Headbasket design

Just found a guy with cnc-machine willing to make some microphone bodies for me.
I've drawn something like RuudNL did http://groupdiy.com/index.php?topic=54440.0
or something I've made by hand(pic attachment.)

But since that 4-axis machine can as far as I understand make almost anything,
I was wondering is there any point of milling somekind of headbasket "mesh" into the body,
so I wouldnt need to solder/glue mesh into the body?


I tried to find info on designing headbasket but didnt find anything great,
Should I treat that headbasket mesh as a resonator?whats the best hole size (and shape)?
whats the best compromise between dense/sparce holes(thinking faraday-shield and capsule "protection" without affecting the sound much.)
I guess making something like oktava mk219 grill is bad, since everyone is cutting them apart.

Making the grille open in the Oktava 219 is the result of one Michael Joly stating that it is somehow better. Do not believe everything you read in the internet...

Might be good, might not be.

I have some info somewhere on maximum hole size as regards maintaining effective screening. I'll see if I can dig it out.

The more solid the grille the more it acts as a resonant chamber. That may have been part of the percieved issue with the old Oktava, part of it may also have been actual physical vibration resonance of the metal (as opposed to the air within the metal). Dampening is quite important in microphone metalwork.

Non-parallel sides much reduce resonance effects - see the U67 / 87 style grilles, and C414 grilles. The cylindrical ones can set up standing waves with the diaphragm and with themselves - though a small effect, it is a part of the characteristic sound of some microphones - one of the significant differences between a C12 and an ELA-M251 for instance.

Matt Nolan said:

I have some info somewhere on maximum hole size as regards maintaining effective screening. I'll see if I can dig it out.

Click to expand...

That would be nice to get hold of!

So basically i should think same Way as room acoustics but in a smaller scale.
Asymmetrical shapes etc.

been on holidays but did some googling.
Found this pic:
http://www3.evaluationengineering.com/features/0101emc1.gif
so basically I dont have to worry about emc if my holes are smaller than ~1cm,
now looking for info on mesh(hole size) vs. airflow vs. sound attenuation.

kooma said:

been on holidays but did some googling.
Found this pic:
http://www3.evaluationengineering.com/features/0101emc1.gif
so basically I dont have to worry about emc if my holes are smaller than ~1cm,
now looking for info on mesh(hole size) vs. airflow vs. sound attenuation.

Click to expand...

I'm not quite sure how to interpret that graph - do you have a link to a page with text where it is from?

I think your conclusion is wrong though. Holes need to be about an order of magnitude smaller, around 1.5mm max.

kooma said:

now looking for info on mesh(hole size) vs. airflow vs. sound attenuation.

Click to expand...

Seriously, don't worry about this. Think about a pop filter. Tiny holes, but do you hear any sound attenuation?

I think the more relevant issue is the internal reflections - how strong they are, what frequencies they are at, etc.

kooma said:

http://www3.evaluationengineering.com/features/0101emc1.gif
so basically I dont have to worry about emc if my holes are smaller than ~1cm,
now looking for info on mesh(hole size) vs. airflow vs. sound attenuation.

Click to expand...

You need to do some real life tests before you settle for ~1cm holes.

They need to be MUCH smaller.

For hole size vs sound attenuation, it is the relative open area which is important.

I suggest the big mike makers know more about this subject than many pseudo mike gurus on the web with their, often really stupid, mods.

Removing the inner mesh WILL change the sound ... but not to something which the original designers want.

Of course if you like that sound ... 

Matt Nolan said:

I think your conclusion is wrong though. Holes need to be about an order of magnitude smaller, around 1.5mm max.

Click to expand...

You are absolutely right, found that graph somewhere, but didnt find the info later.
Just googled "emc hole size" or something
Somewhere they talk about 1/4 wavelength and somewhere 1/20, with the later I get 20dB shielding @ 1GHz on 1.5cm hole
and for 40dB hole of 1.5mm. Does that sound about right?

