Topic: Omni's In the Wind

[stevetoney]

I know this subject line sounds like the title of a taperssection gearslut song  , but it's actually the basis for a technical question. 

I was taping outside yesterday and decided to put my selectable MK5 capsules in the omni position.  There was a breeze, but nothing significant, so I left the windscreens off.  That got me wondering why omni's aren't as affected by wind-noise as non-omni's.  I know the simple answer is that omni's are pressure transducer mic capsules, while the other patters are pressure gradient, but my question is more related to the actual technical details of why wind creates noise in one type but not the other.  Just how does a pressure transducer work differently than a pressure gradient?  (Perhaps I've asked two questions here!)

Thanks!

[audBall]

I think it is due to the fact that pressure-gradient microphones (i.e. cardioids) have both the front and back of the diaphragm exposed to sound pressures.  Whereas, a pressure transducer (omni) is sealed and only the front of the diaphragm is exposed. 

[SmokinJoe]

I was under the impression the reason is that omni mic diaphragms were generally tensioned much tighter than cardioid mics, and so the tighter diaphragms don't flap in the breeze as much.  This would mean that the omni advantage only applies to "real" omni caps (everything from cheap panasonic omni mics, to CK62's and MK2's) but not to switchable mics like my AKG414's which have dual diaphragms and control the pattern by voltage.  I'm not sure where MK5's fit in there... they may be the exception to the rule.

[page]

I was under the impression the reason is that omni mic diaphragms were generally tensioned much tighter than cardioid mics, and so the tighter diaphragms don't flap in the breeze as much.  This would mean that the omni advantage only applies to "real" omni caps (everything from cheap panasonic omni mics, to CK62's and MK2's) but not to switchable mics like my AKG414's which have dual diaphragms and control the pattern by voltage.  I'm not sure where MK5's fit in there... they may be the exception to the rule.

I'll cast my vote with this explanation. It's what I've heard as well.

Knowledge by democracy. 

[shayne]

Not to hijack the thread but is there a best pattern for heavy wind with a mic like the lsd2 that has the switchable patterns? I taped on the beach last week and went with a board patch because the wind was so strong.

[aaronji]

I'll cast my vote with this explanation. It's what I've heard as well.

Knowledge by democracy. 

Well, DPA casts the same vote...

Directional microphones need to have a much softer diaphragm than an omni. This softness results in handling, pop & wind noise which puts a limit to how close you can get to a vocalist, even when using pop-filters.

[page]

I'll cast my vote with this explanation. It's what I've heard as well.

Knowledge by democracy. 

Well, DPA casts the same vote...

Directional microphones need to have a much softer diaphragm than an omni. This softness results in handling, pop & wind noise which puts a limit to how close you can get to a vocalist, even when using pop-filters.

yay, I love being a winner. 

Not to hijack the thread but is there a best pattern for heavy wind with a mic like the lsd2 that has the switchable patterns? I taped on the beach last week and went with a board patch because the wind was so strong.

the LSD2 has three methods of operation via 3 patterns:
XY (Card/Omni),
Midside (Card/Omni,
Blumlein (fig8s)
AB w/ about 4cm of spacing (all 3).

Two problems happen in wind; wind noise and phasing of everything else. So I'd get a jumbo dead rat, and run the omni pattern. At that point, you have two choices, run it vertical as normal, or horizonal (and perpendicular to the stage). Vertical will do a better job at killing the phase issue at the expense of stereo seperation (you can run the caps at like 60 degrees and get some HF seperation/swirl but thats about it), or horizonal in AB and accept whatever phasing you get that the omni pattern doesn't help with. Just make sure to properly weight the XLR end so the cap end sticks straight out instead of suffering from ED (depending on shockmount design, this may not be an issue or concern).

[DSatz]

The explanation that includes the (generally) higher diaphragm tension of pressure transducers is the right answer in most instances. But the Schoeps MK 5 is a special case because its pattern-switching mechanism doesn't affect the diaphragm tension at all.

The backplate of this type of microphone has tiny holes in it, and there's another chamber arranged behind the backplate. That chamber is sealed off when the capsule is set to omni--while when it's set to cardioid, an acoustical "delay line" (friction pathway) connects that chamber to the sound inlets (slits) on the side of the capsule.

