Topic: Cardoid vs Omni

[dominar]

Can someone explain or point me to a description, in laymans terms the benefits/differences of cardoid vs omni mics?

In the simplest terms possible.  I've read alot of info on this site and quite honestly alot of it is over my head!

Thanks!

[Todd R]

Omni mics record sounds coming from all directions equally, cardioids record mainly sound coming from the front and reject sound from the rear.  So cardioids are better suited (speaking in very, very broad terms) of recording the music coming from in front of you and rejecting the crowd noise of people behind you.

With their rear-rejection, cardioids can easily be used in tightly-placed pairs to record in stereo and re-create a stereo soundfield.  2 Omni mics since the record equally from all directions, will need to be separated (say a meter or more) or baffled in order to record in stereo.

This all is highly simplified, but gives the gist of it.

Excellent information is available over at the microphone university at www.dpamicrophones.com

[dominar]

So, what's the point of stealth omnis?  Wouldn't they be too close together while recording a concert?

What is the prefered type of mic for stealth recordings?

I thought I read it was omni because it gave fuller sound with more room information, whereas cardoids were flatter.

[dorrcoq]

So, what's the point of stealth omnis?  Wouldn't they be too close together while recording a concert?

I think your head acts as the baffle.

[Gutbucket]

So, what's the point of stealth omnis?  Wouldn't they be too close together while recording a concert?

2 Omni mics since the record equally from all directions, will need to be separated (say a meter or more) or baffled in order to record in stereo.

In that case your head or body acts as a baffle.  You can make good recordings with closer spacings without a baffle too, but more space or a baffle is optimal.

What is the prefered type of mic for stealth recordings?

I thought I read it was omni because it gave fuller sound with more room information, whereas cardoids were flatter.

Because they pick up sound equally from all directions (unless you mount them on or near a baffle ), omnidirectional mics do record more room information.  That's great if the room information sounds good.. maybe not so great if the room sounds boomy or there are lots of people blabbering behind you.  Omni mics are easier to arrange properly since it doesn't matter as much if they are pointed just right, whereas it's important to point cardioid pattern mics properly.

When you say "cardioids are flatter" you are probably referring to the frequency response of the microphone which determines how sensitive the mic is to each range of sound from the lowest pitches to the highest treble sounds.  All mics are different, but as a very gross generalization, omni mics are usually flatter in frequency response, picking up all frequencies evenly.  Cardiod mics can be and are often very flat too for sounds coming at them directly from the front, but are less flat for sounds arriving from the sides.

However, a perfectly flat frequency response isn't always desired.  Many microphones are well regarded because they either impart a pleasant coloration to the sound or are less sensitive to 'problem sounds' like too much bass.

This all is highly simplified, but gives the gist of it.

[dominar]

So, if you're going to an event at a new location how do you know what you should bring?  I'm guessing omnis would be best for a beginner (like myself) since the pointing is not as critical as with cardoids.

[Gutbucket]

Very hard to say.  You learn your rig, you learn how different recording situations usually work, you determine if its a loud raucous bar or a quiet respectful audience, you ask questions here... figuring that out is half the fun.

You'll have to record a few things to figure out how to run your rig before you know how to use it well, so don't worry about throwing out your first few attempts.

[BC]

Very hard to say.  You learn your rig, you learn how different recording situations usually work, you determine if its a loud raucous bar or a quiet respectful audience, you ask questions here... figuring that out is half the fun.

You'll have to record a few things to figure out how to run your rig before you know how to use it well, so don't worry about throwing out your first few attempts.

agree here. I prefer cardioids, even with a quiet audience I like to try to reject ambient noise like coughing, page turning, etc.

[DSatz]

dominar, a cardioid microphone discriminates somewhat in its pickup of sound, since in the back of the microphone it has almost no response at all (in fancier terms it has a "null" in its response at 180 degrees). Any sound that arrives from the front or sides of the microphone will be picked up, though.

An omnidirectional microphone is similar except that it also picks up sound from the back of the microphone, in addition to the front and sides. Cardioid and omnidirectional aren't opposites, in other words; "directional" and "non-directional" would be opposites, but those terms aren't quite the same as what you asked about.

