Am I correct in thinking that large diaphragm capacitor mics are so much more liable to be noisy due to their sensitivity to the slightest dampness?

I have a AKG C414 xls mic and I suddenly had bubbling noise after I had left it on a stand with an open plastic bag as dust cover, for about two weeks. During this time the temperature had dropped 3 to 4 C and although it had been fine over the last year it was suddenly noisy. I left it on for about an hour with Phantom power on, and it was a bit better but not much. I then put it in the case with silica gel for about a week and it was then OK. None of my other mics have done this. (Ribbons, dynamics, electret capacitors etc).

The same thing happened just over a year ago when this same mic got noisy and was put away in it’s case, only to be fine about six weeks later.

I’m beginning to think it best to only have dynamics, electret capacitors, SDC’s and ribbons as they seem so much more reliable and not prone to such problems.

I have plenty of LDCs that are absolutely fine and have been for decades... so it's not a problem which is endemic to all LDCs.

The fact that the mic gets better whey you actively dry it out suggests to me that that particular 414 is a bit dodgy for some reason.

Either it has a contaminated diaphragm (or diaphragms) which is trapping moisture from the air and allowing the bias voltage to leak resulting in audible noise, or there is an issue on the circuit board causing the impedance converter to become noisy. It could be a failing FET or capacitor, but can also be breakdown of the sealant they often use to maintain the giga-Ohm impedance around that part of the circuitry.

It would probably be worthwhile having the mic thoroughly checked over by a competent mic technician.

H

Thanks for the answer Hugh.

The mic is pretty new - just over a year old. It's always been (mostly kept in its case and has never had anything to contaminate it and has been always used in a clean environment and has never been out of the house.

The other possible causes are rather worrying. Will look into getting in touch with AKG and see what they say.

Bringing something from a cold environment quickly into a warm one can cause condensation. If you have a bag over the mic while the room is being heated, the bag will act as insulation, trapping the cold air in. The mere act of removing the bag could cause a quick increase in temp for the mic and maybe condensation will be formed.

Tim Gillett wrote:If you have a bag over the mic while the room is being heated, the bag will act as insulation, trapping the cold air in.

Aristo said: "I had left it on a stand with an open plastic bag as dust cover..."

The open bag itself won't provide anything significant in the way of insulation and, since the open end of the bag must be facing downwards, any cold air originally contained within will naturally tend to fall out (seeing as it's heavier than any warmer air in the room), and be replaced by said warmer air... ;-) There will be a degree of circulation involved...

So I don't really think the bag is likely to be the problem -- added to which this protective dust-cover arrangement is quite commonly employed in a great many professional studios which don't have the same issues.

I agree, though, with your thinking that condensation due to changing temperature or humidity levels are the issue -- the fact that the mic goes quiet once dried out proves that... But LDCs aren't normally this sensitive when kept indoors in a domestic room. If I remember rightly, there's a grand piano in this room and the temperature is kept pretty stable...

So I reckon it all points to a defective mic/capsule.

H

You're right, the bag won't offer much in insulation but it will a little, and it will still impede convection. Circulation will occur but not as quickly as with no bag on the mic and we don't know how large a hole there was at the bottom. I' m just suggesting it may not be a mic fault.

Hugh Robjohns wrote:

Tim Gillett wrote:If you have a bag over the mic while the room is being heated, the bag will act as insulation, trapping the cold air in.

I agree, though, with your thinking that condensation due to changing temperature or humidity levels are the issue -- the fact that the mic goes quiet once dried out proves that... But LDCs aren't normally this sensitive when kept indoors in a domestic room. If I remember rightly, there's a grand piano in this room and the temperature is kept pretty stable...
H

Actually the mic is in a different room where I record audio narration, and I use this mic and a Beyer M201 at other times. The M201 is always 100% reliable and may become my preferred mic for this sort of thing. It's a small room which has some home made sound dampening - blankets and duvet, although I've mainly dispensed with those at present as I'm using the M201 instead of the AKG C414. The room is heated at times so I would not say the temperature is always stable and can vary by 3-4C.

EDIT: The bag was a freezer plastic bag and there was quite a big opening at the bottom.

It's simply not viable that a 4°C difference between rooms would cause condensation in the UK at the moment. Assuming a minimum temperature of 18°C inside - which is cold for a house - you'd need 75%rH or greater in a 22°C room to get condensation forming. But the warmer room %rH is far more likely to be around 50%rH or lower, so unless Aristo was doing a load of close-up heavy breathing recording sessions, I'd rule out straightforward condensation as being the issue.

