Microphones


by Flawn Williams

Flawn Williams has been a production engineer and trainer for NPR since the late 70's. His credits include Radio Expeditions for NPR/National Geographic, teaching with NPR's Next Generation Radio workshops, and stints as Technical Director of "All Things Considered" and "Performance Today". He also freelances on music and documentary projects, consults with independents on digital audio production, and teaches harmony singing.

(This article reprinted courtesy of NPR's Engineering Update Online.)

fwilliams@npr.org
radio@flawn.org

No matter what kind of recorder you use in your reporting or recording work, you'll need to get a good quality microphone to go with it. Although some recorders come with built-in microphones, or are sold with microphones included, rarely are those mikes good enough to make quality broadcast recordings. Choosing the right mike for your application can be a confusing process, but this section aims to give you enough knowledge to make an intelligent choice.

What's a microphone?

A microphone is a transducer. My dictionary says that means "a device that converts input energy of one form into output energy of another." What's all that about energy? In this case, a microphone senses tiny amounts of mechanical energy (sound waves moving through the air) and converts that to electrical energy (an audio signal travelling through a wire).

A loudspeaker would be another example of a transducer, since it receives electrical signals from an amplifier, and converts that electrical energy to sound energy.

Whenever you change energy from one form to another, there's some loss of quality: the transformation is always less than exact. What separates a good microphone from the lesser models is the quality of the transducing, the transformation it can accomplish.

Now, quality can mean many things to many people. In any given recording situation, different kinds of mikes will give you different results: all of them may be "good mikes," but one mike may give you something more useful for your particular needs. Let's get some vocabulary under our belts first, so we can talk about the various attributes of microphones. We usually describe a microphone first in terms of its transducer type and pickup pattern.

Types of transducers found in microphones

Designers of microphones have come up with several different ways of converting sound into electricity. Here's a quick description of some of the more common types:

  • A dynamic mike uses some piece of metal suspended in a magnetic field. In the most common type of dynamic, called a moving coil mike, a coil of wire is attached to a diaphragm, which is suspended elastically in the field of a magnet. Sound waves cause the diaphragm to vibrate back and forth in the magnetic field, and this induces a voltage in the coil of wire. That's the audio signal! If you hear a mike referred to just as a dynamic, chances are it's a moving coil type. Dynamics have been the most common mikes used by reporters, due to their more rugged construction.
  • A ribbon microphone is another kind of dynamic mike, but instead of a diaphragm and coiled wire this mike suspends a thin corrugated ribbon of metal in the magnetic field. Sound waves striking the ribbon cause it to vibrate, and this in turn induces a voltage in the wires attached to the ends of the ribbon. The ribbon has much less mass than its moving coil counterpart--think VERY thin aluminum foil!--so it can respond to delicate sounds more accurately than a moving coil dynamic. But the thin ribbon can be damaged by bumping the mike, or even by wind blowing past the ribbon. Newer ribbon mike designs have improved sturdiness, but these are still only rarely used for field recording.
  • A condenser mike doesn't use any magnets to convert sound to electricity. Instead, it uses an electrical principle called capacitance. Two thin metal plates are placed close to each other. One is fixed in place; this is called the back plate. The other plate, called the diaphragm, is elastically suspended parallel to the back plate. An electrical charge is applied to the diaphragm and back plate from an external power source (see phantom power later in this section). When sound waves strike the diaphragm, it moves relative to the back plate, and this movement varies the electrical capacitance of the circuit. This weak electrical signal is then boosted by a small amplifier inside the mike.
  • An electret condenser mike is similar to a condenser type, except that the diaphragm is made from a material that has a permanent electrical charge applied to it. This eliminates the need for external power to polarize the diaphragm. The little amplifier inside the mike still needs power to run it, but in many models this power can be supplied from a battery. If you see a mike that runs on an internal battery, chances are it's an electret condenser.

Those are the most common types of transducers used in field recording these days. Other transducer types you may have heard of include carbon and piezoelectric or crystal mikes.

  • Carbon mikes are found in most older telephone mouthpieces. A chunk of granulated carbon acts as a variable resistor, in response to being compressed and expanded by the vibrations of sound waves. These are typically pretty low fidelity devices, and in modern telephones they've been replaced by electret condenser mikes.
  • Piezoelectric or crystal mikes take advantage of a variance in voltage between two sides of a crystalline substance being pushed by vibrations. They are found in musical instrument contact mikes, sonar hydrophones, and CB radios, but only rarely in broadcast recording.

