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Acoustic Instruments and Amplification, Chapter 3

If we didn't have amplification, the only distortions we would be dealing with would be the occasional broken brace rattling or our strings buzzing against the frets. With the advent of electricity, we opened up a whole new vocabulary of descriptive terms that attempt to describe the attributes and failures of our sound systems. We just love the sound of our old D-18, but how do we make it as loud as that Strat plugged into a plexi Marshall? In short, we don't, not if we want to hear THAT sound. However, we can get an approximation of an acoustic sound, with the judicious use of pickups, preamps, EQ's, etc. Please keep in mind during this chapter that there are many, much more subtle distortions, than the clipping that is so often associated with electric guitars. If we are trying for an accurate, acoustic tone, these affects will have a much larger effect on our sound than an obvious clip.

Harmonic Distortion
The most often quoted distortion specification simply because it is the easiest to control. This is what our equalization controls effect. For a demonstration, try out your home stereorun the treble and bass controls up and down, you will hear the effect of adding strength to the highs and lows. A graphic equalizer on your guitar gives a better demonstrationplay a note, choose a slider, and run it up and down. All of the sliders above 1500hz don't have much effect on the fundamental , but do effect the upper partials, or harmonics. The distortion comes into play when your amplification chain can't accurately reproduce the proper relationship among the fundamental tones and the harmonics that go along with it. Closely related to harmonic distortion is:

Intermodulation Distortion
Acoustic instruments are extremely complex, harmonically speaking. While a solid body electric guitar derives the lion's share of it's sound from the string, an acoustic instrument not only uses the strings, but filters that tone through the body, and in particular, the soundboard. In purely acoustic terms, this can be noticed fairly easily in a poorly set up or poorly made violin while playing a double stop. There appears a 3rd tone that is not related to either of the others and causes your notes to sound out of tune. Electronically, this can be heard when using a humbucking pickup on an old tube amp, particularly playing chords down low. The tone sounds jumbled and mushy. Typically, the amp is being pushed a bit and the output transformer is not large enough. The intermodulation distortion occurs as the amplifier, and particularly the output transformer loses its ability to damp the loudspeaker and allows secondary currents to reflect back and influence the amp signal. This is one of the few advantages of solid state over tubessince the SS amp has no output transformer, the direct coupling of the output devices with the speaker terminals lets the amplifier more accurately control the movement of the loudspeaker cones. The use of a relatively large SS power amp effectively eliminates the electronic side of intermodulation distortion. On the acoustic side, we need to have a well made instrument.

Transient Distortion
As we travel along the amplification chain, we will begin to pick up distortions of our sound, and not in just the harmonic domain. The most common is the eroding of the attack transient, since we will have many amplification stages to go through.

Every amplification stage adds noise AND limits dynamic range, the range being limited by either the amount of noise introduced or the amp going into clipping. Many times, especially with acoustic instruments, the limiting of the dynamic range is not necessarily a bad thing, as this limiting smooths out our playing. The typically wide dynamic range of acoustic guitars actually needs to be toned down a bit, just so the amplifiers can handle a loud attack transient without clipping.

Tube preamp stages take care of this naturally, ie, a tube compresses a bit before actually being overdriven and producing clipping distortion. The trained ear can hear a tube doing this and the player will learn to react as he is playing. This is a very subtle, almost subconscious reaction. Transistors on the other hand, give no audible warning when they will clip. When you throw in the harmonic complexity of the acoustic instrument, and the accompanying less than predictable attack transient, the clip becomes very difficult to control.

So consider: the fewer preamp stages you pass your signal through, the more likely you will retain your dynamic range, the ability to punch and the capability to retain subtlety in your style, without having background noise creep into your sound. Every time your signal passes through a device(ie, chorus, delay, eq, etc) you will be adding two preamp stages. If the devices are digital, then the problem is compounded, as the signal must be converted to data and then back again.

Phase Distortion
The most common complaint with piezo pickups is the sharp tonality. Many other terms have been used such as thin, quick, rubber band, bees in a bucket, and several others that can't be used in polite society. This tone is essentially a misaligned attack transient: phase distortion. When you are standing in front of an acoustic instrument, all of the harmonic beauty arrives at your ear in the proper time frame. However, once we use a pickup device, the harmonic unity of the wavefront begins to get a bit jumbled. With a piezo, the high capacitance (an ability of the device to hold a charge) slows down the lower frequencies, and lets the high frequencies pass first. That gives us the typically sharp or brittle tone of the piezoelectric transducer.

A coil/magnet pickup does the oppositethe inductance of the iron(magnetic) core tends to emphasize the lower frequencies. As we reduce windings of the coil, and the inductance decreases, the device will begin to have a flatter frequency response and a more aligned attack transient( it will also have less output).

A mic diaphragm will always have a more aligned phase response over a pickup device, for the most part because of the location hearing the instrument from a distance, just like our ears. The electrical phase response in a dynamic device is derived through the use of a low-impedance coil, that is, a coil with a short length of wire. The condenser mic achieves this through charging a capacitive diaphragm with a DC voltage, and allowing the changing voltage of that diaphragm to be read by the amplifier.

The next chapter we get down to applying what we've learned so farwe'll define our particular live sound situation, choose an instrument that will perform well in that environment, and get a start on defining the pickup system that will work best for us.

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.