Sunday, 9 October 2016

Recording Techniques for Acoustic Guitar

A good friend of mine, Oda Kveinå Tonstad, is just about to embark on Leeds College of Music’s Music Production program. She has called me ‘mentor’ for a few years — a title I didn’t request but am very happy to receive! Before departure she wanted to run through some recording techniques for acoustic guitar. We met at my project studio and had a limit of two hours before other obligations kicked in. We ran through four different recording techniques and I’ll give you a brief summary here. These are not meant to be ‘the four quintessential techniques for every engineer,’ but rather a selection of techniques that I like. Other articles may have slight variations over some of the techniques I’ll describe — this is where you should let your personal preferences be the guide.

The microphones we used was a small collection of typical
project-studio microphones.

The Techniques



1. Stereo Pair/ Spaced Pair

The only small-membrane pair I had available was a pair of affordable omni-mics. Ideally, I’d like to use a pair of cardioid microphones in, for instance, an X-Y configuration. There is little point in trying to use a pair of omnis in X-Y so we did a spaced pair configuration (but leaving out spacing-rules since we just wanted to build up some ‘vocabulary’ of techniques). We used a stereo bar, which roughly mimics the distance between someone’s ears. We placed the microphones about half a meter away from the guitar, just above it, physically pointing towards the guitar-body.

While the X-Y, ORTF and similar techniques work by literally pointing the microphones in different directions, omnis don’t ‘point,’ as it were. A spaced pair utilises the distance between microphones —the sound hits the microphones at different points in time and the combination of the two creates a stereo-image. In other words, the sound is delayed between the channels in your mix if the sound hits the microphones at different points in time. This also means that sound that hits at the same time will be in the centre of your mix. While reflections from the surrounding walls will arrive at different times. Sources placed at the side of the set-up will have more delay between the channels (the stereo-image gets wider or tilts to one side). But with sources placed at the side of the set-up you should look out for cancellations of important frequencies. The X-Y technique or other techniques where the membranes of the microphones are placed as close as possible does not have the same problem with cancellations. That doesn’t make these techniques better in themselves, but they might be better suited for certain applications.

For longer discussion on the spaced pair technique, I suggest this link.

2. Mono Big-membrane Overhead

This is technique where a big-membrane condenser in pointed towards a guitar from a medium distance. When I was sitting down with the guitar, the microphone was placed about half a meter in front of me and a little higher than my head. The distance allows the microphone to pick up the whole guitar, plus some of the room, and not just the relatively isolated sound of the instrument. –not unlike a person sitting in front of someone playing a guitar. The mix between the room and the guitar can be adjusted by moving the mic closer or further from the guitar, just like a pair of ears.

3. Mono — 12th fret

When we were done recording the over-head, Oda insisted that we lowered the mic and put it in front of the 12th fret. I had thought about dropping this technique because of limited time, but it turned out to be a great thing that we kept it. The mic was placed at about 30 cm. (or around a foot) away from the guitar. Perhaps the most classic mics to use in this configuration are SM57s and U47s (although they are very different). Placing a mic in front of the sound-hole can produce a lot of bass-rumble. Placing a mic pointing at the body/ soundboard of the guitar can produce a pleasant sound but with very little ‘bite’. Placing a mic in front of the 12th fret is a good way of capturing both the attack of the strings and some of the sound of the body — with just one mic. This is probably the most used recording technique for acoustic guitar in recording-history.

Graham from the Recording Revolution recently did a great video where he shows a variation of this technique:

4. Two microphones — 12th fret and body

This is my personal favourite. It involves two mics that capture two different parts of the guitar. We changed the condenser mic on the 12th fret to an SM57. This produces a sound with a bit more ‘bite’ and a bit less ‘body’. We then put the big-membrane microphone pointing towards the soundboard behind the bridge. We put it just off the corner of the guitar, pointing at an angle towards the soundboard between the bridge and the edge of the instrument. I tend to use small-membrane condensers (Oktava MK-012 and Neumann KM 184 are favourites). Small membrane mics have a cleaner off-axis response, which is something to take into account, especially when placing the mic at an angle. But a big-membrane or another dynamic will also do. One aspect of making the microphones blend well is to avoid phasing. To adjust the position if the condenser-mic, I usually put on a pair of headphones and move the mic around until I find a sweet-spot where the two microphones blend well. (Naturally, you have to hear both microphones in the headphones when performing this manoeuvre. They should not be panned out, but be dead-centre to reveal any phasing if you have stereo-listening.) We didn’t have time for this today and we ended up needing to invert the phase of one microphone in the mix. After applying the phase-reverse the guitar sounded heavier/ deeper and more focused. Using headphones during mic-placement is a couple of minutes well spent for optimising and focusing your sound. In the mix, the microphones are panned hard left and right, or to - 25%, + 75% according to your preference.

