Zen and the Art of Acoustic Guitar Amplification, part 1by Paul Abbott The Fundamentals (and elsewhere) Have you noticed that the pickups you stick in the soundhole or under the saddle of your acoustic guitar sound dull and one-dimensional? This isn't because they're bad pickups but because they mainly reproduce one part of what your ear is accustomed to: the fundamental. To complete the sonic picture you need another source capturing the overtones. Microphones and 'contact' pickups are both good for this. So good that, in many cases, they reproduce too much of the overtones, creating a sound that is acutely 'live', the opposite effect of the magnetic and under-saddle pickup. Hence the need for two separate pickups, one for the fundamental and one for the overtones. Mixed together they create a composite sound that assimilates what's really happening on your acoustic instrument. Pickup Choices Since the focus of this article is the fundamental/overtone relationship as it applies to amplification, not what brand of pickups to purchase, I will leave the extensive critique of pickups until Zen and the Art of Acoustic Guitar Amplification, part 2. But, I believe that even if you choose the least expensive magnetic pickup and microphone on the market, you'd be closer to achieving a true acoustic sound than if you purchase one of the higher quality pickups that only reproduces part of the equation. There are some pickups that try to integrate both elements into one signal, but I feel these are inadequate. You need to have completely separate equalization and volume control over each pickup because they represent totally different dimensions of the sonic picture. Out-of-Body
Experience Next you need to decide on a preamp for each pickup (that's right, each pickup needs its own separate preamp) before you send your signal to the PA. Make sure that the input impedance of the preamp is higher than the pickup you feed into it. Many preamps are simply meant to amplify a weak signal. Others have equalization, notch filter, phase reversal, and effects loop built in. You can also use a self-contained amplifier (similar to an electric amp with preamps built into it), which allows you to generate a substantial amount of on-stage sound. This comes in handy as a personal monitor or if you play with a band and need to compete volume-wise. Good results can be achieved with any of these scenarios. Leaving the Womb If you are interested in learning more about overtones in music, please consider Henry Cowell's New Musical Resources (ISBN 0521499747). Zen and the Art of Acoustic Guitar Amplification, part 2 by Paul Abbott While part 1 spoke broadly on the physical laws of sound and how they relate to acoustic guitar amplification, this article will narrow its focus to the types of pickups and preamps available, highlighting their strengths and weaknesses. To get the most from this article it's important to read part 1 first. Acoustic Pickups from Musicians Friend
Reproducing the Fundamental Under-the-Saddle
Pickups The strong points are that these pickups have a lot of output and do not visually alter the guitar. If you play a custom or vintage instrument, for which the unaltered appearance is important to you, the latter can be a very attractive feature. The drawbacks to these pickups are that, historically, they are difficult to balance from bass to treble (although this seems to be improving with better technology) and they tend to be on the bright-and-twangy-sounding side. However, if you like bright-and-twangy (a staple in country music), this is a plus. If you are into improving the intonation of your instrument and use a wide, compensated saddle (anything significantly wider than 1/8"), it will be more difficult for this type of pickup to 'sit' properly. The effectiveness of these pickups is largely dependent upon a snug fit. If the saddle has been widened it makes this more difficult. Soundhole Pickups The strengths are that they are much easier to install than under-saddle pickups (some you can literally pop-in or take out without removing the strings or using any special tools) and most have movable pole pieces (again, like electric pickups) that can be adjusted to easily compensate for an imbalanced sound. The drawbacks are that, since they're magnetic, regular acoustic strings won't produce balanced volume. This is because bronze (the material that covers the wound portion of an acoustic string) doesn't generate as strong a reaction in a magnetic field compared to steel (the unwound strings). The two steel strings will always be louder than four bronze strings. A simple way around this is to use a heavy gauge set of electric strings (or create your own custom sets from individual gauges). This way all the strings (wound and plain) are steel and have equal reaction in the magnetic field. Also, magnetic pickups are more susceptible to external noise such as hum produced by a lighting system in a club than their under-saddle counterparts. Capturing the Overtones Internal
