The Violin Wolf Tone – Patrick KREIT

Violin wolf note

The “wolf note” may be heard on all instruments of the string quartet. Many violins by Stradivari and Guarneri reputedly have it. It is almost the prerogative of powerful violins. Nevertheless, this defect can sometimes be an inconvenience for soloists.

It is a beat-type self-oscillating frequency, reminiscent of a wolf’s howl that originates in the interaction between the movements of the string and body of the instrument.

Empirical solutions neutralizing the phenomenon do not reveal its true nature. The wolf note is located in the antinodal vibrational region of the top plate’s B1- mode frequency.

FFT real-time analysis software enables you to determine the origin
of a frequency split and to locate a weak point on the top plate.
A region of strong vibration in red at the base of the left f-hole indicates insufficient thickness. This weakness usually triggers the wolf note.

A fingerboard torsional frequency approaching the B1+ mode frequency can trigger the phenomenon. A bass bar that is too weak behind the bridge will produce the same effect (valid for violins, violas, cellos, and double bass).

The blue zones indicate less amplitude with thicker wood.

For a given frequency, the greater the Q quality factor, the more intrusive the wolf note. It is difficult to produce the same phenomenon on low-quality instruments.

Two frequency peaks are clearly visible (split) with a delta of 4 Hz the origin of the wolf tone’s beat.

When a string’s frequency is close to
the pulsation of an isolated thin region on the top plate (high Q factor), it is impossible to obtain a single frequency to make the wolf note disappear.

On the violin, the wolf note is nearly always in the frequency range of the B1+ mode: B⁴ to C#⁵
on the A-string (494 Hz to 554 Hz). It depends on the moisture content in the wood of the moment.

The B1+ mode frequency contributes to the wolf note’s action but is not the cause of the split on this frequency.

For the wolf note to be heard, there must first be a defect in top plate or bass bar thicknesses .

A single instrument can present both of these problems.

This is a complex phenomenon, for several causes may exist that set the string rolling under
the bow: top plate or bass bar thicknesses, a too-lightweight tailpiece, a tailpiece fastener that is too long (plastic) or too rigid (steel), incorrect sound post setting, torsional frequency
of the fingerboard, chinrest, pegs, string tension, bridge thickness, bridge position, bridge height, moisture content in the materials (non-exhaustive list).

At least two conditions must be met to cause a split on the B1+ mode frequency.

The bow catches the string, releases it, then grabs it again and lets it go, approximately 7 to 10 times per second. These vibratory interferences provoke phase oppositions at the level of sound emission. This phenomenon can be reduced, but never completely eliminated, due to its origin in
a construction defect.

To remedy this problem, mass must be added, either at the end of a string (behind the bridge),
or directly on the sounding board (intrados or extrados) on the red area of photo.
In some cases, wolf tone eliminators employed to reduce the problem affect not only the wolf note, but also transitional notes and playability on all notes. Certain cellos lose harmonic richness
and power. The sound is dull.

The wolf note can be activated by an instrument’s variable body frequency, in direct relation
to the moisture content of the materials (instrument in the white or varnished).

Moisture content in the materials modifies all of the body frequencies, as well as the shape
of the nodal regions on the back and top plates coupled onto the ribs. Hence, the wolf note may temporarily disappear after the materials lose or absorb moisture, or it may appear on a different note or frequency.

The delta between modes B1- and B1+ will be modified if moisture content is not identical
in the top and back plates, temporarily eliminating the wolf note, or making it appear on a different note or frequency.

A wolf note eliminator installed on a string requires precise adjustment. Its weight and position must be verified in order to avoid a loss of tone in the instrument. It imposes a direction
of oscillation on the string by changing the coupling between the string and the body. It modifies the body’s own frequency by increasing the mass in vibration.

A wolf note eliminator glued on the intrados of the top plate or on the bar yields good results
by decreasing damping in a limited anti-nodal region of the top plate (red area - see photo).

Magnetic wolf suppressors have an advantage over classical ones: they can be moved about
on the top plate, following changes in the wolf note’s frequency.

A wolf suppressor is a remedy, but still is not the solution to the problem.

The wolf note is most often heard on:

¨ violin: B to C# on the A-string;

¨ viola and cello: E to F# on the C- or G- or D-string;

¨ double bass: G# on the D-string.