Topic: Effect of humidity on microphones?

[tooldvn]

What are the effects of exposure to humidity?  Lots of you guys tape these outdoor swamp festivals in the south where the humidity has to be insane.  What does that do the mics long term?  I see everyone here storing their mics in dry locations, sometimes with desiccant, does that undo the damage of fests?     Would you be able to hear the difference between a set of brand new caps and ones that had seen a decade or more of humid environments?

[John Willett]

AF and RF condensers behave differently, which is why the RF condenser is the preferred one for outside use, especially in damp and humid conditions.

This is an explanation I wrote up a few years back:-

Basically, AF capacitor microphones use the capsule as a capacitor to store charge. With one fixed plate and the other free to vibrate in sympathy with the sound, the capacitance varies, and the charge moves in or out of the capsule accordingly. This is measured by the head preamplifier and an audio signal results. All well and good, but the capsule is inherently in a high impedance circuit (over 1GigaΩ) – it has to sit there with stored charge until the diaphragm moves and any changes in the charge are perceived as audio. In a humid atmosphere the stored charge finds it easier to escape on water molecules in the air rather than through the input of the preamplifier, hence noisy and reduced output, and misery all round. The high biasing voltage also attracts dust particles to the diaphragm, reducing its efficiency and linearity.

The RF system (as used in Sennheiser MKH microphones) uses the capsule (a low impedance capsule) in a completely different way: as a tuning capacitor for an RF oscillator – which inherently employs it in a low impedance circuit where a high frequency signal is being passed through the capacitor all the time. Changes in capacitance (caused by sound moving the diaphragm) alter the resonant frequency of the circuit (circa 8MHz) and so its frequency becomes proportional to the audio signal. A simple RF demodulator restores the output to a conventional audio signal. More complex and sophisticated (but still very rugged), this system is highly immune to the effects of humidity and is thus the preferred design to be used out of doors (or when moving from outside to inside on a cold day!).

Soundfield microphones (though not the SPS200) have a heater in the microphone to keep the capsules dry and still working effectively in damp conditions, and, some measurement microphones like Gefell have the option of adding a dehumidifier section to some microphones to keep them dry and working to spec.

New and clean AF condenser mics can be better than older and dirty ones as dust and contaminants attracted to the diaphragm by electrostatic action can absorb moisture and make the mic. go poppy and crackly.  I know Schoeps used to recommend to sound recordists that they return the microphones for cleaning occasionally.

I hope this helps.

[flipp]

AF and RF condensers behave differently, which is why the RF condenser is the preferred one for outside use, especially in damp and humid conditions.

This is an explanation I wrote up a few years back:-

Basically, AF capacitor microphones use the capsule as a capacitor to store charge. With one fixed plate and the other free to vibrate in sympathy with the sound, the capacitance varies, and the charge moves in or out of the capsule accordingly. This is measured by the head preamplifier and an audio signal results. All well and good, but the capsule is inherently in a high impedance circuit (over 1GigaΩ) – it has to sit there with stored charge until the diaphragm moves and any changes in the charge are perceived as audio. In a humid atmosphere the stored charge finds it easier to escape on water molecules in the air rather than through the input of the preamplifier, hence noisy and reduced output, and misery all round. The high biasing voltage also attracts dust particles to the diaphragm, reducing its efficiency and linearity.

The RF system (as used in Sennheiser MKH microphones) uses the capsule (a low impedance capsule) in a completely different way: as a tuning capacitor for an RF oscillator – which inherently employs it in a low impedance circuit where a high frequency signal is being passed through the capacitor all the time. Changes in capacitance (caused by sound moving the diaphragm) alter the resonant frequency of the circuit (circa 8MHz) and so its frequency becomes proportional to the audio signal. A simple RF demodulator restores the output to a conventional audio signal. More complex and sophisticated (but still very rugged), this system is highly immune to the effects of humidity and is thus the preferred design to be used out of doors (or when moving from outside to inside on a cold day!).

