Recording Acoustic Instruments Tutorial

Whether you’re capturing a full choir, a solo performer or just a few steel-string overdubs, the process of recording and mixing acoustic instruments offers some unique challenges. Mark Cousins helps you over the hurdles.

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Acoustic instruments have been the driving force behind music for hundreds of years, and despite the prominence of electric guitars, synthesizers and samplers nowadays, they remain a key ingredient in many forms of music. Whether it’s an intimate steel-string guitar or a full-size symphony orchestra, the addition of ‘real’ acoustic instruments in your music can bring an important lifelike dimension and performance quality that’s impossible for samples to replicate.

However, there’s a distinct art to recording acoustic instruments, often in marked contrast to more common techniques (such as copious overdubs and close-mic’ing) that we’re used to following when tracking a band.

In this month’s feature, therefore, we’re going to explore the process of recording and mixing acoustic instruments, exploring everything from single-instrument overdubs through to large ensembles with multiple players. While there’s plenty of advice on individual instruments, it’s worth remembering that many of the principles explained here are transferable to a range of different instruments and groups, whether you’re recording an upright piano, for example, or a gamelan ensemble.

Although there’s always a lot of energy present at an acoustic guitar’s sound hole, it doesn’t always deliver the best tone. Remember that an acoustic guitar resonates across the entire shell of its body, so it’s worth listening out for other sweet spots to record

Going Acoustic

Arguably the key objective behind all acoustic recordings is capturing the instrument/ensemble in a natural and accurate way. Unlike electric or electronic instruments, we have a clear point of reference as to how acoustic instruments should sound, born from hundreds of years spent listening to music.

When recording an acoustic instrument, we need to understand how to capture the essence of what we hear, representing the source and performance in as lifelike a form as possible. While this objective seems simple enough, it will stretch your skills and equipment in some unique and interesting ways.

To successfully capture the sound of an acoustic instrument, we need to observe some overarching principles. Firstly, the timbre should be as wide and as flat as possible, capturing the detail and harmonics of real instruments, but at the same time negating artificial colour such as the proximity effect. We also need to understand how the room can interact with the instrument or group, especially in the case of ensembles or orchestras, when the hall or cathedral becomes an intrinsic part of the overall sound. Another vitally important entity is dynamics – ensuring your recording has plenty of transient detail and accounts for the wide dynamics that occur in most acoustic performances.

For the ultimate in sonic realism, we also need to consider stereo recording, replicating the ‘spatial’ experience of listening with two ears. Although it won’t benefit all solo acoustic instruments, it’s a vital part of ensemble recording.

When placing spot mics, try to observe the 3:1 rule: leave three times the distance between the microphone and performer and other adjacent mics. This will minimise phase issues

Solo Instruments

Before we delve into groups of players, it makes sense to understand the key principles of acoustic recording when capturing just a single instrument. With any acoustic recording it’s essential to take some time to fully consider how each instrument behaves. Even if it’s an instrument you are very familiar with, each performer, each room and each design of instrument will bring a unique set of variables. Ultimately, therefore, it’s worth spending a few minutes listening to the performer and instrument in question, actively moving around the room to understand how the instrument and the room it is being performed in ‘ticks’.

In particular, one of the biggest misconceptions about acoustic instruments is the role and input of the sound hole. On the whole, all instruments radiate sound in multiple directions rather than it emanating from a single point-source. An acoustic guitar, for example, resonates across its entire body, with the sound hole actually working as a form of bass reflex port. The common mistake, therefore, is to place the microphone where you think the sound is coming from – in this case, the sound hole – which often has undesirable results. Use your ear, therefore, and try to find the spot where you feel that the instrument broadly sounds its best.

