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AMPLIFYING YOUR ACOUSTIC GUITAR WITHOUT FEEDBACK

Copyright 2007 Steve Carmody

Manufacturers of pickups for acoustic guitars are at the top of their game right now. I don’t know of any pickup systems currently made by major manufacturers that aren’t high quality. But still, getting a good live amplified acoustic guitar sound often remains elusive for many players. One issue is getting enough volume without feedback. Having a good pickup system and an amplifier designed specifically for acoustic guitars are absolutely essential to getting a good sound, but often not sufficient to avoid feedback problems.


There are a number of factors I can think of which can contribute to feedback problems, and a couple of additional solutions to suggest if all else fails.

1. THE ROOM

2. OVERALL VOLUME

3. PRE-AMP SETTINGS

4. MECHANICAL ISSUES WITH THE GUITAR

5. ADDITIONAL SOLUTIONS

THE ROOM


The setting in which you are playing impacts on your ability to increase the volume of your guitar without getting feedback. A small room can certainly be an issue. The signal coming from your amp can bounce off walls and windows, head right back into the box of your guitar and trigger feedback. If you are playing a small room, set the amp and all speakers such that the path back to the guitar is as complicated as possible. Think of a pool table. If you shoot the cue ball straight at a rail – it bounces right back. But if you shoot the ball at the correct angle, it may never have the energy to return where it started. With this in mind, you should orient your guitar amp and any monitor speakers so that they point to your ears, and not the soundhole of the guitar. Any reflective surfaces, bare walls, and especially windows, can be a problem, so if you control the room, padding the walls with blankets,foam, even egg cartons, will do wonders; but if you show up at a gig and the room is small – think angles


OVERALL VOLUME


No matter how good your guitar pickup is, if the overall volume in the room is high enough, your guitar may generate feedback. The box of your guitar is a resonant sound chamber. The energy from the strings drive this box, but so do sound waves coming from the room. Drums, other amplified instruments, screaming fans, all generate sound frequencies that can resonate within your guitar, and be amplified through your pickup.

(This is one of the advantages of semi-hollow and solid body guitars- they reduce the impact of ambient sound on the guitar pickups by stiffening the resonant chamber. The down side of course is that they don’t sound as nice un-amplified. On the up side, when the impact of resonance on the guitar body is out of the picture you can actually use feedback as an effect ! Controlled feedback, though, just isn’t embraced as much by acoustic players.)


The bottom line is that, with most acoustic guitar pickups, keeping the overall volume of all the instruments in the room as low as possible helps reduce the potential for feedback.

PRE-AMP SETTINGS


In my opinion, anyone with a pickup in their acoustic guitar should have a pre-amp, either mounted on the guitar or as an outboard unit between the guitar and the amp. Wise use of the volume and tone controls on your pre-amp can help to reduce feedback.

Here is some advice:

1. Set the volume at the pre-amp as low as possible and get most of your volume from the amplifying system. This makes your pickup less sensitive to the overall volume in the room.

2. Use the tone settings on your pre-amp to control feedback. Every guitar has a harmonic resonance point. This resonance point will vary depending on the size of the sound chamber and the stiffness of the construction. On a given guitar, increasing the volume in this tonal range can generate feedback. Conversely, reducing the volume in this specific tonal range enables increasing the volume of the remaining tonal spectrum. So, if volume adjustment alone isn’t enough to stop feedback, experiment with your tone settings. Often, dropping mid-range frequencies is a good place to start. Also, a given room will react, sonically, in its own way; so you may have to experiment with these settings each time you play in a different venue.

MECHANICAL ISSUES WITH THE GUITAR


Both the structure of your guitar and the correct installation of the pickup can impact its performance when amplified.

GUITAR STRUCTURE

At higher volumes a guitar box with light construction may react more to ambient sound waves, which can lead to feedback issues in guitars with soundboard transducers. From the point of view of feedback resistance, guitars with light construction may be better served by under-saddle type pickups since under-saddle pickups, contained as they are in the wood of the bridge, are more isolated from the resonance of the guitar box. If combination systems are installed, an under-saddle pickup or magnetic pickup should always be included as an option. Unfortunately, it may not always be easy to make the judgment about how likely a given guitar will be to react this way. In my experience I have seen this issue more in small shop guitars because there is often more attention paid to creating a responsive acoustic sound, which usually translates into a lighter structure, but this is not universal.

CORRECT INSTALLATION OF THE PICKUP

Pickups which are not installed correctly can lead to feedback problems.

