Mixing Tips: How to Improve Your DI

Acoustic Guitar Recordings askaudiomag.com May 6, 2015

Sometimes recording acoustic guitars and finding that sweet spot can be tricky.

Joe Albano explains how to improve the quality of your recordings depending on

the pickups used in the mix.

Everybody knows that the best way to capture the sound of the acoustic guitar is

by placing a high-quality mic (or two) about a foot in front of the instrument,

finding the sweet spot, and hitting record. But thats not always an option, and

for a variety of reasonswhen you sit down to mix, you may find yourself

confronting an acoustic guitar track with a less-than-stellar sound quality.

One common cause is a DId recordingthe use of an on-board pickup to record,

instead of a mic. This might be done in a variety of situations. It could be a live

recording, in front of a loud band, where a mic would have picked up too much

leakage to be usable, or a rehearsal performance, where the playing captured

something special, and was used as a keeper. Whatever the reason, youre left

with a recording that lacks the richness and air that an acoustic guitar track

should have, and you need to massage that dry, direct signal into something that

will do justice to the instrument. Fortunately, there are a number of things you

can dosome tweaks may be more effective than others, and the level of

effectiveness may range from slight to dramatic improvement, but the result will

always be better than just using the DId signal as is.

Pickups: Piezos & Problems

There are lots of different types of acoustic guitar pickups in use nowadays, from

simple add-ons to multi-element built-in systems with electronics, and sound

quality varies considerably.

Fig 1

Various types of acoustic guitar pickups.

One of the simplest options is a basic magnetic pickup, like the kind used on

electric guitars, but voiced for a more full-range response. These can be mounted

in the sound hole of most acoustics, without any alterations to the instruments,

and are cheap and popular. But despite being designed to have a wider frequency

response than electric guitar pickups, thanks to the technology involved, they

still tend to have a more limited bandwidth than a mic, with a pronounced

midrange emphasis.

Fig 2 A typical

mag pickup mounted in an acoustic guitars soundhole.

Audio Example 1An acoustic guitar recorded with a mag (magnetic) pickup:

The most widely-used pickup type is probably the piezo pickup. This is a

pressure-based design thats mounted under the bridge, and responds to

vibrations. Piezos tend to have a very bright sound, and this makes them popular

for live use, where the extra brightness can help the guitar cut through, but that

same brightness lends a sometimes-unpleasant peakiness to a recordingthe

sound emphasizes string vibrations, but lacks body resonance. Piezos also tend

to emphasize picking transients, and this combination of characteristics is

referred to as the piezo quack, describing the overly-bright string-heavy sound

and over-emphasized attack.

Fig 3 A

typical piezo pickup.

Audio Example 2A guitar recording with a piezo pickup (actually a piezo

combined with a body pickup, as well):

A third pickup typeless common, but still often usedis a contact pickup,

which mounts on the body of the guitar. This design can provide a sound richer

in body resonance, but often lacking in clarity and high-end.

The most popular choices are piezos and mags, but many on-board systems

combine elements. Its common to find an acoustic guitar with a piezo in the

bridge (for brightness and clarity), combined with a contact pickup (or even a

mini-mic mounted inside the body) for warmth and resonance (Audio clip #2

was from such a combo system, though you can still clearly hear a little piezo

quack).

Though the sound of these pickups varies, from the mags midrangy thickness to

the piezos infamous bright quack, the approach to improving the sound of any

of them involves the same three elementsEQ, compression, and resonance.

EQTweak the tone

The most obvious aspect of any DId acoustic guitar recording is the tonal

difference from what a good mic would have provided. Naturally, EQ is the

answer, though the specific curves youll need to dial up will depend on the type

of pickup/system in use.

If a mag PU was used, the resulting recording will usually have a broad,

pronounced midrange peak which will have to be tamed, and a lack of highs,

which will have to be added. The starting point would be a broad cut, centered

between around 500600 Hz and 23 kHz, combined with a boost around 812

kHz, and low-end adjusted to taste. The exact settings will be completely

dependent on the particular PUs response, and the sound youre going for

(bright & thin, full & woody, etc).

With a Piezo PU, the over-bright quack will often be centered around 34 kHz,

and a sharp cut in that range will be a good starting point, with additional low-

and high-end tweaks for balance.

Fig 4 Typical EQ settings for DId acoustic guitar: Mag PU (Top); Piezo PU

(Bottom).

Audio Example3 EQ applied to a DId AG: (1) mag PU (as recorded, then

EQd); (2) Piezo PU (as recorded, then EQd):

While you can do wonders just EQing by ear (assuming you have a good sense of

what a proper acoustic guitar recording should sound like), theres a more hi-

tech way to go, if you have the tools available. Specialty EQs called Matching

EQs are available from a number of companies (iZotope, Voxengo, FabFilter),

and may even come with some DAWs (Logic). A Matching EQ will analyze the

frequency response of two recordings (mic and PU), and automatically create an

EQ curve to match the tone of one to the other. This would be especially useful if

you have a properly miked-up recording of the same guitar, but any miked-up

acoustic guitar track can be used as the reference. Most Matching EQs will create

a curve with a greater number of bands than a typical EQ, more precisely

capturing characteristic acoustic guitar resonances. In Audio Example 4, I used

Logics Match EQ to impart the tonal balance from a miked-up take (of the same

part) to a take recorded with a mag PU (which had been designated a keeper for

some reason long forgotten).