Pickup Patterns

Often when you use a mike, you may want it to pick up sounds coming from one direction more prominently than sounds from other directions: you want it to help discriminate some voices or sounds from others. It's like putting blinders on the mike.

Microphone designers have attempted to accomplish this feat with minimal audible side effects (forgive the pun). But even the best mikes are examples of the art of compromise in this matter. A common term for describing microphones' directional sensitivity is pickup pattern. Sometimes this is also referred to as polar graph, a reference to the graphic charting of a mike's varying sensitivity by direction.

  • The omnidirectional mike, or omni for short, is one that (in theory) picks up sound equally from all directions. In practice, because high frequency sound waves don't bend around corners in the mike as easily as low frequencies do, even mikes that are labelled as omnis will be somewhat more sensitive to high frequency sounds coming from in front of the mike than those coming from the sides and rear. Omni mikes are the most common mikes used for field interview recording.
  • Beyond the omnidirectional, most other mikes are referred to as unidirectional, or picking up sound from one direction only. This seemingly straightforward definition actually involves a lot of wishful thinking!

The polar graph you see for a directional mike paints a picture of a mike that is less sensitive to sounds arriving from the sides and rear than from the front. But in real life this directionality varies with frequency: at frequencies in the bass and lower midrange, almost all mikes are essentially omnidirectional. It's only when you get up into the treble range, into the frequencies that carry what we call the "crispness" or "clarity" of the sound, that these mikes are able to suppress sounds from some directions.

This is also one place where the quality/cost issue comes in. Better quality mikes have smoother response curves...which means less variation in sensitivity...for those off-axis sounds. So the sounds being picked up from the sides by a good quality mike will sound quieter but more natural. With a lesser-quality mike, off-axis sounds will sound muddy and indistinct.

Within the unidirectional category are a variety of pickup patterns ranging from mildly to extremely directional. These are termed cardioid, hypercardioid, supercardioid, and shotgun mikes.

  • Cardioid mikes achieve some suppression of sounds coming from the sides and rear of the mike by surrounding the diaphragm of the mike with some kind of shield with tuned openings in it. The term cardioid comes from the upside-down heart-shaped pattern that the polar graph of such mikes shows. The strongest sensitivity is at the front of the microphone, and it gradually gets weaker as the sound source moves toward the rear of the mike.
  • Hypercardioid mikes are somewhat more directional than cardioids, but there's also another difference. Rather than having their greatest suppression at the rear of the mike (180 degrees from the front), they suppress best those sounds arriving from about 120 degrees from the front of the mike, and have a bit of a lobe of increased sensitivity for sounds arriving at the rear of the mike.
  • Supercardioid mikes are even more directional than hypercardioids, but their greatest suppression is for those sounds arriving from about 150 degrees from the front of the mike, and they have less of a sensitivity lobe at the rear of the mike than hypercardioids do.
  • Mikes can be made even more directional by adding a slotted cylinder called an interference tube in front of the capsule. These long slender mikes are referred to as shotgun mikes. These mikes are extremely directional, but may have several small lobes of spotty sensitivity at the sides and rear as well. Some shotgun mikes may be as much as several feet long! The ones under 14 inches or so are referred to as short shotgun mikes. These short shotguns are increasingly popular with some reporters, but they definitely have their drawbacks, as detailed in the next section.

Less common in field recording, but still used in some situations, the bidirectional or figure 8 microphones use a pickup element which reproduces sound equally well from the front and back, but dramatically suppresses sounds arriving from the sides. These are most often made with ribbon dynamic transducers, which have a natural bidirectional characteristic. They can be very useful when you need to record two voices facing each other.

Note: bidirectional mikes are almost always "side-fire" mikes, meaning that the greatest sensitivity is perpendicular to the length of the mike. Most other directional mikes, particularly for hand-held use, are "end-fire": they have their greatest sensitivity extending from the end of the mike.

Which mike is best for field recording?

The intuitive answer to this question would be, "Sure, I want a mike I can point and shoot with, and know that I'll get the best sound." But with microphones it's not that easy. Consider the following problems which plague all mikes to some extent, but affect directional mikes much more than omnis:

  • Handling noise: Will you be hand-holding the mike during recording, or perhaps placing it on a table or podium where it can be bumped? Even the least little bump or finger-rubbing will be audible with a directional mike recording conversational audio, but omnis are usually more forgiving in this regard.

