I mentioned yesterday the power of music is typically below 1kHz if we're talking about an orchestra. Of course a trumpet, a flute, a voice would stand out in loudness and be at a higher frequency than what I mentioned. Several of you asked me about this. There was a great picture I had of the spectral distribution of sound an orchestra produces but, for the life of me, I can no longer locate it. I did find one that's 'close' and gives you an idea of that which I am referring to. In this picture you at least get the idea. Frequency is going from left to right, the left side being the lowest. Amplitude, loudness, is going from top to bottom, the highest being on top. Note how the loudest area of the orchestra is centered around 200Hz. Yesterday I left us hanging with an unanswered question. "there are no thick steel cones and there are many paper cones, and everything in between. Why is that?" The answer turns out to be rather simple. Mass. A thick steel cone has lots of it and that's hard to get moving; even harder to stop moving once in motion. Think of a heavy truck vs. a light bicycle. The first one is tough to get moving. You need to put in lots of energy to do so. The bicycle, not so much. And, as we know, once the heavy truck gets going it takes just as much energy to stop it from moving. Our high-mass woofer cone would have the same issues of starting and stopping. To make our perfect piston move the air, so we hear bass, we want the cone to contribute nothing of its own; which is why a thick steel cone would be nice, only it's too heavy for the tiny motor on our woofer to start and stop it accurately. We turn to, instead, the opposite of a thick steel cone: paper. Paper is light, has low mass, is cheap, maleable, yet if made properly can be surprisingly strong. In the early days most woofer cones were made of paper. Sure there were some wooden ones as well, but for our discussion we'll stick with paper. Our goal using a paper woofer cone will be the same as the thick steel one. We don't want it to flex or change its shape so we get a perfect piston; only, paper isn't too good at that. This means we need to treat the paper. We'll start with a fairly heavy piece of paper for our woofer. Then, we're going to shape it into the form of a cone and, at the same time, let's add some structural elements like folds to strengthen it. Lastly, we might even paint some tar or heavy material to strengthen it even further. Take a look at this example of an older paper woofer cone. See how they added the multiple rings or ridges to it? They are actually folds in the paper used to minimize flexing. How does that work? Take a sheet of paper in your hands and flex it. It bends quite easily. Now, fold the paper in three places, as you would to make a 'W' or the start of a paper airplane. Holding the paper length ways, try and bend it again. It's now quite difficult. That's the idea behind the folds you see in this woofer picture. Lastly, note two more things in this picture: first is the rounded half tube on the very outside edge of the cone. This is called the 'surround' and it is typically made from rubber or foam. Its purpose is to center the cone and stop it from flopping around as it moves. Second, note the similarly ribbed area behind the cone in the picture. It's orange colored. This is called the 'spider' and it has the same function as the surround, only it is holding onto the back of the cone. Tomorrow we'll look at why most woofer cones of today are actually NOT paper.
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