Since the shape of my grille is kind of predetermined to something like in the first post,
I got stuck on thinking the hole size's and shapes, when in real life that (round) shape of the grille is dominating factor
not that mesh hole size/shape?

Then what about adding slanted mesh inside headbasket as a some sort of diffuser?
ie.make a mesh box around capsule..
it's that plain crazy or something worth trying?

ricardo said:

For hole size vs sound attenuation, it is the relative open area which is important.

Click to expand...

Are there any % rules to this or what should I aim for?

kooma said:

Matt Nolan said:

I think your conclusion is wrong though. Holes need to be about an order of magnitude smaller, around 1.5mm max.

Click to expand...

You are absolutely right, found that graph somewhere, but didnt find the info later.
Just googled "emc hole size" or something
Somewhere they talk about 1/4 wavelength and somewhere 1/20, with the later I get 20dB shielding @ 1GHz on 1.5cm hole
and for 40dB hole of 1.5mm. Does that sound about right?

Click to expand...

That graph looks like it is to do with holes with a significant depth - i.e. more like tubes. I'd ignore it and not use holes any larger than those you find on an old C12, preferably smaller to help keep crap out of there - dust, spit, lint, etc.

Since the shape of my grille is kind of predetermined to something like in the first post,
I got stuck on thinking the hole size's and shapes, when in real life that (round) shape of the grille is dominating factor
not that mesh hole size/shape?

Then what about adding slanted mesh inside headbasket as a some sort of diffuser?
ie.make a mesh box around capsule..
it's that plain crazy or something worth trying?

Click to expand...

You have a cylindrical chamber which is only very slightly reflective - because most of it is holes and much of it, being round wire, is not actually parallel and a bit more like a diffuser than a reflector, but mostly it is no obstacle to the sound at all. So the resonance is only a small effect. You're not going to disrupt it much by putting a diagonal mesh on the inside. It is not worth the bother.

ok, thanks!

I'll guess I make one body with mesh milled in the body (with small holes, close as possible)
and one body with cutouts and glue/solder mesh to it,
so I can test if I hear any difference

I have often noticed that it is advised to remove one or more layers of mesh from commercial microphones, as a 'mod'.
I have experimented with mesh sizes and number of layers, but I noticed that a single layer of mesh with a wire spacing of 1.5 mm is simply not good ennough to form  a reliable Faraday shield.
It works, that is sort of, when you are far enough away from the microphone.
But if you come closer (to the capsule), an audible level of hum is introduced.
It is not without reason, that professional microphone manufacturers (like Neumann) use three layers of mesh.
(Different wire spacings and under different angles.)
And I guess that they know what they are doing...
The problem nowadays is that almost everybody is parroting what they read on the Internet...
If one layer would be good enough, why do manufacturers use more layers?

RuudNL said:

I have often noticed that it is advised to remove one or more layers of mesh from commercial microphones, as a 'mod'.
I have experimented with mesh sizes and number of layers, but I noticed that a single layer of mesh with a wire spacing of 1.5 mm is simply not good ennough to form  a reliable Faraday shield.
It works, that is sort of, when you are far enough away from the microphone.
But if you come closer (to the capsule), an audible level of hum is introduced.
It is not without reason, that professional microphone manufacturers (like Neumann) use three layers of mesh.
(Different wire spacings and under different angles.)
And I guess that they know what they are doing...
The problem nowadays is that almost everybody is parrotting what they read on the Internet...
If one layer would be good enough, why do manufacturers use more layers?

Click to expand...

Thanks!
I started this topic so I wouldnt have to be parroting internet
But this is much lager thing than I initially thought!
So whatever I mill I'll still need a mesh or layers of it with large open area and small as possible holes?
that takes care of the emc.

Then soundwise; what is the smallest (hole/cutout) diameter that affects sound?
length of a waveform or parts of it(1/4 etc.)?

and since I'd like to bust some of those internet myths;
Does those "bars" in front of grille have fx in sound?
thinking oktava 2/319 or thiele m4/5

This might be interesting to read: http://www.oktavamodshop.com/product_info.php?cPath=3&products_id=240

I am surprised about the single layer headbasket of the C12! Very 'open' mesh!...