Upshot is, wind tends to create large amounts of force on one side of the diaphragm that aren't correlated in any way with what's happening on the other side. Since the diaphragm motion responds to the difference in force on the two sides, there can be large excursions due to wind or breath noise.

Does that explanation make sense to people? Keep in mind that any cardioid is a mixture of 50% pressure response and 50% pressure gradient response. So a pattern such as wide cardioid is generally less sensitive to wind than cardioid, but more sensitive than omni--while super/hypercardioids are more sensitive to wind (and mechanical shock) than cardioids, and figure-8s are the most sensitive of all.

--best regards

P.S.: Someone asked about the LSD2, which is a coincident stereo microphone with dual-diaphragm capsules. Schoeps' capsules are all single-diaphragm, and the switchable-pattern capsules use mechanical (acoustical) pattern switching. With dual-diaphragm capsules you have essentially two back-to-back cardioids, and the patterns are switched by setting the polarization voltage on each half of the capsule, which controls its sensitivity. In the omni setting of a capsule like that, you still have a lot of wind sensitivity because both the front and back halves of the capsule are busy behaving like typical cardioids, with their outputs summed electrically. So that type of microphone isn't really functioning as a pressure transducer, and generally will need a bigger, more effective windscreen for outdoor use even when its capsules are set to omni than a Schoeps (or other single-diaphragm) microphone would need in its omni setting.

[stevetoney]

DSatz...I'm a professional engineer that works in the nuclear power industry (if you have any questions about this subject, go ahead and ask I'll try to reciprocate).  I'm not sure if others appreciate knowing how audio gear functions, but I really do.  To be honest, when I started this thread and posed the question, I was hoping you'd respond and discuss how the switchable patterns differ from conventional capsules...so I thank you for doing so in the previous response.  As usual, thank you very much for sharing your extensive knowledge.

[612]

Thank you for the thread. I am wondering if the AKG 414's are single-diaphragm microphones? Next week I have to record in Loring Park on the edge of downtown Minneapolis. I've talked with a friend about getting a loaner pair of dead rats so I'm excited about that. I'm thinking I should just run them on the omni setting?

Here's another "interesting" fact I'm considering. This band has very little vocal activity. What is heard is more of an instrument than a vocal singing. The two vocalists run their mics through synthesizers so the vocals can be "bent" and distorted. It's a great time! Maybe I should shoot for cards or wide cards on stage or stage lip? Here's a couple samples of said band indoors;

07/28/10 - 414 cards
07/21/10 - 414 wide cards

[yug du nord]

If the vocals are coming through the PA system, staglip/on stage could make the vocals thin on the recording.  Unless you can pull the vocals from the vocal monitors....  if they have em......  but a good rule of thumb.....  if vocals are being run through a synth or something, stagelip probably won't capture em very good.

[sparkey]

So are sub-cards any less susceptible to wind noise than cards?

[audBall]

Keep in mind that any cardioid is a mixture of 50% pressure response and 50% pressure gradient response. So a pattern such as wide cardioid is generally less sensitive to wind than cardioid, but more sensitive than omni--while super/hypercardioids are more sensitive to wind (and mechanical shock) than cardioids, and figure-8s are the most sensitive of all.

[sparkey]

Keep in mind that any cardioid is a mixture of 50% pressure response and 50% pressure gradient response. So a pattern such as wide cardioid is generally less sensitive to wind than cardioid, but more sensitive than omni--while super/hypercardioids are more sensitive to wind (and mechanical shock) than cardioids, and figure-8s are the most sensitive of all.

[slapping self in forehead]

Thanks

[SmokinJoe]

So are sub-cards any less susceptible to wind noise than cards?

I think that's true if they are "made" as subcard caps... examples are AT853 subcards, Peluso CEMC6 MK21, Schoeps MK21... these are all  single diaphragm fixed subcard caps.  If you achieve it using a multipattern selector on an LD (like my 414's or ADK-TLs), I think that answer is NO.  In essence those LDs have 2 Cardioid diaphragms inside each mic pointing front and rear and then dialing in a pattern by adjusting how those signals are added together.  So in this case, it's it's susceptibility to wind noise is a constant, regardless of what you set the patterns to.

I ought to perform another living room wind tunnel test to confirm, but I doubt I'll ever get around to it.