Microphones

Chapter 2. From Analog Sound to Digital Audio

2.2. Microphones

What is a microphone?

An acoustic microphone is made of at least an electroacoustic transducer (also called capsule), containing the membrane or diaphragm mentioned earlier and a case to hold it which acts as a windshield. When the sound waves hit the microphone, it transmits its energy to the diaphragm and displaces it by tiny amounts: 4 µm (four millionth of a millimeter) at 1 kHz!

How does a microphone work?

The way a microphone then transforms this mechanical (pressure) energy into electrical energy is what determines the type of microphone; however, all microphone types function in the same manner: air molecules hit the diaphragm; those collisions make the diaphragm move. The moving diaphragm creates an electrical current by varying the electrical tension (or voltage) in a circuit (more on that later; note that to express differences in voltages, you can use dBs as well, but the reference level and how they are measured will be different). The current is then transmitted to some device in the chain.

This process is in fact a form of encoding mechanical information into electrical information. Please note that the created current varies with time; this is important because electrical components in the path of a varying or alternating current (AC) must be described not with resistance but with impedance as we will see in here.

Those membranes capturing pressure changes are mechanical devices; this means that they move, they age, they resonate, etc. That is one of the reasons why audio equipment in general (microphones, microphone preamplifiers and speakers) can be very expensive: the quality of the design, the quality of the materials and the processes used to manufacture them can make a difference in the final product. Whether that difference in quality is large enough to warrant the (usually very) large difference in price, you must find out for yourself, but I have heard very respectable people say that you can make great sounding music with cheap equipment. This is a technical book about audio, but I (and many others with me) am a firm believer in the notion that the musician, the performance and the instrument (in that order) count much more in the quality of the music than the equipment.

The transformation of pressure into electricity is not instantaneous because the diaphragm has a mass which must be moved for the corresponding current to be created; this means that for very high frequencies, if the mass of the diaphragm is too large, the diaphragm will not move fast enough: it will not be sensitive to those frequencies. How quickly the diaphragm moves determines the microphone sensitivity , usually measured in dBV (simply another relative scale, this time to measure sensitivity in Volts per Pascal, or V/Pa: the worlds of mechanical pressure (measured in Pa) and electricity (measured in V) nicely meet in this unit!).

Microphone Types

There are three main acoustic microphone types: condenser, dynamic and ribbon. The first two read the pressure while the last one reads the difference in pressure with respect to distance – that is why ribbon microphones are sometimes called pressure gradient microphones. All three microphone types need to be amplified so that the signal is strong enough to be processed later: the output level of the microphone is called microphone level, while the desired processing level is called line level. How much amplification the microphone amplifier proposes is important because if it is not large enough, the outgoing signal’s level will not be high enough.

Condenser Microphone

A capacitor is an electrical component used to store electrostatic energy; how is this useful? Well, for starters, capacitors used to be called condensers; the name stuck, even though it is technically obsolete, but they mean the same thing. Furthermore, if we want to produce electricity from sound pressure, we need a device that can vary its production depending on how much pressure is coming in. That is exactly where the capacitor comes in: the condenser microphone, also called capacitor microphone, is a microphone where one of the plates of the capacitor is the diaphragm; the air pressure variations move the diaphragm, making the capacitor larger or smaller in volume, hence modulating the tension in the circuit proportionally to the frequency of the air pressures changes, in turn representing the frequency of the originating sound.

What is phantom power?

One of the reasons why the condenser microphone needs a source of energy to properly function is the fact that the incoming voltage variations due to the varying sound pressure only module an existing voltage; this source of energy is called phantom power. The other reason is that most of the time, the condenser microphone needs an amplification of its signal close to the source of electricity to prevent loss of certain frequencies due to electrical effects (see Section 2.2). I say most of the time because another type of condenser called the electret condenser has special material inserted between the diaphragm and the capacitor back plate so that the outside energy source is only needed to operate the amplifier. Condensers are known to reproduce high frequencies very well because of their design: their diaphragms are very light and move quickly under air pressure.

How do we categorize condenser microphones?

