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Figure of Eight Microphones in Near Co-incident Stereo Arrays

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Figure of Eight Microphones in Near Co-incident Stereo Arrays

Postby Bob Bickerton » Mon Nov 30, 2020 12:32 am

Succumbing to another Robjohn's review I recently bought a pair of AEA N8 ribbon microphones and I'm really enjoying them. They've already proved their worth in the studio, but I'm keen to try them as a main pair for orchestral and chamber music concerts.

I had only heard about ribbon pairs being used in a Blumleim array (co-incident at 90º, for which I'd have some difficulty rigging), or the Faulkner Phased Array (A/B @200mm), which I seem to recall is recommended when recording in an overly bright acoustic to attenuate side reflections.

That is until I heard this recording on the AEA website: https://youtu.be/FegliwLQHrw

They're a pair of N8s in an ORTF configuration (Yes, I know they're not really ORTF because they're not cardioid microphones).

Anyway, I had a last minute request to record a local orchestra at the weekend (archival purposes only) so I thought I'd give the above a go. I really like the tone of the recording, but I think I'm hearing some instability in the centre of the stereo image and some angular distortion - some elements being thrown out further left and right than one would hear naturally.

Half way through the concert I downloaded this app: https://schoeps.de/wissen/image-assistant.html, which I highly recommend.

So I'm wondering if reducing the spacing between mics to around 12cm and also reducing their angle of incidence to 80º from 110º may help.

Don't often have time to experiment too much in live concert recording, so I'm curious to hear what others have done in this respect.

Thanks as always.

Bob
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Re: Figure of Eight Microphones in Near Co-incident Stereo Arrays

Postby Tim Gillett » Mon Nov 30, 2020 5:52 am

To my ears the YT sample while nicely played seems to have a "hole in the middle" and tends to favour the front and side instruments. For example the violinist directly in front of the mics (and obscured by the mic stand in the wide shot) seems almost inaudible.
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Re: Figure of Eight Microphones in Near Co-incident Stereo Arrays

Postby Hugh Robjohns » Mon Nov 30, 2020 1:33 pm

Bob Bickerton wrote:I had only heard about ribbon pairs being used in a Blumleim array (co-incident at 90º, for which I'd have some difficulty rigging), or the Faulkner Phased Array (A/B @200mm), which I seem to recall is recommended when recording in an overly bright acoustic to attenuate side reflections.

It's not a 'phased array' -- you'd need at least four mics (and ideally a lot more) to qualify for that description. In reality, it was simply a modified spaced array of omnis but with side-rejection (and reduced bass extension) thanks to to the substitute fig-8s. It was just a quick and easy means of averting a very specific problem in a very specific location, rather than some methodically devised, calculated and developed microphone technique! It's really not very useful, as stereo arrays go...

They're a pair of N8s in an ORTF configuration (Yes, I know they're not really ORTF because they're not cardioid microphones).

That's bonkers! It's inherently doomed to failure in conventional recording environments as a moment's thought will reveal. For starters, the accepted working angle for most fig-8 mics is something less than +/-45 degrees. So setting an array with a mutual angle of +/-55 degrees obviously places central sound sources so far off-axis that their timbre and amplitude will both be degraded dramatically. To be fair to the N8, it has a relatively wide pattern and its sensitivity falls to -6dB at about 60 degrees off-axis.

Secondly, the SRA of that configuration is going to be around +/-20 degrees, which means for any practical mic placement, instruments more than about half-way out will all appear hard left/right, giving enormous angular distortion. Moreover, the mutual angle and spacing of the mics means that the rear fig-8 lobes will capture quite a lot of the opposite side instruments, in reversed polarity... adding further to the stereo imaging chaos and confusion.

So for all those stereo mic array basics, it's a complete non-starter to use fig-8s with such wide mutual angle and spacing....

So I'm wondering if reducing the spacing between mics to around 12cm and also reducing their angle of incidence to 80º from 110º may help.

Yes, it would. A lot! ...But still not enough!

Referring to the fig-8 SRA charts in Michael William's superbly informative books*, a capsule spacing of 12cm and mutual angle of +/-40 degrees creates an SRA of about +/-35 degrees, So not a great deal more helpful, really.

Fig-8 Williams.png


You'd really need to get the mutual angle down to about +/-25 degrees with a 12cm capsule spacing to give an SRA of +/-50 degrees to have an array that is practical from a placement point of view. (Or a wider mutual angle with further reduced capsule spacing).

Obviously, the narrower mutual angle brings central sources into the usable on-axis window of the fig-8 mics, and also reduces the likelihood of wide sources being picked up at significant levels on the rear lobes. All reducing the angular distortion, improving the timbre of central sounds, and increasing the stereo image stability.

Personally, though, while I have used fig-8s as widely spaced outriggers in occasional situations where I needed to minimise pickup of lateral sources or reflections, I've not found any near-spaced stereo array configuration that works for fig-8s. It really is pretty much XY (or the MS equivalent) or nothing!

