These pages are devoted to understanding microphones, covering their types, techniques and characteristics. We have tried to present the basic guidelines in a simpler, practical way, especially geared toward home recording. The first time I came across an explanation of the different types of microphone, they were classified by their acoustic-mechanical and mechanical-electrical transducers and by their pickup pattern (pointing out which ones worked by pressure gradient and which didn't), all mixed in with parameters such as impulse response, sound pressure level, frequency response... The truth is, I didn't understand a thing.
Once you absorb all those concepts, you come to the conclusion that this really is the most orderly way to explain the different types of mic, but for a beginner so much variety may be a bit overwhelming. This article is aimed at those users, so the theory will be approached in a simple way, focusing on what you can expect from a given type of mic rather than on why that type of mic has those characteristics.
Types of microphone
The two most common types of mic in a "home studio" are moving-coil dynamics and electrostatic condensers, better known simply as dynamic and condenser mics. Other types, such as ribbon dynamics or electret electrostatics, are less common in these studios and will be discussed briefly. Photo: Shure SM57
Moving-coil mics tend to be rugged and able to handle high sound pressure levels (that is, they can pick up instruments that "play very loud" without overloading), but they often have a not-very-linear frequency response (they have peaks or dips that emphasize or attenuate certain frequencies), and they usually don't reach the highest frequencies.
That doesn't mean they aren't good; they simply sound a certain way. In fact, our ears are so used to the sound of these mics (because they are so widely used) that using another type on a particular source can often make the instrument sound strange.
Common studio applications include (but are not limited to) miking percussive elements such as the kick, snare and toms of a drum kit, or guitar and bass amps.
Condenser mics have a more linear and extended frequency response and deliver a more detailed sound. However, they are more delicate and need power to work, either through the well-known phantom power or through batteries.
Some common applications include recording vocals (both lead and backing vocals and harmonies), acoustic instruments and drum overheads.
Again, it is important to stress that these are just some common applications. Experimentation should be almost mandatory. Trying types of mic where they are "not supposed" to be used often leads to pleasant surprises.
The differences in behavior between the two types of mic are largely determined by the transducer (the element that converts sound into audio). Photo: Beyerdynamic MC 840.
Without going into detail, moving-coil dynamics work by means of a coil that moves within a magnetic field. Because of its own weight, the coil takes a little time to respond to the waves striking it and, for the same reason, keeps oscillating after the waves have stopped. This behavior means the mic can't follow every variation of the wave exactly (hence their lack of rich detail) and that rapid pressure changes (high frequencies) are not picked up by the mic.
The moving element in condenser mics is much lighter and provides a more faithful impulse response—and therefore more detail and better high-frequency response—but it also makes them more delicate.
Ribbon dynamics replace the coil with a corrugated ribbon—much lighter—so they have a better high-frequency response than their moving-coil counterparts, but for that same reason they are more delicate. Photo: Cloud 44 ribbon.
Electret electrostatic mics are similar to condensers, but they don't require a power supply, since they use a material that provides the polarization the mic needs to work. While this is an advantage, it also has the drawback that this polarization is weaker, which affects the frequency response.
Directionality
A microphone's directionality indicates how its response varies depending on where the sound comes from. Omnidirectional mics pick up equally in all directions, while directional mics favor some directions over others.
Among the latter, the most common patterns are cardioid, hypercardioid and figure-eight (or bidirectional).
When recording a single instrument, the choice of one pattern or another determines how the direct sound blends with the sound of the room. Although cardioid is usually the most widely used, other patterns can yield interesting sounds, and you shouldn't be afraid to experiment.
When there is more than one instrument in the recording, the choice of pattern is decisive for achieving good separation, that is, for getting each mic to pick up only the instrument it is meant for.
The general idea in these cases is to choose a pattern that lets you aim the area of greatest pickup at the desired source while having the areas of least pickup facing the other, unwanted sources.
One example is recording a singer who also plays guitar. The first attempt would probably be to place one cardioid mic to record the voice and another to record the guitar, but this setup won't give good separation, since the vocal mic will pick up a lot of guitar and vice versa.
Switching the mics to a hypercardioid pattern will achieve better separation, since the pickup lobe is narrower, but don't forget that the rear lobes will pick up more room ambience.
Proximity effect
Any directional microphone (whether condenser or dynamic) exhibits what is known as the proximity effect. This effect is a boost in the low-frequency response as the source gets closer to the microphone. The distance at which this effect becomes noticeable depends on the microphone itself. In some cases it may be several centimeters, and in others, designed for close use, the effect was already taken into account in the design, and in fact you notice a lack of bass if the source isn't very close to the mic (as with the Shure SM58).
