Topic: 4-mic Phased Array Recording

[DSatz]

No question--nearly any microphone configuration _can_ produce good-sounding results at least some of the time, according to someone. And good-sounding results aren't to be argued with. I'm just saying ... well, what I said: If you're going to mix the signals from two or more microphones into a single channel, and those microphones are picking up substantially the same material, they had best be coincident, for the reasons I explained.

Gutbucket, I fully agree with your statement that "some complexity in the phase relationship between channels is often desirable can be good and of course.  Else we would all gravitate to only using coincident setups where both channels have identical phase at all frequencies." These days, the only time I record with coincident microphones is M/S. However, my point was really about what happens within one channel when signals from two or more microphones are mixed together, if those microphones are spaced apart and are picking up substantially the same sounds.

And that's enough repetition from me for one day. And that's enough repetition from me for one day.

--best regards

[Gutbucket]

Well, I kind of rhetorically cheated a bit there with the bit about liking ORTF more than X/Y because of it's phase interactions (talking main pair/audience recording, not close-up, spot stuff).  That's true in most cases, but when listening in stereo, not necessarily mixed to mono. 

[As an only semi-relevant aside, I also prefer spaced surround configurations rather than coincident ones such as dual M/S or ambisonics for the same reasons.   In stereo, the exception is Blumlien, and crossed super/hypers with sufficient angle between them, so M/S can satisfy that requirement.  But I haven't found a similar exception for coincident surround recording, I speculate that 1st order patterns just aren't tight enough to allow for a coincident arrangement to work for more than a minimal number of playback speakers, like 3 or maybe 4 max.  I'd love to try a coincident array of exotic mics which can exceed 1st order directivity, like 5 Schoeps SuperCMIT shotguns, or the Trinnov SRP.]



As for repetition, below is what I was typing while Jon & D were posting..  enough repetition from me for the next week!  Out of town and off line till next Tuesday.

Regarding the time delay causing said phase interference, I think it's worth noting that in the mic placements I'm describing the flanking omni mics are obviously not coincident but are still somewhat close -  in the experimental one I've posted here and some of the other spacings linked, each omni mic is 10cm from the inner subcardiod.  I know that is close to the wavelength of 3400 Hz, but I'm not sure if that means that's the frequency above which there would be phase interference between each "side" pair.

The "still somewhat close" region is where the dangers lie.  Neither fully coincident or far enough apart to avoid (theoretically) or sufficiently minimize (practically) the interaction problems.  That's the "tread carefully, and it's good, or suffer problems" zone.  At least for the midrange and above.

I'm more a spatial geometry guy than a numbers guy, but let me see if I can get this right-

Things get interesting at half that frequency. Lets say one 3400Hz wavelengh cycle = 10cm (not exactly, but for purpose of discussion).  For a sound of that frequency arriving from directly in front, perpendicular to the array, the arrival time is identical at both microphones and you get in-phase summing (which is the case for all frequencies, not just 3.4k).  You also get in-phase summing if the 3.4k source is directly off to either side, 90-degrees to your mic array, upon completion of one cycle.  In that case the sound arrives at the second microphone delayed by exactly one cycle.  But at an angle somewhere in between, and here's were my trigonometry gets rusty, the path length difference will be exactly 5cm, the resulting microphone signals will be 180-degrees out of phase, and you get a cancellation notch.  That notch is symmetrical to the pair however (the same for sound arriving from 90 degrees left or 90 degrees right) so you get one cancellation notch pointing left and a mirror image of it  pointing right.. Actually that's a simplification to flat plane, in reality it's 3-dimensional and forms the surface of a cone pointing left and and identical cone pointing right.  For the angles of arrival between those positions you get a smoothly varying gradient shifting between cancellation and reinforcement.

At 1700Hz you still get full summation from directly ahead (no time/phase delay), but from 90 degrees left/right you get 180-degree phase cancellation.  If you plot all angles of arrival for the 1.7k sound, you get a nice, neat, forward facing figure-8 response, but only at that single frequency.

Below that frequency you still get forward reinforcement directivity, but without a deep notch directly to the sides. The response broadens out until it eventually becomes omni-directional. 