Some models of mike are better than others in terms of the amount of mechanical isolation between the outer body of the mike and the transducer inside. And you can further lessen handling noise by using a shockmount along with a pistol grip or other support for the microphone.

  • Wind disruption: Imagine having a sense of touch so delicate that you can sense sound vibrations in the air with your fingers. Now imagine what a hard blowing breeze, or even a moderate puff of air, would feel like to something that delicate. More like a hurricane or a tornado!

That's the way it is with microphones. If outdoor wind, or even a direct breeze from an air conditioning vent indoors, blows directly on the microphone, a loud ripping or rumbling noise will obscure the rest of the sound going into the mike. And again, directional mikes are affected much more by wind than omnis are. You'll need much more windscreening on a directional mike, which in turn will muffle the quality of the sound that makes it through the screen into the mike, and it'll also decrease the directionality of the mike!

  • Plosive distortion: This is a specific form of wind disruption. When you pronounce consonants like P, B, T, K, or other hard-edged sounds, part of your mouth closes down momentarily and then releases a little blast of wind along with the sound. If this "locally heavy windstorm" hits the microphone diaphragm, it will cause a distracting noise commonly referred to as a P-Pop or a Plosive.

You can lessen the effect of plosives dramatically by changing where you place the mike relative to the mouth of the person speaking; about 45 degrees off to the side of the mouth is better than directly in front for most speakers. But you'll also have fewer problems with plosives if you're using an omni than you will with a directional mike.

  • Proximity effect: Most directional mikes exhibit this rather bizarre phenomenon, although a few designs manage to mitigate it. When you place a directional mike closer than about ten inches to a speaking person, that person's voice will sound louder through the mike. That part pretty much follows intuition: when you get your ears closer to a sound, it sounds louder. But besides sounding louder through the mike, the voice will sound bassier, with a boosting of the low frequencies. This is what is called proximity effect. The closer you place the mike, the bassier the voice will sound! Now, many people rather like this effect: some rock singers in particular love the "ballsy" bass boost, and will work their mikes so close that it's hard to tell where singer leaves off and mike begins. But proximity effect is just that, an effect; it's not what I'd call "natural" sound. Worse, it changes drastically depending on how you vary your miking distance, so it puts much more responsibility on you to be consistent about where you hold a directional mike. Omnis are a good deal more forgiving in this regard.

Some directional mike designs start out with the assumption that they'll be placed very close to the voice. These designs incorporate a filter or some other means of cutting down on the bass energy coming out of the mike, or boosting the high frequencies. All this can make a closely miked voice, replete with proximity effect, sound more "natural". But if you stray more than a few inches away from the voice with one of these mikes, the resulting recording will sound unnaturally thin, because the filtered bass or boosted top end isn't being offset by the proximity effect.

A few others, notably Electro/Voice's RE10, RE11, RE15, RE16, RE18, and RE20, use a series of tuned ports that run down the body of the mike behind the capsule to help cancel out some of the proximity effect as well as enhancing directionality. These models can sound more natural when worked fairly close--as long as those ports are kept exposed! But if you place your fingers over the ports, or cover them up with a stand adapter clip, watch out! The sound from those mikes will get muddier and less directional.

So, what's a miker to do?

Considering all the factors listed above, my first advice will be no surprise: the most dependable, most versatile mike for a reporter gathering interviews and sound will be a good quality hand-held omnidirectional mike. There are a number of popular models in this category, listed here alphabetically without any implied ranking:

  • Beyer's M58 is a long-handled dynamic omni with reasonable protection against handling and wind noise.
  • Electro/Voice's 635A "family": the basic 635A is a dynamic omni, a rugged standard for decades; the 635L is a longer barreled version of the 635A; the RE50, essentially a 635A enclosed in a larger housing to cut down on handling noise; and the 635 N/D and RE50N/D, lighter, higher output versions of the 635A and RE50.
  • Shure's most popular offering in this category is the VP64 dynamic omni.

If you find that a dynamic omni is not giving you enough electrical output to drive the recorder of your choice properly, consider using an electret condenser omni. These have stronger output than their dynamic brethren. One example is Audio-Technica's ATM10a. It requires a AA battery to power its internal circuitry, but an alkaline AA will last hundreds of hours.

Once you're satisfied that you're handling and using the omni mike well, it may be worth your while to experiment with directional mikes such as cardioids or short shotguns.