Again, this effect doesn't have to be negative. You just need to know it exists and take advantage of it. A typical example is placing a singer with a "thin" voice closer to the mic than a singer with a deeper voice, or varying that distance during a performance to get different vocal textures.
In any case, if the proximity effect becomes a problem, many mics include a switch that engages a high-pass filter to remove the excess bass.
Tying in with polar patterns, if a microphone doesn't have a high-pass filter, you can use an omnidirectional pattern to make the proximity effect disappear, but again, keep in mind that this pattern will pick up more room sound.
Getting to know your mics
It's a good idea to experiment a lot with mics. Whenever you have some free time, try each mic on different sources, at different distances, with different patterns. Your ear will gradually get used to the small differences, and when it comes time to mic an instrument you'll have a clearer idea of the effect a particular mic can have on the source.
Some people take this practice to the extreme and even record the sound produced by scratching the mic's protective grille (with a fingernail, for example), since they maintain that this way they can hear the sound of the grille and later distinguish how that sound subtly becomes part of the mic's overall sound.
Mic-based EQ
Perhaps without going to those extremes (or, why not, going there), the lesson is that it's very important to know the sound of each mic intimately. Once you know how each microphone interacts with reality, it's easier to choose the right mic.
And this brings us to the legendary phrase "we'll fix it in the mix." Although everyone has their own way of working, ideally you should be completely happy with the sound you're getting before you start recording. If you don't, you're creating a new task for later: fixing the sound of the recording. Isn't it better to get it right from the start?
Spending some time looking for a good sound before you start recording almost always pays off better than putting a mic anywhere and then trying to get a good sound in the mix.
So, along with choosing the right mic, there are other aspects that directly affect the recorded sound and should be taken into account:
First, it's a good idea to choose a good spot for the instrument in the recording room. If you have time, you can try different locations to decide where the instrument sounds best.
Once the instrument is in place, you need to find the spot to put the mic. Even if you have some ideas beforehand about where to do it, it's always worth walking around the sound source to hear how that particular instrument sounds in that room.
Since a microphone "hears" in mono (we'll look at some stereo techniques a little later), to get a clearer idea of how the mic will perceive the instrument, we can cover one ear with our hand and place the other ear at the different positions where we're thinking of putting the mic.
All of these factors—from the choice of mic and pattern to the placement of the instrument and of the mic itself—influence the result of the recording, and they should be used to make the sound you're going to record as suitable as possible.
Stereo techniques
Once you're clear on the large number of possibilities when working with a single mic, you can expand your resources and move on to using two mics to capture the source, thus achieving a stereo sound.
In general, stereo techniques capture the tone and spaciousness of instruments better, making them sound bigger. Recording an instrument in stereo can also help if you intend to place it in the background of the mix as a "pad."
When you use more than one mic to record the same source, you need to start worrying about the phase relationship between them. Since the sound will reach each microphone at a different moment, summing the two signals can cause cancellations.
This summing happens, for example, when you listen to the two mics in mono, and depending on the degree of cancellation the effect will be more or less obvious. In general, you should watch out for a change in the sound, which will start to seem hollow and thinner in the low end. The 3-to-1 rule helps avoid these problems. Essentially, this rule says that each time you add a mic, it should be placed away from the first at a distance equal to three times the distance between the first mic and the source.
Even if you follow this rule scrupulously, you should always check the phase between the different mics before recording. Below are some of the best-known stereo techniques:
X-Y technique (coincident)
In this technique, two cardioid mics are placed with their capsules together (hence the term coincident), angled at 90 degrees to each other. Since there is hardly any difference in the arrival time of the waves at each mic, this technique doesn't offer a great sense of spaciousness, but mono compatibility is very good. When two mics with a figure-eight pattern are used, the technique is called Blumlein.
MS technique (coincident)
The MS (Mid-Side) technique uses a cardioid mic to capture the center (Mid) and a figure-eight mic to capture the sides (Side), so that by varying the ratio between the two signals you widen or narrow the stereo image. Mono compatibility is also very good, since the signal from the figure-eight mic cancels out when the signals are summed to mono, leaving only the cardioid signal.
ORTF technique (near-coincident)
This technique is similar to X-Y, but differs in that the microphones are spaced about 17 cm (near-coincident) apart, simulating the distance between the ears. The angle also changes slightly, to about 110 degrees. It creates a better sense of space but can present minor phase issues.
AB spaced pair (non-coincident)
In this case the mics can be 20 or more centimeters apart (non-coincident), and the stereo image is more convincing. However, mono compatibility isn't always good, and you need to pay special attention to the phase of the microphones.