If the guitar has an internal mic or pic-up I usually add this as a separate line. It often turns out redundant, but it adds an extra back-up or option for the mix. In the mix it can be used for layering — panned out opposite another layer, or with processing/ amp-simulation if desired. Though if it is intended from the beginning to feature opposite another layer in a section of the song, I would usually record this track separately to get true double-tracking.

The two microphones used for this technique: one dynamic
pointing at the 12th fret, and one condenser pointing at an angle
towards the soundboard behind the bridge.

The Verdict

Assessing the recordings. Oda was the Pro Tools-operator for
today's session.

1. Spaced Pair

We found the sound to be a bit boxy and bright, which partially reflects the affordable home-studio microphones. We tried to soften, sweeten and focus the sound by gentle use of eq., compression and reverb. –but fixing things in the mix have their limitations. We both agreed that the clear, bright omnis would be a more interesting option if used as room mics in a multiple-mic set-up. Oda felt some bass was lacking. This cannot really be alleviated by moving the mics closer to the source (as it usually can), since omnis have no proximity effect.

2. Mono Overhead

The microphone captured the natural sound of the guitar nicely. Some of the room bled into the recording (which you may or may not like). I have had good success with this technique on bright and rattly guitars with tube-mics before. However, on this particular recording we felt that the sense of the guitar’s physical presence was not as strong as we had hoped for in the mix. So we muted it and moved on to:

3. Mono — 12th fret

This was one of our two favourites today! The sound was focused, large enough, bright enough and very-very mix-friendly. Processing and placement in the mix would have been the easiest to do of all the recordings. The result was a good testimony to how much you can do with a simple home-studio microphone.

4. Two microphones — 12th fret and body

This was the other favourite. The stereo-image is good, but in some mixes it can compromise a clear sense of the sound’s ‘location’ (this can be improved somewhat by finding the sweet-spot when placing the second mic as described above). Oda said that this was the technique she wanted the most to experiment further with, while we both agreed technique number 3 was the safest option to get good results quickly. An added bonus of this technique is that if the two channels are panned out in stereo, it helps to clean up the phantom-centre for vocals or other lead-sounds. But if you have a busy mix and you want the guitar to be exactly in one location, technique number 3 will be the easiest to work with. Technique number 4, on the other hand, can also be used as a mono-technique where the two microphones provide ‘bite’ and ‘body’, to construct a more complete sound. This can be bounced to mono or mixed down to a group-track on a mixer (that is, an aux-track in ProTools), and subsequently be placed in the mix in one particular location. –just like with technique 3.

* * *

To create the most amount of diversity between the techniques, in this article we have convered:

·      Two mono-techniques (respectively close and distant)
·      Two stereo-techniques (respectively close and distant)

If you’re starting out doing recording, my advice would be to get to know technique 3 first. The video from The Recording Revolution will show you a way to expand on this technique.

If you’re used to close-micing guitars in mono, you might want to try technique number 4, as this will broaden your toolbox quite a bit.

Alternatively, you could also experiment with adding a pair of room-mic or more closely paced overheads (or a single microphone for that matter), to the mono-technique you are already familiar with. This can add more depth and room to the sound, but this is also subject to having a nice sounding room. With more than one microphone you should stay on the look-out for phase problems. Also remember that room mics are a compliment to the close-sound and can be brought up and down according to the need, just like with an artificial reverb.

If you want some fresh thoughts on how to set up microphones in a way that plans ahead to the mix, see another one of my blogposts: ‘Mixing with Microphones’.

Have fun recording, and feel free to leave your own recording-experiences or questions in the comments below!

Me pretending to be a Greek Philosopher at the
university campus later the same day. (Photo: Oda)

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