Click here to purchase the entire book in PDF format. So what? So, what do these three dinner plates have to do with acoustic resistance, acoustic reactance and mechanical reactance? Well, let's think about a loudspeaker driver - the woofer, for example. It looks a bit like a dinner plate, and if your speakers are big, it's about the same size. The woofer is pushed in and out by the amplifier (we'll talk later about the details of this miraculous transformation of energy). When it moves, it moves the air molecules next to it just like the water molecules in your swimming pool. It isn't easy for the speaker to move air molecules (just like it's not easy for you to move the water in the pool with your plate) so the speaker sees some acoustical resistance to its movement. At the same time, the air outside the woofer is a little ``springy''. This is a bit of an odd concept, but think of a woofer in a sealed cabinet, sitting in a sealed room. If the woofer pushes out of the cabinet, then it is reducing the air pressure inside the cabinet and increasing the air pressure inside the room. That means that the room's air pressure is trying to push the woofer back into the cabinet (and the cabinet's air pressure is trying to pull it back in at the same time). This is the same as your plate on a spring and is known to the geeks as acoustical reactance. Finally, the woofer, just like your heavy plate, has some mass and therefore some inertia. So, once your amplifier starts moving it outwards, the woofer wants to keep moving outwards, and something has to stop it. This is known as mechanical reactance.