Topic: Quick omni question

[Gordon]

Going to an outdoor show tonight.  I'll be running my ak40's nos and will also run my ca 11 omni's just for the heck of it.  I have no bar for the omnis and don't think I'll be able to clip anywhere to get a good split.  So I plan to just rig them onto my nos bar.  Which of these two setups would be best?




or





obviously I can rig them anywhere on the nos bar.


also I've read that omnis don't pick up much wind noise.  the dpa screens fit over the church screens but I doubt that's a good idea.

[bryonsos]

At that distance, it won't matter much. You need to get them a couple feet apart to get a stereo image from omnis. You could just put 1 in the middle and mix/fix it in post.

[Gordon]

not planning to mix with the nuemann's.  just want to play around with them.

[lsd2525]

You could try the Healy method on the ends of the bar, but I'm not sure how bad those BAS's would block the sound. Would have to be a pretty stiff wind for you to pick up any wind noise.

Given a choice between the two pictured, I'd go for the outside mounting

[ScoobieKW]

FYI - Healy method is 17cm spacing with the capsules facing outward. 180 degree microphone angle

[Ultfris101]

can you tape a thin rod like a tent pole section or one of those fiberglass sticks you use to mark curbs in snow to the bar or something? and then tape omnis to that?

Just about anything to get 24" or so of split and you'll be in good shape. they're so small they hardly need any support. even a coat hanger or wooden dowel or something.

Tape some pencils together or cut a small branch out of tree. Channel your inner Macgyver! 

[zupanic]

for about $5 you can go to most any "dollar store" and still buy television rabbit ears. I don't even have omnis yet, but picked up the rabbit ears for this purpose. Just cut the telescoping sections from the base and you can rig it up for up to 5 or 6 feet of spread.

[lsd2525]

can you tape a thin rod like a tent pole section or one of those fiberglass sticks you use to mark curbs in snow to the bar or something? and then tape omnis to that?

Just about anything to get 24" or so of split and you'll be in good shape. they're so small they hardly need any support. even a coat hanger or wooden dowel or something.

Tape some pencils together or cut a small branch out of tree. Channel your inner Macgyver! 

TV antenna.......

[Gordon]

all good ideas but I'll have to save them for another time.  I'm pretty sure I'll be the only taper.  taking my 3 1/2 year old (its free shows downtown).  I'll have my hands full as it is so not much time to rig something up.  another thing splitting these the cables are only 5 feet.   I'll have to have the pre and recorder taped to the stand as it is, that would put them up real high on the stand!

[Gutbucket]

Late to the party, but since this is a secondary recording anyway, consider trying this perhaps radical seeming idea:

Space them the 2 to 3 feet apart they really need to be.. vertically on the stand, one above the other. Point one left and the other right, just because they have some directionality at high frequencies.  That should be easy to rig, requires no extra bar, and won't block sightlines or be unwieldy.  Make sure the lower mic is high-enough that it isn't sight-line blocked to the PA or sound source.  You'll get much of the open and spacious sound of spaced omnis from the frequency-related decorellation the spacing provides even though it won't image Left/Right as if they were spaced the same distance horizontally.

And if you wanted to play around with it, it may work surprisingly well mixed with the main pair.

[achalsey]

That's a pretty cool idea.  Curious to know how that would turn out.  Most because I have little understanding of how sound works vertically.  Or horizontally...or at all really, but that's a different story. 

Totally off topic, but, in playback there isn't really any 'up' and 'down,' is there?  I'm sure I'm looking at this very simply (ignorantly) and there's a technical explanation totally over my head, but I wonder from a practical sense how vertically spaced mics sound.

Do you pan them left/right?  That seems almost counter-intuitive, but my intuition is pretty limited in these kinds of things.  Double the channels and blend both a little left/right?  I guess I need to experiment myself, maybe with some cards as don't have omnis, but it does sound interesting....

[TSNéa]

Space them the 2 to 3 feet apart they really need to be.. vertically on the stand, one above the other. Point one left and the other right

Very tempting... Why bother with the first pair? 

