A microphone's job is simple: it converts an acoustic sound into an electrical signal that corresponds to the original waveform as closely as possible. That signal then can be processed, mixed with other audio signals, and recorded.
Acoustic sounds occur when something vibrates at a frequency between 20 Hz and 20 kHz in air or another medium. That creates small regions of high and low pressure around the vibrating source. As the molecules in those regions move in response to the changing pressure, they jostle nearby molecules, and that causes the regions of high and low pressure to expand outward.
Once the regions of changing pressure reach a microphone, they impinge on a flexible diaphragm within the mic, causing it to vibrate in response. That physical vibration is then converted into an electrical signal, which is sent to a mixer, a signal processor, or another device. The difference between types of mics is the specific manner in which the conversion is performed.
Before I discuss the various mic types, here are some concepts that are common to all. As mentioned previously, all microphones include a diaphragm, which is usually mounted in something called a capsule. The capsule is mounted in an outer case along with any support electronics. Some cases can be handheld or standmounted for stage use, whereas others must be mounted on a stand. Standmounted mics sometimes include a shockmount, which isolates the mic from unwanted vibrations in the stand.
In most cases, the capsule is located behind a screen of some sort that lets the acoustic sound enter while protecting the diaphragm from physical damage. That screen often includes a layer of foam to reduce wind noise and vocal pops, though an external pop screen is usually more effective in the latter application.
Once the acoustic sound has been converted to an electrical signal, it is conveyed to another device along a cable. Some inexpensive mics include a permanent cable that terminates in a 2-conductor, ¼-inch or ⅛-inch phone plug. The cable includes one central conductor that carries the audio signal surrounded by another conductor (called the shield) that connects to ground. However, that type of cable is susceptible to induced hum and other environmental noise. As a result, most professional and semipro mics use a 3-conductor XLR connector at the end of a balanced cable.
FIG. 1: A microphone''s frequency response is depicted in a graph of frequency versus relative output level. If the sound source is close to most directional mics, the proximity effect boosts the low end. Also notice the presence peak between 2 and 10 kHz.
The frequency range within which a microphone accurately translates the sound-pressure level (SPL) of acoustic sounds into electrical signal levels is called its frequency response, which is measured in decibels (dB) over a range of frequencies. But what does “accurately” mean? For a given SPL, the output-signal level typically varies by no more than ±3 dB from its nominal level. That is normally depicted in a graph of frequency versus output level (see Fig. 1). A mic with a flat frequency response generates the same audio signal level for a sound of any frequency within the specified range at a given SPL.
However, most mics don't exhibit a flat frequency response, partly because making such a mic is expensive and partly because a frequency response that's uneven can be of some benefit. For example, many vocal mics boost the upper frequencies; that is the presence peak shown in Fig. 1, and it helps improve the intelligibility of words. However, a presence peak can exaggerate a shrill upper vocal range.
At the low end, the frequency response of a vocal mic often falls off below 100 Hz. Because the human voice can't produce frequencies that low, there is no reason to make a mic that reproduces them accurately. Instrument mics generally fall off below 50 Hz. However, the low-end response of many mics can be greatly enhanced by moving the sound source close to the mic. That bass boost is the proximity effect in Fig. 1, and it helps radio announcers achieve their characteristically deep sound. However, moving too close to the mic increases breath noises and vocal pops.
All mics exhibit a pickup pattern, which determines how the mic responds to sounds at different frequencies coming from different directions. An omnidirectional mic responds more or less equally to sounds coming from any direction. That pickup pattern is particularly well suited for ambient mics, which are used to pick up the sound of the room in which an acoustic source is radiating.
In many cases, omnidirectional mics are not used in live performance because they pick up sounds from all directions, which can lead to feedback. However, omni mics are generally less susceptible to wind and breath noise, and they tend to have a relatively flat frequency response with no pronounced peaks, which can actually help to avoid feedback. Omni mics also tend to have excellent low-frequency response, and they do not exhibit the proximity effect.
If a mic does not respond equally to sounds from any direction, it is called a directional mic. There are several types of directional mics, most of which respond best to sounds coming from directly in front of the mic's capsule. (The main exception is the middle-side, or M-S mic, which contains two capsules. M-S mics pick up sounds from both sides as well as the front.) Sounds that strike the mic at its most sensitive spot are on-axis; sounds from any other direction are off-axis. Directional mics are prone to the proximity effect, and their frequency response is normally less flat than in omni designs.
FIG. 2: Notice that the pickup patterns—(a) cardioid, (b) supercardioid, (c) figure-8 —are slightly different at different frequencies. In these examples, the left half shows the curves for 125 and 500 Hz; the right half shows the curves for 1 and 4 kHz.
