Secrets Q & A
- Written by Jim Milton
- Published on 07 August 2014
Speaker cones seem to be made out of a lot of different materials. Hasn’t some particular material emerged as the best to be used or is it just the manufacturers want to have something unique to their own speakers?
- Tim, L
One of those debates which had been raging for decades is about metal vs. paper or various exotic diaphragms. Each has its merits and every manufacturer has their preferences. Let's discuss the basic function of the loudspeaker and how the diaphragm (and we are only discussing cones here) is supposed to function and what is actually happening in the “real world”.
The ideal loudspeaker diaphragm would never flex and would exhibit perfect pistonic motion. It would be infinitely rigid, well damped (stops on a dime), and the speed of sound through the material would be unlimited. The mass would ideally be zero and the break up mode frequency would be infinity. Unfortunately, the “ideal” does not exist in the real world, so speaker designers select materials that allow them to get as close to that ideal as possible.
The best thing about plastic (polypropylene) cones is that they perform pretty well and their manufacturing uniformity is very high and inexpensive to produce. These cones have a major advantage in that they are highly damped and lossy. This means they can have a controlled break-up, which results in a smoother high frequency roll-off. Some people argue that plastic cones tend to not sound as snappy or lively as stiff paper cone counterparts. This can be attributed to an almost capacitive storage effect inherent in soft cone type drivers. However, if well engineered, a good, well damped plastic diaphragms can excel at midrange frequencies and at a fraction of the cost of producing a quality paper cone driver.
Paper formulations have all the good points spoken about above, but the paper cone can actually absorb moisture from the air, changing its mass and damping characteristics. Cost, manufacturing complexity-uniformity issues, performance considerations, etc. make diaphragm materials selection a very difficult aspect of loudspeaker design. Paper cones, made in part from felt and wool have a combination of stiffness, low mass and loss inherent in the material that often make the best sounding cones. Sometimes rubber can be added to the paper to increase its durability. Tweeters have to vibrate at very high speeds (20kHz range) and are often made from metals such as aluminum, titanium or silk. They must be very stiff while remaining light weight. Mids and woofers are often made with paper (with additives), Kevlar, ceramics or exotic items such as wood. All of these different materials are selected by the designer to achieve that elusive goal: the “ideal” speaker. Frankly, these variables allow the comsumer a variety of choices and keep us all from buying the same style boring box speakers. Viva la difference!