The directionality of a cardioid isn't nearly as sharp as many people seem to imagine. It isn't anything like a flashlight beam, for example; no conventional microphone has that narrow a pickup pattern. Even sound that arrives from 90 degrees off axis (all the way to one side or other, and/or above or beneath the mike if it is horizontal) is reduced only barely enough to notice. When people here talk about the use of cardioids to reduce audience noise, it is because they are recording clear enough stereo that the listener's brain can filter out some of the unwanted sound in playback. Beyond enabling stereo pickup to occur, though, cardioid microphones themselves don't really do much "filtering out" like that. Cardioid is still a big, round 3-D blob of a pattern.

In practice there are some things that go along with a microphone's being directional or not. Directional microphones are sensitive to wind, breath noise (popping on consonants with closely miked singing or speaking voices, for example) and handling noise or physical vibrations; omnidirectional microphones, if they are of a type known as "pressure transducers," are nearly immune to these problems. And directional microphones emphasize the bass and lower midrange of most sound sources that are in close proximity to them while pressure transducers don't have any such "proximity effect."

Just speaking of condenser microphones, since they're the type most commonly used for high-quality recording and broadcasting, directional microphones have a low-frequency rolloff that generally starts somewhere between about 50 and 100 Hz, while pressure transducers (remember, all pressure transducers are omnidirectional but not all omnidirectional microphones are pressure transducers) can have full sensitivity down to the lower limits of human hearing and even beyond.

Finally, for reasons of physics, "omnidirectional" microphones of normal size aren't omnidirectional at high frequencies, so you still have to take care which way you aim them--strange though that may seem at first.

--best regards

[Gutbucket]

^^^^
Well put and concise as usual.

Can someone explain or point me to a description, in laymans terms the benefits/differences of cardioid vs omni mics?
In the simplest terms possible...

[George]

So, if you're going to an event at a new location how do you know what you should bring?  I'm guessing omnis would be best for a beginner (like myself) since the pointing is not as critical as with cardoids.

Depends on where your seated or standing.  If your close up to the stage/pa, go with omni's...farther away (lets say 20 rows back or more, go with cards).  In my experience, omni's sound hollow and distant when taping from a great distance away from the stage.  If you plan on attending concerts where you can obtain good seats, go with omni's then, but I find cards to be more versatile.

[Gutbucket]

So, if you're going to an event at a new location how do you know what you should bring?  I'm guessing omnis would be best for a beginner (like myself) since the pointing is not as critical as with cardoids.

Depends on where your seated or standing.  If your close up to the stage/pa, go with omni's...farther away (lets say 20 rows back or more, go with cards).  In my experience, omni's sound hollow and distant when taping from a great distance away from the stage.  If you plan on attending concerts where you can obtain good seats, go with omni's then, but I find cards to be more versatile.

Regardless of mics, try to get in the best sounding spot in the room you can.  Generally closer is better, though if there are vocals over a PA, or if there is almost no stage volume and everything comes over a PA it is possible to get too close. Ironically, I was listening just last night to a recording I made of some acapella vocal harmonies made from the board position 4 rows from the very back of a theater with omni's that I figured would be way too far back at the time and it turned out very nice and foreward sounding.  Remember that these usage guidelines are just that, guidelines and you'll find exceptions to all these generalizations, whereas the info DSatz posted on the working principle of cardioid/omni mic types is factual info that describes how the mics function.

[Dede2002]

dominar, a cardioid microphone discriminates somewhat in its pickup of sound, since in the back of the microphone it has almost no response at all (in fancier terms it has a "null" in its response at 180 degrees). Any sound that arrives from the front or sides of the microphone will be picked up, though.

An omnidirectional microphone is similar except that it also picks up sound from the back of the microphone, in addition to the front and sides. Cardioid and omnidirectional aren't opposites, in other words; "directional" and "non-directional" would be opposites, but those terms aren't quite the same as what you asked about.

The directionality of a cardioid isn't nearly as sharp as many people seem to imagine. It isn't anything like a flashlight beam, for example; no conventional microphone has that narrow a pickup pattern. Even sound that arrives from 90 degrees off axis (all the way to one side or other, and/or above or beneath the mike if it is horizontal) is reduced only barely enough to notice. When people here talk about the use of cardioids to reduce audience noise, it is because they are recording clear enough stereo that the listener's brain can filter out some of the unwanted sound in playback. Beyond enabling stereo pickup to occur, though, cardioid microphones themselves don't really do much "filtering out" like that. Cardioid is still a big, round 3-D blob of a pattern.