Microphones Given the choice, no sane recording engineer would ever place a microphone inside the body of a guitar for a recording session. This is because it's a small wooden box where standing waves and reflected noise abound. Using a hypercardioid pattern mic can help block out some of these problems. Finding the exact location for the mic takes some patience and care. A windscreen that covers the mic can help reject some of the reflected frequencies that may not be so pleasant. It's also possible to get a mic that clips onto the outside of the instrument. This eliminates some of the above mentioned problems but cuts down the volume level considerably (ie - it's louder inside the box than outside). Theoretically, this is the best place to put a mic, but it's not as effective if you are playing with a band. The sound generated by the other performers' amplifiers will be picked up and bleed into your sound source. Contact Pickups The strengths are that they are small, have virtually no bleed compared to a microphone, are not susceptible to the reflected sounds created inside the body of the instrument, and have a very dramatic, present sound. When you strum hard or whack the guitar, the audience not only hears it, they feel it. The downside is that these pickups need to be placed strategically on the guitars top. As with microphone placement, finding the right spot may take some time. Also, contact pickups are among the weakest in signal and have the highest impedance. This can be a deadly combination. To compensate, they must be matched with an extremely high-impedance, low-noise preamp. Another thing to consider is the material used to affix these pickups to the instrument. My experience has led me to use silicon glue. It can easily be found in any hardware store and is designed to create a solid bond that will last for decades without ever becoming brittle. It begins to bond within a few minutes and is cured inside of 24 hours. The great thing is that it creates a rubbery type bond that, even years later, can be unmounted fairly easily without damaging the pickup or the instrument. Preamps If you are an electric guitarist, a certain amount of noise is accepted, even expected, in the amplification process. In acoustic guitar, however, only the cleanest, most pristine signal is acceptable. If this weren't a tall enough task, acoustic pickups are usually lower output than their electric counterparts, so the preamp must be of the highest caliber to achieve great results. Check the specs on signal-to-noise ratio. There are certain terminology that can tell you things. Preamps that are discrete-component are usually higher quality and lower noise than integrated preamps (and usually more expensive). Application When the signal leaves a pickup it is high-impedance and unbalanced. In this state it is weak and susceptible to noise (ie - external noise sources such as halogen lights and radio frequencies can degrade your sound). A preamp will convert it to low-impedance. Some, but not all, preamps will take the unbalanced signal and convert it into a balanced one. To have your signal leave the preamp as low-impedance AND balanced is the best scenario. This means that, after leaving the preamp, the signal will be less susceptible to any external noise as it travels through the cable and into the PA system. Many preamps will produce a low-impedance UNBALANCED signal. This is still much better than the original pickup signal, but not as good as a balanced signal. Check the specs on any preamp closely. Just because it has an XLR output does not mean it's producing a balanced signal when it leaves. Many of the better preamps available have built-in EQ, phase switching, ground lift, and effects loop. These are all elements that will come in very handy when trying to deal with a live performance situation. The equalizer will allow you to modify the tonality of your sound. It's very handy to have this at your disposal when performing. If you need to roll off a little bass, you can do it much more effectively (knowing how much you want rolled off) than trying to convey it to the soundperson. Phase reversal can stop low frequency feedback without ever having to adjust the EQ. A ground lift can come in handy in a club with old, noisy wiring. And, even if you think "...I'd never use effects on my acoustic, I'm a purist," you'd be surprised how much definition a little compression can give to the low end of your sound, without ever sounding like your using an effect. A good, clean effects loop can maximize the use of any pedal. Live Sound Tips As I mentioned at the closing of Part 1, live sound is an art. First-hand experience is the best way to learn. By dealing with bad sound situations you learn how to overcome them. If I had to give a few pointers that I thought to be universal in the scheme of acoustic guitar amplification, they would be as follows:
Final Thoughts When assembling your amplified setup, there are a lot of options. The best thing you can do is assess your needs and see which products best fit them. A lot of companies offer fully integrated systems, from pickups to preamps. These will save you a lot of time (and possibly money) when it comes to creating an entire system, but they usually offer fewer options than a mixed-and-matched system you construct yourself. Assembling a system that leaves room for growth is definitely a smart way to go.
His mastering web page is: |
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Paul Abbott is a composer, guitarist, performer and mastering
engineer who lives in San Diego, CA. Since 1990 he has written, performed
and recorded his original music all over the country. Paul has released
one instrumental acoustic guitar CD called "Unsung" and is
currently working on his second CD, as well as assembling a compilation of
San Diego instrumental acoustic guitarists which will be released on his
label, Rhythmicon Records.