More seriously, is there any possibility to feel a distorted horizontal plane? (I'm sorry I can't be precise enough: I lack some English vocabulary in psychophysics of perception). Maybe 3 ft vertically apart is not such a big problem?
I'd be curious to try...

I see that achalsey answered while I was writing: we seem to have the same kind of feeling!

[Gutbucket]

Play it back normally.  One channel left, the other right.   Forget about the configuration orientation when you play it back and just listen for how it sounds.

Though verticality may sometimes be conveyed by frequency response differences and early reflections (either on the recording end or the playback end), and probably even more often by listener expectation, 2-channel stereo is 2-dimensional and flattens interchannel differences to the horizontal plane on playback, regardless of the plane of the microphone configuration.

[TSNéa]

OK, thanks Gutbucket!
So no need to put my (heavy) loudspeakers one above the other to enjoy the sound... 

I think you're right about "listener expectation": if you're lying on your side, listening to ambient sound or to some music, wether in your bed or at the beach, the world around stays stable. Fortunately. Just because we interpret/analyze everything that comes through our senses. 

[bryonsos]

I'm going to call horsefeathers on pointing an omni regardless of whether you align them vertically or horizontally. If it's a proper omni, then the only thing that matters is its spacing from its mates. Distance creates the difference in signal arrival that creates the stereo image. If they're vertical instead of horizontal, then you have eliminated the space needed to create the image, so I guess I'm calling horsefeathers on that part too.

[Tom McCreadie]

..., so I guess I'm calling horsefeathers on that part too.

Me too. I think it's a winning recipe for audio 'pea soup'.
Ach, pea soup, opaque as it is, can be tasty.  :-)

[aaronji]

I'm going to call horsefeathers on pointing an omni regardless of whether you align them vertically or horizontally. If it's a proper omni, then the only thing that matters is its spacing from its mates.

Omnis definitely have directional characteristics at higher frequencies.  The frequency at which this starts occurring is related to the diameter of the microphones.  Just look at the polar pattern graph for a DPA 4006 (or an MK2, if you prefer).  Pretty obvious and can certainly be used to your advantage...Omnis with very small diameters (like a 4060) are much closer to being perfectly omni, but even they exhibit this tendency at very high frequencies.

[Gordon]

I ended up just taping them to the nos bar maybe 5 or 6 inches apart.  I about forgot to even do it    my 3 yo was begging for ice cream the entire time I was setting up and thus had me a bit distracted.  anyway while there is not the "spit omni sound" it sounds very good to my ears.  a bit more crowd than the neumann's but that is to be expected with omnis.  these little mics really impress me!  going to figure out a way to do a proper 2-3 foot split at yonder next weekend.

[capnhook]

I'm going to call horsefeathers on pointing an omni regardless of whether you align them vertically or horizontally. If it's a proper omni, then the only thing that matters is its spacing from its mates. Distance creates the difference in signal arrival that creates the stereo image. If they're vertical instead of horizontal, then you have eliminated the space needed to create the image, so I guess I'm calling horsefeathers on that part too.

Most omnis' frequency response curves differ on-axis vs. off-axis.  You might actually point them to help get the response you want at a particular time.

[Gutbucket]

I'm going to call horsefeathers on pointing an omni regardless of whether you align them vertically or horizontally. If it's a proper omni, then the only thing that matters is its spacing from its mates. Distance creates the difference in signal arrival that creates the stereo image. If they're vertical instead of horizontal, then you have eliminated the space needed to create the image, so I guess I'm calling horsefeathers on that part too.

If the omnis you are using are larger than DPA 406x, they will be noticeably directional at the upper end of their frequency range.  You needn't take my word or that of other TS'ers for it.  Just look at the manufacturer's published polar patterns, or better yet, play around with the microphones and listen for yourself.   

The larger the size of the microphone housing, the more directional they will be up top and the lower in frequency that directionality will be noticeable.  That's why microphone manufacturers like Schopes, Neumann, Naiant and others offer optional attachments for their pressure-omnis which change the size and shape of the microphone housing around the capsule which allow users to modify the pickup pattern and frequency response of their microphones in predictable ways.