When it comes to the choice of microphone, the most obvious has to be the condenser, given its wide frequency response, uncoloured sound (by comparison to dynamic or ribbon mics) and its sensitivity. You also have the choice of a large-diaphragm condenser (which will tend to deliver a more flattering sound) or a small-diaphragm condensers, which will tend to deliver a more natural, ‘flat’ response. Although it’s tempting to opt for a large-diaphragm design (principally because they look good!), you’ll often find that a matched pair of small-diaphragm mics provides much more flexibility in the long run.

Another benefit of many condenser microphones is that it will often come with a range of additional switches that enable you to get the recording right ‘at source’. The trick here is to trade the proprieties that you want to capture against sonic traits you want to tame. Use a cardioid polar pattern, for example, to make the mic more directional and reject other instruments in the room (assuming they’re behind the capsule) or use an omni pickup if you want the sound of the instrument and the room. Use bass roll-off to tame low end, especially if the instrument doesn’t output frequencies in the bass spectrum.

The Distance Equation

One of the most important points to consider in respect to placing the microphone is the impact of distance. Put simply, few acoustic instruments sound their best when they’re close-mic’ed, with the results being coloured by proximity effect (a bass-lift produced by the mic being too close to its source) and the fact that the full resonances of the instrument aren’t given space to ‘mingle’. One suggestion is that a good initial placement can be found at a distance of one-and-a-half times the length of the instrument. In reality, of course, it’s best to use your ears rather than rely on a tape measure, so observe how the tone of the instrument changes as you move the microphone away.

The balancing point to the distance argument, though, is the negative effects that can occur as you move the microphone further away from the source. First and foremost, you’ll need to raise the gain applied to the microphone, potentially increasing problems such as room noise, floor rumble, preamp noise and noisy neighbours. By moving the mic away from the instrument you’ll also change the relationship between direct sound coming from the instrument in question and reflected sound bouncing off surfaces and walls. If you’re recording in a pleasant-sounding room, reflections might be a positive addition. Otherwise, move the microphone closer to the instrument to negate the room sound.

 Like a number of acoustic instruments, an upright bass might have a pickup output. A pickup is worth recording, especially as it can add some interesting midrange articulation to the overall sound.

Recording Ensembles

In most cases, recording a single acoustic instrument shouldn’t present too many challenges as long as you can find a good microphone and a proficient performer. Things get trickier, however, when we start to consider the process of recording a group of musicians – whether a string quartet, a choir or a symphony orchestra. As with solo instruments, the principles are transferable to a range of situations, so if you can master a couple of players, you can easily scale-up the proceedings to ensembles consisting of tens or even hundreds of players.

On the whole, most acoustic ensembles have a ‘natural balance’, with each instrument/section of the group contributing to the overall sound in a musically effective way – in other words, instruments don’t stick out of the mix, and if played correctly, everything should be audible. Given good natural balance, therefore, your job as an engineer is considerably easier. If the ensemble isn’t naturally balanced, you’ve got a harder job on your hands, and you might consider screening or even separate overdubs to make the sound more controllable. Ultimately, though, the best performance often comes from all of the players performing at once, so try not to over-complicate unnecessarily.

Larger ensembles are recorded using a combination of spot microphones and a main stereo pair. In this orchestral session, a Decca Tree is used as the main stereo microphone array. 

Pairing Up

Given a well-balanced ensemble, the most important mics in the room will be your main stereo pair. In effect, the stereo pair forms recording’s equivalent of the ‘best seat in the house’ – capturing a well-balanced ensemble, the full width and dimension of the soundstage, and the room that the players are performing in. Placement of the stereo pair tends to be around the same position as the conductor, although you may want to bring the mics back if you need to get slightly more ‘room’ into the equation. Of course, if you want more control over the acoustics, consider a separate stereo pair as room mics.