At frequencies higher than 3.4kHz a 10cm path length begins to span multiple wavelengths, so you get more than one notch and multiple lobes, until you end up with a fan-shaped array of lobes and notches by the time you get up to 15kHz and beyond.

[voltronic]

^ This is all very helpful.  I appreciate the thoughtful conversation.

I am trying a low-pass filter on this recording right now, and I'm listening while varying it between 400Hz and 100Hz, which is the rolloff point of the CM3.  For this particular recording, using the lowpass on the omnis at any frequency setting, even much higher up, kills all sense of space and presence.  It is actually not that different than just muting the omni track entirely (except for the bass region obviously).  This leads me to two conclusions:

1. The treble range of the omnis is what is most beneficial to this particular recording, and if there are significant phase interactions with the subcards in the treble range, they are doing for more good than harm.

2. It would seem that in order for a recording made this way to sound good with the omnis low-passed, my center pair of mics would have to be set with the spacing a bit closer (NOS, DIN, etc.) to create a wider SRA.  In other words, the pair of directional mics would have to "stand on their own" so to speak in terms of a proper stereo image, and I knew going into this that would not be the case and that the omnis would be needed to fill things out.  As I said in my original post, the SRA of that subcard pair at this spacing is very narrow, but I was just experimenting with what others have tried and was fully prepared for it to crash and burn.

If the subcards were set with a wider SRA as I said above, would that potentially create more undesirable interactions with the omnis?  My thinking is that the resulting "combined pickup pattern" (for lack of a better term) of the subcard pair would then be sharing more perceived directional sound arrival characteristics with the omnis.  I realize that you're not changing anything about the pickup pattern of the mics individually, so this may be a flawed line of thinking.

Again, thanks to all for your patience with me here - trying to take all this in.

[voltronic]

Well that's where the "identical microphone" bit falls apart--the X-Q is based on a flat-response omni cap, but in its case it will have a bit of a HF lift.  I don't know the FR of the CM3, but the X-Q is likely hotter above 15 or even 10kHz.

That seems to be the case.  http://www.lineaudio.se/CM3.html

[boojum]

Allow me to explain how I arrived at the JNorman/Boojum array about the same time as Jim Norman.  I had been recording a rock band for fun with ORTF CMC64's.  It worked alright but did not have the bottom end that omni's offer.  For this reason classical sessions will have ORTF flanked by omni's, usually at a spot about halfway between the edge of the sound field and the center of the stage.  John Eargle was a proponent of this technique.  However, the Williams research indicated that a spacing of ~37cm for omni's will yield an SRA of ~80° and a cm less ~90°.  So, by combining Williams charts with Eargle's classical "center plus flankers" array I could put the whole shebang on one mic stand and get it placed quickly for those "quick and dirty" situations that tapers are faced with so often.

Playing off the omni's against the ORTF, or MS, could adjust the room tone and recorded sound to suit me in post.  Also, the omni's, DPA4061's, not only had a good bottom end but were also clean all the way up to the top end.  They played well with the Schoeps mics.  I am not an engineer but the array seems to work OK for me and for others.  And it is easily placed.

DSatz suggests two mics with a figure-8 for MS.  I have done this using a CMC64, CMC68, DPA4061 array.  I place the 4061 under the CMC68 and the CMC64 above.  It works well and, again, the card against the omni allows work in post.  It also allows the bottom register to be heard which is not always the case with the CMC64.  I low pass filter can be applied to the 4060/1.

However, for novel ideas and experimentation I cannot hold a candle to Gutbucket who has been doing some great stuff as long as I have known him.  Rabbit ears and DPA mini omni's are a fertile ground for him.  The Merlin of micro mics.

Cheers

PS -  TonyF has been credited with two "Faulkner Arrays" that I know of:  the two figure-8's forward facing and parallel and also the 67cm omni's angled out with and ORTF/NOS in the center.   

[2manyrocks]

As far as I know, the most recent TF commentary on this is in the first thread Voltronic linked:  https://www.gearslutz.com/board/remote-possibilities-acoustic-music-location-recording/930912-three-mixes-boojum-jnorman-case-study.html

Among other things, TF commented " my 4-way system was arrived at to get reach and detail from a distance."  apparently without having to resort to set up several omnis closer in on separate mic stands. 

