Today's topic is speakers. I'll explain how speakers work and highlight some of the characteristics of different speaker sizes as well as some of the physics behind a speaker's behavior.
 
We al know what a speaker does. But how does it do it? basically, a speaker moves air. Over the years, inventive minds have come up with a number of ways to make working speakers. Some of these include moving coil, electrostatic and piezo-electric.  But we guitarists use speakers of the most common design, the moving coil.
 
A moving-coil speaker uses the power of magnetism to make sound. All speakers of this type have some sort of diagphragm or cone to interact with the air and a motor assembly consisting of a permanent magnet and an electromagnet. The electromagnet consists of a coil of wire that is attached to the diaphragm or cone. This is called the voice coil. When an electrical signal is sent to the voice coil, it creates a magnetic field which interacts with the magnetic field of the permanent magnet. This causes the voice coil to move, which pushes and pulls the diaphragm which moves the air creating sound.
 
On one level, speakers are pretty simple devices. The cone moves and sound is created. But, as with so many things, it gets more complicated. Cones and voice coils have mass and obey Newton's laws. Futhermore, the parts of the speaker that support the cone and the voice coil and which allow them to move have mechanical resistance. Voice coils can get hot and become distorted. The list goes on and on. For now, let's look at how speaker size affects sound.
 
But first, Here's something that needs to be said right up front: in the hi-fi world, we want speakers to have as few sonic characteristics as possible. If the speaker adds or subtracts anything from the signal, we view that as a bad thing. In the guitar world, this behavior is part of our arsenal of tools get the tones we're looking for.
 
Speaker size affects tone. We all know this from experience. What's interesting is the why of it. Some reasons are obvious. Larger speakers tend to have fewer highs. Because of their greater inertia, larges cones and voice coils just can't move quickly enough to produce highs as well as smaller speakers. On the opposite end of the sonic spectrum, larger cones move more air and, as such, produce more bass. There are some exceptions but, in general, this is true. But here's something that may seem counter-intuitive: larger speakers need less power to make a given volume level. Huh? Let's go back to the voice coil and the permanent magnet for an explanation. The voice coil has a range of movement within the magnetic field of the permanent magnet. Magnetic fields vary in intensity with distance. To move a given amount of air (for a given volume level) a larger speaker needs to move less than a smaller speaker. As the voice coil moves away from the focus of the fixed magnetic field, the force acting on it decreases. The further the voice coil moves, the more electric power it need to maintain the same force. The larger speaker has an advantage because it doesn't have to move as far.
 
Another difference that affects tone is the size of the cone. A speaker's cone is supported at two places. There's a surround around the outer diameter and there's an accordian-pleated support called a spider at the point where the voice coil is glued to the center of the cone. As we put more energy into the cone by increasing volume, the stiffness of the cone starts to play a part. If the cone isn't stiff enough, it begins to distort in shape and vibrate along it's length. This is known as cone breakup. It definitely affects how a speaker sounds. These additional vibrations are form of distortion which affects tone. Cone stiffness is a function of cone material and cone size. If you have two speakers which are identical in every aspect of cone construction (material, thickness, etc.) yet one is 10" and the other 12", the 12" cone will break up at lower volume levels than that of the 10" cone. Because of the added length, the voice coil has some more mechanical advantage.
 
That's it for now. There's a lot more to cover such as power-handling and the effect of cabinets and impedance on how speakers work and sound. We'll get ot all of these subjects in the entries to come. As always, comments, suggestions and even corrections are welcome.