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Acoustic Instruments and Amplification, Chapter 1

Contrary to popular belief, fine acoustic instruments are not necessarily the instruments to have when we are looking for higher gain levels. That being said, in this series of articles we will try to determine not only what are the best instruments for your application, but why some systems sound better than others and why sometimes we just put up with an inferior tone.

We will cover the guitars themselves, the rooms we use them in, the sound systems we plug into. We will talk of the basic attributes and the detrimental characteristics of each of the more common amplification devices and methods. This will involve some technical detail, so be prepared to do some outside study, if for nothing else, to keep me on the straight and narrow. I invite all of you to jump in on this discussion by sending me criticisms and questions on the subject. In this way, we all can learn what will work best in each situation.

The Acoustic Guitar

In this first article, we will discuss the guitar itself and how it reacts both acoustically and in a live sound field. A guitar is essentially an air pump. You hit the strings and air is pumped in and out of the cavity in conjunction with the movement of the top. An arch top guitar has the simplest movement of the acoustic guitars because the strings tend to push the top up and down, producing a relatively simple harmonic overtone series. The flattop and classical work in an entirely different manner. The bridge acts as a fulcrum, ie, as the area behind the bridge moves up, the area in front of the bridge is moving down. So there is some phase cancellation going on as the two parts work against each other. Actually, phase cancellation plays a major role in the tone quality of any given instrument.

So, what exactly is phase? To understand it, try an experiment

Run about 3" of water in a bathtub. Putting your open palm in the water, and at one end of the tub, you can set up wave motions that will either:

A. Reinforce each other so each succeeding wave gets larger

or

B. Cancel each other so there appears to be a lessening of motion.

A = feedback
B = phase cancellation

A is what happens when your guitar starts to ring in the PA system. B is what happens when you find the frequency of A and pull it out in the EQ. The equalizer simply inverts the feedback frequency in the signal and sends it on to the main signal path. Feedback goes away. This all sounds so simplebut the problem is, the baby goes out with the bathwaterso to speakwe lose some of the NATURAL signal from the guitarsome of the tone that we so dearly love.

"A good archtop has no problem in keeping up with a horn section, piano and drumsbut you'll never find even the best D-28 with that ability."
But we're getting ahead of ourselveslet's back up to the phase cancellation going on in our guitar. You have probably heard the term "harmonically complex" to describe the tone of a fine acoustic guitar. Simply put, this is just a way to describe the various phase cancellations and breakup modes of the vibrating soundboard. And that adds to the harmonic overtones of the instrument. Now, if you imagine the area just behind the bridge and the area just in front of the bridge you will realize that much of the time, they are moving in opposite direction from each other. More phase cancellation. So this is desirable in the guitar itself as it adds to the tonal color. Also, other various areas of the top tend to move in opposing directions from each other, contributing even more to the harmonics in the instrument. And realizing that a larger string fretted up higher on the neck than the same note on a smaller string produces differing harmonics and overtones, we begin to understand some of the rapture we have with the guitar: so many colors and overtones by just hitting the strings in a slightly different manner

Many times, to the uninitiated, the jazz archtop just doesn't sound quite as good as its flattop cousin. Because the top is moving in and out, in a pattern similar to a piston, it is not as harmonically rich. But, what we give up in harmonics we gain in penetration and clarity, since the top is moving in a much more unified manner. In otherwordscutting power. A good archtop has no problem in keeping up with a horn section, piano and drumsbut you'll never find even the best D-28 with that ability. Of course, the depth of tonethe lower fundamentals, just aren't as rich sounding in the archtop. Speaking of the D-28..look at your typical bluegrass group, ie, violin, mandolin, banjo, dobro, string bassall of these other instruments work on the same principle as that carved guitarbut not the flattop, so until the advent of good mics and speaker systems, the guitar was pretty much left out as a lead instrumentuntil Doc Watson and Clarence White showed the way by getting right up to that microphone.

In Chapter 2 we will discuss early amplification methods.

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.