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Acoustic Instruments and Amplification, Chapter 5

Now we start getting to the meat of the matter, and the decisions made regarding transducers will bring on not only the most controversy, but also the most disappointment. We must understand that the selection of the pickup device is always a compromise for live sound applications. If you dont need to have high gain levels, then a very close approximation of your guitars sound can be had. If you need those rock and roll volume levels, you have to start making some hard decisions regarding budget, tone quality, and getting your "space" in the mix. The same can be said for the size and kind of rooms you perform in, ie, small coffeehouses, your church, the concert hall, the rowdy roadhouse. Now, lets get to it

The elementary transducer, comes in three varieties of two flavors. The first, and typically unusable for acoustic guitars, is the carbon or crystal mic. The frequency response is much too rough and harsh sounding; the output easily overdrives the input preamps. This is the mic of choice for harmonica players, though. Typically plugged straight into a small, old, ratty tube amp

We have left, for our purposes, the dynamic and the condenser mics. These will come with two polar pattern responses, omnidirectional and cardioid. The omni mic really isnt suitable for live sound applications, unless no monitoring is used. An old time bluegrass band might use one of these for the whole group to perform in a low gain environment. We will focus our attention on the directional (cardioid) variety.

With all cardioid mics, the frequency response is altered by several factors depending on the distance from the mic, and the position of the guitar relative to the polar axis(imagine a line running through the center of the mic). Most microphones are supplied with a chart indicating the polar and off-axis frequency response. And, all directional response mics have a "bass boost", more often called "proximity effect" that occurs as the source is moved closer to the diaphragm. This is exactly what we dont need as the bass frequencies are what feed back first. There are mic designs that can reduce the proximity effect a bit; you can identify these by a long row of slots along the body of the mic.

If you are using monitors on the floor in front of you, you may have reflections bounced off of the guitar top or from your body that gives you feedback fits. It is very critical to have correct monitor placement when using microphones. Just a shift of a few inches in any given direction(either the monitor, the mic, or you) can really help squelch those feedback nodes. I always do this before I start resorting to electronic equalization.

The most important factor to consider when using a "traditional" mic setup(ie, in front of the soundhole) is this: lower frequencies are essentially omnidirectional and your mic doesnt really start to reject off axis sound until you start reaching into the mid range response of the microphone. And this is above the box resonance frequency of the guitar!! It is for this main reason we cant get the kind of high gain response we need from a microphone setup.

So you will need to know the polar response pattern of your mic to use it effectively. But the flip side of this is mics become very intuitive to useyou just play with the mic positioning until it sounds good.

Dynamic mics use a coil/magnet to derive the signal. They are simple and rugged. The Shure SM-57 is the most popular of the mics used for our purposes. It is inexpensive, has decent feedback rejection properties, and if the gain levels arent too high, can be reasonably accurate to the guitar. Most dynamics are limited in frequency response in the upper registers, although the newer rare earth magnetics based mics have much better high frequency response. Warm and harmonically rich, the "color" added by a good dynamic mic can be very desirable.

Condenser mics use a charged diaphragm to generate the signal. This requires a power supply, either an onboard battery, "phantom power" from the mixing board or a dedicated power supply. The condenser usually has a more "open" and "transparent" tone quality to the sound, and works quite well with acoustic guitars and other instruments. You will find you hear more pick and string noises from a condenser setup.

If you are planning on using microphones, remember that the mic setup is more influenced by room acoustics and monitoring than any of the other transducers.

One of the more common ways we see microphones used is in an interior(inside the instrument) setup. While this would seem to allow us a "close" miking technique(to the extreme) there are problems introduced when using this setup. As the condenser mic is very suited to miniturization, this is the more preferred. This of course means you will need a power supply to charge the diaphragm. And, there is the phenomenon described as "comb filtering" that results as sound inside the instrument is reflected into the mic at relatively erratic intervals. This results in "phase cancellation" to the extreme, where desirable tones are completely eliminated due to arriving at the mic 180o out of phase. As octaves are simply a doubling of frequencies, this can occur at every octave above the lowest cancelled frequency. The result is a response curve that appears on a chart as a "comb". The tone you get is either muddy, inarticulate, honky.

As a result of the comb filtering effect, the interior mic used by itself is very rare. More often than not, the installer will play with several different placements in the guitar to arrive at the most "natural" tone; then it is up to the user to determine the mix between the other pickup and the mic. More on this dual system later.

The traditional pickup. Generally clipped into the soundhole, and works by the strings vibrating in a magnetic field. The tone is almost always associated with an "electric" tonality, although much better than 20 years ago. There are two basic designs of this pickup, that is, single coil and hum-cancelling. The single coil has a more bright tone but suffers from the typical radio frequency interferencein otherwords, it buzzes with transformer hum from your amp, cheap lighting dimmers really set it off, and even electric motors and fluorescent lights can give you problems. The only solution to this is to run a ground drain to the strings. This will require modification of the guitar. Very simple to use, just plug into your guitar amp if you dont really care about an "acoustic" sound. If you are using one of the more modern designs, there are several things you can do to enhance the natural tones.

While the hum cancelling design successfully reduces the hum and buzz, it also seems to reduce the high frequency response. Even here, a ground to the strings will help reduce unwanted noises. The better of these designs can produce a very "neutral" tone with even string response. Since bronze wound strings, the standard for acoustic "steel string" guitars, have magnetically unbalanced strings, the plain 1st and 2nd strings will generally be louder than the wound strings. While these designs claim a "balanced" tone while using a bronze wrapped string, much improvement in string to string balance can be achieved through the use of nickel or steel wound strings.

Another consideration when looking at the magnetic pickup is the ability to drive a long cable; if your situation includes a cable run of more than 25 ft. or so, careful consideration should be given to an appropriate preamp or direct box. This will add at least another $50 to your purchasing budget. You may be able to get by with a simple line transformer although the less expensive ones will get you frequency response anomalies and a reduced signal level.

Rare earth magnetics are now being applied to the traditional soundhole pickup. As these magnets have extremely high coercive force, a very small magnet can be used, offsetting the higher cost of these materials. This reduction in mass of the magnetic core allows for a much flatter frequency response. When used in conjunction with a low impedance coil, flat and extended frequency response will occur. However, a preamp will be required to drive any inline effects. It is possible to plug directly into a mic cable, and then use the channel insert on your mixer for your EFX. More commonly, the manufacturer will "require" the purchase of a dedicated preamp for the pickup to work correctly.

While you may be able to "get by" with a $50 soundhole pickup that you plug directly into your electric guitar amp, you will be sacrificing "acoustic" tone for convenience. If you are willing to spend the money for a more pure tone(upwards of $200), a very reasonable facsimile of that acoustic tone can be had. The main advantage of the magnetic pickup is high gain levels before feedback occurs, and the better pickups can get you to extremely high levels if used in a properly chosen guitar. By far and away the most important components of your magnetic system will be the guitar, the preamp stage you plug into and the speaker system you choose.

Next chapter: Piezo electronics, other transducers and dual-hybrid systems

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.