80 J. Audio Eng. Soc., Vol. 58, No. 1/2, 2010 January/February

The typical starting approach at theguitar amp: Shure SM57 microphone,slightly off center of one of the conesof a driver, up close and almost touch-ing the grille cloth. Oh, and angle themicrophone a little.

Ask veteran engineers why thismicrophone placement strategy is socommon and a range of justificationsfollows, from seemingly scientificexplanations, to vague guesses, to anhonest, “I have no idea. I’ve alwaysdone it that way. Everyone does.”

Sure, we change it up a little. Itcould be a different microphone, butmoving coil dynamics at the affordableend of the price range are commonchoices. It’s not always the thicknessof a thin guitar pick away from thegrille cloth; we might back the micro-phone away from the amp an inch,maybe even two. As for the angle? I’venever worked in a studio with aprotractor (until last night), so I couldonly guess the range of angles we use.

It’s time to quantify the close micro-phone, electric guitar ritual. Let’smeasure the effect of these classicrecording gestures one-by-one—offcenter, up close, and angled a little.

INTO THE LABA guitar amp is set-up–—in this caseit’s a currently available open-backFender tube amp with a single 10-inchdriver. The single-driver amp keeps ourtests simple, and any insights are easilyapplied to multidriver amps. On a 4-by

12-inch cabinet, for example, any oneof the four cones is approached the waythis single driver is.

A tasty, clean tone that might beappropriate for any generic rock or poprhythm part is dialed up. And then wedo something rather rude, but edifying(Fig. 1). The guitar is unplugged, and acomputer is hooked up. We boot up thesame sort of software used to measurethe frequency response of loudspeakersor microphones, and we measure theelectric guitar amp, as miked. Wearen’t in search of a flat frequencyresponse, of course. We want a spec-tral snapshot of this real-world guitarsetup, as seen by the SM57, when theguitar sounds good.

If the guitar output is to be replacedby a test signal (a maximum lengthsequence or a swept sine wave, in thisexperiment), we must make sure thesignal electrically looks like a guitar

signal. We re-amp the test signal. Thecomputer output is a balanced, line-level, low-impedance source. Raisingit’s impedance and unbalancing it, thesignal may be fed at very low level tothe electric guitar amp input on aninstrument cable. This measurementapparatus now lets us document thetonal implications of placing thatdynamic microphone off-center, upclose, and angled. Three tests are run.

DISTANCE FROM CENTERIn the first test we measure the effectof having the microphone slightly off-center of the loudspeaker cone. Start-ing with the microphone up close,almost touching the grille cloth, aimedstraight at the amp, we measure thefrequency response dead center of thedriver, and then work our way hori-zontally, left to right, from the centerto the edge, in one-inch increments

Fig. 1: Measurement setup

Alex Case is an assistant professor atUMass Lowell, chair of the AES Educa-tion Committee, and author of SoundFX–Unlocking the Creative Potential ofRecording Studio Effects.

By Alex Case

Recording Electric Guitar—The Science and The Myth

Electric guitar tone,you know it’s right when you hear it. How is it achieved?

J. Audio Eng. Soc., Vol. 58, No. 1/2, 2010 January/February 81

Recording Electric Guitar—The Science and the Myth

(Fig. 2). The distance from the amp isunchanged; it remains at the grillcloth. The angle of the microphonestays perfectly perpendicular to theamp. The only change is the horizontaldistance from the center of the driver.The change in spectral content basedon off-center placement is shown inFig. 3.

The result is a complicated alter-

ation—primarily a reduction—of highfrequency content in the guitar tone.As the microphone moves from centertoward the edge of the cone, the highend rolls-off and becomes choppy. Tobetter highlight the changes caused bythis change in microphone placement,the measurement data can be normal-ized to the center placement (Fig. 4).While placement of an SM57 deadcenter of the amp leads to a sound thatis far from flat, Fig. 4 shows the spec-tral content of each placement off-center relative to this starting point.

The changes to spectral balance asthe microphone migrates away fromthe center can be substantial. Just oneinch off-center leads to pockets ofattenuation that are a good 4 to 14 dBdeep, beginning at frequencies as lowas 2 kHz. Further off center, at 3

inches, the guitar tone is reshaped withsome 20-dB alterations to spectralcontent. While the effect is shown tocontinue upwards in frequency,beyond 10kHz, it is important to note(Fig. 3) that there is little content in theguitar tone that high to begin with. Theperceptually significant part of off-center placement is likely in the middleto upper-middle frequencies.

The familiar mental image of acardioid pick-up pattern explains muchof what is likely going on with the elec-tric guitar amp. Think of the speaker ashaving a cardioid-like radiation pattern.There is a general trend in the radiationpattern of the loudspeaker driver that itbecomes increasingly directional athigher frequencies. While it mayapproach omnidirectional behavior atlow frequencies, it grows more cardioid(and even more focused still) at higherfrequencies. To move the microphoneoff-center is to move it out of the moredirectional high-frequency beam.Meantime, there is relatively littlechange at low frequencies with off-center placement as the radiation patternhas less bias in any direction down atlarger wavelengths.

Figs. 3 and 4 also make clear that thehigh-frequency roll-off is quite irregu-lar. This is likely due to the modalbehavior of the cone at higher frequen-cies. While the designer might intendto build a loudspeaker in which thecone moves as a single, rigid piston ofunchanging shape, the physicsconspires against the guitar amp. Justas the soundboard of a piano or anacoustic guitar bends and moves incomplicated patterns depending on thenotes played, the cone of the loud-speaker flexes into unusual shapes,with small regions of resonance. For amicrophone very close to the cone,localized pockets of spectral colorationoccur, resulting in highly complicatedplots of the frequency response.

