Bass guitar

Bass Bench: Positioning of the piezo pickups on an acoustic bass guitar



Photograph 1. Photo courtesy of basslab.de.

When we previously surveyed several types of acoustic bass guitar pickups [“Amplifying an Acoustic Bass Guitar”], we paid special attention to the popular under-saddle transducer. Let’s continue this line of thinking and see what else we can do with a piezo.

Not all piezo pickups under the saddle are the same. If you’re not happy with the way yours sounds, you might consider looking for a replacement. But before spending money on buying a new one, it’s a good idea to check if yours is set up correctly. This may involve removing dirt from the saddle slot, making sure the piezo has enough space in its slot, adjusting the angle at which the strings break on the saddle, and conquering the buzz. There are many installation and troubleshooting guides on the web that cover these common issues.

Although the number of piezo products is not as huge as with magnetic pickups, it is still difficult to choose the right one. Although the piezo material itself does not vary much, there are some differences in quality between the devices. For example, some piezos are wrapped in a simple heat-shrink tube, while others have hard and thin conductive coatings.

Here are some of the often mentioned disadvantages of piezos under the saddle:

• Unbalanced volume of the strings. This can happen easily and often takes a long time to correct.

• When the piezo is subjected to high pressure (or rather intense contact), the resulting sounds can be harsh, unusually direct, and too punchy.

• Piezos sense the movement of the string, not the soundboard, which can cause unnatural sounds.

When the piezo is under high pressure, the resulting tones can be harsh, unnaturally direct, and too punchy.

Some of these reviews are compelling, while those regarding sound quality are subjective. You can’t judge quality by price alone anyway. (For example, the heat shrink tubing on an inexpensive model can soften these harsh, punchy qualities.)

One obvious solution is to change the position of the piezo, moving it from under the saddle towards the soundboard. On a bass with acoustically balanced strings, this ensures an even string volume. (And once mounted inside the body, the piezo is as invisible as a mic under the saddle.)

Speaking of volume: Without direct contact with the strings, soundboard transducers tend to produce lower levels, in part because of the smoother initial attack. Some manufacturers compensate for this with preamps or by using two or more transducers. These soundboard transducers should be mounted either directly below deck or in areas that vibrate as much and as freely as possible, so stay away from structurally “dead” bracing areas. Experimenting with the positions can be time consuming, but it’s not as boring as trying to balance the volume of the strings by sanding a piezo under the saddle.

Because the soundboard is the natural source of acoustic sound, the soundboard solution would seem like a clear winner, right? Not so fast: it is a little myopic to claim that the transducers under the saddle do not take into account the soundboard. The piezo is a pressure sensor, so if you move the base of the saddle (the soundboard) the mic will surely pick up the difference signal, not just the string above it. Also, if you stick a pressure sensor on the soundboard, it will move with the soundboard. Can you then expect it to provide the full sound image?

There is a third option, let’s call it “mid-saddle”. Here, the piezo is a fixed part of the saddle itself. This approach maintains some string pressure on the pickup element, which can reduce the abnormal percussion attack of the construction under the saddle.


Photograph 2. Photo courtesy of chrislarkinguitars.com.

Instead of using a single piezo element that runs the length of the saddle to simultaneously pick up all the strings, mid-saddle systems include a set of single-string pickups. Check it out: Photo 1 shows a single string ETS saddle with a built-in piezo. In this type of design, each single-string piezo is glued into the nut or embedded in resin, which protects the electrical and mechanical contacts. Another rarity on acoustic bass guitars, the fully adjustable bridge (Photo 2) on this instrument Chris Larkin contains five single-string RMC pickups.

Ah, but we’re not done yet. If you wire a set of single-string mics in parallel and send the combined signal through a preamp, you can only hope that the manufacturer has carefully pre-selected the mics for uniform volume, as there is no way to change the volume of each mic yourself. To avoid such problems, you can go for the more expensive route of sending the signal from each chain to a separate adjustable preamp. Of course, once you have single chain outputs, you also have the option of using MIDI.

And as if that wasn’t complicated enough, there are plenty of solutions that combine various approaches that we have discussed. And we haven’t even talked about microphones yet, although we will soon.