... as Neil Sedaka wrote, "breaking up is hard to do". He must not have had woofers in mind because that's apparently something that happens easily, unless enough care is taken in their design.
The cone material of a woofer is critical. We do not want that material to do unexpected things. We want it stiff enough to push the air without changing its shape. We want it to be light enough it's easy for the woofer's motor to move it back and forth; it is, after all, an air pump. And most of all, we don't want it to have break up modes occurring within the frequency range we're interested in.
I mentioned yesterday woofers started out life made from paper. Over time and as material science matured, becoming available to people that didn't work at NASA or the military, woofer cone materials began a dramatic shift for the better. Stiff plastics were introduced, graphite, carbon, amalgams of all types entered the scene, each with their plus's and minus's. Then, along came metal.
Remembering my first comment about constructing a woofer cone from a thick steel material? That was, of course, an exaggerated remark intended to paint a picture of strength and stiffness. And while we certainly don't want anything that heavy or immovable, the idea of actually using metal was always on speaker designer's minds. Aluminum would be an obvious choice. Aluminum is strong, yet light and low mass. But a metal cone is susceptible (as is any cone material) to what designers refer to as the 'oil can resonance'. That point where the material buckles as it is being forced to move back and forth to pressurize the air.
Remember the old oil cans of yore? Before the spray cans of WD40? The bottom of those oil cans had a thin metal plate. You pointed the spout where you wished to inject oil and then applied enough pressure to buckle the plate, forcing a squirt of oil out the spout. When this happens in a woofer cone, it will make unwanted sounds. So the trick for a designer is to keep this oil can resonance above the highest frequency required for the woofer to operate at; thus, no 'oil can' happens. All stiff materials are susceptible to buckling, causing break up modes. The challenge is to design the material to match the requirements of the task at hand; making bass.
Tomorrow let's look at a fundamental problem with woofers. One that's shared with every driver of a car.