The pickup pattern of any microphone can be depicted in a polar graph (see Fig. 2). In this type of graph, the mic's axis is defined as 0 degrees (usually located at the top of the graph), and the outer circle defines a flat frequency response. The smaller, inner circles represent a drop in frequency response. The curve within the graph indicates how the mic responds to sounds from different directions. An omnidirectional mic's polar pattern forms a circle. Keep in mind that although polar patterns are conventionally graphed in two dimensions from a bird's-eye view, the mic's actual pickup pattern is three-dimensional.
The most popular type of directional mic is the cardioid (see Fig. 2a). Its polar pattern resembles an inverted heart — hence the name. The mic is most responsive to on-axis sounds, whereas off-axis sounds are attenuated; sounds from 180 degrees off-axis are almost completely rejected. In addition, notice that the polar pattern changes slightly at different frequencies. The extra curves give a rough idea of a mic's frequency response and pickup pattern in one graph.
A supercardioid mic is often used in live performance because it rejects more sound from the sides than a cardioid design (see Fig. 2b). However, it does have some response to sounds coming from 180 degrees, as indicated by the small rear pickup lobe. Another variation of that design, called the hypercardioid, is even more directional.
Some mics exhibit a bidirectional or figure-8 pickup pattern (see Fig. 2c), so-called for obvious reasons. Those mics are most sensitive to sounds from the front and rear, rejecting sounds from the sides. That works well for miking two sources (such as two toms in a drum kit or two singers facing each other) with one mic.
DYNAMIC RANGE AND MORE
Not only must mics reproduce different frequencies coming from different directions but they must also contend with sounds at different levels. If the sound reaching a mic is above a certain amplitude, the signal from the mic becomes distorted, and the diaphragm might even be damaged. This upper amplitude limit is called the dynamic range, and it's measured in dB SPL. The dynamic range of most mics is typically between 100 and 120 dB SPL; some go to 130 dB SPL or higher.
At a given source level, different microphones produce an audio signal at different levels. The relationship between the input level and the output level is called the sensitivity of the mic; the higher the output level at a given input level, the more sensitive the mic.
Sensitivity is usually measured with a 1 kHz tone at one or two levels: 74 dB SPL (the level of the average speaking voice at a distance of 3 feet) and 94 dB SPL (which corresponds to a loud speaking voice at a distance of 1 foot). The output level is expressed in dBV (decibels referenced to 1V root mean square) or dBm (decibels referenced to 1 milliwatt). (See “Square One: Decibels Demystified, Part 2” in the August 2001 issue for more about those decibel types.) For example, a mic's sensitivity might be specified as an output level of -47 dBV at 94 dB SPL. Many mics also include a pad switch, which lowers the overall output level by 10 or 20 dB. That is useful if the sound source is particularly loud.
Microphones also exhibit impedance, another important electrical characteristic. That is the microphone's resistance to the flow of electrical current, which changes as a function of frequency (see “Square One: The Shocking Truth” in the June 2001 issue). Most professional and semipro mics are low impedance (typically between 150Ω and 250Ω but sometimes as low as 50Ω; also called low-Z), whereas most inexpensive mics are high impedance (usually above 20 kΩ; also called high-Z).
Low-Z mics are less susceptible to extraneous electrostatic noise in the cable, such as that caused by fluorescent lights or motors, but they are more likely to pick up hum from electromagnetic interference, such as that from AC power lines. Because they operate with relatively high current levels and use balanced cables, low-impedance mics can drive cables that are hundreds of feet long. High-Z mics, especially those using unbalanced lines, are limited to cable lengths of no more than about 20 feet.
Matching a mic's impedance with the input to which it is connected is important. To connect one type of mic to the other type of input, you must typically use a matching transformer. In a pinch, you can connect a low-Z mic directly to a high-Z input, but you will lose too much level if you connect a high-Z mic to a low-Z input.
There are many types of microphones that use different methods to convert an acoustic sound into an electrical signal. Of those types, three are most common today: dynamic, condenser, and boundary.
FIG. 3: In a moving-coil dynamic mic, the diaphragm is attached to a coil of wire that vibrates within a magnetic field. That induces an electrical signal in the wire that corresponds to the incoming acoustic waveform.
In most dynamic microphones, the diaphragm is attached to a coil of wire, called the voice coil, which is free to move near a permanent magnet (see Fig. 3). As the diaphragm vibrates in response to an acoustic sound, the voice coil moves back and forth within the magnetic field. That induces an oscillating electric current in the wire, which corresponds to the original sound's waveform. The design is also known as a moving-coil mic. Common moving-coil dynamic mics include those in the AKG Tri-Power series; the Audix OM-3xb, OM-5, and OM-7; the beyerdynamic M 88 and TG-X series; the Sennheiser MD 421; and the ever-popular Shure SM57 and SM58.
