Click here to purchase the entire book in PDF format. Moving Coil Dynamic Microphones In the chapter on induction, we talked about ways to increase the efficiency of the transfer of mechanical energy into electrical energy. The easiest way to do this is to take your wire that's moving in the magnetic field and turn it into a coil. The result of this is that the individual turns in the coil reinforce each other producing more current. This same principal can be applied to a dynamic microphone. If we replace the single ribbon with a coil of copper wire sitting in the gap of a carefully constructed magnet, we'll generate a lot more current with the same amount of movement. Take a look at Figures 6.79 and 6.79. Figure 6.79: An exploded view of a coil of wire with a diameter carefully chosen to fit in the circular gap of a permanent magnet. Figure 6.80: A cross section of the same device when assembled. Note that the front of the coil of wire is attached to the inside of the diaphragm. Now, when the coil is moved in and out of the magnet, a current is generated that is proportional to the velocity of the movement. How do we create this movement? We glue the front of the coil on to a diaphragm made of plastic as is shown in the cross section in Figure 6.81. Pressure changes caused by sound waves hitting the front of the diaphragm push and pull it, moving the coil in and out of the gap. This causes the wire in coil to cut perpendicularly through the magnetic lines of force, thus generating a current that is substantially greater than that produced by the ribbon in a ribbon microphone. Figure 6.81: A moving coil dynamic microphone with the protection grid removed. The ``front'' of the microphone shows a second protective layer made of mesh and hard plastic. The diaphragm and assembly are below this. Figure 6.82: The underside of the diaphragm showing the copper coil glued to the back of the diaphragm. This coil fits inside the circular gap in the magnet. See Figure 3a for part labels. Figure 6.83: The same photograph as Figure 2a with the various parts labeled. This signal still need to be boosted, and the impedance of the coil isn't high enough for us to simply take the wire connected to the coil and connect it to the microphone's output. Therefore, we use a step-up transformer again, just as we did in the case of the ribbon mic, to increase the sigal strength, increase the output impedance to around 200$\Omega$, and to provide a balanced output. There are a number of advantages and disadvantages to using moving coil microphones. One of the biggest advantages is the rugged construction of these devices. For the most part, moving coil microphones border on being indestructible - in fact, it's almost diffuicult to break one without intentionally doing so. This is why you'll see them in road cases of touring setups - they can withstand a great deal of abuse. Secondly, since there are so many of these devices in production, and because they have a fairly simple design, the costs are quite affordable. On the side of the disadvantages, you have to consider that the coil in a moving coil microphone is relatively heavy and difficult to move quickly. As a result, it's difficult to get a good high frequency response from such a microphone. Similarly, since the output of the coil is dependent on its velocity, very low frequencies will result in little output as well.