What Makes a Good Tonewood for Musical Instruments?

For hundreds of years, musicians have marveled at how the properties of a piece of wood can make the beautiful sounds of Stradivarius violins and Steinway pianos. In fact, there is plenty of intriguing science behind the acoustics of tonewoods and how a tree grows to produce them.

Music is the art of organized sounds, and sound is made by vibrations. When a guitar string is plucked or a xylophone bar is hammered, the vibrations move through the wood into the air and eventually to your ears. For them to sound pleasant, this means a piece of wood needs to conduct those vibrations as clearly as possible. Have you ever wondered how your ear can tell an E-flat from a guitar from the same E-flat from a piano? The answer is something called overtones. In addition to the note you play, an instrument produces a series of other, quieter vibrations at higher frequencies, and the sound is "brighter" the more of these there are (Fiore, n.d.). But those vibrations carry less energy and are absorbed by thick, heavy wood, resulting in a muddy, muted sound.

As a result, the best woods for guitar soundboards are light, moderately stiff, and yet elastic enough to radiate vibrations to the air clearly. Many guitars are made from Sitka spruce to utilize these properties. Spruce and other softwoods also have a high sap content, which can significantly influence the sound of a guitar. A newly built guitar will still have many small pockets of liquid sap that will dampen the sound when it is first played. However, the more it is played, the more the sap will settle into a crystal structure that transmits the vibrations more smoothly. For the same reason, a guitar that has not been played in a long time will take time to "brighten up" as the sap readjusts to the motion of the sound waves.

For hardwoods, there are only a select few woods that have these properties to transmit sound and are strong enough to withstand the physical demands of an instrument, like tension on a guitar neck. Sugar maple is a popular choice – it is light and flexible enough to transmit a bright, clear note and yet sturdy enough to build piano frames and solid-body guitars. Basswood is also popular for guitar construction for its strength-to-weight ratio. Conversely, instruments such as ukuleles are built to be inherently quieter and are traditionally made with koa wood, which is denser and limits the dynamic range.

Woodwind instruments have many of the same requirements. They focus vibrations into a column of moving air, so a hard, resonant wood is best. Clarinets and oboes are often made from jet-black grenadilla wood, which is highly durable and gives them their unmistakable ringing sound (Uebel, n.d.). Grenadilla is exceptionally hard and slow-growing, so much so that it sinks in water and is carved with metalworking tools (Yamaha, n.d.) Other, softer woods like pear or cherry are also used to give a mellower sound (Frydrysiak, n.d.) Tropical woods such as cocobolo, rosewood, and ebony are highly sought after for their striking visual patterns in professional instruments. The drawback to these species is that they are usually quite expensive, and several of them have become endangered due to unsustainable logging.

Wood moisture is also a critical consideration for instrument makers. Different tree species have different fiber saturation points, or the moisture content at which the cell walls (fibers) are saturated with no free water inside the cells. This represents the maximum size that a piece of wood will swell to with additional moisture. Lumber will have different moisture contents depending on its surroundings and how it was dried, and this will greatly affect its sound. Water in wood dampens vibrations, and significant shrink-swell cycles can slowly warp and crack wood (Binyamini, 2021). High-end tonewoods are usually air-dried for a period of several years rather than the faster kiln-drying method to avoid creating microscopic gaps and cracks (Uebel, n.d.).

But not every piece of maple or spruce will make a good instrument – only a few trees will produce the most melodious of lumber without knots, warps, or voids. Foresters with well-trained ears can knock on the trunk of these trees and hear how clearly the wood resonates (Bourgeois, 1994). You can try this yourself with a basswood tree: tap on the trunk a few times and listen for a hollow thunk, then try it on another tree to see how much less it carries. Basswood is known for its long, strong fibers and is a popular choice for guitars; its name is even derived from the musical term "bass".

So what ultimately draws world-class music out of a piece of wood? The answer is a combination of resonance, fairly high hardness, moisture, and several other factors. Importantly, however, much of it also comes down to the skilled techniques of the craftsman and how they have carved, sanded, joined, polished, and tuned an instrument. The tiniest variations can make a great difference in the final sound, and many musicians say this is one of the greatest joys of trying a new instrument!

References:

Binyamini, Y. (2021, April 19). How to dry wood for guitar building? The Electric Luthier. Retrieved April 3, 2023.

Borgeois, D. (1994). "Tapping Tonewoods". Acoustic Guitar Magazine. Retrieved April 3, 2023.

Fiore, J. (n.d.). Pitch, frequency, overtones, timbre and all that stuff. Mohawk Valley Community College. Retrieved April 3, 2023.

Frydrysiak, F. (n.d.). About wood. Filip Frydrysiak - Oboe Maker. Retrieved April 3, 2023.

Moe-Bleichner Music Distribution. (n.d.). Tonewood. Uebel. Retrieved April 3, 2023.

Yamaha Corporation. (n.d.). How the Clarinet is Made: An Instrument Created From Human Craftsmanship and the Bounty of Nature. Yamaha. Retrieved April 3, 2023.