Acoustic guitars are central to many live performances, but making them sound good through a PA isn't as straightforward as you might imagine.
After vocals, acoustic guitars are probably the most common instruments a live sound engineer has to deal with. In practical terms, they are also often relatively simple instruments to amplify, as performers tend to use guitars with built-in pickup systems, which means you don't even need to bother with a microphone. But plugging them in is only the start of the battle: making them sound good is the tricky bit! In this article, I'm going to share the tips I've learnt over many years of dealing with more strummers, pluckers and pickers than I can remember.
Assuming the guitar you're working with has an onboard pickup system, you'll want to plug it in to a DI box, to better match the guitar's output level and impedance to the mic inputs on your mixer. For more info on DI boxes, I recommend you read Hugh Robjohns' excellent article in last month's SOS: /sos/may13/articles/using-di-boxes.htm.
Most built-in acoustic-guitar pickups are of the piezo variety, which is unfortunate as they almost universally sound horrible. Probably the first thing you'll hear once you've turned up the gain and lifted the fader is a very spiky sound (often referred to as 'quack') that barely resembles that of the guitar being played acoustically. This is because piezo pickups sense sound from directly under the guitar's bridge, so rather than the lovely 'bloom' that happens when a guitar's strings, neck and body excite each other to resonate, you're left with a transient-rich sound where every pluck pokes through your eardrum and such sustain as there is sounds extremely quiet compared to the note's attack. This extremely dynamic sound can be effectively dealt with using compression, but my first port of call is to address the guitar's tonality, using my mixer's channel EQ.
More than any other instrument, I find that piezo-equipped acoustic guitars normally require some pretty savage EQ to combat their rather nasal quality. Mid-range cuts of 12dB or more aren't unusual, and these can be accompanied by some rather unintuitive boosts elsewhere in the frequency range to try and coax a natural sound out of the guitar.
If I have the luxury of a soundcheck, I'll do this in the way we've often recommended in our Mix Rescue articles: boost the mid-range, sweep the mid-frequency until that 'quack' sound is most prominent, and then cut, quite brutally if necessary. There are no hard and fast rules, but the cut usually ends up somewhere between 800Hz and 3kHz.
At events where I'm dealing with several performers and quick changeovers, such as festivals, I often don't have time for a proper soundcheck, in which case I'll either try to use the above method as quickly as possible while the guitarist is tuning up (an often overlooked window of time for last-minute adjustments!), or I'll let the musicians start playing, with the guitar channel's EQ more or less flat. From there, it's all about making a subtle mid-range cut, slowly sweeping the frequency until the sound improves, and then gradually cutting more until I've arrived at a sound I'm happy with. This approach requires a keener ear, as mid-range cuts stand out much less than mid-range boosts do, but I'll almost always find that a decent sound has taken shape before the first song is finished.
And in some cases, a deep mid-range cut is all that's needed! Toning down that egregious spiky sound might well be enough to let you hear some of the guitar's resonances, while creating that 'hole' in the guitar's mid-range often makes a useful amount of room for vocals. Since much of my live sound work involves musicians of the 'singing hippy' variety, this is no small bonus.
More often than not, though, cutting those spiky mid-frequencies starts to highlight other problems with the guitar's sound — most commonly, a congested, lower-mid boxiness. If the desk you're using has a four-band EQ (or you're lucky enough to be using a digital mixer), you can simply use another parametric mid-range band to cut that as necessary — but if your mixer is limited to just one sweepable mid, like the compact desk I take with me when I'm not using a venue's or hire company's console, you might have to think outside the box.
It might seem perverse, but when I find myself in the position of wanting to cut some lower-mid boxiness when I've already 'used up' my only parametric EQ band on dealing with the mid-range 'quack', I'll often find myself boosting both the high and low shelving filters, sometimes quite substantially. The logic behind this is that low-shelving filters tend to be set somewhere around 100Hz or so, while the boomy boxiness I'm often trying to tame lies at around 300Hz to 400Hz. With the lows boosted and the mid-range band working at, say, 1.5kHz, the gap between those two bands is essentially being cut, while the boost at the high end helps to counterbalance the increased low end... hopefully leaving you with a full-sounding and sparkly acoustic-guitar tone!
It's worth mentioning that the amount of low-end you add to (or take away from) the guitar will depend to a large extent on what else is happening on stage. For soloists or duos, for example, I tend to boost the low end quite a bit in order to fill out the sound. Without a bassist, you can get away with a fair amount of low-frequency content in the guitar's tone, as long as it doesn't become overpowering. In a band situation, though, especially when you've got bass and kick drum going through the PA, you'll probably be better off thinning out the sound quite a bit, so that you're left with some of the ringing and the percussive quality of the guitar. Otherwise, you'll find that the low end of the mix gets very busy very quickly, and will start overwhelming the vocals, to the point where they become unintelligible.
So far we've only dealt with piezo-equipped guitars, but some have magnetic pickups — of the same family that electric guitars use — wedged in the soundhole underneath the strings. Though they don't sound particularly natural (like piezos, they react to string vibration rather than sensing the resonances of the whole instrument), they also don't suffer from the extreme sensitivity to transients that blights most piezo systems. In general, this means you don't have to worry about the extreme mid-range cut I mentioned earlier, and can basically EQ to taste, and to accommodate other instruments (and vocals) in the mix, rather than to 'fix' the sound.
