December 2011

audioXpress Magazine

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audioXpr ess By Mike Klasco (United States) Everything You've Wanted to Know About Speaker Cones A look at materials, fabrication, and quality T he cone is the most critical component of the speaker and has a significant contribution to its sound quality. When mounted in an enclosure, it is the only visible component to the end-user. If the speaker design engineer spends much of his precious bill of material budget on a high-performance flat wire coil, no one will know. But, spend a few extra bucks on a woven Kevlar cone and you can't miss the yellow high-tech look. Consider the speakers you have bought (or speaker projects you have put together)—more likely than not, the look of the speaker cone probably was an influence. Most speaker cones are fabricated from a recipe of wood (cellulose) fibers in a paper-making process, but polypropylene, woven glass, carbon fiber, Kevlar com- posites, and metal cones are also popular choices. Getting the soft parts right in a speaker driver is an art. The cone, sur- round, spider, and dust cap are the secret sauce in speaker design and most critical are their ingredients and fabrication. Loudspeaker cones are most commonly formed from paper pulp, but plastics such as polypropylene are also popular, and this material can be thermoformed (like melting hot cheese over a form) or injec- tion-molded into the desired shape. Metal cones also appear on the scene, and other high-tech solutions that have achieved success include honeycomb, foam lami- nates, and composites such as woven and non-woven resins, glass fiber, carbon fiber, and aramids (such as Kevlar). Typically a specialist cone manufacturer fabricates cones for speaker assemblers, al- though some of the larger offshore speak- er companies have their own in-house cone fabrication facilities. PAPER PULP CONE FABRICATION The process starts with sheets of vari- ous types of pulp such as bleached and unbleached Kraft pulp. Douglas fir from Canada or the southern United States is common. Exotic blends which include eucalyptus from Brazil or Australia (very stiff ), other specialty pulps from New Zealand, or the hemp (Fostex and Dai- Ichi like banana leaves which are part of the hemp family), kapok fibers (poor man's Kevlar), or various synthetic fibers are also popular. The pulp recipes, ad- ditives, pulp-slurry beating process, and cone-forming techniques all contribute to the loudspeaker cone's characteristics. Young's modulus (speed of sound), tan delta (internal damping), and mechanical parameters such as tear strength, burst strength, and so on are all factors that separate the toy cones from the audio- phile, studio monitors, electric guitar, or prosound diaphragms. Other considerations are wet strength and moisture regain. Will the cone fail if used in a humid environment or will that studio monitor sound the same on a humid day? What about vulnerability to sunlight (UV), fungus, ozone resistance, and so on? Appearance is still another consideration, and there are many sec- ondary surface treatments that add what cannot be fully achieved within the paper- beating process. Once the recipe is selected for a partic- ular production run, the appropriate paper pulp is soaked in hot water, for a period of time determined by the "chef." Pieces are torn off the wet pulp sheets (by hand or by machine) and thrown into a water- filled pulp beater (see Photo 1). The beating process disperses the fibers while also fibrillating (fuzzing them up) so they will mechanically tangle together, thereby holding the cone together. Aside from the me- chanical bond there is also the oxygen bond from the paper-slurry process. Most of the industry uses the old-style beating machines which offer potentially excellent fibrillation. Sometimes hydro- pulpers are used, which can be faster in producing the slurry but may not do as much fibrillating work on the fibers. Better use the hydropulper for making newspaper instead. Measurement techniques such as Ca- Photo 1: Beater shown making a batch of pulp Photo 2: Canadian Standard Freeness (CSF) tester for monitoring batch-to-batch consistency for pulp fibrillation December 2011 7 speak e rs — "par ts is par ts"

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