In practice there are some things that go along with a microphone's being directional or not. Directional microphones are sensitive to wind, breath noise (popping on consonants with closely miked singing or speaking voices, for example) and handling noise or physical vibrations; omnidirectional microphones, if they are of a type known as "pressure transducers," are nearly immune to these problems. And directional microphones emphasize the bass and lower midrange of most sound sources that are in close proximity to them while pressure transducers don't have any such "proximity effect."

Just speaking of condenser microphones, since they're the type most commonly used for high-quality recording and broadcasting, directional microphones have a low-frequency rolloff that generally starts somewhere between about 50 and 100 Hz, while pressure transducers (remember, all pressure transducers are omnidirectional but not all omnidirectional microphones are pressure transducers) can have full sensitivity down to the lower limits of human hearing and even beyond.

Finally, for reasons of physics, "omnidirectional" microphones of normal size aren't omnidirectional at high frequencies, so you still have to take care which way you aim them--strange though that may seem at first.

--best regards

I've been following this thread since day one.
Lots of great tips and usefull information. Thanks for that. 

[George]

So, if you're going to an event at a new location how do you know what you should bring?  I'm guessing omnis would be best for a beginner (like myself) since the pointing is not as critical as with cardoids.

Depends on where your seated or standing.  If your close up to the stage/pa, go with omni's...farther away (lets say 20 rows back or more, go with cards).  In my experience, omni's sound hollow and distant when taping from a great distance away from the stage.  If you plan on attending concerts where you can obtain good seats, go with omni's then, but I find cards to be more versatile.

Regardless of mics, try to get in the best sounding spot in the room you can.  Generally closer is better, though if there are vocals over a PA, or if there is almost no stage volume and everything comes over a PA it is possible to get too close. Ironically, I was listening just last night to a recording I made of some acapella vocal harmonies made from the board position 4 rows from the very back of a theater with omni's that I figured would be way too far back at the time and it turned out very nice and foreward sounding.  Remember that these usage guidelines are just that, guidelines and you'll find exceptions to all these generalizations, whereas the info DSatz posted on the working principle of cardioid/omni mic types is factual info that describes how the mics function.

Yep, its definitely a generalization, but I seriously consider location and the size of the venue when selecting one of my mics to use.  It's a risk we take everytime though because you don't know exactly what will work best until after you hear your recording...I've had some duds with my dpa's and then some killer recordings with my dpa's. 

[dean]

dominar, a cardioid microphone discriminates somewhat in its pickup of sound, since in the back of the microphone it has almost no response at all (in fancier terms it has a "null" in its response at 180 degrees). Any sound that arrives from the front or sides of the microphone will be picked up, though.

An omnidirectional microphone is similar except that it also picks up sound from the back of the microphone, in addition to the front and sides. Cardioid and omnidirectional aren't opposites, in other words; "directional" and "non-directional" would be opposites, but those terms aren't quite the same as what you asked about.

The directionality of a cardioid isn't nearly as sharp as many people seem to imagine. It isn't anything like a flashlight beam, for example; no conventional microphone has that narrow a pickup pattern. Even sound that arrives from 90 degrees off axis (all the way to one side or other, and/or above or beneath the mike if it is horizontal) is reduced only barely enough to notice. When people here talk about the use of cardioids to reduce audience noise, it is because they are recording clear enough stereo that the listener's brain can filter out some of the unwanted sound in playback. Beyond enabling stereo pickup to occur, though, cardioid microphones themselves don't really do much "filtering out" like that. Cardioid is still a big, round 3-D blob of a pattern.

In practice there are some things that go along with a microphone's being directional or not. Directional microphones are sensitive to wind, breath noise (popping on consonants with closely miked singing or speaking voices, for example) and handling noise or physical vibrations; omnidirectional microphones, if they are of a type known as "pressure transducers," are nearly immune to these problems. And directional microphones emphasize the bass and lower midrange of most sound sources that are in close proximity to them while pressure transducers don't have any such "proximity effect."

Just speaking of condenser microphones, since they're the type most commonly used for high-quality recording and broadcasting, directional microphones have a low-frequency rolloff that generally starts somewhere between about 50 and 100 Hz, while pressure transducers (remember, all pressure transducers are omnidirectional but not all omnidirectional microphones are pressure transducers) can have full sensitivity down to the lower limits of human hearing and even beyond.

