For Better Quality - The Classic Stereo Recording Techniques – Part II

The technique I’d like to show you is the so called ‘Spaced Pair’ or AB microphone array. This is what I use for most of my recordings and what most engineers and home-recordists recommend. You can use this technique in many different variations, depending on the instruments you want to record. First let’s take a look at the classical version.

The Spaced Pair (AB)

The classic AB array consists of two identical microphones with omni-directional polar patterns, which are placed apart anywhere between 15 cm and several meters, both microphones pointing at the sound source which could be anything from a single instrument to a full orchestra.

AB with omni mics (Rode NT55)

Variations of this technique:

• For single instrument recording you can use two different microphones instead of two identical ones.

• You can use cardioid mics instead of omnis.

• You can vary the distance of the individual microphones from the sound source (see phasing!)

• You can put additional microphones in the middle (e.g., Decca-Tree).

Phasing

With this setup we have to pay extra attention to avoid possible phasing problems. As I wrote in the previous article, phasing occurs when the same audio signal arrives at two different microphone membranes at different times. When the sound waves are in different phases, they might cancel each other partly or completely. Let me demonstrate this phenomenon with two pictures.

In the first picture you see the audio waves of a guitar recorded with two microphones. The two signals are not identical – as the different mics picked up slightly different signals due to the different positions – but they are very similar, and what’s more important, they are in phase. You can clearly see this at the similar positions of the waves.

Audio waves - in phase

Now listen to the recording with the two signals in phase.

[audio sample – no phasing]

In the next picture you see two signals that are not in phase (when the top wave goes up, the bottom wave goes down). I did this by moving the bottom signal in the DAW, causing artificial latency.

Audio waves - out of phase

Listen to the corresponding sound sample:

[audio sample – phasing]

The result in this case is a hollow and weird sound. However, phase cancellations belong to most multi-mic recordings and they can often improve the sound, adding ambiance to it or the feeling of dimension and distance. The science may help us understand what’s happening with the signal, but ultimately it’s our ears that ought to decide what’s the preferred sound.

There are some obvious cases, though. One of them is when you record a snare drum with two microphones, one at the top, the other at the bottom skin. In this case the signals arriving at the two different microphones will almost certainly be out of phase. You mix the signals from the two mics and it will sound thin and weak. If you switch the phase on one of the recorded tracks in the DAW, the two signals will get in phase and the result is a fatter and more defined sound. But this is a very clear situation (also, it’s not the classic AB setup). Most of the time we have to use our ears to find the sweet spot for a particular instrument.

Applications

• Recording any single instruments in stereo

• Room ambiance

• Full ensembles, orchestras, choirs

• Drum overheads

Let’s start with a well known instrument, the acoustic guitar.

As being said, the classical AB array consists of two identical omni-directional microphones, preferably feeding the same preamp. Why omnis? Well, mics with omni polar patterns pick up the sound not only from the front of the membrane but from any direction. When you have two microphone set apart from each other, the wider the pickup patterns are, the more signals arrive, and a more coherent, thicker and more realistic sound is the result. With these mics you also pick up more room sound so it’s even more important to treat the recording environment then if you work with cardioid mics.

Having said that, cardioid pattern mics work as a spaced pair as well. In fact, they even have some advantages to the omnis: The result might be a more modern, dry and more intimate sound. Also, if you’re on the budget and you must decide in what gear you invest your money, cardioid mics are always a good choice as they’re more versatile than any others.

You can also break the next ‘rule’ and use two different mics for the spaced pair with different preamps. Think of the microphones as ‘spices’. Each has a different ‘taste’ and depending on the particular instrument, picking style, overall sound, they might add some special character to your recording. Feel free to experiment!

AB Position for Acoustic Guitar

Let’s see now, how I set up the spaced pair for an acoustic guitar. My goal is to get a realistic and balanced stereo image of the instrument. My favorite sweet spots are:

• Mic 1: This mic points straight to middle of the bridge, where (ideally) our picking hand is. From this spot I can get more picking sound and the ‘snare drum effect’ comes through here best.

• Mic 2: I put this mic at around the 12^th fret (for dreadnought) and turn it lightly inwards so that it points to the spot where the neck joints the body. Here we can get more bass and a nice texture. Tipp: This microphone should be closer to the instrument than the other one. This compensates for the latency between the spot where mic 1 is (where the strings have been struck first) and the neck joint (where the sound arrives a tiny bit later). This gives us a more coherent sound, especially if we use cardioid mics. If we don’t do this compensation, the recording might ‘tilt’ toward the picking side.