Moreover, these alterations to spec-tral content along the radius of theloudspeaker cone are a function oflevel. The peaks and valleys infrequency response come and go andchange frequency location as thespeaker is forced into more or lessmodal break-up with increases anddecreases in level settings of the ampand guitar, as well as the perfor-

Fig. 2: Moving off center of driver

Fig. 3: Spectral effect of moving off center of driver

Fig. 4: Normalized to center

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Recording Electric Guitar—The Science and the Myth

82 J. Audio Eng. Soc., Vol. 58, No. 1/2, 2010 January/February

mance dynamics of the guitarist.An engineer can not match the effect

of off-center placement with a simpleadjustment in EQ. It is not as simple asa shelf or roll-off. It would requiremany bands of parametric EQ tomatch, and the EQ would have to beautomated into a constant state of fineadjustments to keep up with thedynamic level-dependence of theeffect. Off-center placement is a wayfor the engineer to soften the bright-ness of the tone, while leaving pocketsof strong spectral character.

Perhaps more interesting still, themechanical overdriving of the loud-speaker cone is revealed as ever chang-ing distortion, which a closemicrophone, offset from the center ofthe driver, is well-positioned to empha-size. Off-center placement reshapes thetone in ways that engineers find inter-esting, and perhaps easier to fit into acrowded mix.

DISTANCE FROM AMPThe next variable to quantify is the dis-tance from the amp. Starting dead cen-ter of the driver, at the grille cloth,with no angle to the microphone, mea-surements are taken at 6-inch incre-ments from the amp (Fig. 5).

As we might expect, moving themicrophone straight back, away fromthe amp, leads to an overall reductionin level. The farther away the micro-phone is, the lower the amplitude ofthe signal. Fair enough.

Normalizing the measurementsagain to the starting point, when themicrophone is at the closest locationpractical (Fig. 6), reveals a bit moreinformation. The signal doesn’t growuniformly quieter with distance. Theattenuation is more pronounced at lowfrequencies. As the microphone used isa cardioid, it possesses proximityeffect. Backing-off the microphone notonly reduces level, but also reducesproximity effect.

Engineers adjust the distance dimen-sion, in part, to tune the low-endcontent of the signal—enough for thepower and fullness desired, but not somuch that it muddies the tone andmakes it hard to hear any interestingelements of tone in the midrange dueto overwhelming bass.

At mid and higher frequencies,

peaks and dips in the spectral contentstart to appear. One would expect theeffects of the room to start to creepinto the measurements as the micro-phone is placed farther and fartherfrom the amp. Specifically, the floorbounce and other reflections eventuallybecome a factor (Fig. 7). As the

reflected sound travels farther than thedirect sound, it arrives at the micro-phone delayed relative to the directsound. If the level of the reflection issimilar to the level of the direct sound,the inevitable result is some combfiltering. The strong boosts and cutsthat appear with increasing distance

Fig. 5: Moving away from driver

Fig. 6: Moving away from driver • normalized to 0”

Fig. 7: Floor bounce introduces comb filtering

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Recording Electric Guitar—The Science and the Myth

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are likely caused, in part, by the combfiltering of this first reflection.

OFF-AXIS ANGLEThen there is the famous tendency toangle the microphone, um, you know,“a little.” Does this gesture have merit?Phase 3 of our test explores this ques-tion.

Starting with the microphone deadcenter of the driver, almost touchingand perfectly perpendicular to thegrille cloth, the angle of the micro-phone is adjusted—15 degrees, 30degrees, 45 degrees and beyond (Fig.8). Yes, it happened: someone broughta protractor to the guitar session.

For each measurement, we keep thetip of the microphone perfectly on axiswith the center of the driver, andalways up close to the grill cloth.While engineers are unlikely to gobeyond a 45 degree angle, the experi-ment continues on to 60, then 90degrees, where the microphone is fullyperpendicular to the firing axis of theguitar amp.

Fig. 9 shows the spectral result. Forthe most part, angling the microphonecauses only very small changes to thefrequency response. Fig. 10, whichagain normalizes the measurements tothe starting placement (close, centered,and on-axis), reveals spectral alter-ations of generally less than plus orminus 2 dB from 20 Hz to above 12 kHz for angles as pronounced as 45degrees. Keep in mind that the bulk ofthe energy for guitar tones generallylives above 80 Hertz and starts rollingoff well before 8 kHz.

The most that can be said is thatangling the microphone introducessome choppiness to the frequency

content of the signal being recorded.There is also a general decrease inlevel as the angle of the microphonediverges from the axis of the loud-speaker. Lastly, we might also observethat, with increasing microphoneangle, the complex alteration tofrequency response includes an overallreduction in level that is slightly morepronounced at high frequencies thanmid or low frequencies.

While many factors may explainthese trends (off-axis coloration of themicrophone, nearfield anomalies of theloudspeaker, asymmetric coupling tothe front and back of the capsule, andmore), the recording engineer need onlyassess the impact on their production.

For angles that might reasonably beused on a session (15, maybe as muchas 30 degrees), the effect is minimal

indeed, showing significant changesonly at very high frequencies which, asFig. 9 reminds us, contain very littleenergy in the guitar tone to begin with.Measurable, yes. Perceivable, lesslikely. We leave it for the reader todecide: angling the microphone, validproduction technique or overstatedurban myth?

BACK TO WORKTime-consuming subjective testing toactually figure out what is perceptuallymeaningful with adjustments to eachrecording variable is the subject of fur-ther research. The first step of thiswork seeks only to share with you thedata that was measured objectively.Let it influence what you listen for andmodify your electric guitar recordingtraditions as much as you dare.

Fig. 8: Off axis orientation

Fig. 9: Off axis microphone orientation

Fig. 10: Off axis microphone orientation • normalized to 0”

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