1. Under Saddle Pickups

For proper function, under-saddle pickups must be sandwiched evenly between the saddle and the bottom of the slot in the wooden bridge. What this means is that, if the pickup element were not in place, the bottom of the saddle would sit flush with the bottom of the saddle slot. The pickup element is manufactured to very close tolerances, so if the saddle and the saddle slot are true, there will be even pressure under each string and the string to string volume should be appropriate. It is the down pressure from each string that activates the pickup element. If there is any unevenness of contact between the saddle and the bottom of the saddle slot in the bridge under a given string, it will not sound as loud as the others. This unevenness may not lead directly to feedback issues, except when increasing volume to compensate for a deadened string, but a well fitted saddle is essential to proper pickup function.

A factor of the fit of the saddle which does play into feedback is the looseness of the saddle in the slot. The saddle should not be floppy ”loose”, but it should be able to slide up and down without pinching. A common feedback issue with under saddle pickups results from the saddle not fully pressing down on the pickup element because it is slightly wider than the slot. The bottom line is that a good fit of the string saddle is necessary to avoid feedback issues.

2. Contact Pickups

There are two factors which commonly affect the quality of sound, and resistance to feedback of contact pickups- location and quality of adhesion.

Most contact pickups currently being manufactured are designed to be installed inside the top directly under the saddle position. This location affords proximity to the focus of the string energy as well as being a fairly stiff place on the top. It should be noted that some contact pickups, as in the Taylor “Expression” system, which combines contact and magnetic pickups, are located on other parts of the top. But most contact pickups which are retrofitted to an instrument will best respond, and resist feedback, when mounted in the area under the bridge.

Contact pickups are generally held in place with either industrial-strength two sided tape, acoustic putty, or super glue. Regardless of the material, the bond to the top must be firm.

3. Microphones

Microphones are either clipped to a brace, attached to a flexible gooseneck, mounted to the pre-amp, or held in place with an adhesive Velcro tab. For a given mike in a given guitar there may only be one “sweet” spot- a place where the sound is reasonably pure and somewhat resistant to feedback. The only way to find this spot is to experiment with locations until you like the results. In high volume situations microphones are the most prone to feedback issues and often cannot be used.

ADDITONAL SOLUTIONS


MAGNETIC PICKUPS


Reducing the overall sound coming into the box of the acoustic guitar, and wise use of the volume and tone controls of your pre-amp will help reduce the potential for feedback issues, but if you have no other choice, magnetic pickups, while more commonly used in “electric” guitars, have their place in acoustic guitars as well. They are usually found as units that clamp into the soundhole. Some companies, most notably Taylor guitars (though Gibson did it back in the 1960’s), have incorporated magnetic pickups into their factory installed pickup systems, usually placing them in or near the end of the fretboard. It should be noted that, while magnetic pickups are rightly appreciated for their ability to largely alleviate the problem of feedback when working in high volume situations, even the best magnetic pickups don’t get high marks for their ability to reproduce the “natural” sound of the guitar. By definition, a magnetic pickup is largely, though not entirely, triggered by the vibration of the metal strings within the magnetic field generated by the pickup. Also, pickups which are clamped into the soundhole both stiffen the guitar top, and, maybe even more importantly, block the soundhole. Therefore, if you use a magnetic soundhole pickup, you may want to have it installed such that you can remove it when not in use.

I would also say that a magnetic pickup, combined with a quality digital processing system can create a wonderful acoustic “sound”, but, and I know we can get into a semantic argument here, many people don’t consider a digitally produced guitar sound to be “real”. (For the purpose of this article I am going to consider analog electronic sound reproduction as “real” acoustic sound, though that point can also be debated).

SOUNDHOLE COVERS


There is one product designed to help reduce feedback which I will mention because, well, it is just a great invention. It is made by the Kaman Co. and is called the “Feedback Buster” (part #FBR2). It is a round rubber disc that fits in the soundhole of the guitar. It is made in only one size- 4” diameter. This will fit most Martin dreadnaughts and many others, but not all guitars. Also, it cannot be used if you have soundhole mounted volume controls.

The concept is simple: It blocks all external sound from coming through the soundhole which, in turn, markedly reduces the potential for feedback in high volume situations. I have also seen a “lute” style soundhole cover advertised which also seems to address this same issue. I can attest to the benefit of soundhole covers, and do not hesitate to use them when everything else about the guitar’s sound system has been addressed and feedback is still an issue.

THE BOTTOM LINE


In summation- to reduce the chance for feedback when amplifying your acoustic guitar: don’t point speakers at your guitar, try to keep the overall volume of all the instruments from being too loud, use the volume and tone controls on your pre-amp to your advantage, make sure your guitar is set up properly and the pickup installed correctly, and, if necessary, cover the soundhole. Good luck!

Steve Carmody has worked full-time doing guitar repair since 1990. Questions? - GuitarRepairShop@aol.com