  • I suggest that you practice with such mikes before tackling anything that HAS to go on the air.
  • Wear headphones while recording with directional mikes, because your eyes can deceive you: the placement that looks logical may not be the spot or the angle that gives the best sound. Let your ears be the judge instead. This also will help you learn to minimize handling noise by providing your ears with some immediate biofeedback.
  • If part of your reason for wanting to use a directional mike is to get more direct sound of some particular person or source, and less of the surrounding ambiance, remember that there's another way to change that ratio: move the mike closer to your subject. You can get the same balance of direct to ambient sound with an omni as with a cardioid, with less of the unwanted side-effects of directional mikes, if you move the omni about a third closer to the subject.

And if you're comparing miking distances of omnis and short shotguns, remember that while the actual pickup capsule diaphragm in an omni is close to the front of the mike, in a short shotgun the diaphragm is several inches down the barrel! So in a close miking situation you can actually get an omni's diaphragm closer to the sound source than you can with a short shotgun mike!

Other factors in choosing a mike

Environmental factors: If you're out in extremes of heat, or dampness and humidity, dynamic mikes will be more dependable than condensers. The latter tend to have trouble keeping their diaphragms polarized in damp conditions, as the electrical charge can dissipate into the moist air. When this happens the mikes start making electrical crackling and popping sounds.

An exception to this rule is Sennheiser's MKH series of condenser mikes, which handle hot humid situations very well, but are very expensive compared to other mikes I've mentioned so far.

Power requirements: Condenser and electret condenser mikes require some sort of electrical power source in order to operate, whereas dynamics do not. This power may be in the form of a battery within the mike, which typically would last tens to hundreds of hours. Or, in the case of most non-electret condenser mikes, some external power supply may be needed.

If your recorder is designed to provide it, this phantom power can be delivered right up the same balanced mike cable that carries audio from the mike to the recorder. Various condenser mikes need between 9 and 52 DC volts of phantom power, but the most common supplies are 12 volts (typical for most Shure mikes like the SM81 cardioid condenser) and 48 volts (the most common phantom voltage).

If your recorder can't provide the right power, then you'll need to get a separate phantom power supply to connect between the mike and the recorder; this increases the complexity of your setup and raises the likelihood of problems developing at the worst possible moment.

Some small recorders provide low-voltage unbalanced DC power through the mike input jack, that they call plug-in power. This allows the use of a range of electret condenser mikes without needing battery compartments in the mikes. It's similar to phantom power but operates at lower voltages and with cheaper mikes. For this to be useful, both the recorder and the mike must be advertised as using plug-in power. Also, this feature can create problems. If the mike plug is at all loose in the jack, the intermittent contact with the plug-in power will create pops and clicks in your audio. The solution is to place capacitors in the mike cable to block the DC voltage from reaching the microphone.

Expense:

Good quality microphones typically start at about $100, and go up from there into the stratosphere. The increased complexity of directional mike construction makes those mikes more expensive than their omni brethren for a particular grade of quality. (This is offset somewhat by the fact that manufacturers are able to sell many more units of cardioid mikes than other types for music recording and PA system use, permitting greater economies of scale in manufacture and marketing of cardioids.)

Special Applications Mikes

The mikes described above are generally intended for hand-held or stand-mounted use. But there are other specialized mikes which can make a difference in particular recording circumstances. Among these are wired lavaliers and wireless mikes.

Lavaliers

Lavalier or ‘lav' mikes (sometimes called tie-tack or lanyard mikes) are designed to be attached to the person being recorded. The most typical lav is a small mike mounted on a clasp that can be slipped onto a lapel, collar, or other part of clothing. Other useful accessories may include small pin mounts to pierce through clothing that has no accessible folds to clip onto. In theatrical use a popular mounting technique is the hair clasp, which allows the mike to be hidden in an actor's hairline just at the top of the forehead!

Lavaliers have several advantages, but many countervailing disadvantages, for radio recording.

  • A lav is unobtrusive: instead of being held by the reporter in an aggressive stance, the mike sits passively out of the line of sight of the person speaking.
  • Its smaller size can also make it less intimidating to the wearer, and easier to carry around for the recordist, than a larger mike.
  • When coupled with a wireless transmitter or a small recorder placed on a person, the lavalier mike can give complete freedom of movement, while allowing a recording to be made from the consistent perspective of that person.

