How to get more high end out of this omni capsule?

[Murdock]

Hey,

I have these two quite interesting Rojac Omni mics from the late 60s. I'll make another thread about the rebuild but I wanted to ask, if somebody knows how I could get some more high end out of this design?
The way it sounds now is really great and smooth but it's lacking a little bit high end. Now I have to be closer to the source to capture high frequencies but would like to back off a little.
I played with the backplate spacing a bit. Varied between 70 and 120pF. But this did not make a big difference in high end. Diaphragm tension is quite high. (No, the diaphragm pictured here is not the one I use for testing... this one is not tensioned yet)
I used 5 µm aluminized mylar diaphragm.
Is it because of too much damping?
It seems similiar to the KK53 assembly. But this one is one solid brass backplate whereas the KK53 is hollowed out on the back. But my capsules have a larger air cavity between backplate and outer rim where the diaphragm gets stretched over.

The Backplate is 18mm in diameter and 3mm thick. Has 84 holes with around 1mm diameter. All holes are through holes.
The air cavity between capsule assembly and housing gets closed with the grill.

Here are some pictures of the whole assembly.

 

[Murdock]

I read the B&K microphone handbook and there is this interesting chapter about high frequencies.


"Some main design parameters are the stiffness and mass of the diaphragm system.
These two parameters determine the diaphragm resonance frequency which sets
the upper limit of the microphones frequency range. The fact that the microphone
sensitivity is also a function of the stiffness, makes the stiffness an especially im-
portant design parameter."


"Other major design parameters are the diaphragm diameter and the diaphragm
damping resistance. For condenser microphones, in contradiction to many other
types of transducer, an optimal diaphragm damping may be obtained and main-
tained over time. Therefore, such types of microphone may be used in the frequency
range around and even above the diaphragm resonance frequency.

The damping is caused by the movement of air in the slit between the diaphragm
and the back-plate. Diaphragm movements lead to air movements in the slit which
cause viscous loss. The damping resistance may be controlled by holes in the back-
plate. By changing the number and size of holes and by varying the back-plate's
distance to the diaphragm, various degrees of damping may be obtained. "

And there is this graph with "high, critical and low" damping.
The low damping curve exhibits a boost, critical is flat and high is a rolloff.

So would I lessen the damping when I reduce the air cavity around the backplate?

 

[kingkorg]

You need to increase the amount of air between the diaphragm and backplate. However if you increase the space between the two, you will also loose sensitivity. As i remember those are pretty bright already. If i had to guess, i'd say the diaphragm is too heavy. So maybe use thinner and lighter diaphragm. Maybe increase the tension. Or try to add a spacer between the transparent backplate and perforated metal one.

That diaphragm looks busted btw.

 

[Murdock]

Thanks Kingkorg! Hoped you would chime in.

I thought reducing the diaphragm spacing increases high end? And I already tried reducing it but didn't make a big difference.
Maybe I have to increase it even more.

What diaphragm thickness would you recommend? I don't know where I can get thinner material than 5µm...

Like I said, the diaphragm pictured is obviously not the on I use. This one is the second which I have not yet tensioned.

Do you know these mics? These are not Neumann M55k! They look like them from the outside but the M55k is a glued PVC membrane with center screw.
I would be suprised if you know them. They are pretty rare. Never seen any other and I only found something about them in an old "Funkschau" magazine from the 60s.

 

[soliloqueen]

you know, if you want a fun project and you don't want to change the backplate, you could make a plastic sphere and mount the capsule flush with one of the edges. that's what the engineers at neumann came up with to fix exactly the same issue you're having

http://recordinghacks.com/microphones/Neumann/M-50

 

[k brown]

40mm sphere of the M50 only affects the mid treble, not the very top. It's top-end rise is from the capsule itself.

A 50mm sphere will raise the entire treble range, but it's effect starts at a lower frequency than the 40mm; well into the upper midrange.

 

[kingkorg]

Thanks Kingkorg! Hoped you would chime in.

I thought reducing the diaphragm spacing increases high end? And I already tried reducing it but didn't make a big difference.
Maybe I have to increase it even more.

What diaphragm thickness would you recommend? I don't know where I can get thinner material than 5µm...

Like I said, the diaphragm pictured is obviously not the on I use. This one is the second which I have not yet tensioned.

Do you know these mics? These are not Neumann M55k! They look like them from the outside but the M55k is a glued PVC membrane with center screw.
I would be suprised if you know them. They are pretty rare. Never seen any other and I only found something about them in an old "Funkschau" magazine from the 60s.