Condenser microphones are sometimes categorized by the size of the diaphragm. Small diaphragm condensers (SDCs) tend to have their diaphragm to be less than 1 inch / 2.5 cm in diameter (see reference [1] and the RecordingHacks Microphone Database), large diaphragm condensers (LDCs) covering the rest of the range. The different advantages and disadvantages of using LDCs and SDCs along with the explanation of why they behave differently are covered in details here, but there is a trade-off between sensitivity and fidelity.

Fidelity, especially at high frequencies, comes with smaller lighter diaphragms, while sensitivity comes with larger diaphragms. That is because larger diaphragms have a higher capacitive reactance (it is proportional to the volume of the condenser) and thus a lower impedance, which means a lower sensitivity (see Section 2.2). SDCs are mainly used when high fidelity at high frequencies is desired and the loss of sensitivity is not too much of an issue; LDCs are mainly used in the opposite case of high sensitivity and lower requirements in fidelity.

Condenser microphones can also be categorized by how the original electric signal is amplified: via a tube (or a valve), or via a field effect transistor (FET). Much like for guitar amplifiers, the different components have an impact on the final sound because they modify the electrical path through which the voltage-encoded sound information transits. Without going into details, tube amplifiers are thought to add pleasing harmonics to the captured sound and distort the sound better when pushed to the limit of their capabilities; you will hear engineers talk about a “tube sound” for guitar amplifiers and microphones, for example. Whether you care or not is up to your ears (and your budget, as always).

Dynamic Microphone

The dynamic microphone, also called moving-coil microphone, uses induction (see Section 2.2) to create an electrical current when the diaphragm (under varying air pressure from the sound) moves a magnet with a coil of wire wound around it. The larger the move, the larger the induced electrical current. Because its transducing system (diaphragm + magnet + coil) is much heavier than that of a condenser microphone, the dynamic microphone cannot reproduce high frequencies like the condenser can. However, it is more rugged, sturdier, and does not need phantom power to operate since there is no component which needs power inside the microphone.

Ribbon Microphone

The ribbon microphone, also called velocity microphone, contains a thin corrugated (wrinkled, or bent into folds) metal foil, usually aluminum, placed in a magnetic field, which will move around under changing air pressure. The ribbon acts like a coil and uses the induction phenomenon to create the current, exactly like the dynamic microphone. Because the coil only has one turn, the output voltage is smaller than for the condenser and dynamic microphone, making amplification an absolute must. Also, the ribbon being very thin (in the micron range µm), ribbon microphones are more fragile than their two counterparts.

Recent designs and ribbon materials have made ribbon microphone sturdier, but it is still not recommended to put them in front of a kick or a snare drum! Older ribbon microphone designs did not incorporate amplifiers and did not require phantom power to operate; worse, phantom power could damage the ribbon with an overcharge. More recent models now incorporate an internal amplifier to be used with phantom power.

Sound Pickup Patterns

What is a pickup pattern?

Microphones, depending on their capsule design, can capture air pressure coming from certain areas better than from others. For example, ribbon microphones can pick up sound from the front and rear because the ribbon is exposed to sound waves on both of its sides. A condenser microphone is directional in nature since sound pushes on the diaphragm from only one direction. Of course, microphone manufacturers have come up with designs which allow for microphones to have other pickup patterns than their natural one, or even better, microphones which possess multiple switchable patterns! Sound pickup patterns can be engineered by multiple means, including multiple capsule designs and phase shifting (or delaying) the sound coming from certain angles.

Figure6a Microphones pickup pattern: cardioidFigure6b Microphones pickup pattern: omnidirectional Figure6c Microphones pickup pattern: figure 8

Figure 6 Microphone Pickup Patterns (1)

The line delimits the area in which the microphone picks up sound the best, usually at 1 kHz (see below)

Pickup patterns are usually represented in the form of a polar diagram, representing its sensitivity from all angles around the capsule, from maximum pickup (0 dB) on the outside and less pickup on the inside of the diagram.

What do the different patterns do?

The first is called cardioid (C) and picks up sound better from the front of the microphone (usually indicated by a small sign on the front of the microphone; here, the front of the microphone is the top of the picture). The second is called omnidirectional (O) and picks up sound equally well from all directions. The third is called figure 8 (8) and picks up sound well from the front and rear. One pattern not depicted here is the super cardioid pattern (SC): its shape is the same as the cardioid but the microphone picks up less sound from the sides (90° and 270° angles in Figure 6).

