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Condenser Mic Noise: Possible Causes

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Condenser Mic Noise: Possible Causes

Postby Anonymous » Mon Nov 28, 2011 9:10 pm

Hello.

I've recently had problems with a continuous noise from a (Rode NT2-A) microphone and wondering about the cause. It's like a rustling sound that starts whenever I begin to record, especially after high frequency sounds at close proximity.

Here's an audio sample: http://soundcloud.com/tonespectra/rode-nt2-a-noise

I've eliminated cables, sockets and the audio interface itself as the cause, so I'm not sure if the problem is dampness because I did record an instrument that involves blowing hard on a quill attached to a string (lesiba), but then why would the noise be triggered by sound? The sound level seems a little reduced also.

I left it under a light bulb for an hour and had the fire on not to far away for about another hour yesterday. Then I left it in its case with crystals overnight. The problem is still there.

So I'm wondering... how long might it realistically need to dry out for, if breath vapour has caused it?

I don't have central heating so I'm thinking of taking it to a friend's who has shelving above a radiator. However, there's another possibility: I noticed some wood I brought in from the garage had tiny white bugs crawling on it, so I'm wondering if one has got inside. I suppose a thorough drying it out would cure this anyway.

The mic has 10-year warranty, but rather than subjecting it to an unnecessary journey and handling (only to be returned after drying out) I'd like to see if it's a simple problem. Any advice would be appreciated.

Thanks
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Re: Condenser Mic Noise: Possible Causes

Postby Guest » Tue Nov 29, 2011 2:22 am

I've just read a lot about how these Rode mics are durable and hardy compared to other capacitor mics. I can't see how I could've damaged it so easily. It could take weeks to get it back from repairs or get a replacement. Damn!

Has anyone else heard this sound caused by damp from a condenser...?

http://soundcloud.com/tonespectra/rode-nt2-a-noise
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Re: Condenser Mic Noise: Possible Causes

Postby matt keen » Tue Nov 29, 2011 10:49 am

Its likely to be damp
In any case thats the easiest thing to eliminate

Somewhere warm for as long as you can - I dont know - over the weekend, a week?
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Re: Condenser Mic Noise: Possible Causes

Postby James Perrett » Tue Nov 29, 2011 10:56 am

It could take a fair amount of time to dry out. Did you dry out the desiccant crystals before you left them in the case with the mic? If you didn't, the crystals could well have introduced more moisture if they had been in a damp place beforehand. If you don't have central heating, I'd maybe make a box with an old style 40W incandescent bulb at the bottom and a shelf for the mic a few inches above. Leave it with the bulb on for a few days and then try it. Or leave it in a small box with one of these.

One precaution that I always take is to make sure there is a windshield of some kind in front of the mic whenever there's a likelihood that someone will be breathing in to it.

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Re: Condenser Mic Noise: Possible Causes

Postby Hugh Robjohns » Tue Nov 29, 2011 12:19 pm

It does sound like a humidity problem. This kind of thing is made infinitely worse if the capsule diaphragm is dirty which tends to happen if the mic is used in a smoky environment, or is sung into at close range a lot without a pop-screen.

In these cases the dirt on the diaphragm tends to 'hold on' to water vapour more stubbornly and so it takes far longer to 'dry out' and is more prone to future repeat bouts.

The mic should dry out and be noise-free within a day or two if you leave it in a warm and dry environment. I generally find the airing cupboard is ideal, placing the mic within a polythene bag to keep dust off, but with the end of the bag open so air can circulate easily.

But if you use the mic in a naturally humid environment it will be a recurring problem, as it will if you take a very cold mic from storage and start using it in a warm environment, or subjected to warm humid breath at close range.

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Re: Condenser Mic Noise: Possible Causes

Postby ef37a » Tue Nov 29, 2011 1:56 pm

Hmm? I was immediately struck by your comment " especially after high frequency sounds at close proximity." A sharp transient electrical signal can often induce a dry solder joint to start fizzing, a fault that is likely to be confused in a microphone with dampness since it will likely come and go with temperature variations.

I would not have thought such a good brand name would have been that moisture sensitive? My experience is very limited with cap' mics, I have a LDC Sontronics and two AKG P150s, I do not have central heating either and the room they live in does get cold (tho' not damp!). The mics have been used for voice, clarinet and trumpet and never given any noise problems at all.

An email to the maker is my first suggestion.

Ah! Try a (sensible!) tap with pencil or similar, on the body not the grill. If that cures/induces the fault...Ain't H2O!

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Re: Condenser Mic Noise: Possible Causes

Postby Hugh Robjohns » Tue Nov 29, 2011 2:07 pm

ef37a wrote:A sharp transient electrical signal can often induce a dry solder joint to start fizzing...


Yes, it's a possibility -- and one that could be proven simply by tapping the mic body sharply to see if the fault sound changes. A humidity issue won't be affected. A dry solder joint (or losse or dirty valve base pins/sockets) in the electronics will.

I would not have thought such a good brand name would have been that moisture sensitive?


Nothing to do with brands, and everything to do with the environment and the way the mic has been stored and used. Every DC-biased capacitor mic will suffer from humidity to some degree, regardless of brand or cost. Only RF-biased capacitor mics (basically just Sennhesiers MKH ranges, and Rode's NTG-3 which is remarkably similar to the MKH416), and back-electrets are immune to the problem.

The mics have been used for voice, clarinet and trumpet and never given any noise problems at all.