Magnetic pickups can actually sound pretty decent in the context of a full band. They have a slightly saturated quality that makes them sound almost like an electric guitar going through an amp, and while this might not be ideal for solo acts, I find them much easier to work with than piezo-equipped guitars.
Of course, not all guitars come with a convenient jack socket. At some point in your engineering life, you'll almost certainly encounter one without. If you're lucky, it'll be owned by someone who's too afraid to drill the necessary holes in their vintage guitar to fit a pickup system, rather than someone who's never performed before and has never needed a pickup! Whichever is the case, you're going to have to rise from that comfy chair to get a mic and mic stand ready...
If at all possible, you should be using a hypercardioid or cardioid small-diaphragm capacitor mic — avoid omnidirectional mics, as using one is just asking for trouble in the feedback stakes. Dynamic mics are great for many things, but they're not particularly sensitive, and, guitars being relatively quiet, this means you'll need more gain — increasing your risk of feedback. Dynamic microphones also tend to have a fairly restricted high-frequency response, which can leave you with a very dull and muddy sounding guitar. I have had to mic up an acoustic guitar with only an SM57 to hand, but it's not something I'd recommend to anyone, and I ended up spending the entire time riding the monitor send so that the performer could hear himself without getting blown away by a nasty dose of feedback.
As far as positioning the mic is concerned, I always opt for the 'vanilla' placement, aiming the mic at the point where the guitar's neck and body meet. This is mostly a pragmatic decision, as time is always of the essence when setting up a gig, and I don't think I've ever had time to faff about experimenting with different microphone positions when engineering a show.
It helps to try to get the mic as close to the instrument as possible without the risk of the performer hitting it with the guitar as he or she becomes more animated during the louder numbers. This is easier if your guitarist is sitting down for the show, obviously, but even then, many performers (singer-songwriter types, especially) have a habit of swinging their guitar back and forth, which means you may have to deal with some pretty wild variations in volume. Aside from exercising your fader finger, there's not much you can do about that, although more experienced performers may be quite accommodating if you simply ask them to keep their guitar fairly still as they're playing.
Compressors are wonderful devices, and, set appropriately, they can solve all kinds of problems — including our guitar-swinging singer-songwriter! Although you shouldn't expect a compressor to do all the work when dealing with massive volume jumps, a relatively low threshold and a low-ish ratio (say, 1.5:1), combined with medium to long attack and release times, should help even things out a bit, and you'll find your fader finger shouldn't have to work quite so hard. Beware of feedback, however: if you're close to ringing already, applying compression may well take you over the edge, so before you start squeezing things, make sure you've got plenty of gain to play with.
Another particularly good use for compressors is taming some of that nasty piezo sound. With your compressor's attack and release times set almost to their fastest, and with a more aggressive ratio, the compressor will act to reduce the level of string attack relative to the guitar's sustain. When setting the compressor up, keep your eyes on the gain-reduction meter: when you see it briefly bouncing between 0dB and 3dB or so, and then only when the guitarist is strumming or picking, you should have it set about right, and the guitar will sound considerably smoother and less spiky.
The advice given here should help you out with the most common acoustic-guitar-related problems on stage, but it's important to remember that there's no one-size-fits-all approach. Every guitar is different and every guitarist has a different way of playing, and this will invariably affect the tricks you have to employ to create the best sound out front. It's also important to stay on your guard: you never know when a delicate fingerpicker will magically conjure a plectrum from nowhere and start piercing the audience's (and your own!) eardrums, or when an innocent-looking folkster will produce a violin bow and treat you to their best Jimmy Page impression.
Incidentally, both of these things have happened to me before — and I've also had one innocuous singer-songwriter start his performance innocently enough, only to turn his guitar upside down and use it like a hand-drum, throwing my carefully judged EQ decisions out of the window! That said, armed with these tips and a bit of experience, you should find that, before long, there isn't a six-string strummer or plucker you can't help to sound their best.
Don't be tempted to ask the performer to adjust the EQ settings on their guitar if the sound you're initially hearing out front isn't what you want to hear: they've more than likely either got a favourite setting that they'd rather not change, or they're tweaking so that the guitar sounds right to them through the monitors — and as far as the on-stage sound goes, the performer is always right! Leaving the musician in charge of their monitor sound also saves you having to do much to the guitar's monitor send, leaving you to concentrate on the front-of-house sound.
In some smaller venues, where the sound from the PA is significantly affecting what the musicians are hearing on stage, I have occasionally found that the guitarist is compensating for my front-of-house desk EQ changes by making the exact opposite moves on their pickup system's EQ! This can be frustrating, but it's one of the inevitable compromises when putting on a gig in a small room, and the only thing you can do about it is be subtle with the tweaks you're making, and to apply boosts and cuts slowly so as not to throw the performer off.
Like most instruments, the acoustic guitar is designed to resonate, which makes it perfect for generating feedback! The solution to this is to keep the monitor levels as low as possible (while still giving the performer enough volume to hear themselves adequately), and tackle any particularly troubling frequencies with a graphic EQ inserted in the monitor send. Our March issue article on dealing with feedback on stage (/sos/mar13/articles/live-feedback.htm) contains many tips on coping with howl-rounds, most of which can be applied to acoustic guitars.