Does the Wood Matter? What a Guitar Is Made Of and What It Changes

The mechanical properties that matter for an instrument top are stiffness along the grain, density, and how much energy the material absorbs as it vibrates. A high ratio of stiffness to density means a plate can be made thin and light while remaining rigid enough to hold its shape under string tension, which lets it move readily in response to small forces, and low internal damping means vibrations persist rather than being absorbed as heat. Spruce scores well on all three, which is why it has been the standard top material for centuries across guitars, violins and pianos. Back and side woods are selected differently, for hardness and for the ability to reflect rather than absorb, and for appearance and workability, which are not trivial considerations in an object people look at.

Certain species became standard for reasons partly acoustic and partly historical:

• •Spruce for tops, being light, stiff along the grain and low in damping
• •Cedar for tops on classical instruments, softer and quicker to respond
• •Rosewood for backs and sides, dense and hard, and now restricted by trade controls
• •Mahogany for backs, sides and necks, stable and moderate in density
• •Maple for violin backs and for some guitars, hard and visually figured
• •Ebony for fingerboards, being extremely hard and wearing slowly

The disagreement concerns how much of the sound comes from the species and how much from everything else. Controlled work generally finds that the top matters most, since it is the primary radiating surface, and that within a species the variation between individual pieces is comparable to the variation between species, which undercuts confident claims about particular woods. Back and side material has a smaller measurable effect, with several blind tests finding that listeners and players cannot reliably distinguish instruments differing only in that, and at least one well-publicised study using instruments with backs of different materials found no consistent preference. What plainly does matter is the thickness and bracing of the top, which the maker controls directly and adjusts to the particular piece of wood, so a skilled maker's work with ordinary material outperforms poor work with celebrated material.

Selection is about stability over decades as much as about acoustics, and that constraint shapes the trade. Wood moves with humidity, swelling across the grain in damp conditions and shrinking in dry ones, and an instrument glued together from pieces that move differently will crack or distort, which is why quarter-sawn material is used, since it moves less across the width. Seasoning, meaning drying slowly to a stable moisture content over years, is essential and is why aged stock commands high prices. Grain must be straight and even, and a top is bookmatched from a single billet split to ensure the two halves behave identically. Knots, runout and reaction wood are rejected. Much of what a maker pays for is therefore not the species at all but the specific quality and preparation of the piece, which is consistent with what the acoustic tests find.

The woods traditionally favoured are increasingly difficult to obtain legally and sustainably. Several rosewood species are listed under international trade controls, with restrictions introduced in 2017 that initially caught instruments carried across borders by touring musicians and were subsequently relaxed for finished instruments. Old-growth spruce of the quality historically used is scarce because it comes from slow-grown trees in specific conditions. Ebony harvesting has been heavily depleting. The responses include certified sourcing, use of alternative species that perform comparably, laminated and composite materials that are stable and consistent, and reclaimed wood from old buildings and from logs recovered from riverbeds. Makers report that player resistance to alternatives is a larger obstacle than any acoustic shortfall, which is consistent with what the blind tests find.

Stiffness along the grain, low density and low internal damping are the properties that matter, and spruce scores well on all three, which is why it tops most instruments. Variation between individual pieces of one species rivals the variation between species. Blind tests find back and side material hard to distinguish, while top thickness and bracing, which the maker controls, plainly matter.