Finally, for reasons of physics, "omnidirectional" microphones of normal size aren't omnidirectional at high frequencies, so you still have to take care which way you aim them--strange though that may seem at first.

--best regards

Well done - thanks!

Can you expand on the "directionality" of omnis at high frequency?  Or perhaps point to a resource written in as close to layman terms as possible?  We have a raging debate amongst the MN Tapers on this exact subject.

[dominar]

Thanks to everyone for the help!!  There is alot of info to digest here.  For now I only have omnis so that will have to do.  If I get cards they'll be from Church or Sound Professionals.  I won't be dealing with Giant Squid anymore.

[DSatz]

deanlambrecht, sound at high frequencies is made up of shorter waves, and shorter waves are more severely affected by obstacles of their own size that are in their path (absorption, reflection or diffraction being the main career choices for a sound wave at that point).

Longer waves can more or less ooze around smaller obstacles, so they're correspondingly less affected. Waves that are considerably longer (= lower in frequency) than the size of an obstacle are almost completely unaffected by it. But when an obstacle in the sound field is half the wavelength of a sound wave or larger, it can significantly disturb the sound field at that frequency and above.

Take a look, for example, at the frequency response of a Neumann KM 183 on axis (there's a very nice interactive display on Neumann's Web site), and do the math, or I'll do it for you in a moment--you'll see that the big response peak centers just above where the diameter of the microphone equals half a sound wavelength. 1100 ft./sec. = 33528 cm./sec.; 33528/2.2 cm. (the diameter of the microphone) = ca. 15.2 kHz for a full wavelength, and ca. 7.6 kHz for half a wavelength. Since the designers at Neumann craftily decided to place the diaphragm inside the microphone, logically enough it has a diameter somewhat smaller than the microphone casing. So the peak is centered a little higher than 7.6 kHz, but not by much.

Now, please notice that the on-axis response increase is quite narrow in terms of angle. Look at the KM 183's 8 kHz polar diagram. (No, really, look at it--I'll wait here.) The response at 90 degrees or at 180 degrees doesn't peak around 8 kHz like the 0-degree response does. Now compare this to the response of their KM 131 microphone, which is free-field equalized. It has identical polar response to the KM 183 (or its counterpart in the modular series, the KM 130), but the frequency response on axis is flat at high frequencies. As a result, you know that the high frequency response at 90 degrees or 180 degrees must roll off to some extent.

In terms of Schoeps microphones, the MK 3 capsule (which hardly anyone ever uses any more) is somewhat similar to the Neumann KM 183 or KM 130, while the MK 2 is somewhat similar to the KM 131 (or perhaps the reverse should be said, since Schoeps developed and introduced both of their capsule designs first). The MK 2S and MK 2H capsules, which are so popular, are in between. That's because most people don't usually place omnidirectional microphones either purely in the direct (free) sound field or purely in the reverberant (diffuse) sound field for stereo recording.

Does this help?

--best regards

[Gutbucket]

Does this help?

Certainly!

Sounds to me that response peak at certain frequencies on-axis due to resonant peaks corresponding to the diameter of the capsule is closely related to the boundary layer effect when scaled up to larger surfaces.  At least that is what I have always assumed.  Would that be correct or am I totally off base?

So the resonant on-axis peak of a very small capsule omni (like say a DPA 4060 at less than 1/4" diameter) would be... calculating and assuming .25" diameter..  52.8khz for a full wavelength  26.4khz for a half wavelength.  Basically insignificant, and the small amount of high frequency roll-off far off axis on that mic would be due solely to reduced diffraction around the front of the mic by very short wavelengths from the rear. Yes?

[DSatz]

Gutbucket, just to be sure that we're clear, the on-axis rise of the diffuse-field capsules (the ones with the on-axis peaks, e.g. Neumann AK 30 or Schoeps MK 3) is purely due to the pressure buildup in the air in front of the diaphragm, and has essentially nothing to do with the capsule's own mechanical resonance, which occurs over an octave higher than the peak shown in the response graphs.

