you conclude whether a soft or hard earpad is better?  Does the thickness of the earpad have any impact on the sound?  A few of my friends and I like thicker pads better than thinner ones.  We find that thicker pads give better bass.  But I don't know why.

 

Wachara C.

post #593 of 1477

Hi Wachara,

 

Thank you for your kind comments. Unfortunately, there's no easy answer I am afraid. The simulations I have run show that the earpad materials have a large influence on the low frequency extension of the response. Although I haven't visited this yet (I originally planned to but ran out of time), the earpad thickness most likely also has a significant influence, so does the mounting conditions (glued or not, using a thinner or thicker leather skin, how compressed it gets in seated position).

 

As for rigid vs. soft foam, the simulation for the geometry above, and a particular PU foam used as baseline (there are so many variants of PU foam actually), the harder foam was beneficial to the low frequency extension (no roll-off, invisible fundamental diaphragm resonance, the lowest excursion of the diaphragm below 200Hz which should decrease distortion). But again, use a different material, different thickness, different "skin" and results will shift drastically.

 

If anything, this reinforces why earpads are so meticulously manufactured by Stax and eventually so expensive... Note also that all the simulations above neglected the leakage from the earcup (improper seal or by design), this is another important variable (although I believe only a good sealed design will ensure a clean / extended bass).

post #594 of 1477
Thread Starter 

Apart from the pads and their construction materials, do you think that the distance between the driver and the ear has a significant impact on the bass as well?  This simulation should be quite interesting to see too.

 

Wachara C.

post #595 of 1477

The baseline model (prior to meshing) is parametric so I will be able to investigate earpad depth and thickness without too much trouble. Maybe next week end...

post #596 of 1477

I've got most of the parts made for my new headphones. Even though it's all CNC at this point, it still takes lots of time to get everything made, so it's been slow going with school work.

 

Pretty much a copy of Wachara's design concepts.

 

 

 

Active area is 88mm diameter.

 

Still lots of work to put into them before they're making any sound. Finding access to a router for the stators really took my design possibilities to the next level from what I've been making in the past.

post #597 of 1477
Thread Starter 

Wow, those look super cool!  With active area of 88 mm, you are going to need a lot more tension on the diaphragm.  But, you'll get a huge sound stage and unbelievably deep bass.  Let us know how they go.

 

By the way, are you not going to etch away the unneeded copper around the stators? 

 

Wachara C.

post #598 of 1477

I was debating whether or not to get rid of the extra copper. I have an 0.5mm lip of copper removed to avoid leakage around the edge, which I did on the router. I didn't want to take away a bulk of copper on the router since that'd take away some thickness of FR4 which would make squeezing the package less reliable. That just leaves etching. The problem there is at this point I'm not sure how to get an etch resist on it in any repeatable manner. Typically I'd user a laminator with toner transfer, but it wouldn't really be repeatable positioning. I guess I could just eyeball it and get it pretty close.

 

Do you think it's definitely worth doing? My thought is driving them with a blue hawaii, it won't make much if any difference.
 

post #599 of 1477
Thread Starter 
You might have noticed that I etch out the unneeded copper on all my later version stators. The amount of copper really makes a big difference in high frequencies. I think it is worth it to remove it.

I have a DIY T2. It's more powerful than a Blue Hawaii, and yes, I still can hear the difference.

Wachara C.
post #600 of 1477

Cu foil can be peeled off too - don't always need to etch

 

try a compass cutter to scribe thru the foil, then peel up an edge with a exacto blade - with enough width to support the stress you can sometimes get a flap that can be grabbed with pliers and pull

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