At the back of the guitar, there’s a butt wedge that follows the aesthetic appointments of other parts of the instrument (the neck pocket, the headstock back veneer). In this instrument, it is finished in beautiful bird’s-eye maple, but this is a customization that offers a wide range of possibilities:

This other model showcases a striking ETS cover crafted from zebrawood.:

This detail is actually a magnetic cover that hides the External Tuning Slot. Using a dedicated suction cup, the cover can be opened to access the slot, which allows for adjustments to the lower bout.

Now, let’s take a step back and discuss the phenomenon of wolf tones.

Wolf notes are generated by the interaction of a string resonance (a specific note) with a body resonance (usually one of the first two modes). The sensitivity of the first main modes is at its maximum at their resonance frequency, where the mechanical impedance is minimal. When a played note has a frequency close to or corresponding to that of one of these resonances, the energy provided by the strings is absorbed quickly because the mechanical impedance is very low. That note will have a fast decay, less sustain, and will dampen quickly.

Moreover, when energy is supplied by another resonant system (such as a string, which resonates at a frequency determined by tuning and position over the fretboard), these two resonances interact. If the two frequencies are identical or similar, they tend to repel each other, increasing or decreasing their resonance frequencies—their pitch. The result of these two phenomena combined is the production of a wolf tone: a note that is placed on a resonance and sounds bad, without sustain, and does not stabilize on the tuning, oscillating around the frequency at which we would like to hear it play.

This phenomenon is more evident on guitars with lighter soundboards, while guitars with lower mobility generally have a more homogeneous sound and are less critical in optimizing this parameter.

Let’s take a look at this graphic, which shows the frequency response of the Iulius Orchestra (in black), together with 4 single notes plucked on the guitar at contiguous frequencies:

As you can see, the two peaks of the frequency response are perfectly centered between the notes, and even if the interaction of the main resonances with the notes is clear, the notes look peaky and clean. These notes will sound at their best, with maximum sustain and tonal clarity, both at their fundamental frequency and at their second harmonic.

What if the resonances were not perfectly centered?

I simulated incorrect tuning by lowering the tension of the 6th string, bringing the F# to 90 Hz, exactly over the main air resonance:

Now we have a perfect wolf note: the peak is split, the note is chunky with very low sustain, and the whole body of the guitar resonates in my hands with a strong feeling of dissonance.

Taking care of resonance placement is necessary to obtain the best sound from every acoustic guitar, and the ETS does exactly this—and much more.

By acting on the decoupling of the soundboard with the sides (via a precise added mass at the lower bout), the ETS allows you to lower the resonance frequency of the soundboard monopole by a few Hz, on the fly, without the need to remove the strings, in a couple of seconds.
Here an example of its effect, “on” in the green curve:

It is important to not understimate the importance of the final fine tuning of main resonance modes: it allows the instrument to express its maximum potential, with a coherent, harmonic and juicy sound, plenty of sustain and balance across the fretboard, as you can hear it here:

Each Iulius Guitar model is offered with an individual specification sheet that also reports the frequency response. This data allows you to keep track of the evolution and aging of the instrument over the years, and the ETS system will enable you to fine-tune the main resonances if necessary, to always keep the guitar at its maximum acoustic potential!