Speaker Cones — Fabrication, Materials, and Performance
It is taken as a basic truth that most speaker factories can duplicate an existing design’s frame and magnetic structure, but getting the soft parts right is an art. The cone, surround, spider, and dust cap contain many of the secret ingredients in speaker design. Selecting the right soft parts is tough, with the most critical aspects being the materials, process, and fabrication. Speaker engineers know that three important physical properties determine a material’s suitability for use in loudspeaker diaphragms — stiffness, low density, and internal damping. Stiffness, in particular, determines the bending wave velocity, and for any given design, the frequencies at which the first break-up resonance occur. This resonance in a diaphragm determines the transition point above the frequency point at which the diaphragm loses piston motion and response becomes rougher. The high Young modulus (and the steepness of the cone’s body angle and the diameter) determine at what point things become nasty. The degree of internal damping, or loss factor, determines the material’s effectiveness in suppressing such resonances, which is especially important near and above the upper resonance. Endumax absorbs extremely high amounts of energy. While this property was designed for ballistic protection products, the properties also make the material especially suited for speaker diaphragms as there will be less energy reaching the surround edge to reflect back.
The best materials will have high velocity and high damping, but for those who have used titanium cones and domes, you know that in many cases these properties are mutually exclusive. Endumax, which is similar to aramids (e.g., Teijin Technora and DuPont Kevlar), offers a great combination of both damping and velocity. Materials such as Endumax have low density so using multiple layers will result in a stiffer and stronger cone with low weight. Loudspeaker cones are most commonly formed from paper pulp, but plastics (e.g., polypropylene) are also popular, and sheet material can be vacuum thermoformed (like melting hot cheese over a form), compression molded, positive/negative molded (or injection-molded) into the desired shape. Metal cones are also used. Other high-tech solutions that have achieved success include a sandwich construction with skins and honeycomb or foam cores.
In the last years, a number of notable composites for speaker cones and diaphragms have been introduced to the speaker industry. These include woven, cross-ply layers and non-wovens, glass fiber, carbon fiber, and aramids (e.g., Kevlar), thermoset, and thermoplastic resins. Typically, a specialist cone manufacturer fabricates cones for speaker assemblers, but the largest offshore speaker companies often have their own in-house cone fabrication facilities. Over the last 25 years, I have worked with many of the suppliers that provide materials to the cone industry and have visited most of the cone factories in Asia and the US.
At first glance, everyone is using the same techniques, but if you dig deeper, the subtle differences can be significant. In this article, we take a look at the basics of making different types of cones — but what I discuss here does not begin to touch on the secrets that the cone companies have learned from decades of experience.