The classic Blumlein (or Stereosonic) configuration does seem to give a wonderful sense of front-back depth that is quite hard to achieve with other arrays -- assuming the room acoustics are suitably sympathetic. That said, though, I rarely use a 90 degree mutual angle, usually favouring a narrower angle of between 65 and 80 degrees... or better still, set them up in an MS configuration where it's possible to dial in the optimal equivalent mutual angle at will!

* Michael's superbly informative books on stereo (and multichannel) mic arrays are reviewed here:https://www.soundonsound.com/reviews/microphone-arrays-stereo-multichannel-sound-recording

And you can get a taste of the content from an article available for free on the Mic Data website here: https://www.microphone-data.com/media/filestore/articles/Stereo%20zoom-10.pdf (although this one doesn't include the SRA charts for fig-8 arrays).
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Re: Figure of Eight Microphones in Near Co-incident Stereo Arrays

Postby Bob Bickerton » Mon Nov 30, 2020 8:21 pm

Thanks Hugh, I’ll abandon the notion.

Interesting that AEA quite strongly promote the concept of ORTF N8s on their website https://www.aearibbonmics.com/how-to-record-string-wood-brass-sections-and-orchestras-with-the-n8/ and even talk about a bespoke ORTF mounting kit for them.

I’ll just have to rough it with my TLM193s ;)

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Re: Figure of Eight Microphones in Near Co-incident Stereo Arrays

Postby Hugh Robjohns » Mon Nov 30, 2020 9:05 pm

Oh dear... That's tough if you've only got the 193s... :lol:
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Re: Figure of Eight Microphones in Near Co-incident Stereo Arrays

Postby Bob Bickerton » Thu Dec 03, 2020 9:29 am

Just re-read the article on the The Gerzon Array in the November issue, which sounds like it might be worth a crack, though I'm guessing my MKH8040s might be a better choice than the TLM193s.

I like the idea of experimenting with the 'Shuffle' technique explained in the article, working the mics closer than ORTF and then opening things up a bit in post, but I'm also thinking the N8s may work as the outrigger A/B pair instead of omnis - maybe not.

Up until my recently failed experiment I was using a standard ORTF rig with Omni outriggers, which has sometimes been described as one of the Faulkner arrays. However, one reason I was keen to explore the figure of 8s was to enjoy a sense of room, but to attenuate some of the ceiling to floor reflections.

Also, in reading up several old posts, you seem the be quite skeptical of the Faulkner array Hugh, even though Tony seemed to be using it on quite a regular basis at one point, and not just in a specific difficult acoustic (as can be read in the original 1981 article, the pages of which can be found scattered here: http://www.sengpielaudio.com/TonyFaulknerPhasedArray06.htm).

I'd have thought his concept of summing left and right channels to a mono bus and adding some in for more focus (Zoom?) might give some options in post, but I haven't tried it, so maybe there are deficiencies with this approach.

Anyway I should have the opportunity to experiment somewhat for a Festival I'm recording in February as we have 4 days of rehearsals leading in - and yes, I'll probably revert to the ORTF/AB set up.

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Re: Figure of Eight Microphones in Near Co-incident Stereo Arrays

Postby Aled Hughes » Thu Dec 03, 2020 9:52 am

I started a similar thread a while ago, some interesting insights from Hugh here too: https://www.soundonsound.com/forum/view ... 13#p691110
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Re: Figure of Eight Microphones in Near Co-incident Stereo Arrays

Postby Bob Bickerton » Thu Dec 03, 2020 10:52 am

Yes, interesting thread that one.

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Re: Figure of Eight Microphones in Near Co-incident Stereo Arrays

Postby Hugh Robjohns » Thu Dec 03, 2020 12:05 pm

Bob Bickerton wrote:I'm guessing my MKH8040s might be a better choice than the TLM193s.

Definitely. The mics are angled very widely, so you need high sensitivity and a flat response well around to the sides. So SDCs are a much more appropriate choice.

The 'Gerzon' array as I called it is an interesting application of cardioids, and makes a change from 'ORTF with everything'.... I wouldn't rate it as a favourite of mine, but it has some useful properties, particularly in terms of its imaging.

I like the idea of experimenting with the 'Shuffle' technique...

Shuffling is a fascinating thing and can work wonders... and these days it's fairly straightforward to do without causing more harm than good (which was always the limitation in early analogue attempts). There are some good explanations and demos here: http://www.phaedrus-audio.com/intro_to_shuphlers.htm

Also, in reading up several old posts, you seem the be quite skeptical of the Faulkner array Hugh, even though Tony seemed to be using it on quite a regular basis at one point...

Tony has massively more experience, and better ears, than me, so I'm quite sure he managed to make great recordings with it. As always, it comes down to whether the benefits of the array in a specific situation outweigh the inherent deficiencies of it.

http://www.sengpielaudio.com/TonyFaulknerPhasedArray06.htm).
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