Likewise: I have many capacitor mics, some of which are twenty or more years old, without any issues. But as I said, the environment and the way they are used has a big effect. Smoke (of all kinds) is a major contributor to humidity problems, as is using it on a vocalist at very close range without a pop screen. It's scary how much crap is emitted from a human mouth when singing -- and I'm not talking about lyrics! I once had to clean the grille of an SM58 after it had been used by a vocalist who has just eaten a chinese meal! Not nice, but very instructive!

Also, moving the mic from a cold environment to a warmer one (or even just placing a cold mic in a cold room in front of the warm air from a vocalist's mouth can generate sufficient condensation to cause this kind of problem.

The higher the biasing voltage and the impedance converter electronic's impedance, the mosr susceptible the mic is to the problem as well.

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Re: Condenser Mic Noise: Possible Causes

Postby ef37a » Tue Nov 29, 2011 3:10 pm

Thanks Hugh,
Yes, I was aware of the usual precautions and problems but this mic does not seem to want to dry out!

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Re: Condenser Mic Noise: Possible Causes

Postby SimonZ » Tue Nov 29, 2011 4:28 pm

The OP implies the mic was stored out in the garage (with the bugs and wood?)
I have known this sort of thing to take up to a week to dry out fully.
The audio sample does sound right for damp.
It can speed things up if the mic is kept powered up, but often getting a warm dry place and phantom power set up in the same spot for a week isn't that easy.
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Re: Condenser Mic Noise: Possible Causes

Postby Guest » Tue Nov 29, 2011 4:32 pm

Thanks for all the responses! It gets me down when stuff goes wrong with equipment because I'm so reliant on it. I'll leave it to dry out another day or so above a radiator and if it recurs I'll try the pencil test.


One thing... "A sharp transient electrical signal can often induce a dry solder joint to start fizzing..." If this is the problem, do I send the mic back for repairs?

My flat is quite dry I think, but I have recently fixed double-glazing (I found in a skip) to the windows, so perhaps it's increased humidity. I'll be careful when cooking things like pasta to keep the door shut and maybe eat in the kitchen from now on. I don't smoke anymore but... I've recently had my gas fire fixed and perhaps all the burning dust that had accumilated over the summer had gone into the air and formed on the mic.

There's not too much detail on the web about humidity and mics (such as how long it can take to dry) or alternative causes, so hopefully others will find this useful.

Regards,

José

RE: The OP implies the mic was stored out in the garage (with the bugs and wood?)

No, I just brought some wood in to use for my windows. The mic is left in the stand with a dust cover, though I admit I hadn't put the crystals in as they advised. Thanks
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Re: Condenser Mic Noise: Possible Causes

Postby . . . Delete This User . . . » Tue Nov 29, 2011 5:59 pm

gas fires chuck a fair bit of moisture in to the air, (one by-product of burning gas in open atmosphere is water.. ) and fitting double glazing will mean that less of it escapes, so the relative humidity will very likely have gone up significantly recently.....

add any dust settlement on the capsule membrane, and the coming of winter, and you have a recipe for sudden onset of moisture noise....
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Re: Condenser Mic Noise: Possible Causes

Postby Guest » Tue Nov 29, 2011 6:55 pm

The gas fire is ventilated via the chimney so I'm not sure if that avoids the problem. I've only used the fire twice yet, so it is probably down to dust and/or breath vapour.

Otherwise I might have damaged it through an accidental hard blow, but I'm not sure how that compares to a jumbo jet SPL they claim it can handle.

Cheers
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Re: Condenser Mic Noise: Possible Causes

Postby Hugh Robjohns » Tue Nov 29, 2011 8:48 pm

It's almost impossible to damage a capacitor mic with a 'hard blow' -- unlike a ribbon!

Everything seems to point towards a damp capsule and/ir damp electronics to me. Leave it somewhere warm and very dry for a good few days and see what happens.

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Re: Condenser Mic Noise: Possible Causes

Postby Guest » Thu Dec 01, 2011 5:22 pm

I tested it last night after two days and nights on a shelf above a radiator, in a room which is very dry anyway. The shelf gets slightly warm but not hot. It worked for longer this time before the noise returned again and continued. It recorded a few minutes of audio that seemed Okay, if perhaps a little quieter than normal.

I tried gently tapping it and although it affected the noise, it didn't cause it to stop.

I'm not sure if to leave it for longer in extra-dry conditions, say a week, but I can't understand why the noise would start after a few minutes rather than straight away. This time it wasn't triggered by a high frequency or loud level, it just started seemingly without a trigger. Perhaps it has something to do with the effect of phantom power on the circuit after a time. Don't know whether it's best to send it back or try again. I don't fancy sending it from Manchester to London unnecessarily.

Thanks again for the advice.
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Re: Condenser Mic Noise: Possible Causes

Postby Hugh Robjohns » Thu Dec 01, 2011 5:43 pm

J.A.S wrote:It worked for longer this time before the noise returned again and continued.

The fact that it was initially noise free does support the idea that it is a damp issue, and an extended drying out might be worthwhile....

But equally, there are other possible causes for noise of this type, and which might not appear until phantom power has been on for a while, such as dying capacitors and/or dodgy FETs in the impedance converter electronics.

I had a CAD M179 mic a while back that exhibited similar noise issues and that turned out to be electronic failure, so it's certainly not unknown.

So I would suggest that if the problem doesn't disappear after a few more days drying out ti will need a return trip to its maker (or their appointed agent).

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