ETS
External Tuning Slot
Highly responsive guitars are susceptible to wolf tones, a defect that manifests as “chunky” notes with reduced sustain and incorrect intonation, particularly on the 4th and 6th strings. The instrument feels uneven, and no solutions seem to improve it.
Preventing wolf tones requires precise tuning of the instrument’s main resonances and the ability to make adjustments to compensate for instrument aging or significant shifts in humidity and temperature. Crucially, the guitar must be designed and built to position these resonances accurately between contiguous notes.
Each instrument I build has a target frequency response and a target placement of main resonances, essential for achieving optimal tone according to the specifications.
The frequency response of the Orchestra model. Here more info about how to perform this measurement.
Additionally, I developed a unique device, the External Tuning Slot (ETS), which allows me to fine-tune the frequency of the main modes quickly and non-destructively. This is integrated into the guitar’s design.
At the back of the guitar, a butt wedge, finished in materials such as beautiful bird’s-eye maple (a customizable feature), covers the ETS.
This detail is a magnetic cover that conceals the External Tuning Slot. A dedicated suction cup is used to open the cover, providing access to the slot for adjustments to the lower bout.
Now, let’s take a step back and discuss the phenomenon of wolf tones.
To illustrate proper resonance placement, consider this graphic, which displays the frequency response of the Iulius Orchestra model.
You’ll observe that the main resonances are carefully centered between the played notes. This deliberate placement results in notes that are clear and well-defined, with distinct peaks, harmonious character, and excellent balance and sustain.
The guitar’s overall sound is rich and captivating.
Wolf notes instead are generated by interaction of a string resonance (a specific note) with a body resonance (usually one of the first two modes). 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 rapidly absorbed, resulting in rapid decay, reduced sustain.
Moreover, when energy is supplied by another resonant system, such as a string resonating at a specific frequency (a note), these two resonances interact, and they repel each other, increasing or decreasing their resonance frequencies—their pitch, or generating a split peak.
The combined result of these two phenomena is the production of a wolf tone: a note that sounds undesirable, lacking sustain, and failing to stabilize in tuning, oscillating around the intended frequency.
The entire body of the guitar resonates in the musician’s hands, producing a strong feeling of dissonance.
Addressing resonance placement is necessary to obtain the best sound from every acoustic guitar, and the ETS facilitates 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 quickly, without removing the strings.
This operation can be performed in a couple of minutes, without removing the strings.
It is important to not underestimate 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 track 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 maintain the guitar at its maximum acoustic potential.