Fig 5 Logics Match EQ analyzes a Reference recording (miked-up AG) [Top] and

a Target (DId AG w mag PU) [Center], and creates an EQ curve to match the

tone of the DId track to the miked-up version [Bottom].

Audio Example 4 Logics Match EQ applied to fixing the tone of a DId acoustic

guitar (recorded with a mag PU): 4 Bars Each: (1) the Target: the original DId

AG (w mag PU); (2) the DId AG w Match EQ applied; (3) the Reference track

(miked-up AG); (4) the DId AG w Match EQ (again):

Part of the dreaded Piezo quack can be an over-emphasized attack. Often a

little compression (in combination with EQ, as above) can help to tame this

particular quality, over and above any compression you might decode to add

later, as part of the mix. Id set it to mostly catch the sharpest attacks, if possible,

but a few dB of overall gain reduction will probably help, in any case.

Add some bodyconvolve

One of the main things lacking with DId acoustic guitars is the rich resonance of

the body. Both mag and Piezo PUs tend to capture more of the string sound, and

even even combing a Piezo with a contact (or internal) body mic doesnt really

capture the open, airy quality that a typical miked-up recording has (even at the

standard distance of around a foot). Of course, youll likely be adding general-

purpose reverb and ambience in the mix, and this will help considerably, but

theres another option available these days, thanks to convolution processors.

Convolution reverbs capture, via sampling, the response of a real acoustic space,

and impose that on signals running through them, making the audio sound like

its in that specific space. These samples of real spaces are called IRs (Impulse

Responses), and while we typically think of them when it comes to rooms,

convolution can capture the sound of anything (theyre commonly used to

provide the distinctly colored tonality of guitar amp cabinets), and that includes

the characteristic resonance of an acoustic guitar body. To add a little richness to

the string-heavy sound of a DId acoustic guitar recording, you could run it

through a convolution reverb with an IR of an acoustic guitar body, imparting

that extra woodiness to the otherwise overly-dry PU sound. Many convolution

reverb plug-ins include a way for you to make your own IR (hold a mic up to the

sound hole and generate a transient pop to stimulate the guitar bodys response),

or you could hunt up suitable IRs on the internetthere are many free IRs

floating around, theyre just normal audio files.

Audio Example 5A miked-up acoustic guitar recording vs the same part, DId

via a mag PU, processed with (Matching) EQ and added guitar body resonance

(via a convolution reverb plug-in): AG-DI unprocessed (4 bars); AGMiked (4

bars); AGDI processed (4 bars); AGMiked (4 bars); AGDI processed (4

bars):

Cool Toys

There are also commercial solutions for DId acoustic guitar issues. Quite a few

companies make DI boxes specifically for acoustic guitar that include features to

ameliorate the usual flaws of mag and Piezo pickups. One of the best-sounding of

these comes from Fishman (a well-respected maker of AG pickups)the Aura

system. This combination DI-processor included what Fishman calls images

tone-prints of the sound (the characteristic resonance) of various types of

acoustic guitars (dreadnaught, jumbo, parlor, etc), combined with the

characteristics of various high-end mics. You simply match your mag- or Piezo-

equipped guitars body type to images of that same type, and what comes out is

the sound of the guitar as it would be had it been miked up, with the appropriate

tonal balance, body, and air (you can hear examples of this system

athttp://www.fishman.com/videos/fishman-videos).

If youre already stuck with an already-recorded DId acoustic guitar, Universal

Audio has a plug-in that lets you dial in missing body resonance, and

microphone tone & air, after the fact, courtesy of their new Sound Machine

Wood Works plug-in. This also offers you choices of various acoustic guitar body

types, along with the ability to balance simulated neck & body micsyou can

hear examples (comparing the original DId signal to the processed version)

athttp://www.uaudio.com/store/guitar-bass/sound-machine-wood-works.html.

Fig 6

Fishmans Aura and UADs Wood Works acoustic guitar processors.

Products like the Fishman Aura system and the UAD Wood Works plug-in

probably use techniques similar to the ones I outlined above, as well as more

sophisticated processing like modeling, to achieve the rich, full sound they can

provide from a typical DId acoustic guitar track. But even if you only have the

basic tools available in your DAW, you can still turn that dry, lifeless DI into a

reasonably good-sounding track, with a little creative massaging, and a careful

earof course, if you have the option, the best bet is still to record with a mic in

the first place!