For centuries wood has been the essential, irreplaceable material in the construction of guitars, violins, harps, pianos, marimbas, xylophones and many other musical instruments. In the marimba's case, particularly Mesoamerican instruments with their entirely wooden parts, the contribution to the resulting sound comes not only from the bars, but also from the resonators and mallets. Each of these components uses particular woods, according to their function. For the designer and builder of marimbas, it is important to understand the properties that make wood an irreplaceable material —at least until now— to make his/her instruments.
First, wood is an orthotropic material, that is, its mechanical properties depend on the direction in which they are measured with respect to the grain, in mutually perpendicular orientations. This is because the cells that make up wood have an elongated shape, roughly parallel to each other. The longitudinal direction is given along the fiber or grain of the wood, oriented along the trunk of the tree. The second direction is radial, from the center of the tree towards the bark, perpendicular to the growth rings. The third direction is tangential to the growth rings. In the case of marimba bars, the elastic properties in the longitudinal or grain direction are the most relevant in the selection of the most appropriate wood, together with lateral toughness or resistance to impacts. The choice of suitable woods for various instruments has been made over many centuries of experimentation, using wood locally available.
The acoustic properties of wood are a function of two basic material properties: the density or mass of a given volume of material, and the modulus of elasticity in the grain direction. The latter is also known as Young's modulus, in honor of English physicist Thomas Young. The modulus of elasticity indicates how easy it is to compress the wood with a given force, and depends on the orientation of the applied force. Naturally, another property required for the marimba bars is the lateral hardness of the wood, or impact resistance.
The modulus of elasticity and the density of wood are not independent: denser woods typically also have a higher modulus of elasticity. A third property, the coefficient of internal energy loss, is a measure of how much vibration energy is dissipated or converted into heat during vibration. A wood that dissipates vibrations a lot is not very loud, that is, its sound decays rapidly. The dissipation of the vibration energy in the wood increases with the water content. Hence, before using, the wood must be dried slowly, until the water content is reduced to a small fraction, but without splitting. On the contrary, the organic chemical compounds contained in wood (for example, lignin) tend to decrease energy dissipation. These two properties of wood —density and modulus of elasticity—, together with the coefficient of internal energy loss, give the wood its acoustic properties.
The acoustic radiation coefficient is derived from the above mechanical properties of the wood, and indicates how much energy dissipation occurs due to radiation of sound into the environment. For example, a wood with high acoustic radiation produces a strong but short initial sound, as it transmits energy to the air too efficiently. Thus, such material will be ideal for guitar tops or piano soundboards, but a poor choice for marimba bars.
The wood for resonators —as in a guitar tops, violin bodies, piano soundboards or marimba resonators— should be light and flexible (in acoustic language, should have low impedance). Both properties depend on the modulus of elasticity and density. Woods that meet this requirement are maple, fir, cypress, pine and cedar. In Mesoamerica, marimba resonators are made out of the last three wood types, unlike world "industrial" marimbas, manufactured with metal resonators.
The wood for the marimba bars must have an intermediate radiation coefficient, so that vibrations do not decay too quickly. It must also have a low internal energy dissipation coefficient, especially if a "brighter" sound is desired that includes multiple overtones. But it must also have a high lateral resistance to withstand the impact of the mallets. This last requirement further limits the type of woods for marimba bars to those with very high density.
Trees that produce high-density wood grow mainly in the tropics. The Mesoamerican region has been particularly blessed, as it grows three of the world's preferred woods for the marimba keyboard: the rosul or Honduran rosewood (Dalbergia stevensonii), "hormigo" (Platymiscium dimorphandrun) and granadillo (Dalbergia granadillo). The fourth species used for marimba bars is padouk (Pterocarpus soyauxii), native to the western tropics of Africa. Honduran rosewood is also becoming popular for the sides, back and fingerboard of guitars, after its cousin, Brazilian rosewood, went commercially extinct.
Hormigo (Platymiscium dimorphandrum) has been the choice for musical instruments in Guatemala for centuries, beginning with pre-Columbian percussion instruments, such as the tun-teponaztli or slit drum.
Maya musicians playing drum, rattles and an unidentified mallet instrument on the floor, Maya Classic Period. Rollout photograph of Maya vase by J. Kerr, catalog number 3007.
Currently, virtually all the marimbas manufactured by the large global companies use Honduran rosewood for the bars, a wood species found only in the border region of southern Belize and Guatemala (departments of Petén and Izabal.) In addition to guitar parts, this wood is also coveted to make luxury furniture in Asia. Hence its limited supply, high price, and endangered status. It is a species that needs protection.
For the mallet sticks, the wood should be resilient to repeated bending, keep its shape with use, and not be too flexible, allowing good control for the player. Woods with these properties are birch, maple, oak, guayacan and huitzizil. The latter (said to have been used to make arrow shafts in pre-Columbian times) has been traditionally used in the Mesoamerican area. Birch is the preferred type for mallet sticks elsewhere.
All-wood concert marimba designed and built by Belizean-Guatemalan marimbaist Fernando Morales Matus, Universidad Veracruzana, Mexico. The bars, tuned in triple harmonics, are made of Honduran rosewood and the resonators of cedar (courtesy of the Morales Matus family).
Efraín Figueroa Lemus is the author of "The Mesoamerican Marimba: An Illustrated Story," Editorial Piedra Santa, Guatemala, 2016 (in Spanish.)