[arnaud]

Edit: consolidated result graphs into the original post for clarity
 
Quote:

Hi Arnaud,
Do you know why the FR fall so sharply even before 10K?
Wachara C.

 
Hi Wachara,
 
Not yet but I am suspecting these are just acoustic resonances in the earcup with null pressure at the mic location. Having the mic centered on the earcup does not help and we're bound to see such stuff. Once you have the actual ear in there / average a few positions, these would likely disappear. At this stage, I am mostly looking for some calibration data for the simulation and will investigate the rest numerically if I can get things to line up.
 
I have updated the graphs in the original post and added comparisons with the Omega 2 (SR007A): http://www.head-fi.org/t/498292/my-diy-electrostatic-headphones/435#post_8607847
 
And a few pictures of the measuring rig:
 

 

 

[ujamerstand]

I like your rubber gasket. I couldn't find one appropriate, so I used caulk instead.

 

[arnaud]

Quote:

I like your rubber gasket. I couldn't find one appropriate, so I used caulk instead.

If you ever come to Tokyo, "Tokyu Hands" store is really a DIYer's friend! BTW, you caught a typo, I got confused with the labels... The "PUF" backing is actually wetsuit stuff (neoprene, 3mm). Am correcting the graphs and adding a few comparisons...

 

[ujamerstand]

Ah, you are using the same material as I am then. I am using a much thicker piece though. 5/8 inch stuff from ebay. 

 

[tranhieu]

Quote:

If you ever come to Tokyo, "Tokyu Hands" store is really a DIYer's friend! BTW, you caught a typo, I got confused with the labels... The "PUF" backing is actually wetsuit stuff (neoprene, 3mm). Am correcting the graphs and adding a few comparisons...

May I ask which software you used for plotting those graphs? I'm playing around with ARTA atm but I have no idea if it's a good one to use.
Perhaps you might want to replace that acrylic plane with something else to dampen the low frequencies. That huge peak at 200hz looks a bit weird.

 

[arnaud]

Quote:

May I ask which software you used for plotting those graphs? I'm playing around with ARTA atm but I have no idea if it's a good one to use.
Perhaps you might want to replace that acrylic plane with something else to dampen the low frequencies. That huge peak at 200hz looks a bit weird.

 
Homebrew using Octave, really helps to get clean results (long averaging, fine resolution...) and opens a lot of doors for subsequent processing (extract of impulse response, CSD, step response, or whatever you fancy from the same raw data). Did the recordings using the equipment in the pics (Berhinger ECM8000 + M-Audio Firewire Solo) using Audacity.
 
For the huge peak at 200Hz, this is the open air resonance of the diaphragm, there's no backing plate in this case. We seldom see those graphs (open baffle would also be good) and I was also surprised to find it to be so high. Not quite sure what to make of the resonance shifting down / not up in frequency once the earpad seals it off since I assumed the earpad cavity acted like a damped spring in that frequency range. I guess there's also very significant mass load from the entrapped air (esp. relative to the diaphragm weight), which isn't felt when front and back waves acoustically cancel out. I am going back to the simulation soon so hopefully I can get a handle on this...
 
arnaud

 

[arnaud]

The plot is thickening a little... Short story is that I've been banging my head against the keyboard trying to reproduce the shift in resonance between free air and loaded conditions for the 009 and got nowhere. Went back to quick and dirty measurement this morning, placing the mic on the outside grill this time and, there you go, the earcup sealed cavity does act like a spring until a kHz or so (e.g. below the first acoustic resonance in this tiny volume).
 
By looking at this graph below, it would seem like the diaphragm resonance is pushed up from ~180Hz to about 700Hz, which would make sense and is also visible in the previous data I sent (but to a lesser extent because very damped). This is getting interesting as this is challenging my previous beliefs (that there were zillions of modes in the membrane) .
 
Also interesting to see the effect of the ear pad and the dust pad on the inside...
 