As long as it's clear that we aren't talking about capsule resonance, though, you're exactly right: the pressure increase at a rigid boundary of sufficient size is what makes "PZMs" or "BLMs" (or whatever you want to call them) work. A boundary layer microphone has to be mounted on a sufficiently large surface so that the transition frequency is below the range of interest, while an omni microphone should be as small as possible so that the transition frequency would (ideally) be above the audible range--except that if you really make the microphone anywhere near that small, it will be intolerably noisy.

That's why most people put up with "omni" microphones that are directional at high frequencies. At least there are no fundamentals in the range where the peak might occur. And microphones with somewhat reduced off-axis high-frequency response are still a lot better than microphones that have peaks in their off-axis high-frequency response.

--best regards

[Gutbucket]

Understood.

I wasn't thinking of the mechanical resonance of the diaphragm in this case, but thank you for pointing out that potential point of confusion which I would likely have stumbled into (and still may).

I can also see how this principle applies to what's happening with ball shaped devices surrounding the mic capsule, in for example, Neumann's M-150 mic and add on attachments such as their SBK130 "sound diffraction sphere" and DPA's various "acoustic pressure equalizer" ball attachments that make omni pressure capsules more directional than they would be alone.  That's perhaps more directly relevant to this discussion than PZM/BLM mic's utilization of the principle.

Thanks again for explaining this so clearly.
Regards

[DSatz]

Gutbucket, right, those accessory spheres for pressure transducers (or the built-in sphere of a Neumann M 50, its successors and/or imitators) work along the lines we're talking about--except that the spherical shape makes the "disturbance" (as a function of frequency) much more gradual, and the effect more subtle.

In their 0-degree response, microphones of this type show a rise in the "presence" frequency region (ca. 2.5 kHz and above), while off-axis, something of the opposite occurs. Schoeps has a graph on their Web site (http://www.schoeps.de/E-2004/specs-ka.html) showing the effect of a 40 mm sphere on their flat-response (on axis) MK 2 capsule. It's the combination of this added directionality and the altered frequency response that does the trick.

In practice you wouldn't use a sphere attachment on a free-field omni capsule, but the Schoeps graph is the easiest way to see what a sphere does to the 0-degree frequency response. These microphones are characteristically diffuse-field equalized regardless of the sphere--the original M 50 had a very pronounced on-axis rise in its high frequency response overall. Three different capsule types (K 50, K 53, K 63) were used over the years, so the result depends to some extent on that, of course.

The two Neumann graphs attached below are from the "middle period" of the M 50, while the K 53 capsule was in use; as far as I can tell, this is the type which most connoisseurs seem to favor. Note the big rise centered around 9 - 10 kHz; that's the capsule itself talking. The elevation before that--from about 2.5 kHz on up--is due to the sphere.

--best regards

[dean]

First off, Gutbucket and DSatz, I'm learning an awful lot from you both. I really appreciate it.  Technically I'm in quite a bit over my head, but I'm doing my best to understand what I can.  I'm going to enroll in some sound classes in the fall (I can go for free because my wife is a professor in the university system here!), where I hope I'll be able to make much more sense of this data.  In the meantime I'm going to pick up some books by John Eargle to see if I can start to make sense of it a bit sooner.

In any event, +T to you both.  This is extremely helpful for me.

I think I understand how I'd get falloff at the 90 & 180 axes, so, a I have a practical question in 2 parts (& feel free not to answer).  I have the MBHO 603a series and love to run the corresponding omni capsules. 

1.  I very frequently run Healy Method omni method, both to capture a PA in good circumstances, as well as on stage when I have limited space.  Clearly in these situations my 0 axis is perpendicular to the sound source.  What's the impact of directionality in this case?  I've been very pleased with my results.

2.  Additionally, I also frequently run split omni's on stage, with my 0 axes typically aligned off either side of the drum kit, in order to gather as much sound as possible from the non drum instruments.  What's the impact of HF directionailty in this case?  Again, I've been very pleased with my results.

Thanks in advance,
Dean

[Gutbucket]

I've taken a look at the pressure transducer sphere attachment response graphs at the Schoeps, Neumann & DPA websites. I can see the correlation of increasing sphere size (and the corresponding larger surface area around the capsule) to increasingly lower frequencies where the pressure increase and resulting boost start to take place in all the response graphs. Yet, there is one thing that I do not understand from looking at the graphs. Why does the "pressure build-up effect" appear to diminish on-axis in the highest frequency regions? Those frequencies have even shorter wavelengths compared to the size of the sphere surface. This seems to be the case for all instances regardless of sphere size.