Some common instruments
Electric guitar
Probably the most widely used technique for recording an amp is to place an SM57 right up against the grille, pointing at the speaker cone (at 90° to it) and slightly off the cone's axis (that is, off center). Variations such as subtly changing the angle, or placing the mic on axis or at the edge of the cone, are also common.
If the amp has several speakers, it's a good idea to first find out which one sounds best: one of them will probably stand out. If they all sound the same, one way to decide is to choose the one farthest from the transformer that powers the amp, to avoid electrical noise. (If the transformer isn't visible from the outside, it's probably on the side where the power cord comes out.)
To have more sonic resources, it's common to place two mics on the speaker, one on axis and another slightly off axis or at the edge, and to play with the blend of the two signals. A notable pair for this job is an SM57 and a Sennheiser 441.
You can also mic two different speakers, or swap the second mic for a condenser placed somewhat farther away (a room mic), use both close mics plus the room mic... each combination will provide different textures.
Acoustic guitar
With acoustic guitars, techniques vary even more. Depending on the sound you want, you can use dynamics or condensers, and the placements can be very varied.
Placing a condenser mic between the sound hole and the end of the neck is a good starting point. You get a natural sound, and the tone can be varied by angling the mic slightly toward the sound hole (more bass) or toward the neck (less bass).
Another common spot is to place the mic to the left of the sound hole, level with the bridge and slightly above it. This gives good tone, but the sound of the pick can be accentuated.
A less common technique is to use a large-diaphragm condenser mic placed parallel to the floor and pointing up toward the bridge from below, so that the mic sits right under the guitar's body. This achieves a natural balance, without excess bass or pick noise.
You can also use two mics at different points and blend them until you get the sound you want, or use any of the stereo techniques discussed. The possibilities are wide, and once again experimentation will let you decide which setup is best for a particular job.
Electric bass
The electric bass seems to be one of the instruments where there is the most consensus about how to record it. Many producers/engineers like to record both the amp sound and the signal from a DI box. That way you can use, for example, the amp signal to get a powerful low end and the DI signal for the highs.
When placing the mic on the amp, the same points apply as for an electric guitar. You can use either condenser or dynamic mics depending on the sound you're after, although if you use the former it's advisable to use a special mic for low-frequency instruments such as the AKG D112 or the Shure PG52. These mics have a frequency response designed to capture this type of instrument.
Vocals
Although it may seem simple at first, the voice is one of the hardest instruments to record. This is probably because, on the one hand, our ears are very used to "how" a voice should sound and, on the other hand, this instrument has a huge variety of nuances and textures.
A large-diaphragm condenser mic (set to cardioid) is usually used. That's the usual choice, but it doesn't hurt to try a few mics (even some we wouldn't normally use in a million years) to see which one suits that voice best. It wouldn't be the first time that a big production passed over extremely expensive mics for recording vocals in favor of "much worse" mics.
A common problem when recording vocals is plosive consonants such as "p" or "t." Pronouncing these consonants sends a puff of air into the mic that can produce an annoying noise. To avoid this, a pop filter (named after the onomatopoeia describing the noise produced) is placed between the singer and the mic.
This filter is also very useful for showing the singer the distance at which to sing, and thus controlling the proximity effect.
Drums
This is undoubtedly one of the hardest instruments to record. A popular saying holds that a record can't sound better than its drums, because of how hard this instrument is to capture.
Although you can try techniques with two mics, three mics, four mics, etc., the usual approach is to use at least one dynamic mic for each component of the kit, plus a stereo pair of condensers as overheads.
That means one mic each for the snare, kick, tom one, tom two and floor tom, plus the two overheads. In addition, you might place a mic on the hi-hat, ambient mics, mics for the cymbals, or more than one mic on each drum, such as a dynamic inside the kick and a condenser outside, one on top of the snare and another underneath... the possibilities are endless.
Whatever technique you use, when recording an acoustic drum kit you need to be very careful with the phase of the mics. A common practice is to take the snare mic as a reference and align the phase of the others to it, since it's the instrument that bleeds the most into the other mics.
In the home studio
And all this brings us to a question many ISP readers ask themselves: which mics should I buy for my studio?
When the budget is limited, choosing which mics to buy can be very complicated, since you're looking for affordable mics that allow a wide range of applications. Photo: Shure Motiv
There are different combinations, but in general it's worth investing in at least one good large-diaphragm condenser mic, since this mic can be used on any instrument. Prices have dropped a lot these days, and you can find good mics of this type for less than 200 euros. This investment should be complemented by a dynamic mic or two. Since these mics are usually cheaper (the ubiquitous Shure SM57 costs less than 100 euros), having at least a couple of them (preferably of different brands or models) is very useful for a more varied palette of sounds.
With these three mics you can start recording the most common instruments (even drums), and from there your real needs will dictate how to expand your collection.