 

 

[tranhieu]

Quote:

The plot is thickening a little... Short story is that I've been banging my head against the keyboard trying to reproduce the shift in resonance between free air and loaded conditions for the 009 and got nowhere.

does that mean when loaded the peak shifts somewhere else or it's simply not there?

 

[arnaud]

does that mean when loaded the peak shifts somewhere else or it's simply not there?

Unclear at this time and hard to guess as it's all a matter of relative mass/stiffness between the tensioned diaphragm and the earcup cavity / pad system.

From the graph about, I thought the free air resonance was shifted up by several hundred Hz but actually, the 700Hz resonance is visible in both free air and loaded data (looking at the SPL on the outside). Conclusion is that I am as clueless as before I started .

 

[AmarokCZ]

Just finished and installed DIY Stax male Jack, it's quite ugly, but better than nothing. 

 

 
Each channel has capacity of 91pF including 3m cable, which is quite nice.
The difference between 230V and 580V bias is (to my ears) only efficiency (8dB higher), efficiency is now 98dB/100V @ 1kHz.
 
I also compared 230V and 580V bias using mic.
 

White curve is normal bias and the difference between two curves is certainly effect of bias, because I didn't even get close to the phones while measuring them.
Output level of amplifier was changed so the curves are at the same level.
The bass bump is present because earpads was pressed to the desk only by weight of earcup = quite bad seal.
 
Anyone got idea what's causing the sharp change in response near 30Hz? (same thing is on responses of SR-009 measured by arnaud)

 

[chinsettawong]

I can't comment anything about the FR curves.  I just don't know anything about them, unfortunately.  

 
Wachara C.

 

[dude_500]

Got my 6uM mylar today, I made some new diaphragms using the same tensioning weights I used on my stable 2uM diaphragms, and the new ones both almost immediately went unstable. 
 
Interestingly, the FR is practically identical to the 2uM (I managed to get this reading before it went unstable and stuck to a stator):
 
 

 
Any differences are meaningless since I rebuilt the headphones between takes of course. 
 
 
So, this would imply thinner is better: The same tension is unstable with 6uM, but stable with 2uM, yet they have the same FR. This means it needs more tension with a thicker material, which will push the free-air resonance up and lower bass response.
 
This seems like the opposite of what everything I've read says though, since bass panels are almost always thicker materials.
 
 
Also, in doing this test I took out my 2uM diaphragms which seems to have drained them. They now sound like absolute garbage compared to what it sounded like before all this. It's been about an hour and they're still crap, which implies it takes days to charge my headphones.

 

[ujamerstand]

Would be interested to see distortion and CSD graphs in addition to FR. There might be changes in distortion levels with the thicker diaphragm that is not shown in the FR graph.

 

[chinsettawong]

You know what?  I just did a test a few days ago, and I too didn't notice a difference between going from 3 micron Mylar to 6 micron either.  However, going from 2 micron to 3 micron, for me, the sounding difference is huge.  
 
If you can afford to go with thicker spacers, thinner Mylar does sound superb. Since you don't need to put a lot of tension on it, the bass from thin Mylar is very good while the high frequencies are very nice too.  I still have a set of headphones using 0.6 mm spacer and 0.9 microns Mylar, and they sound lovely. 
 
Wachara C.

 

[dude_500]

So a few things... first off, I had one of the drivers mounted upside-down in my headphones, which was leading to the awful sound. I can't stand phase-reversed channels!
 
 
I turned down my bias and went back to 6-micron to try them out again, now they're stable (about 450V bias vs. 600). They sounded about the same as the last week of listening on the 2 micron. 
 
Randomly, I thought let's take off the sweat screens (I still just don't like the concept of them being there). Within seconds, I was absolutely blown away. Felt like I was being kicked in the face by the low bass.
 
Blue - 6 micron with sweat screen (sealed)
Green - 6 micron without sweat screen (sealed)
Red - 6 micron without sweat screen (a few inches away, unsealed)
 
 
 
 
 

 
So, it looks like I have free-air resonance at 100hz, and sealed it pulls it down to an incredible 30hz. 
 
 
6-microns it is! I'm going to sit back and enjoy the sheer beauty.