Here are all the graphs:

Neumann's is flash based so I can't directly link the photo, but the sphere response graph and the naked KM 130 response is there.

Schoeps-

62U (CCM62 = CMC6+CCM2 ?)


62U with 40mm sphere:


62U with 50mm sphere:


DPA-

4006 with no sphere (free field response grid fitted):


4006 with no sphere (diffuse response grid fitted):


4006 with 30mm sphere:


4006 with 40mm sphere:


4006 with 50mm sphere:


The response curves without the spheres indicate either a flat or elevated on-axis high frequency response up 20khz or so.. Yet all the graphs showing the on-axis boost provided by the spheres, regardless of diameter, indicate that boost diminishing at higher frequencies well before any high frequency roll-off inherent to the capsule design.

Dean, I just saw your post above as I was previewing mine & I'll defer to the more experienced rather than attempt a 'therory as I understand it' based answer - I've never run a Healy config although I am familiar with the setup.

[DSatz]

Gutbucket, I'm not an acoustician and there are aspects of this situation that I'd like to understand more clearly, too. Give me some time and I can ask some people, if no one else here fills in the blanks in the meantime.

Meanwhile my best guess as to the likely answer to your question ("Why does the 'pressure build-up effect' appear to diminish on-axis in the highest frequency regions?") is that the greatest degree of reinforcement occurs when the size of the obstacle is exactly one-half wavelength of the incoming sound (i.e. the waves "coming" and "going" are in phase), with correspondingly less reinforcement on either side of that frequency--like a peaking filter with Q = 1. However, as I said, that's only a guess; it could be grotesquely mistaken.

deanlambrecht, this is the first I've ever heard of a Healy method, so I need to do some homework before commenting on that. But it sounds to me as if "it ain't broke, so don't fix it" may apply to your situation, and if so, that's what I like to hear--I'm not into urging other people to be dissatisfied all the time. (The ones most in need of improvement wouldn't listen anyway.)

The frequency range in which the orientation of the mike matters most with normal, small cylindrical omnis is in the region above 6 - 8 kHz, so it's too high up to be "presence"--it's somewhere between where "brightness" lives, on the one hand, and "sheen" or "sparkle" or "air" on the other. So that's definitely a "season to taste" situation rather than a "follow my rulebook" one as far as I'm concerned.

--best regards

P.S.: Two notes on the Schoeps graphs that Gutbucket posted above: Again, the CMC 62 is a free-field omni, and wouldn't normally be used with spheres. Distant miking is what these sphere thingies are all about. The design is from the mono era, and in those days, such microphones were placed (one per recording!) at distances greater than almost anyone today would ever think of using. In terms of Schoeps capsules that approach calls for the MK 2H, the MK 2S or even the MK 3. But the MK 2 is shown precisely because it has very flat response on axis, making it easy to see the sphere's effect by contrast.

The second thing is that it's my understanding that the KA 50 sphere will be phased--no pun unintended--out shortly if it hasn't been already. The sphere size originally specified by the NWDR, when they put forward the design that was ultimately implemented as the Neumann M 50, was 40 mm. I have pairs in both sizes (for no particularly good reason) and agree that it's not much of a loss; as you can see, there's hardly any difference. Sound waves flow around a smooth, spherical shape more readily than they do around a shape with sharp corners and flat, "confronting" surfaces. A sphere this small is more of a distraction than an obstruction.

[stirinthesauce]

Healy method is more commonly known (at least outside taperland) as and A-B pair, 17cm, 180degrees

http://members.aol.com/mihartkopf/lexicon.htm

Again, thanks for all your input, DSatz and Gutbucket.

[dean]

More on healy method:

From the Oade's, though I prefer to space @ 17 cm whenever I have enough room:
http://oade.com/Tapers_Section/streicher-healy.html

Here's a ts.com discussion where Moke come down firmly on the side of directionality:
http://www.taperssection.com/index.php?board=3%3baction=display%3bthreadid=3536%3bstart=msg30936#msg30936

Here's another good Moke description with diagram:

The u89's aren't what you want to use for this technique, or the TL's for that matter. Anything w/ dual diaphragms won't cut it.
The technique calls for a pair of single diaphragm omni's to be used, with each mic aimed 90* off axis (away from the soundstage). 180* opposite to each other.