But watch out for some of the pitfalls of using lavs:

  • Contact noises-the person being miked is in contact with the mike and its cable, so any movement of clothing can cause an obtrusive rustling sound to be picked up by the mike. This can be minimized by careful placement of the mike and cable, but may require taping the cable to the inside of the clothing to prevent rubbing. Wardrobe selection can help; ask your interviewee to wear natural fibers, as they generally make less noise when rubbed against a mike cable. And in extremely dry circumstances, static electricity generated by the clothing can cause clicks as it discharges into the mike.
  • Wear and tear-lav mikes use very thin cable, which is more subject to damage than conventional mikes, so you may also start hearing electrical crackling and intermittent audio as frayed cables get moved around on the body.
  • Plosives-a typical placement for a lav is on a tie or elsewhere on the central chest of the speaker. In this position you're unlikely to hear plosive problems with pronounced P's and B's and K's. But T's produce a sharp blast of air that goes straight down from the mouth to the chest. So your lav mike may pick up popping T's! You'll need windscreening of some sort, even indoors, if you mike in this position. Off to the side a bit will yield better results.
  • Chin Shadowing-one way to avoid those plosives is to move the mike higher up the chest, so the plosive blasts are blocked by the chin. But this positioning also sacrifices most of the direct pickup of the consonant sounds which give a voice recording its crispness and clarity.
  • Perspective shifts-if you move the mike to one side of the chest, you can also get out of the plosive range. But the quality of the recording will vary greatly depending on the head position of the speaker. Let's say you put the mike about halfway out to the left side of the chest, just below collarbone level. When the person is looking to her/his left, your mike gets a very close, clear (and perhaps plosive) perspective of what's being said. But when that person swings around to her/his right side, your mike suddenly hears a more muffled, distant version of the words.
  • Chest resonance-placing a mike on someone's chest picks up much more of the sound resonating directly from the chest, and less from the mouth and nasal passages where consonants are formed. This is typically much higher in bass content, and deficient in clarity. Many lavalier mikes are designed to boost high frequencies drastically to offset the dullness of chest sound.
  • Wireless problems-any signal being sent through a wireless transmitter will have more noise and distortion, and more chances for catastrophic loss of signal, than a wired connection. There are now wireless lavs available for about the same price as regular mikes...but high quality ones still cost hundreds or even thousands of dollars. Short battery life, antenna interference from reflections from the building's metal superstructure, and occasional verbal intrusions from your local cabbie are among the problems plaguing wireless users.
  • Ratio of direct to ambient sound-because lavs are typically placed farther from the mouth than a handheld mike, they pick up more ambient sound compared to the direct sound. If you're using directional lavs as opposed to omnis, you may get a more on-mike sound from the voice, but the ambiance being picked up off-axis can sound hollow and unnatural. And if you're using two lavs to record an interview-one on yourself and one on your subject-that hollowness will characterize both voices.

So, how come these mikes work so well for TV?

Television and film production thrives on the use of lavalier mikes. They can be unobtrusive or even invisible, hidden within clothing. But consider some of the differences between the typical TV interview and your simple radio reporting forays:

  • The forgiveness factor-if you have a visual image to go with the sound of someone's voice, you'll tend to rely less on the sound for intelligibility than in an audio-only situation like radio. Lip reading can help ease the loss of consonant clarity. That buzzing refrigerator in the background is less noticeable.
  • Vocal projection-television interviews are typically done with interviewer and subject farther apart than those done for radio. To span that conversational distance, people tend to talk louder, they project their voices. This gives a better balance of direct to ambient sound in a lav mike, and a better signal to noise ratio, than the quiet, intimate level of speech typical of radio. If you're going to use lavs for radio, "get spaced out"...move farther away from your interviewee.
  • Combining lavs with boom mikes-much TV and film work is done with multiple mikes, combining the chest resonance quality from the lav with the crisp consonant pickup from a boom-mounted overhead shotgun mike. If you don't see the boom shadow, you may be misled into believing all that sound is coming from the lav.
  • Gating and switching mikes-audio production for television can combine many mikes using automatic mike mixers (which turn mikes down or off when other mikes are spoken into) or noise gates (which turn down the signal from a mike when the input level drops below a certain threshold). This type of processing reduces the amount of background sound, making the voices more present. But in doing so it produces an uneven, shifting background ambience, which is more obtrusive on radio than it is on a TV soundtrack.