Topic: Cardioids, Omnis, and Wind Noise.

[javertim]

It has been ages since I've recorded with omnis, so I'm trying to remember just how much less sensitive they are to wind noise than their cardioid counterparts.  I have recently tested my SP-CMC-8s (AT943s) outdoors without windscreens, as well as with mini screens, standard screens, and "premium locking" screens. ... In every one of the results, the recording was overwhelmed with wind noise, even though the wind at hand was little more than a draft (a delicate, light breeze at best).

So, I was wondering -- could please explain to me why cardioids are less wind-resistant than ominis?  Are there any tricks other (other than using windscreens) to avoid the wind noise with my cardioids?  Of course, I will purchase omni caps for my SP-CMC-8s for outdoor use as soon as the wallet permits, but I generally always record with cardioids indoors, and some of the venues I frequent can be a little drafty.  Since super-stealth is my only means of taping, I run the cards under a shirt collar into an elastic loop that points the mics upwards.  They are a little recessed under the collar, but not too much.  I figured the recession would block a little bit of the wind noise, but I guess it doesn't do too much of that.

Thanks, guys!

[Belexes]

So, I was wondering -- could please explain to me why cardioids are less wind-resistant than ominis?

I believe you have that backwards. Omni's are more forgiving for wind noise than Cardioids.  I use Omni's outdoors most of the time.

As for the technical reason, paging Dsatz.

[javertim]

By referring to cardioids as "less wind-resistant," I am trying to imply that omnis are MORE wind resistant, meaning more forgiving of wind.

[Gutbucket]

^^^
Moke beat me to it while I was typing.

The microphone pattern least sensitive to wind is an omni which converts fluctuating air pressure into voltage. 
The microphone pattern most sensitive to wind is a figure-8 which converts fluctuating air velocity into voltage.

All other microphone patterns can be thought of as combinations of those two extremes in various amounts, so generally the more directional a microphone is the more sensitive it will be to wind noise.

If there is a breeze you'll want some kind of windscreen with any pattern.  All wind protection devices work the same way - by trapping a layer of air near the microphone element to keep it from moving turbulently, but still allowing it to vibrate back and forth with the sound you want to record.  To trap that air near the mics you could use your dirty socks, the fabric of a shirt, typical foam windscreens, or basket style windscreens which work particularly well because they effectively trap lot of dead air without much other material for the sound to penetrate - all with various degrees of effectiveness and trade-offs in sound.

[ozarkbilly]

not to be a piggybacker, but I've got a related question:

Is it a general rule that one should use omnis outdoors, as opposed to cards?  I'm going to be taping outdoors for the first time next week, at Summerfest in Columbia MO.  Generally speaking, would omnis be the best option?  I've got the Church CA-11 mics with both sets of caps.

[Gutbucket]

Is it a general rule that one should use omnis outdoors, as opposed to cards? 

Not a general rule.  Omnis can sound very open and natural and have great bass response so they can make great recordings, but they can be tricky to use indoors because they will accurately record all the sound bouncing around in a mediocre sounding room.  It's just oftentimes easier to get a good recording using omnis outdoors than it is indoors because there aren't so many room reflections. That festival would be a great opportunity to try omnis, just be aware that you will not be rejecting the sound of the people around the mics.

told ya someone would explain it better, ^^

Moke's explanation is right on too.  He's coming from the angle of what is actually happening in the capsule, I was talking more about the general principle involved. Ain't neither 'un better.

[Gutbucket]

I think it has to do with the turbulence and eddys in the wind stream behind the mic vs the smoother air flowing around it head on.  I notice the same thing at the beach with my meat mics, I mean ears.  If I turn my head so that one ear is facing the wind there is less wind noise than if I look into the wind and it blows past my head with lots of turbulence generated by these big goofy ears.

[tgakidis]

Isn't it also true that onmi's are more forgiving with stand movement (phasing?) caused by wind?