_____________________ stage



        o----  ----o  -> capsule facing this direction, opposite direction for other mic

----o = mic body and cap.

And some pics of my healy config at 10klf last summer (That's me in cowboy hat in the second shot)

Matching dead rats (Dean's MBHO's in Healy omni config, and my Superluxes)



deanlambrecht

[Gutbucket]

DSatz, your guess sounds like a reasonable explanation to me, thanks again for helping me think this through.

The 'Healy method' as other have mentioned is two near-coincident omnis angled 180 degrees apart (back to back) and was a technique developed by live sound engineer Dan Healy working with the Greatful Dead. I believe he used it on-stage, possibly for feeding stage ambiance to in-ear monitor's but I may be wrong on the original application.

There is something else that struck me while looking at these response graphs - not concerning the mics themselves but the way the data is presented.  In the polar response plots that typically accompany the frequency response graphs of any manufacturers' microphone including the sphere attachments graphs posted above, none of the polar plots indicate any change of level on-axis at any frequency.  In each polar plot, all of the overlayed frequency plots line up perfectly at 0-degrees, which I would read as indicating a perfectly flat on-axis response.  I suppose each overlayed frequency plot is normalized on-axis to 0db.  In that case the polar plots reveal directional response information for each individual frequency measured, but you cannot easily make meaningful polar response level comparisons between different frequency lines on the same plot.  In other words, if a cardioid pattern mic has a high frequency response peak and a low frequency roll-off when measured on-axis and some unwitting soul tries to compare the mic's rearward rejection at various frequencies using the polar plot, they are bound to be lead to believe the mic has more rearward rejection at low frequencies and less rejection at high frequencies than it actually does. I guess this is an 'industry standard' way of presenting polar response data, but in my mind it is quite misleading if the on-axis frequency response of the mic is anything other than flat at the frequencies selected for the plot.  I can see no good reason for doing this other than making the plots 'look nice'.  If the polar response plots instead reflected the actual level of each frequency in each direction they would be much more useful.  You could actually compare the amount of rejection at various frequencies in any given direction, or look at the plot and roughly determine the frequency response curve of the mic in any direction, including on-axis.  The frequency response chart and the polar response graph would then actually agree with each other!

[/rant off]

Dean, nice top.  A smart man wears a hat in the sun.  Have a good weekend all, I'm out 'till next week..

Lee

[DSatz]

Gutbucket, the realization you've just described concerning polar diagrams (that the 0-degree response at each frequency is normalized to 0 dB) is one which everyone has to think through at some point: Polar diagrams don't say anything about the frequency response of a microphone as such; they only show you its directional pattern, and let you judge the relative consistency of that pattern at various frequencies.

But the typical free-field (0-degree) frequency response plot says nothing about the directional characteristics of a microphone, either. They're two independent axes--I mean the plural of "axis"--of information. We're meant to integrate the information from the two types of diagram in our minds' eyes (or maybe ears).

An alternative approach would be to plot the frequency response of the microphone at various angles of sound incidence as a set of more or less parallel-running curves on the same graph. You can sketch a set of curves like that once you have the microphone's 0-degree response curve and polar diagram, though; I recommend trying that some time. Maybe we can even do that together here as a joint exercise--I'll gladly show you one example that I wish I'd plotted out before making a certain rather harsh-sounding recording a few years ago.


> [ ... ] if a cardioid pattern mic has a high frequency response peak and a low frequency roll-off when measured on-axis and some unwitting soul tries to compare the mic's rearward rejection at various frequencies using the polar plot, they are bound to be lead to believe the mic has more rearward rejection at low frequencies and less rejection at high frequencies than it actually does.

Why do you say that? The polar diagram of a cardioid microphone will indicate its rear rejection at each frequency directly. There's nothing tricky or ambiguous about that. No special interpolation or computation is necessary; there's no money down and no salesman will call.

Take for example the Neumann KM 85 response curves attached below. At 125 Hz the 0-degree response is down about 6 dB from the microphone's nominal sensitivity at 1 kHz, since this is a "speech cardioid." The polar diagram shows that the front-to-back ratio at that frequency is about 15 dB. So the net 180-degree sensitivity at 125 Hz is 6 dB below that, or about -21 dB relative to the nominal sensitivity of the microphone, but you don't need to do any addition or subtraction to read the front-to-back ratio at a given frequency from the graph; you only have to find the particular dotted or dashed line that you're looking for.