And if the storage room/mic was warmer than 18°C, it's even less likely.

Another thing, once I was aware of the noises I would immediately turn off the polarising voltage to try and avoid damage to the mic. I'd only reinstate the phantom power after giving any moisture in the mic more than enough time to dry.

I did put 2 silica gels into the oven at low heat for 5 minutes to make sure they were dry and then put them with the mic in the case for a week. The mic sounded absolutely fine then and no noise whatsoever. It also worked fine for 11 months from last year when always put away in the case. It was only after leaving on mic stand for two weeks in the change from summer to autumn (early October to mid October) that it developed the noise.

That's why I wondered if these mics with a large charge on the diaphragms are much more likely to have problems, as against back electrets, ribbon mics, etc., that always work every time?

Also, is it correct that passive dynamic mics and ribbon mics have very low self noise due to their having no amplification circuits built in, and when used through a good quality pre-amp or pre-amps on a quality recorder, give excellent results?

Ariosto wrote:It was only after leaving on mic stand for two weeks in the change from summer to autumn (early October to mid October) that it developed the noise.

Perhaps the room temperature variations or humidity are greater than what might be considered 'normal' for a domestic room. Or you have particularly humid breath! ;-)

Is the room heated by a gas fire perchance?

That's why I wondered if these mics with a large charge on the diaphragms are much more likely to have problems, as against back electrets, ribbon mics, etc., that always work every time?

It's not the size of diaphragm that's the issue, It's the operating principle. A voltage is applied between the diaphragm and backplate -- typically around 30-60 Volts -- to put an electrostatic charge in the capsule. The impedance converter senses voltage changes that occur as the capacitance varies when the diaphragm moves, but it only works if the sensing is via an extremely high impedance to stop the charge leaking away. Anything which allows the charge to leak away results in the kind of unwanted noise you've heard, and the most usual cause is contamination bridging the junction between different parts the capsule housing -- the contamination tending to attract and hold moisture which is electrically conductive.

Electrets are constructed in a different way, and the required electrostatic charge is created internally from the embodied materials. There is no need for an external biasing voltage and so no risk of external leakage. And RF capacitor mics operate the capsule in a low-impedance circuit, so are again immune to the leakage problem which can afflict DC-biased mics. Ribbons and dynamics operate on a completely different principle again and are immune to this issue as well.

Also, is it correct that passive dynamic mics and ribbon mics have very low self noise due to their having no amplification circuits built in, and when used through a good quality pre-amp or pre-amps on a quality recorder, give excellent results?

Yes and yes... Their self-noise is essentially defined by the resistance of the voice-coil (or matching transformer). In a capacitor mic (or active dynamic mic) the active elements in the impedance converter circuitry generates some inherent electronic noise. But against that, the signal level is usually significantly higher than a dynamic mic, so less gain is required from the mic preamp... and so the combination may produce similar or less noise than a dynamic mic for the same end level.

As for quality, the much lighter diaphragm of a capacitor mic typically allows a greater high frequency extension and more accurate transients than a dynamic mic.

H

I'm generally about 12 inches from the mic and often use the pop screen and I don't think I exude much breath anyway, as I try and control breath from my diaphragm, and was told by a wind player that I use my diaphragm quite well ...

The heating is from a radiator and often only comes up from the lower floor if I have the radiator off. But maybe there is more variation in temperature than I'm aware of as I don't have a thermometer in the room. Maybe I should get one and monitor it more closely!

The strange thing is that the mic works perfectly quietly for weeks and even months on end and the problem is only spasmodic.

"As for quality, the much lighter diaphragm of a capacitor mic typically allows a greater high frequency extension and more accurate transients than a dynamic mic."

But for voice production for narration the dynamic mic such as the M201 might be better in some ways as high frequency extension and transients are produced less - especially for the male voice? Or have I got that wrong?

I'm also wondering about the new Aston dynamic mic that has different voice settings and also the possibility of increased output with the option of phantom power.

Ariosto wrote:But for voice production for narration the dynamic mic such as the M201 might be better...

Obviously a subjective decision, but the BBC employed M201s in discussion studios for years very effectively indeed before someone decided to switch to C414s, and 4038 ribbons were the mainstay at World Service for decades too...

I'm also wondering about the new Aston dynamic mic that has different voice settings and also the possibility of increased output with the option of phantom power.

It's a good-sounding mic, with useful tonal variation. I'm not convinced about the mounting spigot, and I think the internal preamp has excessive gain, but these issues are minor. If you're contemplating the Stealth mic should should also try the electrovoice RE20 and the shure SM7B.