Last month's article dealt with the selection of the principal audio system components such as amplifiers, interconnects, loudspeaker cables, and power cables. In this article, Part 5, the fabrication process will begin, starting first with the overhead electronics console. Since there are plenty of how-to articles available regarding the fabrication of fiberglass-reinforced composites, the emphasis of this article will be on materials' properties, mechanical design, and how they correlate to improved audio system design and implementation.
List Of Materials
Fiberglass
Plywood
Aluminum
Steel
Materials Science 101
Before delving into the fabrication process, it's necessary to discuss the fundamental properties of materials common to the car audio industry. Knowing the basic mechanical properties of these materials will aid in their selection and optimize their implementation. "Stiffness" and "strength" are two of the most fundamental and important properties of materials. Stiffness is a term used to describe a material's resistance to deformation when subjected to a load. The simplest characterization of a material's stiffness is known as the "modulus of elasticity" or "tensile modulus of elasticity," or "Young's modulus." The modulus of elasticity is usually derived from the slope of a stress-strain curve. Since strain is defined as the change in length divided by the original length, the units for strain are dimensionless. By definition, a stiffer material has a higher modulus of elasticity. The International System of Units (SI) for modulus of elasticity is the Pascal, abbreviated Pa, and is defined as N/m (Newton per square meter). The traditional non-SI units used in the U.S. are psi (pounds per square inch).
Strength describes a material's ability to withstand loads. Stress is defined as the load per unit area. The term strength usually encompasses such properties as "ultimate tensile stress," or "tensile strength," or "shear strength." Sometimes, however, it's experimentally advantageous to create tensile stresses in beams by subjecting them to bending. The problem in doing so, however, is that this method of testing doesn't impose pure tensile stresses on the test specimen, but rather imposes a complex mixture of tensile, compressive, and shear stresses. The stiffness and strength of MDF and plywood are determined and reported from bending mode tests. Data reported in this manner, especially with regard to strength, can be somewhat misleading. For example, the estimated tensile strength, sometimes called the "modulus of rupture," of MDF derived from a standard bending test is estimated to be 23 MPa (3.48 kpsi). However, another kind of test called "Internal Bond Strength EN 310" subjects an MDF test specimen to an almost purely tensile load and reveals that MDF can only withstand about 0.75 MPa (109 psi). The impact of this material property is widely known and understood by professional installers. First, this property limits MDF's screw-holding capability. Using T-nuts sandwiches the MDF, thus subjecting it to reasonable compressive stresses, a state of stress that MDF can better handle. Second, professional installers know MDF works well for loudspeaker enclosure construction, since the panels of the enclosure are subjected to primarily bending stresses.
Density is defined as the weight of material per unit volume. The SI unit for density is N/m (Newton per cubic meter). The traditional non-SI units used in the U.S. are lb/in (pounds per cubic inch). It's commonplace in the U.S. to express weight in kilograms, which is technically incorrect. The kilogram is the SI unit for mass, not weight. A balance or force transducer that determines weight in kilograms is actually determining the mass in kilograms of force. Kilograms of force multiplied by the acceleration of gravity, 9.80665 m/s, will yield the weight in N.