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Old 1st June 2009, 10:36 PM   #1
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Default Building a 'Wachara' headphone

This week I received the stators and cups for a pair of headphones from Wachara, you may have seen pictures of his work in the thread 'Has anybody made an ELS headphone?'.

The stators and spacers are beautifully CNC machined pieces of 1mm fr4 circuit board and the cups are machined from some type of plastic. The holes in the stators have a 2mm diameter and are spaced 4mm center to center.


Click the image to open in full size.

As can be seen from the picture I removed a bit of copper in four area's on the stators so I can assemble the elements with metal bolts without shorting the thing.

Here's a picture of all the parts involved

Click the image to open in full size.


Before I assemble the headphones I want to do some experiments and take a couple of measurements:

  • resonance frequency
  • maximum polarization voltage
  • drive voltage required for adequate volume (sensitivity)
  • need for diaphragm damping
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Old 1st June 2009, 11:24 PM   #2
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Here's a picture of the experimental setup.


Click the image to open in full size.


I chose to use 6 micron Hostaphan and a coating with a surface resistance of 1E8 ohms/square.

The polarizing voltage for the experiments is 1500 Volts, way higher than I'd dare to wear on my head, but this is what I had available and to my surprise the diaphragm was stable even with moderate tension.
The drive signal is delivered by a 1:75 stepup transformer of good quality.


The first measurement is the frequency response at 1.5cm from the stator:

Click the image to open in full size.

The first resonance mode can be clearly seen at 235 Hz. The small peak around 600Hz is caused by a reflection between the stator and the microphone. Around 1800 Hz there is another small resonance. Otherwise the frequency response looks fairly smooth with a slightly upwards tilted slope. Note that these measurements are only slightly smoothed with 1/24 octave. Theoretically the frequency response would be flat (discarding the resonance) when measured right at the diaphragm, and we expect a more upwards tilted slope when moving away from the diaphragm because of the increasing directivity with frequency. I have a measurement at 5.5cm that clearly shows this that I will post later.

I did some experiments to try and damp the resonance with felt, BAF and nylon mesh and got the best results with a 2mm thick piece of felt on the front (top) of the element. Damping at the back of the diaphragm did not work because of reflections.

Click the image to open in full size.

As a comparison a measurement of the Sennheiser HD650 at 1.5 cm:

Click the image to open in full size.

Clearly the sensitivity of the Sennheiser is much (10dB) higher and the output extends much further down. We also see high frequency aberrations caused by reflections of the cup/ear cushions. In real life (e.g. with the headphones positioned on the head) these don't effect the frequency response like that.

By comparing the output of the HD650 I'd estimate the sensitivity right now to be around 93dB/1Volt.

I'm not too worried about the resonance being in the pass band and the sharp drop off in the low frequencies because these are free field measurements. As a circumaural headphone, the diaphragm will see a completely different load and the low frequency response should theoretically extend all the way down to the subsonic range when loaded by the pressure chamber formed by the cup, the cushions and the head/ear.

Next step would be to emulate such a pressure chamber including the microphone and see if we can indeed extend the bass to below the resonance frequency.
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Old 2nd June 2009, 05:21 AM   #3
vaughn is offline vaughn  United States
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Well I certainly will be following this thread with interest as I too have a set of parts
from Wachara to build these headphones...

I do notice that you have two additional white plastic spacer pieces that I don't have. I'm
not sure exactly where they would go in the housing as the current stack of stators and
spacers milled from circuit board material fill the space inside.

With that diaphragm stretcher I have to assume that you have done this before! I will make a
much smaller version for the headphone.
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Old 2nd June 2009, 05:37 AM   #4
vaughn is offline vaughn  United States
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A quick note on the housings, they are made from a plastic called Sintra. It is a vinyl material with a slightly foamed interior and skin on each side.
It is widely used in sign making and bonds easily with cyanoacrylate.

I used Sintra frequently back when I made scale models for product design firm.
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Old 2nd June 2009, 06:15 AM   #5
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Wow! Arend-Jan, you are really a professional. With your stretcher and measurement, I'm sure you will be able to build the best pair of headphone. Since I can do all the parts you need, if you feel that there is other things that can be done to make it better, do let me know.

I'll be eagerly waiting to see your final result.

Wachara C.
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Old 2nd June 2009, 06:25 AM   #6
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Quote:
Originally posted by vaughn
Well I certainly will be following this thread with interest as I too have a set of parts
from Wachara to build these headphones...

I do notice that you have two additional white plastic spacer pieces that I don't have. I'm
not sure exactly where they would go in the housing as the current stack of stators and
spacers milled from circuit board material fill the space inside.

With that diaphragm stretcher I have to assume that you have done this before! I will make a
much smaller version for the headphone.
Hi Vaughn,

Are you sure that you don't have the white plastic spacers? I'm sure that I sent them to you together with other parts. I use them to glue extra diaphragms to put them against my ears for sweat protection.

Wachara C.
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Old 2nd June 2009, 06:48 AM   #7
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Hi Arend-Jan,

What do you think make the frequencies drop so sharply at 200hz? I'm very sure that I can hear a lot lower than 200hz.

This is so fun. Let's make the best!

Wachara C.
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Old 2nd June 2009, 02:56 PM   #8
vaughn is offline vaughn  United States
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Quote:
Originally posted by chinsettawong


Hi Vaughn,

Are you sure that you don't have the white plastic spacers? I'm sure that I sent them to you together with other parts. I use them to glue extra diaphragms to put them against my ears for sweat protection.

Wachara C.

You are absolutely right! The spacers fit so perfectly inside the housing I thought they were glued in as part of the structure. My mistake
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Old 2nd June 2009, 03:21 PM   #9
Calvin is offline Calvin  Germany
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Hi,

nice work indeed ;-)
The fundamental resonance is a bit high for my taste, but real listening situation should ease the bass-loss-problem considerably. Even though I assume that the mechanical tension is a bit too high. Using a heatgun to tension solely by tempering might just give a Fs around 100Hz which is sufficient for the bass when the HP sits on the ears with a cushion sealing the ´chamber between ear and membrane.

@Wachara: What would be the conditions to get a set of Your stators and spacers?

jauu
Calvin
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Old 2nd June 2009, 08:10 PM   #10
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Hi all,

I agree the resonance frequency is a bit high, but to be honest I have no idea if it's going to be a problem or not. But it's easy enough to lower the tension if required.

Like discussed the addition of the cups and a head will change the bass response considerably. Loading the diaphragm with the closed area formed by the diaphragm, cup, cushions and head should do mostly two things:

1. The compliance from the volume of trapped air will dominate the compliance of the diaphragm (it's effectively a series resonator with the diaphragm mass) and this will raise the resonance frequency, perhaps even into the kHz region.

2. Theoretically the super light diaphragm powering a pressure chamber should be able to transfer all energy into pressure, which should extend the bass into the subsonic area (couple of Hertz).

So the next thing will be to produce such a closed chamber and put the microphone in there and see what happens. If theory holds up it might prove beneficial to raise the resonance frequency even further...

@Wachara: The drop off below the resonance frequency is normal for all resonant systems. The natural response for an ESL is to drop off with 12 db/oct below the resonance frequency. But as explained above, the situation is quite different when you put the headphone on your head. You may be able to verify this effect by slowly moving the headphone away from your head while focussing on the low frequencies. They should drop off very fast.

@vaughn: actually this is my first headphone. I build the stretcher to refurbish Quad electrostatic speakers. Thanks for the identification of the cup material. You probably have a lot of experience with this Sintra then. I'm thinking about spray painting the cups. Any tips to get a good result?
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