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Acoustic Instruments and Amplification, Chapter 4

We will need to define the operating parameters of our acoustic amplification system; let us first name the parts.
  1. The acoustic guitar itself(or whichever is your acoustic weapon of choice).

  2. The input transducer(pickup, microphone, etc.) This is much more than the term used to commonly describe the "piezoelectric transducer" (how many of these would the piezo manufacturers have sold if they called them "crystal pickups"?).

  3. The input stage preamplifier. This brings the relatively low output current of the input transducer up to a level that is compatible with all of our tone shaping stages, and the power amp. I cannot emphasize this enoughthis is THE most important component in your chain, after the instrument itselfmore important than the type of pickup. On a modern, saddle ducer equipped guitar, this is usually onboard the instrument. The FIRST amplification stage, so it had better be rightor nothing else that follows will be.

  4. The tone controls section(EQ). This is where we try to put back the damage that is done to the signal from our innaccurate pickup systems, inadequate speaker systems, and cheesy sounding guitar. Or, we use it to help control the feedback that generally occurs from our better sounding instruments. We have a tendency to try to "fix" our tone from this part of the amplification chain(fix it in the mix), rather than having it right in the first place.

  5. EFX loop. It seems we always need to have that reverb, or chorus, on our "natural" instrument. That is one side of the cointhe other type of EFX we use is in transient control(limiting)either to "smooth" out our playing, or to keep our power amp from clipping.

  6. Power amp. This is where we make it loudand, as long as we have enough power, solid state power amps are pretty much the same. But, the fly in the ointment is that acoustic guitars have a HUGE attack transient, and the power supply in our amps is usually not up to the task.

  7. The speaker system. Accuracy within the response range of the guitar is important here, as is efficiency. Wide frequency response(20hz to 20khz) is not nearly so important as we might think.
The guitar itself

As acoustic guitarists, we want to have as much of an "acoustic" tone as is possible, to have that harmonic sweetness that we hear in our living rooms.and have it at gain levels that immerses us in the center of the sound. Weve paid $2000 for this guitarand that is what we want to hear! Now for the reality. That resonant guitar, with all that harmonic richness, responds to your every touch, every subtle nuanceof course, it responds to sound waves in the performing environment also. The more resonant the guitar and the higher the sound levels in the performing environment, the more the instrument will react and the tone, along with the playing response, will change.

We can have three basic reactions to this axiom. The first, and most common, is to believe the advertising hype, ie, "this pickup will give you the acoustic sound of your guitar"and then hide our ears in the sand when we hear the result. The second is to spend a ton of money on gear, mistakenly trying to correct the inadequacies of the pickup system, not realizing the guitar itself is part of that system. Or third, buy a guitar designed to play loud, realizing we will give up some of that response in the process. Once we get past that first reaction, we are left with the two remaining compromises, and then we can begin to grow as musicians again, taking control of our situation.

If you do thorough research on what the pros useyou will soon discover that most of them use a different instrument on stage, than what they keep at home or use in the studio. That is, the right tool for the job. In general terms, the less boomy the guitar, and the higher the resonance frequency, the less problems we will have with feedback and resonance exaggeration.

The first feedback frequencies are in the lower end of the spectrum. The size of the body, and the diameter of the soundhole will determine the box resonant frequency. Essentially, the acoustic guitar is a Helmholtz resonator. Its a big bottle, and when the air in the bottle is disturbed, it resonates at a particular frequency or group of frequencies. Heres an experiment for you.

Get a compressed air hose and blow it across the top of the guitar at a low angle(keep the air pressure low, about 30 or 40 lbs.), much like you would across a bottle openingthe ensuing note is the box resonant frequencyand the note that will give you the most initial feedback problems.

The thickness, stiffness, and wood species will also contribute to the amplitude and color of this resonant frequency. A large rosewood guitar will have more bass response(and feedback problems) than a mahogany parlor guitar. This is not to say that the parlor guitar will be easier to amplify since the smaller guitars(at least the good ones anyway) tend to have much lighter woods and bracing systems, which opens them up to resonance exaggeration in the upper registers. And that brings up the top resonant frequency. With the guitar up to pitch, gently tap on the bridge with a knuckle, right behind the pinsthis is the top resonant frequency. The good designer will try to keep this frequency within a "5th" of the box resonant frequency, and at a bit lower amplitude. So we have found two resonant peaks within the two lowest octaves of the guitar. A D-28(which we will consider as the "standard" guitar) has a box frequency of about an Ab, an octave and a major 3rd below middle C. And all dreadnought sized guitars will be within a couple of tones for the box resonant frequency, depending on wood choice and top/bracing dimensions. A stiffer, thicker top will yield a higher top resonance, which in turn brings up the box resonance a bit while lessening the feedback tendencies somewhat. Accordingly, a smaller box raises the resonance frequency, and can reduce the feedback.

Lets consider the style of guitar we are looking for:
  1. Do you really need the bass response of that rosewood dreadnought? If you are playing bluegrass, or are the only rhythm instrumentthen yes, probably. But particularly when amplifying, too much bass response can not only cause feedback, but muddy up the "bottom line" of your sound. If you have a bass player, then you may want to consider at least a lighter guitar, such as mahogany.

  2. If you are playing solo, you can probably get by with that big box, but you probably dont need to. A smaller instrument usually has better articulation. Of course, Mike Hedges used a D-28 and tuned it low. He needed that bass response.

  3. Again, playing solo, you love the resonance and harmonic complexity of you expensive guitarjust beware that the more responsive it is to your touchthe less gain will be available to you before the guitar begins to resonate with the speaker system and your tone will change.

  4. At higher gain levels, such as used in a loud rock band or a "postage stamp" stage in the typical country roadhousethe guitar should probably be small, laminatedand, well, use a pickup that isolates the tone of the guitar from the amplification system.
This brings up basically two different "attitudes" when amplifyingeither you want to capture the sound of your guitaror you want to get a "reasonable" acoustic tone. The former can be very expensivethe latter much less so, but you wont want to listen to your guitar out of the live sound formatit just doesnt sound natural.

To cope with these attitudes, the pickup and mic manufacturers have come up with 3 different ways to look at amplifying your instrument. A mic or soundboard transducer that attempts to capture the "whole" sound, an "isolation" pickup, such as a saddle ducer or soundhole pickup that essentially captures the tone of the strings, or a "hybrid" system that uses two or more sources.

Next chapter: Transducers

To learn more about Dave and the guitars he builds, please visit http://www.electrocoustic.com

Dave Wendler is a luthier at Ozark Instrument, and holds a US patent in the field of acoustic instrument amplification technology.