quad miking techniques and listening test

In anticipation of an upcoming quad sound library for echo | collective, I've been looking to build a specific quad kit for recording ambiances. 

I ran some tests to help in purchasing decisions that I'd like to share.  To hear everything properly, you'll need a DAW, an MS decoder and a surround setup.  Files are downloadable here.

I really like quad for surround ambiances - four mic perspectives really does give a very cool spacial perspective to ambiances, and for film work they drop into bgfx tracks seamlessly while leaving the center channel open and clear for dialogue and foley.

The biggest question I needed to answer before upgrading the kit was: is it better to run dual ORTF, spaced pair, or dual MS to get the most natural sound?

Each setup has its plusses and minuses outside of the pure sound of the setup, but I chose to evaluate the pure sonics first, and then use that as the jumping off point for any further decisions. 

The main thing I was interested in was in seeing how the imaging would work for sounds that move around within the ambient field.  If a bird or person or whatever moves from front left to rear right, I want the clearest imaging of that movement I can get.  Of particular interest to me is the way the different setups track movement from L to Ls or from Rs to R, as this is the trickier movement to track realistically.

I chose to use four Audio Technica 4050s to conduct the test, primarily because  I have access to four of these mics, and secondly because the AT4050 can switch from cardioid to figure 8, so that meant that I could objectively evaluate the setup on its own merits without having to compensate for listening to different mics for the diff setups.  

I also chose to record in our warehouse because I wanted to test these arrays in a reverberant space with a moderate noise floor, since that'll be the primary type of ambiance that I intend to record.

My methodology was a follows:

1. set up the mic array
2. run all 4 channels direct into a 788t, and set the preamps to 3 oclock.
3. no highpass, no pads, no limiters.
4. suspend a string on a mic stand directly above or in the center of the array.  This string will be used to keep me exactly equidistant from the array as I move around it.
5. record a pass of walking in a circle around the array while holding the string
6. record a pass of walking the same circle while counting
7. record a pass of walking the same circle while jingling some chimes
8. stop and setup the next array

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First up was the dual ORTF array.

ORTF is defined as two capsules about 17cm (6.69 inches) apart and angled 110 degrees from one another. 

110 degrees adds up to 440 total degrees, so adhering to that standard would end up with the wide 100degree  front and rear angles and a more narrow 70 degree angle between the L and Ls and the R and Rs (if all capsules are equidistant from one another)

Since I was so concerned about tracking from front to rear and back, I decided to narrow the angles a bit and go with a more symmetrical four corner design.  here's the difference:

In the recordings, I found this setup to be very natural sounding, and to have the benefit of being relatively compact.  If I were using small diaphragm condensers I'd be able to pack a rig like this into a large blimp or a pair of blimps.  I could also custom build a square framed mic mount that could just pop on top of a single mic stand in an indoor environment.

Tracking around the room seemed to be very stable with this setup, if a little compact sounding.

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The second setup was a spaced pair.  

I left the angles of the mics at 90 degrees and set the mics themselves 6 feet apart at the corners of a square. 

In my final evaluation, this was easily my favorite sound.  the ambiance was super wide, yet the tracking across the various speakers was superb, and far more defined than the other, tighter arrays.  I could really feel where I was in the room, and at no point did I feel any holes or dropouts. 

If I have total control over a quad environment, this is going to be the way that I will record. 

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The last setup was dual MS. 

 I ran each M at 180 degrees from one another, and kept them relatively close to each other. 

After running the test, I came across this photo, which showed a dual MS setup sitting comfortably inside of a single blimp.

In that setup the rear M channel is slightly offset, so if you'd like to approximate that sound with my test, just decode the rear portion using the S from the front portion (discarding the rear S channel I recorded)

Here's what my setup looked like:

Upon listening, I found the dual MS setup to be the least satisfying.  The fridge in the room kicked on during this test, but I felt that after decoding it sounded even noisier than what the fridge added. 

The pattern was also super tight, and it was more difficult to track movement around the room from the distance that I was walking it. 

The dual MS rig is clearly the most convenient, but also the least impressive sounding of the three setups I tested. 

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In the end, I encourage you to pull down the files and listen to them to make your own assessments.  

After hearing the tests I opted for a quad cardioid approach rather than a dual MS one, and if I control things to the degree that I'd like, I'll be spreading those mics out very wide within the rooms that I'll be recording.

Quad four corners is probably the most cumbersome to setup, and it's impossible to do it stealth style, which is a shame because it yields what is to my ears the best overall results.  It'll be my default for quad nature recordings though, and for interiors where I have control over mic positions. 

