Many different kinds of microphones are available, but fundamentally, they all work on the same principle: sound waves move a plate or membrane ( diaphragm ) inside the mic, and this movement is translated into electrical voltage. The exact details of how this is implemented impact the way recordings sound.
While trying to figure out whether a given mic is the right choice for a particular recording chore, we need to look at several qualities:
Different methods of microphone construction have certain shared properties. If you use the wrong mic for the job, you could fail to capture the sound you want, or even damage your mic. For instance, diaphragms respond uniquely when sound waves cause them to move, particularly with short, intense sounds (transients, as shown in Figure 6.2 ). Heavier diaphragms respond slowly to this abrupt change, thus distorting the sound of the transient, whereas lighter ones respond more quickly and are more likely to be damaged by sounds that are too powerful. You would thus match diaphragm sizeand other physical properties of micsto the qualities of the sound source you're recording. Put simply, use a fragile mic right against your guitar amp or kick drum, and you could break the mic! Other mic characteristics don't threaten the health of your mic, but can threaten the quality of your recordings, based on the position, frequency range, and amplitude range of your sound.
Figure 6.2. An example of a transient signal: a 2-second bass drum sample has a sharp transient at the beginning.
We'll observe frequency and amplitude response by different microphone types, but directionality can be easily described in terms of general patterns .
Microphones tend to be most sensitive to sounds that arrive directly perpendicular to the mic capsule, or on-axis ( Figure 6.3 ). You've probably noticed TV reporters pointing their microphones at an interview subject in order to exploit this fact. Different microphones vary in how sensitive they are to off-axis sounds. A mic's relative sensitivity to sounds from different angles is called its polar response or, more commonly, a mic pattern .
Figure 6.3. Directional microphones are most sensitive on-axis or near it, relative to the facing of their diaphragms. (Shure SM-58 image courtesy Shure, Inc.)
A directional mic picks up sounds best from the front, whereas an omnidirectional mic picks up sounds fairly evenly in a 360-degree radius. Different mic patterns are directional to varying degrees ( Table 6.1 ). Keep in mind that microphone response patterns differ at different frequencies. Even if you're using an omni mic, you may find sound coming from behind the microphone to be more hollow than sounds from the front.
Table 6.1. Basic Mic Patterns
Proximity effect and cardioids
Cardioid microphones exhibit another unique behavior: their bass response becomes more pronounced as a sound source nears a microphone. This phenomenon is known as the proximity effect . You can try it with a cardioid mic by monitoring yourself as you get closer to the mic capsule while speaking. In some cases, as in miking instruments, this can be undesirable, but for spoken or sung vocals it's not uncommon to use it to intentionally add bass to the sound.
Choosing a mic pattern
You'll choose microphone patterns depending on the recording application. Journalists might work with omni microphones because they won't lose a critical interview if the subject speaks off-axis. Or they might use a highly directional mic if they need to reduce ambient noise. General music applications typically involve cardioid microphones, which screen out unwanted ambient sound while remaining flexible about positioning to the front. Extremely dense ensemble recordings may require multiple, discretely placed supercardioid or hypercardioid mics to prevent bleed.
Some microphones have a specific, fixed pattern and either are labeled as such or can be identified using manufacturer specifications. Others have a switchable capsule so you can adapt the mic to a specific recording situation ( Figure 6.4 ).
Figure 6.4. By using a mic with switchable patterns, you can change patterns without swapping mics. Neumann's U 47 first popularized this feature. Shown here are the knobs on its current stereo USM-69 mic. (Photo courtesy Georg Neumann GmbH)
Mic Type and Construction
Microphone types or models are families of microphones that share a means of construction and accompanying acoustic characteristics. The two most common types are dynamic and condenser mics, although ribbon mics are popular for recording as well. Traditionally, dynamic mics have been more affordable, but today many high-quality condenser mics compete even in the sub-$200 range; ribbon mics almost always command a higher price.
Because different mic designs produce various trade-offs, there's no perfect mic: even beginners should consider assembling a small "mic cabinet" of models for maximum flexibility and creative options. Typically, this would include at least a matched pair for stereo recording and a combination of condenser and dynamic mics. Even two or three good mics will provide additional choices for recording.
