From the March/April 2021 issue of Acoustic Guitar | By Martin Keith
Q: “I recently had the action adjusted on my acoustic-electric guitar. The plugged-in sound is now very uneven—the high and low E strings are hollow-sounding and quieter than the rest. What happened, and what should I do to fix it? —Roberta H., via email
A: Ah, piezo pickups. They have vastly simplified the task of live performing with acoustic guitars, and over the years they have improved dramatically. Originally notorious for their thin, scratchy quack, piezo systems have evolved into a broad family of acoustic sensors that can be quite musical and robust-sounding, if they are properly installed. However, that last part is the proverbial Big If—and many (many, many!) piezo systems suffer from installation problems that result in less-than-ideal sound.
Most common piezo sensors fall into two categories: undersaddle or top-/body-mounted. Both have their strengths and weaknesses, and some more advanced systems even combine the two in an attempt to get the best of both. The majority of factory guitars have undersaddle systems, largely because they are easy to install and offer solid output and less feedback than body sensors. The downside of undersaddle pickups is that they tend to have a very strong, spiky transient attack, which can make the guitar sound harsh or unappealing, especially when played with a heavy right hand. Body sensors tend to have a more airy and natural response, with more of the instrument’s character in the tone—but they can suffer from lower output, uneven balance, and problematic feedback. For these reasons, top sensors tend to work better for fingerstylists and performers who play at lower volumes in smaller settings.
Without having your guitar on my workbench, I’d be willing to bet that the culprit is the underside of the saddle (the white strip of bone or composite that the strings contact at the bridge). The word piezo comes from the Greek word for pressure, and piezo systems rely on compression from the strings in order to couple with the guitar’s vibration and create a signal. When the pickup is sandwiched between the saddle and the bridge, it needs to be fully and evenly in contact along its entire length. If it isn’t, the result will be what you describe—hollow-sounding strings and uneven volume.
If your tech adjusted your action by sanding the bottom of the saddle (a common approach), then it’s likely that they didn’t sand it perfectly flat and square. Check it by removing the saddle from your guitar (you’ll have to take off the strings, of course!) and placing the bottom edge against a ruler or straight surface. If you can see light or gaps at the outside ends, then the saddle needs to be flattened.
While removing the saddle, take note of how tightly it fits in the slot. My rule of thumb for saddle fit literally involves my thumb, along with my index finger: I’d like to be able to remove the saddle using just those two digits, but it should be just snug enough to stay in place when the guitar is turned over. If the saddle is too tight, it can bind or seize in the slot, preventing the even pressure that the pickup needs. However, if it’s too loose, the saddle can tilt over in its slot, which will also compromise pickup performance (and may lead to a cracked bridge!). If the saddle is too tight, sand the faces lightly with 220 or higher grit paper until it fits as described above.
If you are still experiencing balance issues but the saddle is perfectly straight and flat, then the next possible culprit is the saddle slot itself. Most flat-top guitars are not really flat—some have gently domed tops engineered in during construction, while others were built flat but have bulged forward from string pressure. Some amount of this is normal and is generally considered the sign of a responsively built instrument. However, this top distortion can sometimes result in a flexed bridge, which could mean that the bottom of the slot has taken on a convex shape. If you have a competent luthier nearby, they can often reroute the saddle slot to create a flat surface if the top has flexed too far. It may be necessary to use jacks or other braces inside the guitar to simulate the string pressure on the top, so the slot can be flattened accurately.
A slightly less invasive approach to dealing with curved saddle slots is to build some flex into the saddle itself. In cases where flattening was impractical or impossible, I’ve resolved string balance issues by drilling five small (1/16-inch) holes through the saddle, between each pair of strings. These holes travel through the saddle in the same direction as the string paths, and I drill them just below where the saddle is visible at the bridge, so they don’t show on the assembled instrument. Then, I saw a small slot from the bottom of the saddle up to each hole, creating a keyhole shape. This idea follows the same principle as the kerfed lining inside your guitar—the slots allow the saddle to flex evenly, and help it conform to the curve at the bottom of your saddle slot. This quick approach has solved many a string balance issue for me, in less time than it takes to jig up a Dremel tool and recut a slot.
It can be tempting to try to put a little shim under the weak string, to try and regain a bit of extra pressure and even out the balance. I’ve come across plenty of these when digging into bridge setups to resolve these pickup problems. My experience has been that this approach usually almost works, but never quite resolves things completely. Adding a shim to the weak string will focus more pressure on that spot, but it will create a step in the surface that robs the adjacent string(s) of good contact. If the strings are not responding evenly, the problem is that the surfaces are not flat and even, so making them even less so is not likely to be fruitful. This is a case where one should treat the cause, not just the symptom.
One last point worth mentioning: piezo systems are also very sensitive to the signal chain. I’ve had several clients bring me instruments to solve an apparent pickup issue, only to discover than the guitar sounded great when plugged into a proper input. Any passive piezo pickup will need a high-impedance input (such as an acoustic preamp) to sound good, and even many onboard active systems suffer from poor matches between pickup and preamp. If the tone is thin, scratchy, overly compressed, or lacking in bass warmth, it could easily be the result of a bad combination of guitar and amp. Unfortunately, these same problems can also arise from issues with fit and installation, so it can sometimes take an experienced tech to tell the difference. I’ve spent what feels like years of my life trying to debug uneven piezo systems, and I know how frustrating it can be. If you’re lucky, a bit of quick sanding or filing on the bottom of your bridge saddle should have your guitar sounding good.
This article originally appeared in the March/April 2021 issue of Acoustic Guitar magazine.