There are several different types of wolf tones, but for the sake of simplicity, today we are going to refer to the definition of a wolf tone as a note (or notes) which is, “literally, not absolutely controllable by the player.” (Pickering page 39)
Before we can understand how a note can be out of control, let’s take an elementary look at how sound is created and controlled on a violin, viola, cello, or bass.
Although there are many moving parts that contribute to the creation of sound on your instrument, here we will focus on the following parts and how they play a large role in how notes/frequencies are created/excited:
As the bow moves along the string, the continuous energy supply coming from the bow causes the string to vibrate side to side.
Let's review our simple sound concepts.
No Batteries Required for Amplification.
The body of the instrument (and its reverberations or resonance) is especially important when discussing wolf tones.
Simply put-- your instrument's body is its amplifier.
The instrument body plays such an important part in how pleasing an (acoustic) instrument sounds that violin, viola, and cello bodies are built to be as resonant as possible.
In fact, when making an instrument, a luthier (string instrument maker) will tap in different places on the top and back plates, listen to the note/frequency made when tapping, and then remove wood in different areas to help fine-tune the top and back plates so that, when tapped, specific notes are created.
The better fine-tuned the top and back plates are, the more easily notes will excite the individual plates and cause them to vibrate.
Fun Fact: As described in the post "How Does a Violin Work?" by Professor Joe Wolf at the University of New South Wales, the air inside the instrument can, "vibrate a little like the air in a bottle when you blow across the top. In fact if you sing a note near D4 close to the violin (or the second lowest string on a viola, cello, or bass), and then hold your ear close to the f-holes, you may hear the air in the body resonating. This is called the Helmholtz resonance.”
So try singing into your instrument's F Hole! If you have a violin, sing the same note as the D string. If you have a viola or cello, sing the same note as the G string. Then listen for the Helmholtz resonance!
Here’s What a Renowned Engineer Can Teach Us
In his article titled, “The Wolf Tone,” which originally appeared in Volume 37, Number 3 of Bass World, musician and renowned engineer, Norman Pickering, goes into great detail on what a wolf tone is and offers some suggestions on correcting the issue.
We won’t go through the entirety of his article here, but let’s touch on a few of the more important points he makes regarding the most common causes of wolf tones.
Played Note vs. Strong Body Resonance: Competing Frequencies
Mr. Pickering says, “In the case of an instrument with a strong body resonance close to the string frequency, the (strong body resonance) can steal energy from the string and oscillate strongly enough to overwhelm the bowed note.”
This means that when a bowed note is close in frequency to the strong body resonance of the instrument, the excited, super-strong body resonance fights back and creates interference or a note which is “not absolutely controllable by the player.”
Similar Bowed Note/Frequency + Similar Strong Body Resonance = Wolf Tone!
Why Does the Bow Feel Out of Control During a Wolf Tone?
Many players complain that when they try to play the note which creates/excites a wolf tone, it feels as if they cannot control their bow.
Mr. Pickering explains that when the musician is bowing a note which occurs below the strong body resonance, “the string and bridge are moving in-phase (together).”
Later he continues, “At the start of the (problem) note, the string forces the bridge to move in the same direction as the force it exerts. Coupling (the note played combined with the body resonance) can cause the bridge motion suddenly to reverse phase, opposing the string force. Bridge motion is suppressed, and the tone drops abruptly.”
Remember what we learned about the creation of sound?
When a note that doesn’t have a wolf tone is bowed on the string, the string and the bridge move together. The body of the instrument then happily resonates and acts as an amplifier of sound.
With both forces pushing the bridge in opposite directions, the bridge motion will temporarily be stifled and the tone of the note will drop abruptly.
Once the note drops, the strong body resonance is no longer being excited and the player is able to regain “control” of the note through continued bowing. Then the cycle begins again.
Bowing through a wolf tone becomes even more difficult the quieter (or further from the bridge) the note is bowed.
Other Wolf Predictors
Mr. Pickering also lists the following elements which can help in the prediction of a wolf tone:
Many cello players notice a more pronounced wolf tone on/between the E3 to G3 when playing on the G string than when they play the same note in first position on the D string.
“Derived from the string tension and distributed mass, (the string parameter known as wave impedance) … indicates how much energy the string imparts to the bridge. Typically, violin D strings have nearly twice and G strings more than three times the driving energy of the A string. A similar relationship exists between the lower three strings of the viola and cello.”
Using the violin as an example, Mr. Pickering explains why this happens:
"The same note played in higher positions on a lower string transfers much more energy to the body of the instrument, and explains why wolf tones often show up only on those lower strings.”
Put simply, wolf tones become more obvious in higher positions because lower strings provide more energy to the instrument body which excites strong body resonances.
So there you have it!
The most common wolf tones are created when a bowed note is similar in frequency to a strong body resonance. And the higher up on the fingerboard you play that trouble note, the stronger the wolf tone may be.
There are a few things your luthier may want to check on your instrument to see if they are causing your wolf tone.Gluing
Wolf Eliminators (Suppressors)
If your luthier has gone through all of the items on the list above, but you still have a troublesome wolf, they may suggest a wolf suppressor.
Wolf suppressors are more commonly known as "wolf eliminators" but that term is a bit of a misnomer because, and we cannot stress this enough— wolf eliminators do not eliminate the wolf on your instrument.
A wolf eliminator's job is to simply move the wolf to a note which you do not play often or, even better, between two notes.
The most common wolf suppressors available today are on-string wolf suppressors. They fit on your string between the bridge and the tailpiece. The weight they provide to your string changes how your string, bridge, tailpiece, instrument body, etc. vibrates together which slows (or impedes) the movement of sound throughout your instrument.
Imagine you were jumping on a trampoline with your friend. If you find that special sweet spot, when you jump, you can send your friend flying into the air. If you move just a little bit to the left and jump again, you may not be able to launch your friend with the same force.
In a rough, metaphorical nutshell that is what your wolf eliminator is doing; just moving your jump a little bit to the left so you don't upset your friend.