A guide to microphone construction and applications
The home recording boom of the past two decades has driven – and been driven by – the availability of quality hardware at astonishingly affordable prices. Studio microphones, for example, that previously cost thousands now have competitors selling for one-tenth of the value. Yet despite the major gap in price, the gap in audio performance is minor. Many low-cost mics emerge as valid performers taking place beside the classic studio players.
Despite this equipment revolution, the audio reproduction quality of microphones hasn’t improved since the 1940s. There are essentially two classes of microphones that dominate the recording world. Even with all the new variations, such as USB microphones that directly connect to computers or ribbon microphones with active electronics, most of the contemporary innovations come back to these classes. We will take an in-depth look into both condenser and dynamic mics, as well as their variations and applications, to gain a better insight into the wide world of microphones available in today’s market.
Anyone who understands how a speaker generates sound using a coil moving in a magnetic field also understands how a dynamic microphone works. Not only is the process the same, though reversed, it’s not uncommon for recording engineers to use speakers as microphones, particularly when recording low-frequency instruments, such as electric basses, played through amps, and bass drums in a rock context.
A dynamic microphone usually features a diaphragm attached to a magnetic coil. That coil suspends in a magnetic field. As sound hits the diaphragm, the coil moves in the field and the magnet generates a tiny electrical signal across its north and south poles. This signal is a copy – or analog – of the sound that hit the diaphragm.
The key characteristic of a dynamic mic is that it converts sound energy into mechanical energy – the movement of the diaphragm – and then into electrical energy. Because the mass of the diaphragm and coil must physically move, there’s some energy lost to overcoming the resistance of that mass at rest. This leads to the sound of dynamic mics. While much can be done with the way a mic captures audio or treats it acoustically, dynamic mics aren’t usually recognized for their faithful reproduction of sound sources. Despite this, there are many advantages otherwise.
Perhaps the most significant characteristic is the ruggedness of many dynamic mics. Most people picture a concert vocal mic as a cylinder with a ball into which the artist sings. This describes the Shure SM58, which is probably the most-used stage vocal mic in the world. It’s difficult to say because so many other companies emulate the look of the SM58. It is tough, reliable and predictable. Using the same capsule, the SM57 has no round head. While it’s often used as a vocal mic as well, you’ll find it more frequently miking guitar amps. Both these mics base off the Unidyne III capsule, which dates to the late 1950s.
Dynamic mics aren’t limited to this cylindrical shape. In fact, you’ll find a wide variety of designs, usually due to some acoustic principle used with that microphone.
Classic Dynamic Microphones
- Shure: SM57, SM58, SM7B, Super 55
- Electro Voice: RE20
- Sennheiser: MD421 II, MD441U
Think of a classic studio microphone and you’ll likely picture a larger mic with the artist singing into the side. Just like the SM58, the shape that probably comes to mind comes from a single classic microphone that remains a studio classic, the Neumann U-87.
The condenser microphone capsule works on the principle of capacitance, rather than electromagnetism. Described simply, the microphone’s diaphragm sits near a charged plate. When sound hits the diaphragm, the pressure changes induce an electrical signal in the plate. It’s a one-step conversion process from acoustic to electrical energy, since mechanical energy is negligible.
This results in a fast, faithful conversion, since there’s no physical mass to move, as with the coil in a dynamic mic. The trade-off was, in the early days, a more delicate microphone that was often large and required external power to charge the plate. Today, phantom power is common on mixers and audio interfaces expressly for the use of condenser mics that use external power.
There are condenser mics that vary from the large diaphragm, phantom-powered studio versions. Electret condensers use a plate with a permanent charge. However, many of these use phantom power for an internal amplifier. The output of a capacitance-based mic is quite small, so some amplification usually happens on board the mic to output at a reasonable level.
Perhaps the most faithful type of mic is the small capsule condenser. These are end-address cylinders about 5 inches long and less than an inch in diameter. Featuring smooth frequency response, these mics – called pencil condensers due to their shape – also have excellent off-axis response. This is the property where sound changes as a source moves from the front of the mic to the side. The level and frequency response of the mic may change together or independently. Some of the best pencil condensers exhibit little off-axis change.
While condenser mics are generally studio mics, many are now built for live performance use. As with dynamic mics, there’s no single package for a condenser. Formerly very expensive, all types of condensers can be found at affordable prices.
Classic Condenser Microphones
- Neumann: U87, KM184, TLM103
- AKG: C414 XLII, C1000 S Mk4
- Telefunken: U47
New Microphone Trends
The big change in microphones recently is the addition of USB capability. Demand for quality mics that connect directly to computers and laptops drove this innovation. Essentially, an audio interface gets built into the mic, so either dynamic or condenser mics can accept USB conversion.
Ribbon microphones, a dynamic mic variation, are regaining popularity and arriving on the market in a variety of new packages. Tablet computing is also expanding into the music recording field, and both dedicated microphones and adapter interfaces provide ways to augment built-in mics.
As the home recording scene grows, expect the world of microphones to evolve, affordably.