Page 44 - Fall 2005

Whatʼs new in string instruments?
Joseph and John Virzi, to provide a secondary vibrating sur- face, thereby producing a more complex set of overtones and a more mellow tone. While the original premise is debatable at best, the device does act as an addition mass that could split the lowest body resonance into two resonances.
Building Trends in Hammered Dulcimers: David Peterson, University of Central Arkansas
The hammered dulcimer, a stringed instrument played with two wooden hammers, probably originated in the Middle East, but has become part of the musical culture of many coun- tries. In the U.S., the folk revival in the 1970s sparked renewed interest in the hammered dulcimer as a concert instrument. Today, despite some consolidation, there are still hundreds of builders, mostly amateurs, who experiment with the basic design. The most important design parameters are: soundboard size, shape and composition, internal bracing, bridge shape, string arrangement and composition, hardness of bridge caps, hammer weight and stiffness, instrument resonances due to the unique string splitting and also the stiffness of the body, and soundboard modes.
Hammer/string/bridge interaction—Standard wooden dul- cimer hammers are much lighter (8-12 grams) and have hard- er heads than felt covered piano hammers. Light, hard ham- mers and the playing position facilitate the percussive sound (hammer clicks lasting 20 ms or so are quite pronounced), percussive playing techniques (e.g., double stroke rolls), and very fast hammer strokes that help define the unique sound of a hammered dulcimer. Hammer/string contact time is typ- ically 2-4 ms, depending on string tension (lower pitched strings are at lower tension on a hammered dulcimer). Using padded hammers, (typically leather) changes the sound radi- cally. Hammer contact time is increased and higher harmon- ics are significantly damped, leading to a more mellow sound. In general, padded hammers change the sound of an instrument more than any other design feature.
Dulcimer bridges extend about 1.25 inches above the soundboard. The resulting vertical force (10-30 lb) allows the bridge to be driven vertically and laterally, and also produces a rocking motion. The in-phase mode for paired strings does decay much faster than the out-of-phase mode, but the marked dual decay rate found in pianos seems to be absent or at least less important in hammered dulcimers.
The sustain of heavyweight pre-1970 dulcimers was compara- ble to that of an undamped piano, but is somewhat less on lighter more flexible modern instruments.
Playing range—The traditional American hammered dulcimer had 12 treble courses and 11 bass courses.
The musical weakness of such instruments is an inaccessible key of A and weak bass. Adding three more courses at the low end gives 3 octaves for a standard 15/14 instrument (D3 to D6).
The standard 15/14 instrument has 2x15 + 14 = 44 possible notes to play, but only 30 of these notes are distinct—the others being repetitions of the same pitch available elsewhere on the instrument. It is possible, but inconvenient, to play chromatic
scales. The more expensive professional models have additional notes to extend the chromatic range and/or bass range. Strings—Most hammered dulcimer builders use steel piano wire ranging from 0.016 in. in diameter (#6) to 0.024 in. in diameter (#10). Because of the trapezoidal shape of the instru- ment, string tension decreases as strings get longer and are tuned lower, and this becomes a problem.
String tension (treble bridge, right side):
String freq G5 784 D5 587 G4 392 D4 294 G#3 196
rad. 0.009 in. 0.010 0.010 0.011 0.012
L T 9 in. 35 12 43 15 30 18 29 21 21
lb
% of breaking strength 60%
60 34 33 20
Efforts to improve the sound of lower strings include the use of brass or phosphor bronze strings with approximately 10% higher densities than steel or using single wound strings. Side rails and bracing—Total longitudinal tension string ten- sion is about 1900 lb, with a total downward force on each bridge of about 150 lb. The resulting bending moment must be overcome with relatively deep side rails (3-4 in.) and internal bracing. Before 1970, the instruments were strong boxes with hardwood backs, heavy pin blocks, floating soundboards, and several braces glued to the back.
The current trend toward lighter instruments makes use of the soundboard as a structural element by gluing it to the frame. The pin blocks are minimal, the back is high strength 3/8 in. Baltic birch plywood, zither pins replace piano pins, and the internal bracing is tapered and honeycombed. Soundboards have gotten thicker but less dense through the use of softwoods. Some builders are experimenting with tapered soundboards that are composites with a softwood interior core. The acoustical goal of this design work is a more mellow tone with less sustain.
The miracle bridge cap—Traditional bridge caps were made of thin steel wire, brass, or wooden dowels. About 1985, builders started experimenting with the hard plastic rods made of del- rin, invented by Dupont. Delrin has high mechanical strength and stiffness, good wear and abrasion resistance, and a low coefficient of friction. Thus steel strings can easily slide over the bridge caps without making grooves. Perhaps more impor- tantly, delrin has much higher internal damping than brass and steel so that higher frequency string vibrations have reduced decay times. The positive effect on hammered dulcimers is to reduce sustain and hammer noise.
Bridge design—The most common design is a solid bridge. Saw cuts and individual caps are sometimes used in an attempt to de-couple adjacent courses. Inlaid scale markers make the instruments much easier to play accurately
Materials—The exterior frames are made of hardwoods—wal- nut, cherry, and exotic imports—chosen for appearance, sta- bility, and marketability rather than for acoustic reasons. Soundboards use Baltic birch plywood on less expensive mod-
42 Acoustics Today, October 2005