You are right, my bad, i was thinking Neumann capsule.

Say the space between membrane and backplate is typical. About 40 microns. Increasing it to 80 micron will not increase the amount of air behind the membrane enough to impact hf all that much, but it will reduce the sensitivity considerably. You might want to increase the pol voltage to compensate for that.

It is easier to widen the existing holes and get better effect. However that process is irreversible. This will also reduce the sensitivity somewhat due to reduced backplate surface.

Maybe the easiest solution is to try add a spacer between the clear and perforated metal backplate. This will also increase diaphragm compliance and increase high end. It might though change the shape of the response for better or worse, depending on what you like.

5 micron mylar is just fine. Less than that might make the diaphragm too sensitive and difficult to work with. Less than 5 micron might increase high end but above say 15k depending on capsule type.

 

[kingkorg]

you know, if you want a fun project and you don't want to change the backplate, you could make a plastic sphere and mount the capsule flush with one of the edges. that's what the engineers at neumann came up with to fix exactly the same issue you're having

http://recordinghacks.com/microphones/Neumann/M-50

This would also change the pattern response, converting it to omni/cardioid. The response becomes more cardioid towards high frequencies, remaining omni for mids/low end. Which is something you might or might not want.

Some frequency response measurements are found in this post.

https://groupdiy.com/threads/diy-m50-capsule.73114/

 

[k brown]

40mm APE used with flat-response capsule:

 

[Murdock]

you know, if you want a fun project and you don't want to change the backplate, you could make a plastic sphere and mount the capsule flush with one of the edges. that's what the engineers at neumann came up with to fix exactly the same issue you're having

http://recordinghacks.com/microphones/Neumann/M-50

As far as I know that was not the problem Neumann tried to adress. It was quite the opposite. The usual omni capsules exhibit a top boost of almost 10dB and not a smooth transition... The Sphere has the least and most smooth top end boost because it diffracts the waves instead of the usual pressure build up that happens on other bodies.
You can read about it in some papers from Neumann.

Neumann.Berlin Pressure Microphone With Sperical Acoustic Surface User's Manual | Manualzz...

http://recordinghacks.com/pdf/neumann/omni_sphere_aes_schneider.pdf
As you can see in the last picture, my capsule is in a spherical body.

You are right, my bad, i was thinking Neumann capsule.

Say the space between membrane and backplate is typical. About 40 microns. Increasing it to 80 micron will not increase the amount of air behind the membrane enough to impact hf all that much, but it will reduce the sensitivity considerably. You might want to increase the pol voltage to compensate for that.

It is easier to widen the existing holes and get better effect. However that process is irreversible. This will also reduce the sensitivity somewhat due to reduced backplate surface.

Maybe the easiest solution is to try add a spacer between the clear and perforated metal backplate. This will also increase diaphragm compliance and increase high end. It might though change the shape of the response for better or worse, depending on what you like.

5 micron mylar is just fine. Less than that might make the diaphragm too sensitive and difficult to work with. Less than 5 micron might increase high end but above say 15k depending on capsule type.

Thanks! Will try the spacer between backplate and plexi dish.
It's just that I thought more air will dampen the response even more...

 

[Disco Volante]

More air = bigger air cushion = softer spring = lower damping (and lower resonance frequency).
Not a mic guy though, I'm from the other end, but physics apply everywhere...
Good pictures, love to see the inside of things;-)

 

[Icantthinkofaname]

That diaphragm looks very damaged. Also, why not just use EQ, either in post or added by the circuit?

 

[Murdock]

Like I said in the first post, the diaphragm on the picture is not the one I use... I have two of these mics and I tensioned one of them already.

Concerning EQ. That is an Option but as this capsule is easily modified I just wanted to make it how I like it and learn a thing or two about capsules.

And I got great results by just varying the space between backplate and diaphragm. Had it too close before but now have 60pF and it sounds awesome! Beat an M49 and MV691 with M58 Omni capsule on guitar and overheads.

Thanks Kingkorg!
I also tried the spacer between backplate and plexiglas dish but that was too much high end and other weird effects.
Will try to get nickel foil and experiment with that.

 

[gyraf]

Perhaps put a resonance disk in front of the diaphragme, like GrooveTubes (Josephson?) did back in the 90'es: Groove Tubes GT55 | RecordingHacks.com

/Jakob E.