Click here to purchase the entire book in PDF format. Moving Coil Loudspeakers REWRITE ALL OF THIS CHAPTER Think back to the chapter on electromagnetism and remember the right hand rule. If you put current though a wire, you'll create a magnetic field surrounding it; likewise if you move a wire in a magnetic field, you'll induce a current. A moving coil loudspeaker uses a coil of wire suspended in a stationary magnetic field (compliments of a permanent magnet). If you send current thought the coil, it induces a magnetic field around the coil (think of a transformer). Since the coil is suspended, it is free to move, which is does according to the strengths and directions of the two magnetic fields (the permanent one and the induced one). The bigger the current, the bigger the field, therefore the greater the movement. Figure 6.128: Figure 6.129: Figure 6.130: A cross section of a simplified model of a moving coil loudspeaker. Figure 6.131: A diaphragm is glued to the front (this side) of the coil (called the voice coil. It has two basic purposes: 1) to push the air and 2) to suspend the coil in the magnetic field In order for the system to work well, you need reasonably strong magnetic fields. The easiest way to do this is to use a really strong permanent magnet. You could also improve the packing density of the voice coil. This essentially means putting more metal in the same space by changing the cross-section of the wire. The close-ups shown below illustrate how this can be done. The third (and least elegant) method is to add more wire to the coil. We'll talk about why this is a bad idea later. Figure 6.132: Voice coil using wire with a round cross section. This is cheap and easy to make, but less efficient. Figure 6.133: Voice coil using wire with a flat cross section. This has greater packing density, producing a stronger magnetic field and is therefore more efficient. Loudspeaker have to put a great deal of acoustic energy into a room, so they have to push a great deal of air. This can be done in one of two ways: - use a big diaphragm and move lots of molecules by a little bit (big diaphragm, small excursion) - use a little diaphragm and move a few molecules by a lot (little diaphragm, big excursion) In the first case, you have to move a big mass (the diaphragm and the air next to it) by a little, in the second case, you move a little mass by a lot - either way you need to get a lot of energy into the room. If the loudspeaker is inefficient, you'll be throwing away large amounts of energy produced by your power amplifier. This is why you worry about things like packing density of the voice coil. If you try to solve the problem simply by making the voice coil bigger (by adding more wire), you also make it heavier, and therefore harder to move. There are advantages and disadvantages to using moving coil loudspeakers: