This month, we are going to talk about plywood. Plywood is everywhere in professional audio. It is used in trailers and ramps, speaker boxes and cases, stands and stages. Even though plywood is ubiquitous, we rarely pay it much mind. At least if you ignore the constant complaints about how heavy it is. Plywood is a versatile material, and an excellent introduction to a class of substances known as composites. The behavior of plywood is a good example to delve into in terms of thinking about some of the properties of materials in general. This month, our aim is to uncover some of that in a way that shows why plywood is preferred to solid wood for many structural applications.
What is a Composite?
Broadly, composites are created materials that combine different base materials to produce a final product with properties different than any of the starting materials. Alloys, even though they are made up of different metal compositions, are generally not considered composite materials under the classic definition. There are, however, metal matrix composites (MMC) and ceramic matrix composites (CMC). For instance, MMCs combine a metal and ceramic together into one material with some of the best properties of both starting materials.
In the popular press, references to composite materials commonly refer to one specific material called “carbon fiber reinforced polymer” (CFRP) or just “carbon fiber.” These references are usually in the context of airplanes, advanced cars, and other high performance mechanical structures. But this is by no means the only common composite in modern society. Materials like concrete (cement and aggregate), fiberglass (glass and polymer), pavement (asphalt and aggregate, and
papier-mâché (paper and glue) are common composites. Paper itself is a composite of cellulose and clay. Composites of biological origin include mother of pearl, bone and wood.
Wood is a mixture of cellulose fibers and binder polymers. Cellulose is a polymer, essentially an organic material made up of repeating structures called monomers. The monomers for cellulose are glucose, which is a type of sugar. Trees are built of many sugar molecules bonded together into chains! Cellulose is a very versatile material. Cotton is also composed of cellulose, but the nature of the chains that form give rise to a completely different material than wood. The remainder of wood is bonding polymers like lignin. They act as the natural glue to hold the wood fibers together.
Engineered Materials
If wood is already a composite, then what does that make plywood? Plywood is considered an engineered material, which is to say that it combines thin slices of natural wood (veneer) with man-made adhesives to form a new material. In the case of plywood, the veneer is formed by removing wood from a tree in a rotary fashion, not unlike using a vegetable peeler. The veneers are then laid up in a series of layers to form the familiar end grain look of plywood (see Fig. 1).
There are a number of different grades of plywood. Some are used for flooring, building construction, structural members or decoration. Plywood in birch, mahogany, balsa or basswood was used extensively to build airplanes during World War II. Plywood can be formed into intricate forms using pressure and heat, such as the curved bass horn flare in the classic Altec A7-500 shown in Fig. 2. Marine plywood, which has increased resistance to water and humidity, as well as high performance glues, is used to build boats, docks and stringers inside fiberglass boats.
Baltic Birch
The type of high-quality plywood most commonly used in professional audio is Baltic birch. Baltic birch is made from birch trees and comes largely from Finland. Birch plywood from Finland has a smooth surface, good durability, and fairly tight tolerances for thickness. While Baltic birch is found in the 4-by-8-foot sheet size common in the U.S., perhaps the most common sheet size for these materials is in 6-by-6-foot sheets.
The majority of Finnish birch plywood is bonded with phenol-based glue, which has superior water resistance to the urea-based glue found in less expensive plywood. Finnish plywood can be found in pure birch, or as mixtures of birch and conifer. Plywood is typically graded by the quality of the outer two surface veneers. Each face is given a letter rating to characterize the quality. For instance, one rating system for Finnish birch plywood where the two outer face veneers are of the best quality gives the grade of B/B, with the second best grade being B/S. The next best grade is S/S.
Why Split Wood Into Layers?
Why go to the trouble of peeling a tree like a large carrot and then gluing it back together? The answer to this question turns to be rather deep, and starts with the concept of “anisotropic” materials. Anisotropic is a fancy way of saying that mechanical properties of the material are different depending on which direction a load is applied. This may sound esoteric, but the behavior is quite common. For instance, if you pull on a rope it will resist, but if you push on the same rope, it offers almost no resistance. This means that a rope is anisotropic in tension (pull) versus compression (push). And so it is with wood.
Wood has much different mechanical behavior when measured along the grain versus at some angle to the grain. This is because the strands of cellulose are aligned in the direction of the grain. In some ways, each little strand of cellulose is like the rope in our example above, and behaves differently in tension versus compression. In many circumstances, it would be advantageous to have wood behave in a more uniform manner, and the structure of plywood helps achieve this goal.
Plywood has the thin layers of veneer orientated at different angles. In the most basic configuration, each layer is perpendicular to the layer before. So every other layer has the grain running north to south or east to west. Some types of plywood take this further and orientate each layer’s grain over a range of angles. The material properties of each veneer layer, like strength and thermal expansion, depend heavily on whether they are measured along the grain or at an angle to the grain. Wood, and the wood veneers that form plywood, are anisotropic for many of their properties.
The configuration of the plywood layers at different angles to each other helps to make the overall material’s properties more uniform. While this may be somewhat intuitive, the various mechanisms behind the scenes are involved. In a sense, the natural tendencies of each veneer layer are pitted against their neighbors for the overall good of the material.
Plywood Properties
One way in which plywood has improved behavior is for splitting. Wood tends to separate between the fibers that are orientated parallel to the grain. Each layer of the plywood structure is at a different orientation with respect to its neighbors. Therefore, splitting does not tend to travel all the way through the material, instead stopping at the glue between adjacent layers. And because the layer below is at a different orientation, its natural tendency is to help keep any splitting above it from opening further.
A similar behavior helps plywood exhibit less warping than wood. As one layer tries to expand north and south, the adjacent layer is attempting to expand east and west. The glue between the layers serves to transmit the movement between the layers. This sets up stresses between the layers that serve to reduce the amount of movement that each layer can achieve as it tugs against its neighbor.
The mechanical properties of plywood also depend on the layer structure. Even in plywood with layers at many different angles, the material exhibits anisotropic strength and stiffness behavior. Generally, the properties for plywood are measured in two different ways. One is in the plane of the material, measured on the edges of a sheet. The other way these properties are measured is perpendicular to the material surface, pressing on the outer plywood veneers from above or below. Behavior to any arbitrary angle to the plywood can be approximated.
Strength Versus Stiffness
Before we consider the behavior of plywood further, let’s consider the difference between strength and stiffness. These two materials properties are often confused, and it is important to distinguish them. Stiffness is the resistance of a material to flexing when a force is applied to it. Strength is the ability of a material to bear a force without incurring permanent damage, or breaking. Some materials are strong but not very stiff, such as nylon or Kevlar. Other materials are both strong and stiff, like maraging steel. For many common applications, the material’s strength is sufficient, and the engineer is more concerned with the material’s stiffness. Plywood is a material of compromises. It is not the strongest or the stiffest, but it provides a good mixture of mechanical properties, cost, flexibility in manufacturing and durability. Plywood is generally strong enough for most pro audio applications, and the materials thickness is typically a consideration of stiffness.
Stiffness and strength both derive from underlying materials properties, which result from the atomic, crystal and grain (or fiber) structure of the starting material. Stiffness and strength also depend on the physical structure of the final material. For instance, an I-beam is optimized for stiffness and it derives that performance by placing as much of the material as possible in the top and bottom webs of the beam. In a similar way, the outermost layers of the plywood are the ones most responsible for the materials’ stiffness.
When a load is placed on top of an I-beam, or a piece of plywood, the surface near where the load is applied are said to be in compression, with the surface opposite the load in tension. Compression means that the material’s constituents are being moved close together, and tension means they are being pulled farther apart. The reason for this behavior is that the volume of materials is approximately constant. If the material is stretched outwards in one location, it must be pulled closer together in another. There is a material property, called Poisson’s Ratio, that describes this behavior. As the upper surface is compressed, and the lower surface is stretched, the inner plies try to slide against each other. This sliding behavior is known as shear. The shear tries to separate the layers of veneer in the plywood. And, of course, the shear behavior of each layer depends on that layer’s orientation. While this quickly gets very complicated inside the material, the aggregate behavior of compression, tension, and shear defines the overall stiffness of the plywood sheet. It is the interaction of the veneer layers and glue that provide plywood with properties superior to the original wood.
Conclusion
Stepping back one level further, the configuration of the plywood as it is assembled into a speaker or case determines where outside forces will be applied and how powerful those forces are. The size, shape and choices of joints and reinforcement determine how the bumps and bruises from the gig, or the pulsing of a loudspeaker, are transferred into the plywood. The plywood’s inherent properties then dictate its response.
The internal complexity of plywood, like other composite materials, was not readily simulated on computers until fairly recently. Today, most structural simulation packages come with at least some capability to simulate loads on anisotropic materials. And even in the absence of this capability, isotropic approximations are still useful for identifying areas that need extra stiffness or strength.
For too long, the industry has relied on “sound by the pound.” Today we have the ability to design lighter, less wasteful products while retaining high levels of performance. Computer tools are one aspect of that, but another factor is the willingness of those in pro audio to trust the engineering teams to use thinner and lighter materials in their enclosures, cabinets and cases.
Phil Graham is FRONT of HOUSE’s technical editor and resident scientist. His formal education includes graduate research in high temperature mate
