In short, the outcome was that the body/resonator contact had a clearly noticeable influence on the acoustic tone, but close to none for the electric output. Another variation on this experiment is to put the headstock—rather than the body—in touch with the box. Can we expect the same outcome as before?
One obviously huge difference between the body and neck is their cross sections, influencing stiffness and mass, and therefore providing susceptibility for dampening, resonances, and eigenmodes stimulated by the vibrating string. (For more on how eigenmodes work, see “Killing the Bass, Part 1," from the August 2020 issue.) The main parameters influencing a neck's vibrational behavior are—of course—material, shape, and design of the truss rod, or truss rods. (Yes, dear guitarists, there are basses with more than one truss rod.) There is also the act of our hand grabbing the neck, which will both dampen and stiffen the neck.
An old trick to move dead spots up or down in frequency is to add or remove mass at the headstock—the end of its “lever arm." This is known to work on guitars most of the time, but not so well on basses. This is unscientifically spoken and without further proof, but to significantly influence low bass notes requires bigger changes in mass than most would accept as realistic. There is a bit more about this subject in my column “Bass Necks: Adjustability and Resonance," from November 2012.
Fig. 2 — Here's the spectrum of an E chord on an electric guitar recorded via microphone, with black indicating contact with a box and red without. Graphic courtesy of “Physics of the Electric Guitar" by Dr. Manfred Zollner
Back to the experiment: So, if varying the mass at the headstock can shift these resonances, so should its fixation to the box, right?
Fig. 1 shows our experimental setup. Fig. 2 represents the measured signal of a microphone, while Fig. 3 does the same for the measured signal of a pickup. Both diagrams compare neck contact with the box, and without. It's worth noting that the basic signal for the measurements was an E chord played roughly 50 times by an experienced player.
Opposed to last month's experiment with the body contacting the box—where it was hard to even distinguish the curves in the plot of the electric signal—we can now see at least a few differences in two frequency ranges. Although visually noticeable, however, it was reported that even experienced listeners weren't able to say which is which.
Fig. 3 — Here's the spectrum of that E chord recorded with a guitar's pickup, with (black) and without (red) contact to the box. Graphic courtesy of “Physics of the Electric Guitar" by Dr. Manfred Zollner
An explanation for the results of the first experiment with the body/box contact is that there is a flow of vibrations into the body. Otherwise, we wouldn't hear any acoustic difference when contacting the box. But the reflow from the body back into the strings is zero, at least practically, as we can't measure or hear any differences. On the other hand, the softer neck and its vulnerability for resonances and eigenmodes is able to make a small difference due to the added stiffness and coupling when contacting the box.
All these measurements were done with a guitar, so it would be interesting to see whether there would be bigger differences in the electric signal for a bass. Why? Because we have a higher string mass and overall vibrational energy, and a longer scale length.
There are certainly quite a few more constructional details on our instruments that will alter acoustic or primary tone, but with even less likelihood to make it into the electric signal. So, bottom line: Don't rely too much on acoustic tone when you're evaluating an instrument!
It's pretty common to begin assessing an instrument through its acoustic tone, but how much does this really reveal about the instrument's plugged-in tone? You see it often in reviews, where a player starts out by describing the acoustic tone of a soon-to-be-plugged-in instrument and then draws the first conclusions of what to finally expect.
Think about it: Whenever we pick up an instrument, the first thing most of us do is play it acoustically. It makes sense on many levels, since we want to get used to the neck, overall ergonomics, string spacing, and/or whatever else we need to feel at home before we start annoying (entertaining) our neighbors. The sooner we feel at home, the more likely it is that we are going to like its electric tone. But can we really use an instrument's acoustic tone as a tell for its amplified tone? Not so much! And it's not because we aren't yet familiar with its pickups and electronics.
To be clear, we're talking about solidbody instruments—not acoustics—but the lines can be blurred. With an acoustic bass, the final acoustic tone depends solely on moving the top through the vibrations of the string. With a solidbody, however, only a tiny fraction of the strings' movements or vibrational energy is transferred to the body. Hence, the longer sustain of a non-acoustic bass, since a higher portion of the vibration is kept in the strings.
So, what do we hear unplugged and what can affect it? The first thing to consider is your listening position. The airborne sound of a vibrating dipole consisting of the body and the far more influential and resonant neck will heavily depend on where your ears are. Are they in line with the body's surface in a typical player position or bent over the body with your ears almost in front of it? You can easily hear the difference by rotating the instrument's body on your lap.
Often, the impression of an acoustically loud instrument leads to the conclusion of getting a strong, aggressive, impulsive, dynamic—or whatever you want to name it—electric tone. In reality, there are a lot of construction details that blur the categorical split between an electric and acoustic instrument, so be sure to expect differences in the airborne sound. There could be a regular open pickup routing or a more generous routing that's closed with a floppy pickguard and acting as a sort of a tiny speaker. The same thing goes for a chambered body that—depending how it's done—can give us a sort of acoustic touch, sometimes even with its plugged-in tone.
Fig. 2 — The spectrum of the E chord recorded with a guitar's pickup, with (black) and without (red) contact to the box. Graphic courtesy of “Physics of the Electric Guitar" by Dr. Manfred Zollner
Luckily for us, there are measurements that can show how misleading the direct connection of airborne sound and electric tone can be. A repeatedly played E chord on a guitar is recorded with a microphone, and then via the pickup. In each scenario, the sound is captured while the guitar is in contact with a box, and then without. Fig. 1 shows the acoustically noticeable and measurable change in both the midrange loudness and low-end spectrum of an instrument when in touch with the box, which is caused by the extended radiating area of the box. The graph in Fig. 2 shows the measurements when recording the pickup's output signal with and without the box, where you have to look very closely to see any differences at all.
So, none of the acoustically obvious differences made it into the final pickup signal in a way that even an expert's ear would be able to distinguish. And if attaching a box to a body doesn't alter the electric tone, this gives us a hint of how influential the body wood is, but that's another story for another time.
There are a lot of emotions involved when playing an instrument, so there are surely some qualities one might rediscover in an guitar's plugged-in tone that relate to its acoustic tone. Maybe it's how it inspires you to play in a certain way, how it reacts to bending or different playing styles, or maybe even some of its dynamics. But it's almost impossible to fairly judge an instrument's so-called primary tonal character by just its acoustic tone.