[DSatz]

People here mostly seem to be getting their facts right as far as I can see. Another major part of the reason is that diaphragm tension is generally quite a bit higher in an omni. The corresponding increase in stiffness also affects a microphone's sensitivity to solid-borne sound.

There are many variables in the equation, but for small-diaphragm, single-diaphragm condenser microphones, the difference between the wind sensitivity of a typical omni and a typical cardioid might be 15 - 20 dB. That could make the difference between an unlistenable recording and a listenable one, but it's not generally enough to make wind noise inaudible. If there's wind, you need some kind of windscreen no matter what pattern of microphones you're using.

It's just that with directional microphones the need is much greater to begin with, and then the design of the windscreen also has to take into account the active sound openings on both sides of the diaphragm. It's much better if the windscreen traps some "still air" around the capsule, as in a basket-type windscreen or the special two-layer foam types that have a gap between the two layers.

--best regards

[fotoralf.be]

WARNING: The following is an awful lot of nonsense I've dished out late at night, having just completed two full days worth of work in one single day. Sic tacuisses...  ;-)


There's a very interesting article on this at the Schoeps website, but unfortunately only in German.

In a nutshell: the distinction isn't so much between omnis and cardiods but between pressure transducers and pressure *gradient* transducers as well as dynamic and condenser capsules. Bear with me. You'll see why in a moment.

Pressure transducers are by their very nature omnidirectional. They only react to the air pressure at the front of their diaphragm. If they're condenser microphones, they are equally sensitive to all frequencies, low and high. Condenser pressure transducers could theoretically go as low as DC if they're subjected to a static pressure. This explains why condenser omnis are generally better at reproducing bass frequencies.

Pressure gradient transducers (and all dynamic microphones), on the other hand, only generate an output voltage while their diaphragm is moving due a pressure difference (gradient) between the front and the rear of their diaphragm. The higher the pressure gradient and the resulting speed (velocity) with which the diaphragm moves, the higher will be the output voltage they generate. At lower frequencies, their diaphragm moves slower and their output voltage drops accordingly. Much like a dynamo, in fact. 

Now, in order to make pressure gradient transducers equally sensitive to all frequencies in the desired operating range, their diaphragm has to be suspended very loosely ("tuned" towards the low end) to allow for more excursion (i.e. velocity) at the low-frequency end and keep their output voltage linear. And that's what makes them more prone to wind and low-frequency handling noises. Practically all directional microphones are pressure gradient transducers.

Am I making myself clear in any comprehensible way? Unfortunately, I haven't been able to find any pictures to illustrate this. But feel free to ask if you have any questions. Oh, and please note that English isn't my mother tongue.

Ralf

 

[Gutbucket]

There are some serious errors there Ralf.  I'm not very qualified to correct them but I'll do my best to point to the problems in what you posted.

In a nutshell: the distinction isn't so much between omnis and cardiods but between pressure transducers and pressure *gradient* transducers as well as dynamic and condenser capsules. Bear with me. You'll see why in a moment.

Pressure transducers are by their very nature omnidirectional. They only react to the air pressure at the front of their diaphragm. 

I don't see how dynamic vs condenser type figures in here at all.  If it does I hope someone can explain why.

Pressure transducers react to pressure on both sides of the diaphragm like any microphone.  The difference is the back of the diaphragm is sealed to the outside world (nearly, see the exception below) and being sealed determines the pressure on that side.

If they're condenser microphones, they are equally sensitive to all frequencies, low and high. Condenser pressure transducers could theoretically go as low as DC if they're subjected to a static pressure. This explains why condenser omnis are generally better at reproducing bass frequencies.