One of the biggest points to consider with your stereo pair is the type of microphone arrangement you choose – a pair of near-coincident cardioids, for example, some spaced omnis, or an arrangement such as the Decca Tree, which is favoured by many orchestral recording engineers. Each arrangement brings its own strengths and weaknesses into the equation, so do plenty of research before you make a decision. On the whole, a near-coincident pair tends to deliver the best results in terms of phase (so the final mix will have good mono-compatibility) whereas spaced omnis create a wider stereo image at the expense of phase coherence. A Decca Tree comprises three omni mics (left, centre and right), arguably allowing you to control the width of the recording yet retain a good monaural image.

In addition to the main stereo pair, you’ll want to employ a number of spot mics to help the articulation of each instrument group in the mix, whether you’re lifting an instrument for a solo, for example, or adding detail that the main stereo pair might have missed. The trick with spot mics is not to go overboard. Most notably, it’s easy for the spot mics to create phase issues due to timing differences as one instrument bleeds to an adjacent microphone. As a rule of thumb, try to keep the distance between mics at least three times the distance from the performer (the 3:1 microphone rule) to minimise phase issues.

Try adopting a looser approach to recording drums that captures more of the instrument and the room. Build the sound predominantly from the overheads, adding spots to support the kick and snare. 

Mixing It Up

Mixing an ensemble recording presents many unique challenges, primarily as you can only influence the overall balance rather than radically re-shape it. Always start from the main stereo pair and then lift the spots to add definition and articulation to each instrument. As you blend-in the spots, keep an ear on any ‘phasiness’, and use just enough to reinforce the main stereo pair rather than dominate the mix. If phase is an issue, you can consider applying a small sample delay (around 129 samples for every metre between the main stereo mics and the spots) to better align sound arriving at the two capsules.

Compression and equalisation is best used subtly, although both will be important in establishing a good overall balance and polished sound. Compression is essential to bring some of the wide dynamic range under control, although you’ll often find that a low threshold and ratio (around 2:1, yielding about 3–6dB of gain reduction) provides smoother control than harder compression settings. It’s also worth experimenting with parallel compression – a technique that actually originates from classical recordings – as a means of adding body to the sound without destroying transient detail. When it comes to the spots, you can achieve some interesting results by being slightly more heavy-handed, helping their sound ‘bed-in’ behind the sound of the main stereo mics.

Acoustic EQuation

Light, ‘corrective’ EQ in respect to acoustic instruments always sounds better than aggressive re-shaping. Again, though, you can afford to make some distinction between a minimal approach on the main stereo pair, and a slightly more judicious application on the spots, which should already sound coloured due to the proximity of the spot mics. In particular, you might find that EQ goes some way to remedy bleed between the various mics, although in this respect a live acoustic recording will never have the extreme separation most engineers are used to achieving in an overdubbed band recording.

Even if the room sounds great, you’ll probably find it beneficial to add a small amount of additional artificial ambience back in the studio. Indeed, most orchestral albums, despite being recorded in a big, well-equipped studio, are still recorded relatively dry so that they can edited easily, with reverb added afterwards as part of the mix. It goes without saying that you’ll want to use a good digital reverb, either a convolution reverb that samples a real concert hall or one of the ‘name-brand’ reverbs such as a Lexicon or Bricasti. I also find that using one or two reverbs tends to deliver better results than multiple different sends, especially if you want the players to sound as if they’re performing in a single room or concert hall.

Experiment with a range of stereo mic arrangements when recording a piano. Moving the mics away from the piano will yield a softer ‘classical’ tone: placed closer to the instrument, the sound becomes more percussive

Going Unplugged

The process of recording acoustic instruments is a voyage of infinite discovery, with unique variables presented each time you record with a different performer, instrument and acoustic space. Pleasingly, though, it seems to be a process that rewards simplicity – a few of the right decisions (especially the choice of and placement of mics) seems to pay far more dividends than an over-complicated production process. Ultimately, it’s about understanding the instrument and the music it produces, using your recording skills to capture it in an unobtrusive and elegant way so that the listener almost believes they’re sitting in front of a real performance

This vid samples a few mics and acoustic guitar recording techniques:


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