Dealing with acoustic guitar in a live environment can be tricky business. A lot of external factors — many over which you have no control — influence your options for getting the guitar into the PA system, producing sufficient volume, delivering tone faithful to the instrument, and avoiding feedback. Your ability to effectively mic an acoustic guitar on stage is dependent on the stage volume of the musician(s); the volume of the audience; the acoustic guitar player’s preferred method of monitoring; the type of guitar (steel or nylon string) and your budget.
It’s a Beautiful World
The best situation you could possibly hope for is a solo guitar player on a small stage, with an audience that is there to actually listen to and respect the artist. Think Kazuhito Yamashita at Alice Tully Hall. A situation like that allows you to actually use microphone technique to reproduce the sound of the guitar in the PA. I’d start with a single small-diaphragm cardioid condenser microphone such as a Shure SM81, Neumann KM 184, Audix SCX or DPA 2011, six to 10 inches away, pointing at the 12th fret or where the neck meets the body. Use a shock mount so that the mic does not pick up any vibration transmitted through the floor. The sound hole is generally going to produce a lot of “boom,” so try to stay away from that area, unless you’re using a mic designed to be clipped into the sound hole.
An alternative placement would be facing the mic directly at the top of the soundboard between the bridge and the end pin, but you’ll have to use your ears because some guitars sound thin when miked from this area. You can angle the mic slightly upward so that the rear (where rejection is maximum) is facing a floor wedge, if any. Ideally your guitar player either does not need a monitor or uses in-ears, either of which tremendously cuts down on the possibility for feedback. Oh yeah, and hopefully the player remains relatively still and stays on mic.
Real World Realities
As volume requirements increase, your options decrease. Take for example a singer-songwriter performing in a small club, restaurant or coffee house — where people are paying attention not so much. You may still be able to use a microphone to capture the guitar, but oddly enough, a microphone with a limited response and sensitivity may produce better results. The microphones previously mentioned are all very sensitive; that sensitivity extends to capturing room noise and spill from other instruments on stage. A Shure SM57 (or other dynamic mic such as an Audix i5) can be more effective in such a situation because it may capture less bleed and be less prone to feedback from a wedge monitor. Some coffee house performers place their PA behind them so they can hear the same mix as the audience (thus violating a prime directive of live sound) and avoid the need for monitors. While I wouldn’t say this is wrong (on second thought, I might say this is wrong), it certainly is begging for feedback. If you have to work within those parameters, keep the mic close to the guitar and try to use the guitarist’s body as a barrier between the PA speakers and the microphone. A sound hole cover can be very helpful in avoiding feedback in such situations, but it could also diminish the timbre of the instrument. Good luck.
Notice that we have not mentioned use of multiple microphones. Multi-miking an acoustic guitar may work in the studio, but I suggest avoiding it on stage. Every additional microphone greatly increases chances for feedback, spill and increase possibilities for phase issues (which in turn also increase likelihood of feedback). Stick with one microphone.
Practice Your Pickup Lines
It’s an unfortunate fact of life that most live situations involving an acoustic guitar will require a pickup. Unfortunate because — although pickups solve some of the issues at hand — many of them do not sound natural. We’ll leave most of the “which pickup should I use?” debate for the guitar forums and get to the nuts and bolts. The basic types of pickups include the under-saddle transducer (UST), magnetic sound hole pickup and surface mount pickup. Luckily for sound engineers, there is an entire industry of acoustic guitar pickups with clever designs coming from many manufacturers. One example is the Fishman Ellipse Matrix Blend, a hybrid device combining a sound hole microphone with a UST. A preamp installed in the guitar provides the player with the ability to blend the two sounds, and the engineer takes a standard ¼-inch TS feed from the output jack.
As an engineer, you need to pay attention to the type of pickup that is being used and not just slap a DI on the output of the guitar. Piezo transducers are very sensitive to loading, so you’ll need a DI with very high input impedance. As an example, the PZ-DI from Radial Engineering provides adjustable loading for use specifically with magnetic or piezo pickups — resulting in a more natural sound. The Para DI from L.R. Baggs is intended for use with acoustic guitars pickups and features a five-band EQ with a tunable notch filter for correcting feedback. Some guitar players may use a stage amplifier tailored for acoustic guitar, in which case you can try miking the amp. YMMV.
Processing
Compression on an acoustic guitar is very helpful for the front of house mix but is a no-no in monitor wedges because it makes controlling feedback very difficult. When the guitarist is playing, the compressor holds the gain down. When the guitar player stops, the compressor lets go and gain comes up — often resulting in feedback. It’s imperative that you separate the acoustic guitar feed to monitors and front of house. If you have a mic splitter feeding separate monitor and house consoles, this is easy: each engineer receives their own signal, processing it as they see fit.
In cases where you are doing monitors from the front of house position, this can get sticky. You need to be aware of the signal flow from a channel to the aux sends. You want to take the aux send to monitor(s) after the EQ but before the insert. Most digital consoles allow you to switch this routing via software but hardware consoles may not (or you may have to get under the hood and adjust internal jumpers). If the channel insert comes before the aux sends, then inserting a compressor will affect the signal going to the aux (monitor) sends as well as the L/R house mix. If you can tap the monitor send before the insert, then you can compress in the house mix to your heart’s content without worrying that the compression will promote feedback in the monitors. You’ll have to check the manual and perhaps trace a block diagram.
Generally the same may be said of EQ. The best scenario is separate desks (and therefore EQ) for monitors and house, in which case the monitor engineer can EQ to tame feedback while the house engineer EQs for taste. In any case, the High Pass filter is your best friend when amplifying acoustic guitar. Bring the channel up in the mix (whether it be house or monitors) and engage the HPF. Sweep the filter frequency up until you hear the sound start to thin — then back it down a hair. There’s no point in amplifying low-end muck that doesn’t contribute to the tone of the guitar but may cause feedback or amplify mechanically transmitted sound. A sweepable midrange EQ can be very helpful in correcting feedback. If you start to hear feedback, slowly bring the midrange EQ gain control up a few dB and then sweep the frequency selector until the feedback begins to get worse. Then dial the gain back into negative territory. Don’t be scared if the EQ display on a digital desk looks like you hacked the channel to pieces. Use your ears to make the adjustments, not your eyes.
When blending a pickup and a microphone on separate channels, check the phase relationship between the two. Sometimes it might actually help if the channels are out of phase because this could result in a lower-midrange notch that reduces feedback — just make sure that you are not killing the overall tone of the instrument. A bit of cut in the vicinity of 1 kHz to 2 kHz on the pickup channel can tame that plastic-y direct sound, while a touch of reverb can open it up a bit so that the audience doesn’t feel like they have their ear on the soundboard. Use a relatively short decay (1 to 2 seconds) and high-pass or use a low-shelf cut on the reverb to maintain clarity.
One last word about microphones: mics with the same pickup pattern are not created equal. Some provide higher rejection of off-axis sounds, meaning two things: 1) they reduce the amount of leakage from sounds off to the side and 2) they are less forgiving if your talent is moving around. This is equally important for vocal microphones, because leakage of the guitar into the vocal mic can cause phase issues and — in severe cases — result in flanging.
All of the issues we’ve discussed take time to resolve, so make room in the schedule for a proper sound check. A little bit of fine-tuning will yield high dividends come showtime.