No microphone is equally sensitive to all frequencies.  They all have a limited bandwidth, although some have a flatter response than others.  Dynamic mics are the same as condensers or any other type in this respect.  As I mentioned above and regardless if they are dynamic or condenser based, pressure transducers have a sealed volume on one side.. almost. If the chamber was totally sealed the microphone would be sensitive down to DC and act as a barometer.  You would record a DC offset with atmospheric pressure changes or with changes in altitude. So there is a small hole that equalizes the pressure and that is one factor that effects the lower frequency limit of its frequency response. Same for condensers and dynamics.

Pressure gradient transducers (and all dynamic microphones), on the other hand, only generate an output voltage while their diaphragm is moving due a pressure difference (gradient) between the front and the rear of their diaphragm. The higher the pressure gradient and the resulting speed (velocity) with which the diaphragm moves, the higher will be the output voltage they generate. At lower frequencies, their diaphragm moves slower and their output voltage drops accordingly. Much like a dynamo, in fact. 

Lots of stuff getting mixed up in there.  All mics only generate output voltage when their diaphragms vibrate.  All of them vibrate because of a pressure difference between the opposite sides of the diaphragm, but with pressure omnis, only one side is exposed to the atmosphere.  The higher the pressure gradient between sides, the farther the diaphragm moves back and forth, producing a larger output voltage swing which means a stronger output signal.  The speed at which it vibrates corresponds only to the frequency of the sound exciting it.  The output voltage doesn't drop if the diaphragm moves slower, the frequency drops. The voltage produced only drops if it doesn't move as far, as when it is exposed to a lover SPL sound.

Now, in order to make pressure gradient transducers equally sensitive to all frequencies in the desired operating range, their diaphragm has to be suspended very loosely ("tuned" towards the low end) to allow for more excursion (i.e. velocity) at the low-frequency end and keep their output voltage linear. And that's what makes them more prone to wind and low-frequency handling noises.

This is true and a point that DSatz makes in his previous post.

Practically all directional microphones are pressure gradient transducers.

Well practically all directional microphones are a combination of both pressure and pressure-gradient principles.
The only microphone pattern that uses the pressure-gradient principle exclusively is a single diaphragm type figure-8.

I'm not trying to be overly critical of your post, Ralf.  But I do want to correct those fundamental an misleading errors.  Again, I'm not an expert, I only fake it.

 

[fotoralf.be]

There are some serious errors there Ralf.

Point well taken. I should have known better than to indulge in such an exercise in a language over which I have only limited command, especially late at night after one hell of a working day. You are of course quite right. Is there any way to delete this whole nonsense?

Ralf

[fotoralf.be]

again - welcome aboard.

Thank you. I'll try and come up with some more reasonable stuff next.

Ralf

[Gutbucket]

Your command of English is fine, don't let that keep you from posting.  I hope I didn't come across negatively trying to get the facts straight.  Personally I often learn the most when I have to think a reply like that through, so I owe you thanks for getting my brain working!  A bit unthoughtful of me to not say hello first, though. 

Welcome to this fascinating place. +T

[John Willett]

I'll let DSatz answer this all in detail as he seems to have a way of putting things that is very good.

But - to put it simply - a pressure microphone is always omni and reacts to changes in pressure.  It does not matter where in the room the pressure change comes from as the mic. is sealed (like a baked beans tin with cling-film across the top).  There will be a tiny hole in the diaphragm or the capsule to equalise atmospheric pressure, but too tiny to affect any audio frequencies.

A pressure-gradient microphone is a figure-8 which reacts to differences in pressure and has the diaphragm open on both sides (like the beans tin above with the back of the tin sawn off).

A cardioid is a combination of the two above - wire an omni and fig-8 in parallel and the resulting polar-pattern will be cardioid (which is how the very first cardioids were made) - nowadays they make holes in the back of the beans tin and silk damping (or similar - just to keep it simple).



From the above - a super-cardioid is like an omni and two fog-8s and a wide cardioid is like two omnis and a single fig-8.

This helps explain why prosimity effect is most pronounced in a fig-8, less in a super-cardioid, less again in a cardioid and so on (and none at all in an omni).

I hope this helps.