On the second part, you are correct that when the microphones are spaced one above the other, there will be minimal time-of-arrival differences for sounds arriving along the horizontal plane which is the same height above the ground as the mid-point between the microphones.  Those sounds will image in the center between the speakers.  Sounds arriving from above or below that plane will image toward one speaker or the other. 

More importantly I think, there is a lot more going on in stereo than directional imaging.  The diffuse sound, the reverberant ambience, comes from all directions and the horizontal plane only makes up a small slice of that. Spacing the microphones along ANY axis creates differences between channels for that stuff. 

The obvious experiment is to make a recording with the omni microphones positioned coincidentally, and then with them vertically spaced 3' or so, and listen to see if those recordings sound identical or not.  Even if you aren't inclined to actually do the experiment, just thinking about it probably makes it clear they will not sound identical, even though determining the nature of the differences probably requires actually trying it.

Besides, all two channel stereo (with the exception of binaural recording and playback) is something of a feathered horse anyway.  It's just an illusion and we are the magicians doing the slight-of-hand tricks.

[Gordon]

so as small as the church audios are does it make a difference if they are facing up or straight at the sound source?  Obviously I know nothing about omnis.

[Gutbucket]

A sound source usually sounds a little brighter up top when the microphone is pointed at it.  Somewhat similar in frequency range and degree of difference to the effect of using a foam windscreen or not.  Pointing omnis either up or forward is mostly a treble-range EQ tweak.

With a microphone the size of the miniature DPAs, that effect is minimal and way up above 15kHz.  Monitoring on headphones in a quiet room, and moving around the microphone while rubbing my fingers together a few inches from it and listening to the HF part of the 'swishsa' sound I can hear a slight difference, on-axis verses far off-axis, but it's subtle.  Subtle enough that I don't really worry about the orientation of those when recording music.
 
With a small but not miniature omnis, the frequency range is not really low enough to make a big difference in the range which strongly effects imaging, but an inter-channel difference in that range can improve the sense of 'airiness and openness' in a non-imaging stereo sense when the two microphones are pointed in different directions.

Since I can make pretty much the same subtle overall treble EQ adjustment afterwards as I would get from pointing the mics up or forward (and I tend to EQ things later anyway), I'm thinking primarily about ways of maximizing that subtle inter-channel difference which I can only get from the microphone arrangement.  So with small omnis I'd probably point them directly to each side, opposing each other.  If I wasn't going to EQ things and wanted all the high-frequency brightness I can get for the main sound, I'd angle them less far apart and point each directly at the stacks.

[Church-Audio]

Omni mics need a boundary between them IMO. So I doubt it will be very good unless you can manage that. You would be better off wearing them on a hat than clamping them to a bar with out a boundary between them.

[Gutbucket]

No hat, this was all about a quick last-minute setup of a secondary recording on the same stand without other stuff to assist. 

But Chis has a good point.  A baffle (if you can rig one) IS the best idea when the omnis need to be mounted close together, even if it's only a cardstock music flier or piece of pizza box or something taped to the vertical mic-stand between the two mics.  The baffle makes the microphones directional down through the midrange frequencies determined by the size of the baffle.  It's more or less a further extension of the same omni directional aspects we've been talking about.  That arrangement would definitely be the better choice for Left/Right imaging, and probably what I would choose if I was just running the omni pair and couldn't space them enough horizontally, but it won't get that open, spaced-omni ambience sensation stuff. 

The best choice of what configuration to use is always determined by what you are trying to achieve.  Different priorities suggest different answers, as much as different constraints dictate which are doable.


As Chris is very familiar, measurement microphones are usually small diameter omnis, mounted at the end of a thin tube, and both of those things help to reduce these directional effects.  When used for measuring room acoustics they are often pointed directly upwards, so the response is as even as possible for sound arriving from any direction in the horizontal plane, even though it is pretty much the same for all directions.

[DSatz]

bryonosos wrote:

> I'm going to call horsefeathers on pointing an omni regardless of whether you align them vertically or horizontally. If it's a proper omni, then the only thing that matters is its spacing from its mates.

With all due respect, this is why people who make recordings REALLY need to learn how to read polar diagrams. You would then know right away that this isn't so.

Perhaps when you say "a proper omni" you mean "a microphone that is fully omnidirectional at all audible frequencies." But essentially no professional-quality recording microphones are that way. And it's not because it's hard to build a microphone with this behavior; you can in fact buy suitable capsules ready-made for less than a dollar each.

The crux is that sound waves are reflected, either partially or completely, whenever they strike any rigid object whose dimensions are about 1/4 of a sound wavelength or larger. Sound wavelengths are inversely proportional to frequency, and in air, range from about 55 feet (for 20 Hz) to about 2/3 of an inch (for 20 kHz). One inch (the diameter of many large-diaphragm condenser capsules) corresponds to a sound wavelength at about 13 kHz, and 1/4 of that is just above 3 kHz. For a typical small-diaphragm capsule with about 1/2" diameter (the housing dimensions matter, too, but the capsule dimensions matter even more), the critical frequency is about twice as high. But in the top audible octave, even what we normally call "small" diaphragms aren't acoustically "small"--their size causes them to interact with the sound waves at those frequencies, in a way that alters their directional response (because the degree of reflection and diffraction depend on the angle of sound incidence).

Below I've posted a scan of the frequency response and polar response diagrams for an excellent, small omnidirectional microphone that I used a lot in the 1970s, and that also was used by WGBH-FM for live broadcasts of the Boston Symphony Orchestra back then. It has essentially flat high-frequency response for random-incident sound (i.e. when no particular direction of sound arrival predominates--an acoustically "diffuse" sound field). But as the graph shows, it has a pronounced high-frequency rise on axis, with the 3 dB point at around the 6 kHz that you would predict from the capsule's ~1/2" diameter (1/4 wavelength). So you damn well better aim it if you're going to use it at moderate miking distances! Sometimes you even need to aim it away from the sound sources (as WGBH did).

A capsule of about 1/4" diameter or smaller can avoid the bulk of these problems, and such capsules are favored for this reason in certain acoustical measuring applications. But the sensitivity of a condenser microphone is proportional to the capacitance of its capsule, and smaller capsules have lower capacitance (since capacitance depends on area, it's proportional to the square of the diameter). So sensitivity and noise become critical issues with very small capsules, and that's why they're not generally used for professional music recording.

--best regards

[voltronic]

bryonosos wrote:

> I'm going to call horsefeathers on pointing an omni regardless of whether you align them vertically or horizontally. If it's a proper omni, then the only thing that matters is its spacing from its mates.

With all due respect, this is why people who make recordings REALLY need to learn how to read polar diagrams. You would then know right away that this isn't so.

Perhaps when you say "a proper omni" you mean "a microphone that is fully omnidirectional at all audible frequencies." But essentially no professional-quality recording microphones are that way.

I think the closest you can get are Earthworks QTC mics.
http://www.earthworksaudio.com/microphones/qtc-series-2/qtc30/

[DSatz]

Maybe so. But for stereo recording, why would you want reflected sound to be picked up with full-strength high frequencies? That only smears and confuses the imaging, unless the venue is very dry. If anything, in spaces with typical reverberation, I think it's desirable for omni microphones to de-emphasize the upper-mid and high frequencies even more than a small-diaphragm capsule naturally does, when those frequencies aren't arriving on axis.

The classic example is the Neumann M 50 microphone, which had its capsule (similar in basic response to the KM 83 shown above) embedded in the surface of a 4 cm acoustically opaque sphere--see attached photos, with the screens of the microphones removed. That's the microphone that Decca used most often for their "Decca tree" recordings, although sometimes they used Schoeps M 201s (presumably in the omni setting, where they operated as pure pressure transducers). You can get a very similar effect by mounting a sphere accessory onto a small pressure transducer capsule--both Neumann and Schoeps sell such accessories. The Schoeps MK 2H and MK 2S work particularly well with them.

[Church-Audio]

Maybe so. But for stereo recording, why would you want reflected sound to be picked up with full-strength high frequencies? That only smears and confuses the imaging, unless the venue is very dry. If anything, in spaces with typical reverberation, I think it's desirable for omni microphones to de-emphasize the upper-mid and high frequencies even more than a small-diaphragm capsule naturally does, when those frequencies aren't arriving on axis.