AB - A good position for the guitar

A few more tipps:

• Never point any mic straight to the sound hole! It will make the sound boomy.

• Try to avoid picking over the sound hole! Thus you have a much more clearly defined sound, and again, it’s a good recipe against boominess.

Listen to the two samples I recorded with omnis and cardioid mics.

[audio sample – AB omni]

[audio sample – AB cardioid]

Panorama: Pan the two signals anywhere between L40-R40 to L90-R90. I personally don’t like to pan it fully left and right, as they loose coherence and it might sound a bit weird to the ears.

Positions for Other Instruments

Some of these setups may not really be described as classic AB setup, yet I’d like to include them here because I see them as variations of this technique. Feel free to find the right position to yourself by moving the microphone with a headphone on. These position worked the best for me:.

Mandolin
Similar to the guitar setup but the two mics are closer to each other. Here I don’t hear much benefit from putting the neck mic closer and turning it inwards.

Double Bass

One mic is at the body, under the bridge, on the right side. It should be a mic with good bass response. The other mic is set up higher, pointing at the picking hand. Thus we can add the finger ‘snap’ to the mix. On the second mic you can use a low cut filter. I usually pan them very tight, only L10-R10 (we don’t want to hear a wide and laid upright bass on the recording).

Handpan (Hang)
I had the chance to record this rather special but quite popular instrument a few times. I usually put a classic AB array (omnis) over the pan, high enough that the player doesn’t hit the mics and can play comfortable. I pan the signals between L50-R50 and L90-R90. 

Handpan recording with Andreas Koller (Mandara)

Piano
With the cover open, place an AB array over the strings so that one mic covers the higher strings, the other the lower strings. Panning from L50-R50 to L90-L90. 

AB for piano (Rode NT55)

Banjo 

One mic points to the banjo head under and away from the bridge, the other points at the neck joint. Panning is same as with the guitar.

Electric Guitar
Although it’s quite different from the classic AB technique, I think it’s important to talk about this topic here that otherwise deserves a full article. I try to scratch it for you now.

The principle here is that we take the amplifier as a single instrument. We try to cover the widest audio spectrum possible that’s coming out of the speakers. If your amp has two or more speakers, use two different ones for the two microphones. First try to place both mics as close to the speakers as possible so that both mic capsules are approximately at the same distance from the speakers (see picture). Then try to find the sweet spot for each mic, which is mostly away a bit from the middle of the speaker. Here’s a setup that’s widely used and works well most of the time:

Electric guitar cabinet miking - The distance of the two
mics from the speakers are not identical. I used this setup
after trying different positions.  

About the mics: We use two different mics. One is a dynamic microphone which can be a Shure SM57, Sennheiser MD 421, EV RE20 (these two are costly mics, though) or similar. These mics usually have a more focused sound (a narrower audio spectrum, emphasizing certain frequencies, mostly in the middle region that’s often a bit rough and ‘scratchy’. Mic 2 is some kind of LDC (Large Diaphragm Condenser), like a Rode NT1, AKG c414 (c214) or Neumann U89 (in our case it’s an Audio-Technica AT4033). Their role is to give us a more detailed sound which would be usually too ‘nice’ on its own, because it lacks the roughness we get from the dynamic mic. For panning, I don’t use a wide stereo setup as my goal is to mix the two signals, the two different ‘spices’. I pan them L10-R10 to L30-R30.

Full ensembles, room ambiance
Use omni mics with a distance between them that suit the size of the orchestra. If this distance is too big we might have a ‘hole’ in the middle of the stereo image. To compensate this, a third additional microphone might be applied in the middle of the array. For this there is a well established setup called ‘Decca-Tree’, in which the third (omni) mic in the middle is closer to the ensemble, forming a triangle with the other mics. The two side mics are panned L100-R100 while the third one is panned center.

Mini Decca-Tree

 For room ambiance I usually use a small distance between the mics, still I have a very nice and realistic stereo image. In this case I use a panning of L100-R100. This is actually the classic AB setup. I have to mention at this point that you can always record an additional room ambiance to any single or multi-instrument recordings. In this case you need 4 microphones. Tipp: Put the room mics on any locations in the room where you think the recorded instrument sounds good.

Pros and Cons of the AB setup

Pros:

• Wide & adjustable stereo image

• Much more realistic representation of a single instrument than mono recording

• There are different variations and applications

Cons:

• Phasing problems can occur

• Ideally you need two omni-directional microphones