The only difference between a KM 85 and a KM 84 (standard cardioid, also shown below) is the low-end rolloff: At 50 Hz the standard cardioid is 3 dB down while the speech cardioid is 12 dB down. This difference shows up in the frequency response plots but not in the directional (polar) plots, since there's essentially no difference in the directional behavior of these two microphones. Microphones with identical directional properties should have the same polar diagrams; that's the whole point of a polar diagram.

--best regards

[DSatz]

(more or less replying to myself, I guess)

> An alternative approach would be to plot the frequency response of the microphone at various angles of sound incidence as a set of more or less parallel-running curves on the same graph.

Attached is an example of a graph using that approach. It's nice, no? Is this what you'd all prefer?

[What follows has been rewritten after a little Sunday morning reflection.]

The trick is to look at traditional frequency response and polar diagrams and to see (or hear) this result in your mind. It's a trick that can be learned, but I'm beginning to see that this requirement is an unnecessary barrier to understanding--almost like a hazing ritual (I jumped through the hoops; now I belong to the club, and I say that you must jump through them, too).

I'd still recommend learning to transform traditional diagrams into this kind in your mind--or on paper, if you prefer. But I have to concede that the manufacturers would be doing a real service if they simply printed such composite, multi-angle frequency response curves outright. They're very revealing, but there's no standard method for presenting them, and the leading manufacturers like to follow standards. (The AES has a committee that has been actively developing such ideas for a number of years--chaired by David Josephson, one of my heroes in this meshugginer business.)

Another thing that would be very helpful would be diffuse-field frequency response curves (i.e. the integrated 360-degree response), since that has a lot to do with what we actually hear when we use a microphone at any normal distance in a room. If such curves were available, they would go a lot farther than the 0-degree curve does toward explaining the sound quality of microphones in practice. But it can be rather difficult to produce a reliably diffuse sound field, and probably no two manufacturers would do it in the same way.

--best regards

[Gutbucket]

Very helpful DSatz, I now have a better understanding of what's going on behind these graphs.  What was bothering me is the fact that the polar plot shows the response at various angles around the mic, as referenced to the on-axis response curve, not to some absolute loudness level.  So like you say, to determine the actual sensitivity at a certain angle, I have to do more than just look at the polar plot and read off a value or look at the shape of the curve compared to the other curves superimposed under it, I must read the value off the plot and then add that (typically negative value) to the value from the frequency response curve.  That can produce a chart (mental or otherwise) like the one you posted:

Yes I like this one!  It's very easy to read and understand.  And yes, I agree that a set of these curves above, plotted for both the free and diffuse fields would be extremely helpful.

That same information could also be presented in the form of a polar plot, which is what I was imagining.  I did a quick search and found what I think shows this approach in the plot attached below.  As you can see the curves do not line up perfectly on-axis as in a typical polar response plot.  Instead, it appears that the traces on the plot are referenced to an absolute level (like a frequency response chart) and you can read level differences between frequencies directly off the plot without the calculation (like on the chart you posted above).  In effect, both of these combine the data from a typical frequency response chart and polar plot, allowing someone to look at the curves and immediately see both the relative and absolute sensitivity at each angle without calculation.  It's the same data, whether presented as an x/y graph or wrapped into a polar plot.

That's what I was trying to get at when I said,

..In other words, if a cardioid pattern mic has a high frequency response peak and a low frequency roll-off when measured on-axis and some unwitting soul tries to compare the mic's rearward rejection at various frequencies using the polar plot, they are bound to be lead to believe the mic has more rearward rejection at low frequencies and less rejection at high frequencies than it actually does...

That's incorrect, what I should have said is, "If some unwitting soul tries to compare the mic's sensitivity at various angles using just the polar plot, they are bound to be lead to believe the mic has more sensitivity at low frequencies and less sensitivity at high frequencies than it actually does...", because with a typical polar plot you cannot know the sensitivity at a certain angle just by looking at the polar plot without also referring to the frequency response cart (and doing the mental math).  Your example of the KM84 vs. KM85 illustrates that well. 

Regards again

[edited for typage & clarity]