Click here to purchase the entire book in PDF format. Ribbon Dynamic Microphones The simplest design of dynamic transducer we can make is where the diaphragm is the piece of metal that's moving in the magnetic field. Take a strip of aluminium a couple of $\mu$m (micrometers) thick, 2 to 4 mm wide and a couple of centimeters long and bend it so that it's corrugated (see Figure 6.78) to make it a little stiff across the width. This will be the diaphragm of the microphone. It's nice and light, so it moves very easily when the sound wave hits it. Now we'll support the microphone from the top and bottom and hang it in between the north and south poles of a strong magnet as shown in Figure 6.78. Referring to the construction in Figure 6.78: if a sound wave with a positive pressure hits the front of the diaphragm, it moves backwards and generates a current that goes up the length of the aluminium (you can double check this using the right hand rule described in Chapter 2.6). Therefore, if we connect wires that run from the top and bottom of the diaphragm out to a preamplifier, we'll get a signal. Figure 6.78: The construction of a simple ribbon microphone. The diaphragm is the corrugated (folded) foil placed between the two poles of the magnet. There are a couple of small problems with this design. Firstly, the current that's generated by one little strip of aluminium that's getting pushed back and forth by a sound wave will be very small. So small that a typical microphone preamplifier won't have enough gain to bring the signal up to a useable level. Secondly, consider that the impedance of a strip of aluminium a couple of centimeters long will be very small, which is fine, except that the input of the microphone preamp is expecting to ``see'' an impedance which is at least around 200$\Omega$ or so. Luckily, we can fix both of these problems in one step by adding a transformer to the microphone. The output wires from the diaphragm are connected to the primary coil of a transformer that steps up the voltage to the secondary coil. The result of this is that the output of the microphone is increased proportionally to the turns ratio of the transformer, and the apparent impedance of the diaphragm is increased proportionally to the square of the turns ratio. (See Section 2.7 of the electronics section if this doesn't make sense.) So, by adding a small transformer inside the body of the microphone, we kill both birds with one stone. In fact, there is a third dead bird lying around here as well - we can also use the transformer to balance the output signal by including a centre tap on the secondary coil and making it the ground connection for the mic's output. (See Chapter 6.5 for a discussion on balancing if you're not sure about this.) That's pretty much it for the basic design of a ribbon condenser microphone - different manufacturers will use different designs for their magnet and ribbon assembly. There is an advantage and a couple of disadvantages in this design that we should discuss at this point. Firstly, the advantage: since the diaphragm in a ribbon microphone is a very small piece of aluminium, it is very light, and therefore very easy to move quickly. As a result, ribbon microphones have a good high-frequency response characteristic (and therefore a good transient response). On the contrary, there are a number of disadvantages to using ribbon microphones. Firstly, you have to remember that the diaphragm is a very thin and relatively fragile strip of aluminium. you cannot throw a ribbon microphnone around in a road case and expect it to work the next day - they're just too easily broken. Since the output of the diaphragm is proportional to the its velocity, and since that velocity is proportional to frequency, the ribbon has a very poor low-frequency response. There's also the issue of noise: since the ribbon itself doesn't have a large output, it must be boosted in level a great deal, therefore increasing the noise floor as well. The cost of ribbon microphones is moderately high (although not insane) because of the rather delicate construction. Finally, as we'll see a little later, ribbon microphones are particularly suceptible to low-frequency noises caused by handling and breath noise.