The classic example is the Neumann M 50 microphone, which had its capsule (similar in basic response to the KM 83 shown above) embedded in the surface of a 4 cm acoustically opaque sphere--see attached photos, with the screens of the microphones removed. That's the microphone that Decca used most often for their "Decca tree" recordings, although sometimes they used Schoeps M 201s (presumably in the omni setting, where they operated as pure pressure transducers). You can get a very similar effect by mounting a sphere accessory onto a small pressure transducer capsule--both Neumann and Schoeps sell such accessories. The Schoeps MK 2H and MK 2S work particularly well with them.

What a thing of beauty. This sphere is to control the polar pattern and make off axis response smoother?

[DSatz]

The sphere causes a slight increase in upper-midrange response in the forward direction--basically a "presence" boost--while smoothly rolling off the high frequencies at the sides and back. The graphs on http://www.schoeps.de/en/products/ka40/graphics show the on-axis part of this effect. It's a subtle effect which allows the microphones to focus well when placed at greater than usual distances.

I think some people here might benefit from knowing about this technique, since I get the impression that people often have to record from greater distances than they would otherwise prefer to use.

Originally this approach was used by German broadcasters in the mono era to pick up an entire orchestra with chorus and soloists from a single miking position in a reverberant hall. You have to back the microphone away from the sound sources by quite a distance if you're going to get everything in balance without favoring one group of performers or another.

Schoeps' accessory sphere is called the KA 40; Neumann's is called the SBK 130. Neumann also builds a sphere of this kind directly into their "M 150 Tube" (so you can see the polar diagrams by going to http://www.neumann.com/?lang=en&id=current_microphones&cid=m150_data and clicking on the "Interactive view of diagrams" link), and also as part of the "KK 133 SBK" capsule of their KM A/KM D series.

--best regards

[Gutbucket]

I've never seen such good detailed photos of the acrylic sphere of M50 before.  What is the grey "chewing gum" blob atop the sphere? Some sort of cushion against the mesh basket housing?

Ratio of the pressure on the surface of a cylinder, cube and sphere, of the normal axis to the pressure in the incident sound wave-

This is from Stan Linkwitz's site. He credits- G. G. Muller, R. Black, T. E. Davis, "The Diffraction Produced by Cylindrical and Cubical Obstacles and by Circular and Square Plates",  J. Acoust, Soc. Am., Vol. 10, July (1938)

My own experimentation with similar sized DIY spherical attachments on the miniature DPA 4060-

[voltronic]

The sphere causes a slight increase in upper-midrange response in the forward direction--basically a "presence" boost--while smoothly rolling off the high frequencies at the sides and back. The graphs on http://www.schoeps.de/en/products/ka40/graphics show the on-axis part of this effect. It's a subtle effect which allows the microphones to focus well when placed at greater than usual distances.

I'm a bit confused by the Schoeps link you provided: The Schoeps KA 40 shows the presence boost you described, but then the graphs seem to show the opposite effect of what you said regarding high frequency attenuation off-axis.  I'm looking at the bottom graph showing increased HF attenuation as you move increasingly off-axis, which is labeled "without KA".  The top graph "with KA 40" does not show this off-axis attenuation.  To me, these graphs are saying that the KA 40 is compensating for the off-axis HF rolloff that the capsule exhibits on its own.  The M150, on the other hand, shows the high-frequency roll-off off-axis starting at 2kHz.

Maybe so. But for stereo recording, why would you want reflected sound to be picked up with full-strength high frequencies? That only smears and confuses the imaging, unless the venue is very dry. If anything, in spaces with typical reverberation, I think it's desirable for omni microphones to de-emphasize the upper-mid and high frequencies even more than a small-diaphragm capsule naturally does, when those frequencies aren't arriving on axis.