Needless to say, this isn't just "buying advice." Once you have several microphones to choose from, you'll regularly select certain microphones for different applications, so knowledge of mic types is critical to creating the recording you want.
Dynamic mics use a moving magnetic coil to produce voltage ( Figure 6.5 ), so they're sometimes called moving-coil mics. They operate on the principle that any coil moving perpendicular to a magnetic field will create current, a process called electromagnetic induction . The fact that they require a physical coil to be moved is their primary advantage and disadvantage : dynamics are unusually resistant to air pressure and are the most rugged mics, but they're not as good at picking up high frequencies.
Figure 6.5. Inside the dynamic capsule. (Illustration courtesy Shure, Inc.)
If you want a mic that can take some physical abuse or record loud sources, make sure you use a dynamic mic. You'll find it's invaluable for many other recording applications, too, which is why you won't find a pro studio in the world without an SM57 on hand.
Instead of using air pressure to create voltage by induction, condensers are already charged and use pressure to measure a change in charge in their circuitry . Two charged metal plates, one acting as a diaphragm and the other fixed, react to sound waves ( Figure 6.6 ).
Figure 6.6. In a condenser mic, two charged plates form a capacitor. One of the plates, a light, thin metallic membrane called a diaphragm, moves in relation to the thicker, fixed metal backplate as air pressure from the sound pushes and pulls it. As the distance between the plates changes, current flows to or from the capacitor , forming the output signal. An internal amplifier (either transistors or tubes) makes this signal usable by increasing its strength. (Illustration courtesy Shure, Inc.)
Condenser mics have a relatively smooth frequency response and are ideal for capturing softer instruments and high-frequency sounds, perfect for applications like recording acoustic guitar. Because they're more sensitive, however, they're not as suited to close miking or high-amplitude situations like kick drums. They also require power, either in the form of an internal battery or from an external source (see sidebar "Phantom Power").
Condensers are categorized by diaphragm size; you can immediately recognize them by the size of the microphone head. Narrow, small-diaphragm mics are usually less expensive and can be used onstage as handhelds, or to record instruments. Broader, large-diaphragm condensers are bigger and often more expensive, but offer higher recording quality.
Variations on construction have allowed the production of very inexpensive mics, although they sacrifice some recording quality. If you have a portable cassette recorder, it probably uses an electret mic. Electret condensers' diaphragms are internally charged, making electrets extremely cheap to manufacture, but they have thicker diaphragms and thus sacrifice high-end response. Back-electret condensers use the same principle, but by using a thinner membrane material, they behave more like the expensive models. Condensers for under $100, for instance, use this approach.
Ribbon microphones are closely related to dynamic mics; sometimes they're even called "ribbon dynamics." Like dynamics, they generate signal by disrupting a magnetic field, but they use an extremely thin ribbon instead of a diaphragm and coil ( Figure 6.8 ).
Figure 6.8. Inside a ribbon mic. A single corrugated ribbon stands in for the diaphragm/coil combination in a dynamic mic. The ribbon is suspended between magnets. As it moves with air pressure from sound it disrupts the magnetic field and generates voltage, just as with a dynamic mic. (Photograph of R-121 transducer courtesy Royer Labs)
The ribbon microphone was extremely popular in years past, thanks to classic mics like the RCA 44 and 77 ( Figure 6.9 ) and Shure Model 55. If you've seen a black-and-white photo from the '30s, '40s, or '50s featuring a big, classy-looking mic (possibly with the NBC or CBS letters on the side), it was probably a ribbon mic. That doesn't mean ribbon mics are just museum pieces; their unique characteristics remain popular. Many of the historical designs have been revived in improved replica form: AEA, for instance, makes modern versions of the RCA mic, even aping the logo. New designs from AEA and mics in more compact form factors from manufacturers like Royer Labs are more rugged and compact than traditional ribbon mics.
Figure 6.9. Left: A ribbon classic: the original RCA Type 77-DX. (Photo courtesy Stanley O. Coutant; microphone from his private collection, www.coutant.org) Right: AEA's R84 mic has the looks and figure-8 polar response of an RCA mic, but has been updated with a more rugged design and improved sound. (Photo courtesy Audio Engineering Associates)