Soundfield microphones (though not the SPS200) have a heater in the microphone to keep the capsules dry and still working effectively in damp conditions, and, some measurement microphones like Gefell have the option of adding a dehumidifier section to some microphones to keep them dry and working to spec.

New and clean AF condenser mics can be better than older and dirty ones as dust and contaminants attracted to the diaphragm by electrostatic action can absorb moisture and make the mic. go poppy and crackly.  I know Schoeps used to recommend to sound recordists that they return the microphones for cleaning occasionally.

I hope this helps.

It might if you explained what the acronyms are.
AF = I haven't a clue
RF = Radio Frequency?

[tooldvn]

AF and RF condensers behave differently, which is why the RF condenser is the preferred one for outside use, especially in damp and humid conditions.

New and clean AF condenser mics can be better than older and dirty ones as dust and contaminants attracted to the diaphragm by electrostatic action can absorb moisture and make the mic. go poppy and crackly.  I know Schoeps used to recommend to sound recordists that they return the microphones for cleaning occasionally.

I hope this helps.

Thanks John - that was really informative.   How do we tell which are which?   Schoeps should be AF by that last comment? But I'm guessing they make both types as maybe most mfrs would for different applications?   

[notlance]

RF condenser microphones are rather uncommon, except for the MKH line from Sennheiser.  They have been making RF mics since at least the 1960s.  Their MKH mics are superb and expensive, although no more expensive nor less superb than AF mics from Schoeps and Neumann.  The only other RF mics I am aware of are a couple of Rode shotgun mics, but there may be others.  Rode appears to have used the RF principal specifically for its ability to operate in high humidity.

Here is a list of RF mics currently in production:

http://recordinghacks.com/microphones/tag/rf-condenser

I am not claiming this is an exhaustive list, but certainly the most commonly available RF mics.

It is not too difficult to find stories of Sennheiser MKH microphones being used for weeks in South American rain forests without problems.

[John Willett]

It might if you explained what the acronyms are.
AF = I haven't a clue
RF = Radio Frequency?

Sorry

AF = Audio Frequency
RF = Radio Frequency

[John Willett]

Thanks John - that was really informative.   How do we tell which are which?   Schoeps should be AF by that last comment? But I'm guessing they make both types as maybe most mfrs would for different applications?

All condenser mics are AF except for the Sennheiser MKH series and the Røde NTG3 and 8.

Schoeps did start to develop an RF condenser at some point in the past, I don't know if it actually made it to market; they certainly don't do any RF condensers now.

The only company to really research and develop RF condensers is Sennheiser.  They only started doing condenser microphones after transistors were developed and you *have* to make an RF condenser with transistors.

The traditional AF design is based on valve (vacuum tube) technology.

When FETs came along (which are basically solid-state valves) all microphone manufactures just changed their valve designs into FET ones.  But as Sennheiser never made valve mics, they carried on with their RF transistor designs.  The advantages being their immunity to moisture and having less self-noise at lower frequencies.

I hope this helps.

[John Willett]

Here is a list of RF mics currently in production:

http://recordinghacks.com/microphones/tag/rf-condenser

That list is not complete.

RF condensers are:-

Sennheiser
MKH 20
MKH 30
MKH 40
MKH 50
MKH 60
MKH 70
MKH 800
MKH 800 TWIN
MKH 8020
MKH 8040
MKH 8050
MKH 8060
MKH 8070
MKH 8090
All above are symmetrical capsule mics with ultra-low distortion and self-noise.

MKH 416
MKH 416T
MKH 418S

Discontinued:
MKH 80
MKH 405
MKH 816
MKH 806
MKH 405
etc.....
Any Sennheiser mic. starting "MKH" is an RF condenser

Røde
NTG 3
NTG 8

[notlance]

And in that list above, where is the 8030...?

If Sennheiser came out with it, I know they would sell a lot of them.  I'd buy two and give one away just because I'd be so happy.

We should call the 8030 "The Second Coming" microphone because we have been waiting so long for it, and it's a matter of faith that it will ever happen.

[John Willett]

And in that list above, where is the 8030...?