Pickup patterns depend on the frequency; for example, lower frequencies tend to be picked up from all angles well, no matter what the intended pickup pattern is, because they are less sensitive to obstacles (like a diaphragm) than higher frequencies; also, the method used to construct the pickup pattern might not treat all frequencies equally; those are the reasons why manufacturers might display more than one frequency on their microphone polar pattern plots. If they do not, the pickup pattern shown will be at 1 kHz.

Why do I need different patterns?

Pickup patterns are important in audio recording because choosing the right pattern can make mixing much easier later. For example, if you want to precisely record a snare drum without picking up too much of the cymbal, you will choose a directional microphone, pointing its area of best sensitivity towards the snare – and its blind side towards the cymbal to minimize the spill of sound from the cymbal into the microphone (this is called bleed). If you are recording a classical orchestra, you might want a microphone which picks up sound from all directions to capture the ensemble rather than some instruments.

I want to buy a microphone, help me!

Next is a table giving you a summary of the best microphones in the business, some old, some more recent. Most of these names are known in audio circles; they are usually known for use in certain situations. A “+” sign in the pattern column means that other patterns are available for the microphone, usually through a switch on the front or detachable microphone heads.

In this book, prices are mentioned to give you an idea of the required investment to acquire the item; you can find it used for cheaper – or not at all; I have noticed a general 10% per year price drop for most audio equipment, with some exceptions: some devices have gone up in price in recent years.

Manufacturer Model Type Price Pattern Famous for
AKG C414 LDC $1000 C, O, 8, + Instruments, drums
AKG C451           Condenser(2) $350 C, O, 8, + Drum overheads
AKG D112 Dynamic $120 C Kick drum
Audio-Technica AT4033(3) LDC $350 C Bass, kick drum
Cascade FAT HEAD Ribbon $175 8 Brass
Electro-Voice RE20 Dynamic $400 C Lush instruments
Neumann KM84 SDC $1100 C Strings
Neumann U47 FET LDC $4000 SC Kick, bass, male vocals
Neumann U67 LDC C, O, 8 Vocals, overheads
Rode NT-1A LDC $250 C Tight budgets
Royer R-121 Ribbon $1295 8 Loud instruments, rooms
Sennheiser MD421 II Dynamic $350 C Drum toms
Shure SM57 Dynamic $100 C Guitar cabinets, vocals
Shure SM7B Dynamic $350 C Vocals
Sony C-37A LDC C, O Everything
Studio Projects C1 LDC $250 C Tight budgets

Table 4 Characteristics of Some Known Microphones

You will sometimes see a microphone dubbed as another older microphone’s “successor”. What manufacturers mean is that they intend to produce microphones with the same revered characteristics as their ancestors; unfortunately, these newer models are not always faithful descendants, so make sure to read up on them before thinking you just made a great deal. I will also mention copies which look the same from the outside but are very different on the inside.

Microphone Placement

Finding a good sounding spot and placing a well-chosen microphone there will go a long way in giving you a good sound to work with later : “get it right at the source”, instead of trying to “fix it in the mix”.

Why do I care about microphone placement?

Microphone placement is the secret sauce that many recording engineers use to get their sound. There are no general rules, but the closer to the source the microphone is, the less natural it will sound and the more processing it will need – this is sometimes desirable (e.g. guitar cabinet). Also, when a microphone is placed close to the sound source, it risks favoring picking up low frequencies, an effect called proximity effect. Moving the microphone further out from the source gives a more natural (reverberant) sound, but if the recording space is not acoustically treated, those sound reverberations will influence the recorded sound in a negative manner (see here).

Sometimes, placement distance to the source is not an option because of bleed issues; in those cases, angling the microphone in different ways helps getting a better sound. The best way to find your sound is to start from what the pros do and move on from there. After a while, you will have your own set of microphone placements.

(1) From the Wikipedia Microphone entry, reproduced with permission under the GNU Free Documentation License; credit to Galak76

(2) Size of diaphragm unknown

(3) The AT4050 is the same microphone with two more pickup patterns: figure 8 and omnidirectional

 

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