microphone capsule noise

[MicUlli]

Hello All,
as some members know i am very interested in building microphones with exceptionally low noise. I dont care for LDCs but for SDCs because i cannot find any advantage using large diaphragms.
Unfortunately there are several misconceptions regarding the mic capsule noise (NOT electronically generated noise) and i found no documents how noise origin in capsules can be calculated. Therefore i tried it by myself, have fun to participate. As every time i posted a document: Please feel free to leave your comments and suggestions

 

[abbey road d enfer]

You make a number of shortcuts that make it difficult to follow.
I don't get how you're coming to your S/N ratio figures.
Since, if I understand correctly, you're evaluating the actual acoustic noise level pressure, why not stay in the acoustic domain, where S/N ratio would be measured in comparison with 94dBspl.
Are you familiar with the Philips document Noise in Condenser Microphones by Becking and Rademakers. ?

Your comment about LDC vs. SDC is worthy of some development.

 

[kingkorg]

Hello All,
as some members know i am very interested in building microphones with exceptionally low noise. I dont care for LDCs but for SDCs because i cannot find any advantage using large diaphragms.
Unfortunately there are several misconceptions regarding the mic capsule noise (NOT electronically generated noise) and i found no documents how noise origin in capsules can be calculated. Therefore i tried it by myself, have fun to participate. As every time i posted a document: Please feel free to leave your comments and suggestions

Awesome stuff as always. I have to be honest it will take a couple more reads to get full grip of it.

One question that came to mind. Is the Am parameter adjusted for the hole surface on the backplate? It's not just the diaphragm surface that determines the capacity. Also there are center terminated SDC, how does that fit in the equation?

 

[molke]

The short section in the AIP Handbook of Condenser Microphones (of which I unfortunately don’t understand much as well, e.g. the equivalent circuit is in series not in parallel?) apparently comes to the same conclusion, i.e. resistance/admittance (and therefore membrane radius) is the most important factor. The reference given there is this one and might be available in the B&K Knowledge Centre (I haven’t tried yet).

 

[kingkorg]

You make a number of shortcuts that make it difficult to follow.
I don't get how you're coming to your S/N ratio figures.
Since, if I understand correctly, you're evaluating the actual acoustic noise level pressure, why not stay in the acoustic domain, where S/N ratio would be measured in comparison with 94dBspl.
Are you familiar with the Philips document Noise in Condenser Microphones by Becking and Rademakers. ?

Your comment about LDC vs. SDC is worthy of some development.

Remarkable document. Thanks for sharing. Easy to see what Sennheiser went for with their underdamped capsules, and Neumann with feedback. Duh.... It also makes me wonder how the center termination and highly tensioned diaphragms impact the effect of lowering the distance between backplate and diaphragm. Also golden drop approach used by JZ.

 

[MicUlli]

You make a number of shortcuts that make it difficult to follow.
I don't get how you're coming to your S/N ratio figures.
Since, if I understand correctly, you're evaluating the actual acoustic noise level pressure, why not stay in the acoustic domain, where S/N ratio would be measured in comparison with 94dBspl.
Are you familiar with the Philips document Noise in Condenser Microphones by Becking and Rademakers. ?

Your comment about LDC vs. SDC is worthy of some development.

Thanks for the link to the document (i was not aware of it). The authors point out the same thing (satisfies me). The step from mechanic-acoustic domain to electrical domain is not that difficult (sensivity S) so i wanted to get also voltage levels...

 

[MicUlli]

Awesome stuff as always. I have to be honest it will take a couple more reads to get full grip of it.

One question that came to mind. Is the Am parameter adjusted for the hole surface on the backplate? It's not just the diaphragm surface that determines the capacity. Also there are center terminated SDC, how does that fit in the equation?

No, i did not adjust for that, because of simplicity. Same for center terminated capsules (no idea yet how this can be included).