I have not found this to be the case with these particular microphones.  If you want to hear for yourself, here is a very minimalist independent record label in my area where they only use a single pair of Earthworks omnis for their recordings.  http://www.dtrmusic.com/

[DSatz]

The upper graph on Schoeps' Web site shows only the on-axis response with the sphere attached, while the lower graph shows the response at various angles of incidence without the sphere. So you're right: these graphs don't show the effect of the sphere off-axis. It would have been better IMO if both graphs showed the response at the same set of angles--one with and one without the sphere.

--As for Earthworks mikes and other omnis based on very small pressure transducers, for any given type of microphone with almost any given set of characteristics, a person can eventually learn how to use those characteristics to best advantage, and to choose venues and music so that good-sounding recordings result. I don't mean to disparage that one bit; I have respect for a person who can get a good sound with less-than-ideal equipment. And probably 90% of the LPs ever made, excepting the top classical labels, were recorded on equipment that engineers today wouldn't choose for its sound quality.

Anyway, for most recording with omnis, I think most engineers have developed their way to estimate the best initial microphone placement, and their expectations regarding the sound quality to aim for, based on the way traditional small-diaphragm omnis work, with definite directivity at high frequencies. It's part of the traditional idiom for listeners as well as engineers.

--best regards

[voltronic]

The upper graph on Schoeps' Web site shows only the on-axis response with the sphere attached, while the lower graph shows the response at various angles of incidence without the sphere. So you're right: these graphs don't show the effect of the sphere off-axis. It would have been better IMO if both graphs showed the response at the same set of angles--one with and one without the sphere.

Now that makes more sense.  I had thought they were showing that that the response at all angles with the sphere was more or less equal on that graph; I hadn't considered that they simply chose a graph that doesn't show them compared against one that does.

[yates7592]

Here's a link to quite a detailed paper on how the DPA APE's modify the off-axis frequency response on the 4006 and 4003 mics:
http://www.bgavinsound.com/Reference/Music/DPA%20Microphones/article%20-%20acoustic%20pressure%20equalizers.pdf

Interestingly, section 11 refers to 'increasing stereo separation in stereo-pair miking' - using near-coincident pairs of directional omni's (NOS/DIN??) with APE's to further widen the stereo image. Does anybody have any experience with this? Would you end up with a hole in the middle? I would like to try it but sounds too good to be true!

[Gutbucket]

It would be somewhat like Jecklin-disk baffled omnis with a smaller disk, with seperation at low frequencies limited by the narrow spacing and directionality comming into play a bit higher in frequency than with a standard Jecklin setup.

When I was using the DIY spherical attachments I mostly used a Decca-tree like arrangement with the left and right microphones spaced about 1 meter apart with a 90 degree angle between them (+/- 45 degrees) and a third in the center facing forward.  With the center microphone in play, I could have pointed the left and right mics 180 degrees apart, fully to the sides, without a 'hole'.  That would have ahieved an even better ambient seperation, however more important to me was orientating the left/right mics to take advantage of their on-axis presence range frequency response boost for the primary direct sound arriving from in front.

[Gutbucket]

Interestingly, section 11 refers to 'increasing stereo separation in stereo-pair miking' - using near-coincident pairs of directional omni's (NOS/DIN??) with APE's to further widen the stereo image. Does anybody have any experience with this? Would you end up with a hole in the middle? I would like to try it but sounds too good to be true!

Besides their on-axis presence boost, my reasoning for using the spheres was partly as a way of optimizing around the limitation of a more narrow spacing than I would have prefered to use- in my case a total spread of 3 feet instead of 6 feet apart using three microphones.  Using two microphones those numbers would be smaller, but I wouldn't consider 12" enough without a baffle between the microphones. It would work of course, but I don't think a NOS/DIN arrangement would provide optimal seperation for a main stereo pair using omnis with APE spheres and distance mic'ing like most are doing here, although it might be an appropriate choice for mic'ing a piano or something.

[Gutbucket]

Here's a photo I just came across while looking for something else, showing a spaced pair of the DIY APE baffled omnis oriented 180 degrees apart.  This is a center/rear-facing pair (note the drum kit in the background, which was positioned center stage).  The left/right pair was spaced 3', facing +/- 45 degrees, and is out of frame.