Hello,
I think and made the Experience that not only the Number of Mashlayers  has a  Influence , but also the Quality of the used Material. In Case of the Octava MK 219/319 the Quality is very bad. It s a very hard Material that rings like Hell. This rigging can be heard  in the  Recordings. If You replace the original Wiremesh with a softer, better Quality the rigging gets less. I made the Experience that If using 1 Layer of Mesh, it s Important to have a  perfect conductive  Connection to the Micbody to avoid Hum
I think its a great Idea to dig Holes in the Body to create a Headbasket If You have a solid Mictube. If the Hum is too much, it s easy to sqwees in a second Layer Wiremesh .
Greatings
Lothar

RuudNL said:

This might be interesting to read: http://www.oktavamodshop.com/product_info.php?cPath=3&products_id=240

I am surprised about the single layer headbasket of the C12! Very 'open' mesh!...

Click to expand...

Thanks, that was good read.

tubestation said:

Hello,
I think and made the Experience that not only the Number of Mashlayers  has a  Influence , but also the Quality of the used Material. In Case of the Octava MK 219/319 the Quality is very bad. It s a very hard Material that rings like Hell. This rigging can be heard  in the  Recordings. If You replace the original Wiremesh with a softer, better Quality the rigging gets less. I made the Experience that If using 1 Layer of Mesh, it s Important to have a  perfect conductive  Connection to the Micbody to avoid Hum
I think its a great Idea to dig Holes in the Body to create a Headbasket If You have a solid Mictube. If the Hum is too much, it s easy to sqwees in a second Layer Wiremesh .
Greatings
Lothar

Click to expand...

In those oktavas did you remove those aluminium bars in front of the grille or only the mesh?
iirc. that aluminium was really weak and I understand how that ringing is bad
but if I would make replica of it with proper aluminium would I hear audible reosnances due the shape of the grille?
(alltho these bodies will be round, thinking about bars in front of capsule.)

Hello,
I  removed, cut the Bars because They don t make Sense to me. Maybee If You have the Habbit to throw the Mic on the Ground or If You like to  hammer Nails in Walls with it.
As a Rule, a Soundwave get s  reflected if the physical Barrier is as big or bigger  than the Soundwave.Some People mean twice the Size of the Soundwave.
Soundwave  Size is  1,7 cm for 20kHz  3,4 cm for 10kHz  at 20 Degrees Celsius. and so on. If the Soundwave is bigger  it s get bend around the Barrier. So If You have Bars in the Headbasket there may be a Reflection at certain Frequencys that results in Comp Filter Effects. The velocity Effects are one Thing , the Pressure Effekts inside a Headbasket may have an Inpact  for the Capsule Reaktion as well.
You could dig Holes in the Bars to reduce Comp Filter Effects.
I once made a Test and installed a  porous Damping Material  inside at the Bottom of a round DIY Doublelayer Headbasket ( at the Plate where the Capsuleholder is installed ) . The Materialthickness was 2 cm. For a 100% Absortion at 90 Degrees Angle  of incidence ,( at different Angels less Absortion, even a Reflection can Happen ), You need a thickness of 1/4 Wavelenght ( good Absorption untill 1/8 Wavelenght ) 
It made a audible Difference and the Comp  Filter Effects  where almost completly gone. Sounded much more clean and  natural. I like it so much that i left it inside in one Pair of my G 7 s . In a more viby, colouring Mic , the Damping  may be Contraproductive because the " Sound " of the Headbasket is a good Part of the Sound of the Mic.
The Bodyresonance is a Part too and responsible for the honky Sound in the Recording of some cheap Bodys ( like the MK 219/319 ) , not easy to get rid of. Silicon doesn t help much. You need to bring Mass to the Body to lower the Resonantfrequency out of the Audiorange to get best  Results . If visual  Looking  is not an Issue the Body can be damped from the Outside ( Looks  ugly but more Possibilitys to use Different Material You could not Sqweez inside the Body ).
Greatings
Lothar

tubestation said:

Hello,
I  removed, cut the Bars because They don t make Sense to me. Maybee If You have the Habbit to throw the Mic on the Ground or If You like to  hammer Nails in Walls with it.
As a Rule, a Soundwave get s  reflected if the physical Barrier is as big or bigger  than the Soundwave.Some People mean twice the Size of the Soundwave.
Soundwave  Size is  1,7 cm for 20kHz  3,4 cm for 10kHz  at 20 Degrees Celsius. and so on. If the Soundwave is bigger  it s get bend around the Barrier. So If You have Bars in the Headbasket there may be a Reflection at certain Frequencys that results in Comp Filter Effects. The velocity Effects are one Thing , the Pressure Effekts inside a Headbasket may have an Inpact  for the Capsule Reaktion as well.
You could dig Holes in the Bars to reduce Comp Filter Effects.
I once made a Test and installed a  porous Damping Material  inside at the Bottom of a round DIY Doublelayer Headbasket ( at the Plate where the Capsuleholder is installed ) . It made a audible Diffrence and the Comp  Filter Effects  where almost completly gone. Sounded much more clean and  natural. I like it so much that i left it inside in one Pair of my G 7 s . In a more viby, colouring Mic , the Damping  may be Contraproductive because the " Sound " of the Headbasket is a good Part of the Sound of the Mic.
The Bodyresonance is a Part too and responsible for the honky Sound in the Recording of some cheap Bodys ( like the MK 219/319 ) , not easy to get rid of. Silicon doesn t help much. You need to bring Mass to the Body to lower the Resonantfrequency out of the Audiorange to get best  Results . If visual  Looking  is not an Issue the Body can be damped from the Outside ( Looks  ugly but more Possibilitys to use Different Material You could not Sqweez inside the Body ).
Greatings
Lothar

Click to expand...

Thanks, now I'm getting the info I was looking for
Those Bars look cool, but I also always thought they must have "sound"!now I'm just trying to find proof why so..
So if my bar(&slotted hole) is 5mm high and 40mm wide,
theoretically everything above 68KHz goes straight thru, but all audible frequencies will have some sort of effect?
but are those compfilterings in a 1:1/1:2/1:3/..etc of my hole cutouts wavelenght 5mm 68KHz and 40mm 8.5KHz
or is the math much deeper?

as for pressure in side; could I use some sort of "ported speaker case" calculator
to get approximation of what frequencies to fear?

http://www.mobileinformationlabs.com/HowTo-1Woofer-Box-CAL%20Port%20lenth%201.htm
tried that one with slotted ports, 10 per side(=20 ports, size 5x40mm) enclosure volume something between 1-2dl
and tested frequencies till I got material thickness as a "vent length"
ang got results in 1-2KHz, does that seem legit?

I understand that makin complete and accurate analysis would take aLOT of math,
but I'll settle for close enough approximation

theoretically everything above 68KHz goes straight thru, but all audible frequencies will have some sort of effect?


Hello,
Everything under  68 kHz goes more or less straight thru.
Not sure what happens with a Barrier of 5* 40 mm.
I think the Barrier Must be X * X  (5 mm * 5 mm for Your 68kHz ) to reflect.
It s possible that the Resonantfrequency  of the Headbasket is related to the Helmholz Formula.
But it s more a Kind of a Slot Resonator with a partly Open Back.
How to calculate it s explained here.
http://www.recordingchannel.com/slat-type-helmholtz-resonator-formula-5843010.html
Here You find a good Calculator for a  single Hole Helmholzresonator.
http://www.acousticmodelling.com/helmholtz.php
But I am not sure If it s even possible or sensefull to calculate a Headbasket.because there may be too many unknown and uncalculateable Aspects.
Greatings
Lothar.