If Sennheiser came out with it, I know they would sell a lot of them.  I'd buy two and give one away just because I'd be so happy.

We should call the 8030 "The Second Coming" microphone because we have been waiting so long for it, and it's a matter of faith that it will ever happen.

The 8030 is not there because it does not yet exist.

If everyone who wanted one wrote to Sennheiser in Germany asking for it and asking when it will come out may spur them into action.

Back in 2008 they told me it was planned for 2010 and I told them then that it should be earlier.

Well, 2010 has come and gone and it's now 4 years later ...........

[DSatz]

John W., yes, during the brief era of the germanium transistor, the RF approach was the only known way to build solid-state condenser microphones with acceptable dynamic range, so Schoeps developed and manufactured RF condenser microphones. This began with the CMT 20 series in 1963 and continued with the CMT 200 and the CMT 100 series which were produced until about 1967. Depending on the series, parallel or phantom powering was used. Each series included omni, cardioid, speech cardioid, hypercardioid, two-pattern and three-pattern models.

Schoeps was an even smaller company then than it is today, and these models weren't produced in large quantities. They were distributed and sold mainly by Telefunken. By 1964-65, low-noise FETs were readily available in production quantities. Schoeps changed over to that circuit technology, which both they and their customers seem to have preferred rather strongly. Early RF condensers (from any manufacturer) weren't as reliable in the field as the ones produced decades later; I do credit Sennheiser with bringing that technology a long way forward.

I was still a teenager then, and just starting to pay attention to recording. The people I learned from were naturally still using tube equipment, and mostly they used dynamic microphones, because condensers were expensive, fragile, and difficult to use outside the confines of a studio. From my (American) standpoint it seemed that some Sennheiser dynamic microphones were considered to have practical value (especially in broadcasting) for their particular sound quality, and by the early 1970s there was one Sennheiser omni--again, a dynamic--that was sometimes used for music. But it was a bit of an alien brand, while AKG and Beyer were the mainstream high-quality dynamic microphones for music.

Similarly the success of Sennheiser's early condenser microphones (the shotguns) was built mainly on their functionality, with sound quality taking a second position to put it quite kindly. Of course the company has enormous resources at its disposal, and eventually, major improvements were made.

--best regards

[DSatz]

To respond to the OP's question: Long-term, humidity as such does no harm to any condenser microphone. There's a difference between humidity (below 100%) and outright condensation, but if we're only talking about water vapor condensing into moisture droplets with no other substances being added, the condensation should do no lasting harm to a condenser microphone, either.

In studio use with close-miked vocals, "humidity" becomes a euphemism for spit and other particles (food, tobacco or other types of smoke, etc.) carried by the breath, which can create an adhesive mixture that cakes up on the diaphragm. That causes problems mainly by creating a leakage path for the DC polarization voltage on the capsule. This then makes the capsule sensitive to actual humidity (the signal may cut out entirely). If a capsule with this kind of gunk on it can't be cleaned, it will eventually have to be replaced. Similarly, using any microphone in dusty surroundings risks harm to the microphone, and that potential for harm is amplified by high humidity, especially if condensation occurs. And ribbon microphones are a whole other story which I won't go into here.

Short-term, though, as John Willett says (although I want to make his remarks more precise), some currently available RF condenser microphones are more reliable than many currently available DC-polarized condenser microphones in settings where high humidity can veer over into outright condensation within the microphone.

On the other hand, some DC-polarized condenser microphones could be improved in this regard, so maybe in the future the difference won't matter as much any more. It's largely a matter of the market for microphones; most customers just want the best sound quality, and traditional (DC-polarized) condensers dominate that field. When people need to record in very high humidity with moisture condensation being a real possibility, there are also microphones available to meet that requirement, which aren't necessarily the ones you'd choose for the best sound quality, although they are gradually improving in that respect, too.

--best regards

[stevetoney]

I once ran a pair of LD mics that liked to randomly click outside when it was humid, like for late night sets.  I never got to the bottom to prove conclusively, but I assumed it was caused by phenomenon John mentioned above.