Zen and the Art of Acoustic Guitar Amplification, part 1

by Paul Abbott
1998 Paul Abbott. All rights reserved.

About the Author

The Fundamentals (and elsewhere)

Part 2 (below)

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The phenomenon of sound is a combination of two components: the fundamental and the overtones. On your guitar, the fundamental is the lowest order of vibration from the moment you strike the string, and the overtones are 'related' notes that develop through fractional division of string vibration and sound waves. The fundamental and the overtones combine to create what your ear perceives as one sound. To a large degree, the 'voice' of an instrument is determined by the fundamental/overtone relationship. This is part of the reason why each guitar has a unique sound.

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
If you agree with the above stated, it's time to think about which pickups (and/or mics) to choose. There are so many models currently available the choice can be overwhelming. It's up to you to decide if you're more fond of the magnetic/soundhole type pickup or under-saddle transducers, and whether you prefer internal microphones or 'contact' pickups. Each has its pros and cons. Consider your playing and performance needs. If you're a flamenco or classical guitarist (using nylon strings) a magnetic pickup isn't going to be very useful. Also, check out the technical specs on each product. Things like output volume, signal-to-noise ratio, and price will go a long way to help you make your decision.

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
What happens once the signal leaves your instrument is as important as your choice in pickups. Get the best quality cables you can afford. A good cable will provide clearer sound with less noise. Another consideration is the cable length. Some pickups are active (and low-impedance). For these the cable length is less of an issue. For high-impedance pickups, though, you'll want the shortest cable before it reaches the preamp. A high-impedance signal will lose high-end with each foot of cable length. As a rule for high-impedance pickups, try to keep the cable less than ten feet before reaching the preamp.

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
After all this you will face the ever-present challenge of the gig. Conquering room acoustics with proper equalization is indeed an art which your ear will become more attuned to over time and with experience. But to have a fighting chance I truly believe you need to assemble a high quality, flexible setup that accurately reproduces your guitar's sound and overcomes the environmental circumstances that are part of live playing.

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
1999 Paul Abbott. All rights reserved.

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
Acoustic pickups that fit under the bridge saddle constitute a large part of the market. Within this genre there are a number of manufacturers, all of which claim to have the best sounding technology.

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
Because of the different materials, design, and placement, soundhole pickups have a much mellower sound than under-saddle pickups (read: less twang). These are, in design, similar to electric guitar pickups and, to my ear, sound a lot like a jazz archtop pickup.

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
For great sound, the microphone has always reigned supreme in the recording studio. But, one of the things you quickly learn as you gain experience performing and recording is that these situations are worlds apart.

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
Many companies have produced contact pickups with widely differing results. The better ones rival microphones for their clarity and sonic accuracy while the lesser of the species sound harsh and microphonic.

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 from Musicians Friend

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
The preamp should be the first thing that the pickup reaches in the signal chain. In theory, the shortest physical space (cable length) between the pickup and the preamp is best. Some pickups have preamps built in as a complete system that can be powered by a battery in the guitar. Another popular setup is to have the preamp built into the endpin jack. If, however, your preamp is outboard, keeping the cable length ten feet or less should be satisfactory.

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:

  1. Use as little equalization as possible. The more EQ you use, the thinner (and more 'electric') the sound will become. If you have a good instrument with quality pickups and preamps, you don't want to change that -- just refine it. Inevitably, though, every room you play in will emphasize problematic frequencies. You will need to remove these to keep the sound of your instrument clear. My philosophy is to try and solve the problem with as little effort as possible. For instance, if I notice a low rumbling frequency (maybe a low D), I'll try reversing the phase. Often times that will stop the problem. If it doesn't, I notch out that specific frequency. By being as conservative as possible with equalization, you will keep the fullness and character of your instrument intact.
  2. When working with two sound sources (pickups), sound check each individually before mixing them together. This way, if there's a problem frequency (and there may be a different frequency for each pickup, since they're handling different tasks) you can isolate it first, then move on to putting the finishing touches on your sound.
  3. Stand in different spots on the stage (facing in different directions). You don't want to find out in the middle of a song that if you move your instrument will begin to feed back. Also, try to get out into the audience area and see if it sounds as good out there as you think it sounds on stage.

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.


About the Author

Paul Abbott is a professional audio mastering engineer and owner of San Diego’s ZenMastering. He has mastered over 500 projects for artists in America and 23 countries around the world. Paul has also appeared as a columnist in EQ, Sound on Sound, Music Connection and Tape Op magazines and is a voting member of AES.

His mastering web page is:
http://www.zenmastering.com
.