And we have Gutbucket using, what, 6 specific microphones in a pattern I've never read about elsewhere. 

I feel like I walked into a Phd class after graduating from kindergarden. 

[voltronic]

https://www.gearslutz.com/board/remote-possibilities-acoustic-music-location-recording/997189-tony-faulkner-lso-abbey-rd-shostakovich.html

Abbey Road recording session where TF used his 4-mic setup as the primary array for the orchestra.  Check the video in the first post, and specific descriptions of mics used, balancing etc. in posts 14 and 17.

[voltronic]

Regarding the "reach and detail from a distance" 2manyrocks referenced, I decided to try an experiment the other night to see just how far that could be taken with this array.  Here's a post I made on GS with all the details and a link to samples:
https://www.gearslutz.com/board/remote-possibilities-acoustic-music-location-recording/930912-three-mixes-boojum-jnorman-case-study-2.html#post11020078

[2manyrocks]

That's more like it. 

How much of the improvement comes from using wider spacing on a different mic bar do you think?

Did the drums seem that prominent live on Street Marches? 

[voltronic]

That's more like it. 

How much of the improvement comes from using wider spacing on a different mic bar do you think?

That's hard to tell, as this was the first recording I made with this wider bar available, and thus the first I've made with the wider spacings on the phased array.  Like I said in the GS thread, I was looking to see   

Did the drums seem that prominent live on Street Marches?

Yes, that auditorium is quite boomy and that's pretty much what it sounded like in person.  Even though it's only one bass and one snare, they were in the orchestra pit which is not carpeted, and they also had the hard wood paneling in front of the stage right behind them.  I suspect those two guys were also professionals, whereas several of the brass players clearly are not.  The hall was also pretty much empty for this rehearsal.

[voltronic]

Here's a sample from a couple nights ago of the choir concert I record every year, this time with the 4-mic phased array at the 47/67 spacings.  At the choir director's request, I recorded from center-front balcony, which isn't any farther back in the hall from where I normally have been in the past, and it has the obvious height advantage.  He also asked that I record a second set of tracks using his Schoeps MK4/MK8 mid-side setup, though I told him from that distance it's not going to be the best choice.  I included that sample here for your comparison.

This is a very large (about 250 voices) high school choir with a digital piano / amp.  The concert grand wasn't used for this concert due to logistical issues with an orchestra setup.

Lineage:
CM3 / X-Q > DR-70D

MK4/MK8 > CMC 5U > FP24 > M10

https://www.dropbox.com/sh/mx2683jqrwdqrob/AABVf4LOYFnKOmCgkQF-aIMZa?dl=0

[2manyrocks]

Seems like there was more low end in your array then in the m/s.   Even though the m/s had that Schoeps goodness to it, I would have preferred closer micing. 

Are you going to keep your FP24 now that you have the 70d? 

[voltronic]

Seems like there was more low end in your array then in the m/s.   Even though the m/s had that Schoeps goodness to it, I would have preferred closer micing. 

Are you going to keep your FP24 now that you have the 70d?

Well the array includes omnis, so no proximity effect means they are getting the bass at distance as opposed to the CM3s.  I actually didn't want to use the M/S setup at all; it was the conductor who wanted me to run it even after I told him it's going to be too far back.  I just sent him tracks of both setups but told him that the Phased Array is the one that actually sounds like it did live, as opposed to the M/S which from that distance is predominantly mono sounding.  That Schoeps M/S rig does sound fantastic up close though.

I was thinking about selling the FP24 to help offset the purchase of new mics, but it's such a good pre that I might hold on to it.  It is quite handy to have a second complete rig, especially in situations like this one where my 4 channels of the 70D are occupied and I need to record another 2.  The conductor has a Zoom H6 which I've used to record his concerts in the past, but for this one I decided to use my FP24 > M10 because it was available and probably much better than the H6 pres.

[2manyrocks]

With the FP24, at least you know the meters are going to be accurate.