Topic: Suggestions for a center channel omni to run into my MixPre3

[Gutbucket]

Have fun with it Ed, and thanks for the good excuse!

[Gutbucket]

Hiya Ed,

If interested, I just posted more details on the "why" behind the idea about mounting a non-low-passed center channel omni significantly higher or lower than your stereo pair here- Relationships between spaced omnis and center mic

Warning though, that post along with the rest of my contributions to that thread are pretty much TL:DR for most.

[edtyre]

Here's an example of a pair of forward facing MK4V's along with a rear facing AT 4022 Omni panned center and mixed into the two stereo tracks.
I didn't track out the audio just used it in this video my friend shot.

https://www.youtube.com/watch?v=PtGXNd_EXY4

[heathen]

Sounds great, at least IMO.  It would be cool to hear a comparison clip (even if it's just a minute or so) of just the Schoeps vs. the Schoeps + AT...if you're up to the effort of that.

[edtyre]

Sounds great, at least IMO.  It would be cool to hear a comparison clip (even if it's just a minute or so) of just the Schoeps vs. the Schoeps + AT...if you're up to the effort of that.

Sure, here
https://www.dropbox.com/s/avbgo5u143f3b7x/Archive.zip?dl=0

[Gutbucket]

Hi Ed,

If you end up using that setup regularly, and aren't averse to EQ, you might play around with EQing just the rear-facing omni channel with a curve resembling the equal loudness curve (boosted bass, deeply scooped mids with maximum reduction of the upper mids, boosted highs).  That helps the omni contribute what it's best at (bass extension down low + rear facing ambient air and openess up top) while keeping it out of the way of the MK4 stereo pair through the midrange, which is where the addition of the omni tends to collapse image width and sometimes muddy things up somewhat.    Also helps to minimize the listener's attention being pulled toward the audience members behind the recording position acting up or talking loudly, without overall reduction of omni level.  That way, you may find you are able to dial in more omni level without problems if you want to do so.  Once you find a general curve that works, you can apply the same curve to the rear facing omni each time, only tweaking it further if necessary.  I find this can be quite useful with my own rear facing ambiance mics.

[heathen]

A single center omni will extend bass response of a pair of cardioids, and that's the first thing most folks think about when considering adding an omni. If only interested in that bass extension aspect, a low-pass filter on the omni will keep your near-spaced cardioid configuration from being affected above the low bass region and preserves the stereo qualities you're familiar with using a standard near-spaced stereo microphone configuration.

Sorry to bump this old-ish thread but I'm thinking about trying a 3-channel setup with a card pair and an omni.  The omni will be used solely to supplement the low end, so I will likely use a low-pass filter in post.  My question is, when using an omni for this purpose and with a low-pass filter, does it matter at all where the omni is placed with respect to the cards (within reason, of course)?  Are there no phase issues or the like at the very low frequencies?  And what if the omni is just clamped lower on the stand than where the cards are at?

Another thing I'm thinking about trying is similar to a Straus Packet, with a pair of mini omnis set up right under the cards in the same config...but that's for another thread.

[Gutbucket]

Necessary proximity to the cardioids depends on the frequency ranges being summed.  Low frequencies have long wavelengths.  If you are low-passing the omni you can probably put it anywhere on your stand without phase interaction at the crossover point.  And actually if you want to play with using the omni for higher frequency ambience addition as well as extending the bass response of the cardiod pair and don't want to change your cardioid stereo configuration, I can make an argument for mounting the omni as far above or below your cardioid pair as possible (say a few feet or more).  That will intentionally randomize the phase differences of much of the higher frequency ambient pickup, while not affecting the phase of the longer wavelength bass frequencies.

If you are aiming for an overall flat response - with the omni picking up exactly where the response of the cardioids fall off at low frequencies, you'd tailor the omni rolloff to be the inverse of the cardioid's response so the combination sums flat.  But in the real world who's to say flat is what you really want.  The bottom end would probably benefit from EQ anyway.  Mess with the combination afterwards to find what works.

For making a Strauss-packet (what Schoeps Polar-Flex does), you need the mics as coincident as possible so the phase difference is minimal throughout the entire frequency range.

[heathen]

Necessary proximity to the cardioids depends on the frequency ranges being summed.  Low frequencies have long wavelengths.  If you are low-passing the omni you can probably put it anywhere on your stand without phase interaction at the crossover point. 

Not that I'd necessarily do this, but just for the sake of discussion, could the low-passed omni be below head level of the crowd without any ill effects?

My idea with this is just to supplement the bass to account for the slight roll-off of the cards...flat "on paper," but subject to what sounds best in post.

[Gutbucket]

If you are discarding everything from the low mids up then I don't see why not.  That low bass goes right around human sized things. 

You might notice some increase in low bass level as the mic gets closer and closer to the floor.  That's the boundary effect in action, and the boundary layer is relatively deep at very low frequencies.  Ever notice how much louder the very low bass is if you sit on the ground rather than stand in the same place at an outdoor music fest with large subs?   That's the same boundary layer effect thing going on effecting the lowest frequencies.  Similar to how the omni needs to be to more and more coincident to the other mic(s) if you want ever-higher frequencies to remain in-phase, a microphone taking advantage of boundary layer effect needs to be super-close  to the boundary (ideally flush-mounted to it) if it is intended to leverage the boundary effect without combing up to the high-frequency limits of hearing, but the effect is still active at low frequences up to a significant distance from the boundary, which in this case is the ground.

[heathen]

Thanks for the insight as always.  I'll mess with this at some point in the future and hopefully not botch things too badly.