Moving-coil dynamic mics are often housed in a handheld case. They are also quite rugged and able to withstand rough treatment and high SPL levels. As a result, they are great for miking drums, electric-guitar cabinets, and vocals. Most dynamic mics use a cardioid, supercardioid, or hypercardioid pickup pattern to reject onstage ambient noise. Good dynamic mics have an excellent frequency response, but the diaphragm-voice coil assembly is relatively heavy, so those mics are somewhat less sensitive to fast transients.
A variation of the moving-coil design is called the ribbon mic. Instead of a diaphragm and voice coil, that type of mic uses a thin ribbon of metal suspended in a permanent magnetic field. As the ribbon vibrates in response to an acoustic sound, an electric current is generated in the metal.
Although they're not made much these days, ribbon mics have excellent transient response, and they are famous for their warm sound. However, they are extremely fragile and delicate; you can destroy the ribbon by coughing into the mic. Their output level also is generally lower than moving-coil designs.
FIG. 4: In a condenser mic, the diaphragm is suspended over a parallel backplate, forming a capacitor. As the diaphragm vibrates, the voltage across the capacitor varies, producing an electrical signal, which must be amplified by an internal preamp.
In a condenser mic, the diaphragm is a very thin sheet of Mylar coated with gold or another conductive material and suspended over a parallel conductive surface called the backplate (see Fig. 4). That forms an electrical capacitor, which is sometimes called a condenser — hence the name. A static voltage of 9 to 48 VDC is applied across that capacitor. As the diaphragm vibrates in response to incoming acoustic sounds, the voltage varies slightly as the diaphragm moves closer and farther away from the backplate, generating a tiny signal. That signal must be amplified with an internal preamp before it is sent to the mic's output. Popular condenser mics include the AKG C 414, the Audio-Technica 4033a/SM and 4050/CMS, the beyerdynamic MC 834, and the Neumann TLM 193.
The static voltage across the condenser element and the power required to operate the internal preamp are typically supplied by a battery or by the input of the device (such as a preamp) to which the mic is connected. The voltage from the device's input is called phantom power, because it is sent along the same cable that carries the signal from the mic to the input; there is no separate power cable. That does not interfere with the audio signal, because the phantom power is a fixed voltage, whereas the audio signal changes over time.
A variation of the approach is called an electret condenser. In that type of mic, the diaphragm or backplate is made of a material (such as Teflon) that retains a permanent electric charge, which removes the need for an external power supply. (However, the internal preamp still needs power, which is normally supplied by a battery.) In most cases, the backplate retains the charge, so the diaphragm can be made of Mylar instead of Teflon, which is less sensitive to acoustic vibrations. That design is called a back-electret condenser.
Condenser mics can exhibit different pickup patterns. Many condensers offer several switchable patterns, which is handy. Condenser mics have excellent transient response because the mass of the diaphragm is very low. They are most often used in the studio for vocals, acoustic instruments, and just about everything else. However, they are more delicate than dynamics, so they are not used as much in live performance.
Another variation of the condenser approach is the boundary microphone. In that design, a small electret capsule is mounted in the center of a flat metal plate, and that forms a sonic boundary — hence the name. Incoming sound waves are reflected from the plate and reach the capsule at virtually the same instant as the direct sound waves. That reinforces the acoustic signal at the diaphragm.
FIG. 5: The Sound Grabber II is one of Crown''s PZMs. The electret capsule is suspended over the plate, facing downward.
Crown's version of the boundary mic, called a Pressure Zone Microphone (PZM), has become the most common form of boundary mic in the United States (see Fig. 5). Unlike other boundary designs, in which the capsule faces away from the plate, a PZM capsule is suspended above the plate by a small distance and faces downward, toward the plate. The gap between the plate and capsule is called the pressure zone, where the direct and reflected sound waves meet and add coherently to stimulate the diaphragm.
In most boundary mics, the electret capsule is omnidirectional. However, the plate effectively blocks any sound coming from the side opposite the capsule, so the practical pickup pattern is hemispherical. These mics are often mounted on a large board or wall for picking up room ambience. They also work well on instruments with a large sound-radiation pattern, such as vibes or woodwinds. A few boundary mics have a cardioid pattern, in which case the capsule axis is parallel to the plate rather than perpendicular. These mics are often used on stage floors for actors.
Unless you record and perform instrumental music on synthesizers exclusively, you need at least one mic in your toolbox. However, mics can be expensive. Invest in at least one high-quality condenser for recording purposes and one or more dynamics for recording and live performance.
Each mic has its own unique sound, so experiment with different models until you find the right one for each instrument or voice. Once you find the right mics for your applications, the world of acoustic sound awaits your pleasure.
The studio in whichScott Wilkinsonrecords the voice-over narration for United Airlines' Classical Collection audio channel uses a hypercardioid shotgun mic aimed right at his mouth.