Woofers. Bass drivers. Big cones. All used to produce about 70% of our music’s content in terms of quantity.
If you look at an amplitude spectrum of music you’ll find the vast majority is well below 1,000Hz and most of the energy our power amplifiers deliver to our loudspeakers is used by the woofer to reproduce these lower frequencies. And as you go down in frequency you need bigger sizes and more power to push them.
In our continuing series of subwoofers, we’ll start to take a look at the actual woofer driver itself. While my field of expertise is in designing analog electronics and I am no expert on the design of woofer drivers, I do know enough to get us a bit of understanding. So let’s move forward.
We understand that all speaker drivers are pistons; perhaps better put as ‘air pumps’. They push and pull back and forth, pressurizing and depressurizing the air in the room to make sound. The quality of that piston has a lot to do with how well the air is pressurized in the room.
The amplifier sends out power to move the piston a specific distance, then brings it back again. The expectation is that the piston (speaker driver) will move the air in an exact duplicate of the amp’s output. No deviations allowed. Any deviation from what the amplifier intended is called distortion. We hope for a perfect piston but, alas, nothing is perfect. Imagine, for a moment, a poor piston. A really poor piston. One that perhaps cannot move as far as it needs to, or not as quickly as the amplifier demands, or changes its shape from that of a perfect piston to something else. In this case we wind up with a lot of wrong air pressure in the room and get a noticeably distorted output.
Most modern woofers do not have loads of distortion. Even the cheapest crap out there is pretty good. But we’re not interested in pretty good. At least I am not. We’re searching for varying degrees of really good and, as such, we need to look at several parameters of woofers. How they are constructed, what materials they are made from, and how they are driven.
If we want to make the ‘perfect piston’ or ‘air pump’ we want to make sure, at a minimum, our piston follows exactly the instructions from the amplifier and does nothing on its own. The first requirement, following exactly the amplifier’s instructions, is perhaps the most difficult task for a woofer of all. Let’s put that aside and focus on the second; what materials they are made from.
A perfect woofer might be made from a thick steel cone. Why? Because it is so stiff and unbending that the air it is attempting to pressurize cannot push back hard enough to change its shape. If we make our cone instead out of something more flexible, such as paper, our chances of the air pushing back and distorting that paper go up dramatically.
Yet, there are no thick steel cones and there are many paper cones, and everything in between. Why is that?
Tomorrow we’ll take a look.