When I need to have a smaller footprint the narrower four corners is a nice substitute for public space recording, and proper wind protection is probably achievable. 

I'm a little bummed that the dual MS didn't stack up better because that's by far the most convenient rig to put together (3 channels and it fits in one blimp), but I suppose that's why we do the tests.  :)

In the future I may run one more test where I place the mics in the corners of the room and walk the same circle inside of the mics instead of outside of the array. 

enjoy!

building a ribbon mic part 2: listening test

One of the best pieces of advice I got after building my ribbon mic was to test it through different preamps (thanks John Sanacore).


At my house I noticed some RF buzz, which is actually typical because my house is an RF nightmare.  I decided to ping the designer, Rick Wilkinson, and ask his advice.  Rick responded almost immediately with a comprehensive and detailed troubleshooting list that was incredibly helpful.  The problem ended up being a faulty cable, but I learned that the brass grille was part of the faraday cage from the exchange.

I took the mic to work the next day and ran it through the John Hardy M1, but wasn't very impressed with the sound.  It actually had sounded louder and clearer at the house through the RF buzz.  I then plugged into a sound devices pre similar to the one I used at home and was much happier with the output.  I also began looking into getting a cloudlifter to help with impedance loading and output gain from the mic.  My research was showing that ribbons in general tend to want pretty loud sources, but I have designs on using in more delicate situations and needed get it up to par there.

I emailed Rick again, and again he was again super responsive and detailed.  here's a quick excerpt of what he said to me regarding my experiences to this point:

High end response is determined by the input impedance of the preamp.

Generally, a higher input impedance delivers better high-end response in ribbon mics.  A REALLY high impedance preamp (10k ohms or higher) like the AEA TRP, or my DIY Preamp Kit made especially for my mics (available in another month or so) will really help the top end.  

"loading" is the term for the interaction between the output impedance of a microphone and the input impedance of the preamp.  The correct description of this interaction is something like: "The microphone has to work harder, if the input impedance of the preamp is lower."  This makes sense if the microphone is overly-simplified to a current source: It takes more current to drive a low-impedance load.

A simplified version of this is:

A lower input impedance requires the mic to work harder to deliver a signal. Thus, low-energy, high frequency soundwaves do not get transferred to a low impedance preamp as efficiently as to a high-impedance preamp.

You can see this interaction in my own design, by looking at the impedance curve on the specification PDF on my website.  As the measured frequency goes past 10kHz, the impedance soars - literally off the chart - lowering the effective output of the mic in those frequencies.

That's probably why an Austin Mic through your Hardy sounds like it does... I just looked at the specs, and their Jensen input transformer is 150 Ohms. The current flowing through the ribbon is trying to keep the foil inside the gap, not allowing it to move with low-energy, high-frequency soundwaves.

Supporting that theory, the Sound Devices input shows an input impedance of 2000 ohms - 13x higher than the Hardy.  Better high-end, right?

 And of course, he was correct.  The thing that the cloudlifter provided me was a good impedance load (3000 Ohms) in addition to the extra 20db of gain.  Here's a quickie listening test of the mic through the three different configurations.  All of this audio is 100% as recorded - no gain or eq adjustments of any kind. Note that the gain settings are described in the recordings, and in the first two examples its all the way open, and in the last clip its around 12 o'clock.



So, the impedance loading clearly makes a huge difference with regards to the tonality and the output level of the mic.  IMO it's not very usable straight into the John Hardy M1 unless you're talking about very loud and bright sources, but the sound devices pre and the cloudlifter/M1 combo make the mic much more versatile.

So, given an input chain that I liked (cloudlifter/John Hardy M1) I did a few more little tests with bright, transient things and the mic performed extremely well.



To my ears the mic is certainly mellower on the top end than my usual LDC, the Audio Technica 4050, and its certainly got its own personality.  It handles transients like a dynamic, which is to say that its a little jumpy on the loud stuff and it drops off on the low level stuff more quickly than a condenser.  To me that generally means that its not going to catch as much detail on a highly transient source, and will be happier with something relatively consistent.  It's also got low end for days.

With all of that said, that little strip of aluminum leaf I bought at hobby lobby, cut, corrugated and mounted into that chassis is clearly capable of capturing frequencies well above 20k, which is impressive.  I only ran the instrument tests at 44kHz, but check out how easily it handles all of the frequencies up to the top of that 22k spectrum.

This is the spectrogram of the instrument file linked above:

And here's the zoom in on my voice slate and the tambourine.

This mic is clearly capable of capturing ultrasonic frequencies.

I'll run another test soon where I use it as the S in an MS config on some thick metal movements and see how she does.