 

[Icantthinkofaname]

Can't really comment on LDC vs SDC, but it would probably be easier to get really low noise with an LDC. Some of the quietest ones around are (an advertised) 7 dBA or less, and even cheapies like the CAD M179 have (an advertised) 11 dBA, don't know how accurate that but I don't hear any more noise than my MKH 8050.

 

[k brown]

Can't really comment on LDC vs SDC, but it would probably be easier to get really low noise with an LDC. Some of the quietest ones around are (an advertised) 7 dBA or less, and even cheapies like the CAD M179 have (an advertised) 11 dBA, don't know how accurate that but I don't hear any more noise than my MKH 8050.

Physics WILL not be denied!

 

[abbey road d enfer]

Thanks for the link to the document (i was not aware of it).

All praise to member ccaudle, who unearthed it.

 

[ricardo]

Guru MicUlli, I'm in awe of your work.

I think your Omni analysis is good and backed up by B&K & Philips.

But I'm not sure if the inherent Fig8 noise is 1/f

I'll have to lern 2 reed, rite en kunt to try en unnerstan yaw sums

 

[mhelin]

How about electret condensers vs. externally biased, can electret capsules be noisier than other type of capsules because of the electrostatic field (can polar molecules somehow orient randomly etc)?

 

[abbey road d enfer]

How about electret condensers vs. externally biased, can electret capsules be noisier than other type of capsules because of the electrostatic field (can polar molecules somehow orient randomly etc)?

Electrostatic field, whether it originates from a fixed charge element or an external voltage, has identical effects.

 

[Marik]

MicUllu,

Just a few thoughts in no particular order.

For capsule calculations friend of mine developed a mechanical-electrical analogies sim. He meticulously put all the parameters of the air, holes, depths, distances, thickness, spacing, compliance, etc. When I was developing a capsule for our single diaphragm cardioid TF10 he was helping with that sim for a certain frequency response, capsules output, and pattern. While I could see the correlation the parameters still were quite a bit off, so everything needed ‘real life’ adjustment.

I just skimmed through your paper. It looks like the AIP Handbook is in agreement for the pressure capsules. I am not sure the fig8 condenser is the correct one for comparison. For flat response ideally the single diaphragm fig8 should really be a mass controlled system (as opposed to stiffness controlled pressure capsules), which would be easily achieved with ribbons, but impossible with condensers. Instead, we have a resistance controlled system, the problem is because of the nature of fig8 we cannot really add enough resistance for a wideband response (that would be so easily achieved in the cardioid). That would through things way off, esp. considering the Johnson noise of the trapped air in the cavity behind the diaphragm would be dominating. Such cavity in fig8 is missing.

Best, M

 

[MicUlli]

Can't really comment on LDC vs SDC, but it would probably be easier to get really low noise with an LDC. Some of the quietest ones around are (an advertised) 7 dBA or less, and even cheapies like the CAD M179 have (an advertised) 11 dBA, don't know how accurate that but I don't hear any more noise than my MKH 8050.

Dont get me wrong, i dont want to discredit LDCs. When i look upon solioqueens K47 LDC capsule i am deeply impressed. Its not only a capsule, its a piece of art . My preference for SDCs: Better (off axis) response at higher frequencies, unobstrusive in concert recordings...

 

[abbey road d enfer]

Dont get me wrong, i dont want to discredit LDCs. When i look upon solioqueens K47 LDC capsule i am deeply impressed. Its not only a capsule, its a piece of art . My preference for SDCs: Better (off axis) response at higher frequencies, unobstrusive in concert recordings...

I fully agree. I generally favour SDC's, except with singers, for the wow factor, or when the noise penalty is significant.

 

[Icantthinkofaname]

Dont get me wrong, i dont want to discredit LDCs. When i look upon solioqueens K47 LDC capsule i am deeply impressed. Its not only a capsule, its a piece of art . My preference for SDCs: Better (off axis) response at higher frequencies, unobstrusive in concert recordings...

I also prefer SDCs, I just don't think we're gonna get many ultra quiet ones without spending a small fortune.