"Specific stiffness" is defined as a material's stiffness divided by its density. A material with a high specific stiffness is, in simple terms, both stiff (possesses a high modulus) and light in weight. If the modulus of elasticity is expressed in GPa (Giga Pascals, or 109 Pa) and the density in kN/m3, the units for specific stiffness will be meters x 106. If the modulus of elasticity is expressed in Mpsi (Mega pounds per square inch, or 106 psi) and the density in lb/in, the units for specific stiffness will be inches x 106.
"Specific strength" is defined as a material's strength divided by its density. A material with a high specific strength is, in simple terms, both strong (possesses a high tensile strength) and light in weight (low in density). If the strength is expressed in MPa (Mega Pascals, or 106 Pa) and the density in kN/m, the units for specific stiffness will be meters x 103. If the strength is expressed in kpsi (kilo pounds per square inch, or 103 psi) and the density in lb/in, the units for specific stiffness will be inches x 103.
As an audio system designer and builder it's essential to know the aforementioned materials' properties so you can exploit their properties in your design. You'll most likely need to consider many more physical (viscoelastic behavior, thermal stability, machinability, bondability, and formability, for example) and chemical properties (e.g., resistance to solvents and ultraviolet light, conductivity, magnetic properties, and compatibility with finishing materials) to be certain these materials will perform as intended in your design. You'll likely design parts such as mounting structures for audio components, loudspeaker enclosures, and cosmetic panels. For structures on which audio components are mounted, designing for strength will most likely to be your primary goal, due to the weight of the components and the shock to which they'll be subjected in the automobile environment. For loudspeaker enclosures and cosmetic panels, designing for stiffness will most likely be your primary goal, because most materials are adequately strong for these applications. Loudspeaker enclosures must be designed for maximum rigidity, otherwise they'll vibrate excessively and color the sound of the music they're producing.
Table 1 (page 66) summarizes the aforementioned materials' properties for materials commonly used in the mobile audio industry. The properties listed represent conservative estimates. For example, the properties listed for fiberglass composites were obtained from composites with the lowest reported fiber contents of 25 and 37.7 weight percentage for those comprised of chopped strand mat and woven roving, respectively. Given the practical limitations of hand lay-up techniques, the use of the conservative values was deemed most appropriate. In a likewise manner, the properties listed for Baltic Birch plywood were obtained from Species Group 5, which had the lowest reported modulus and bending strength.
Table 1 also shows the moduli of elasticity of materials commonly used in mobile audio applications span almost two orders of magnitude, with MDF at the low end of the spectrum and low-carbon, cold-rolled steel at the high end. In addition, the data in Table 1 clearly illustrates that steel is three times stiffer than aluminum, and that a fiberglass composite comprised of woven roving is essentially twice as stiff as a fiberglass composite comprised of chopped strand mat. This demonstrates the significant advantage of using woven roving wherever possible to create the stiffest possible structures. Baltic Birch plywood is 63 percent stiffer than a fiberglass composite comprised of chopped strand mat, and 216 percent stiffer than MDF.