Microphone Characteristics


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:

A mic capsule is the construction that houses the diaphragm and other microphone components .


  • Amplitude response: Sensitivity to sounds of different loudness

  • Directionality: Sensitivity to sounds coming from different angles

  • Frequency response: Sensitivity to different parts of the frequency spectrum as well as specific low-frequency characteristics ( Figure 6.1 )

    Figure 6.1. Frequency response curves vary by mic. A smooth response (top; a Shure KSM-32) responds relatively consistently across the frequency spectrum. (The dotted line shows the proximity effect: bass frequencies are boosted when the source is 15 centimeters from the mic as opposed to 60.) A less consistent curve (bottom; a Shure SM-58) might reduce low frequencies and accentuate mid frequencies. Smooth is not necessarily better: the response on the bottom can be perceived as a "warmer" sound and is intended for use with vocals. (Images courtesy Shure, Inc.)

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.


A transient is any sudden change in signal. In audio recording this term is used to refer to the abrupt peak caused by sounds like a gunshot, drum hit, or the attack of a brass stab.


Resources: Microphone Manufacturers

AEA ( vintage reproductions) (www.ribbonmics.com)

AKG Acoustics (www.akgacoustics.com)

Audio Technica (www.audiotechnica.com)

Blue (www.bluemic.com)

Crown (www.crownaudio.com)

Electro-Voice (www.electrovoice.com)

Neumann (www.neumann.com)

R de (www.rodemic.com)

Royer (www.royerlabs.com)

Sennheiser (www.sennheiser.com)

Shure (www.shure.com)

Studio Projects (www.studioprojectsusa.com)


We'll observe frequency and amplitude response by different microphone types, but directionality can be easily described in terms of general patterns .

Mic 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.)


Directionality

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

Polar Response

Name , Width

Description and Applications

Omnidirectional (widest)

Omni mics pick up sound nearly equally from all directions, so they're useful for picking up larger fields of sound or ambient sound, or in situations where direction from the source is uncertain . They're the most forgiving in terms of placement.

Cardioid (narrower)

Cardioid microphones are the most commonly used, basic directional mics, because they respond well to sounds in front of them while rejecting potentially unwanted ambient sounds behind. This makes them especially useful for live applications, since they're less likely to pick up audience noise.

Supercardioid/Hypercardioid (narrowest)

Supercardioid microphones have a narrower pattern and are used to prevent bleed in crowded situations or other recordings that need to reject ambient sounds. Care must be taken in placement, however, because their narrowness is more unforgiving. Since, unlike cardioids, they are sensitive to the rear, you'll also need to keep monitors and other sources out of this area. Sometimes the terms super and hyper are used interchangeably.

"Shotgun" (narrower supercardioid)

Supercardioid patterns are available in varying degrees: the narrowest versions of this pattern are "shotgun/long gun" or simply "gun," which reject more off-axis sounds in both front and rear.

Bidirectional (sides are dead)

Bidirectional or "Figure of Eight" microphones pick up sounds from the front and rear while rejecting sound on the sides. "Front" means the front of the capsule, not the mic head. Because the capsule is mounted sideways , these microphones pick up sound from the sides.

Images courtesy Sennheiser Electronic Corporation. From top: Sennheiser MD 21-U dynamic, MD 421-II dynamic, MKH-50 condenser, MKH-70 condenser, MKH-30 condenser.


Cardioid is not an alien: Think " cardiac ." The cardioid pattern is named for its heart shape.


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.

Tip: Do you want a deeper "radio" voice? Try moving closer to your cardioid mic (you'll want to use a pop filter for the best results). You're now making intentional use of the proximity effect.


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)


Patterns aren't perfect: Mic patterns are only ideal models of mic performance. Real cardioid mics don't completely eliminate sound from the rear (though they do reduce it), and reflections from nearby surfaces will further color your recording.


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.

Tip: Starter mic ideas

A great recording can begin with relatively inexpensive mics; two matched dynamic mics and a condenser mic would be a good starting point. Here are some typical examples, although there are many options:

Dynamic: Shure SM57 / 58 / Beta 58 ($80150 street), Blue The Ball or USB-compatible Blue Snowball ($100150)Ideal for drums, vocals, onstage

Condenser: R de NT-1A ($200, shown), Studio Projects C1 ($200)Ideal for vocals, instruments


Dynamic

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.)


A light, thin diaphragm (usually Mylar) is displaced by air pressure created by sound waves.

A coil of wire attached to the diaphragm moves as the diaphragm moves.

As the coil of wire moves it disrupts a fixed magnetic field, generating current, which is fed out of the mic as output signal.

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.

Dynamic examples: Shure SM57/58, Sennheiser MD421 mk II (shown at the bottom of p. 164, courtesy Sennheiser Electronic Corporation)

Needs power? No

Pros: Rugged, wide amplitude range, often more affordable

Cons: Not as good at capturing high frequencies, tend to "bias" low-end, produce a weaker signal

Best for: General-purpose recording, loud recordings like drums and guitar amps, onstage applications


Genre-bending mics: You can't rely on rigid categories to understand microphones, because engineers are always breaking the rules. Case in point: Blue makes popular, powered dynamic mics like the USB-connected Snowball (pictured, courtesy Blue Microphones), an exception to the "rule" that dynamics don't need to be powered (see sidebar "Phantom Power", p. 167). By powering the mic, engineers are able to mix desirable qualities of condensers and dynamics, and make a mic that can be used in a wider variety of recording situations.


Condenser

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.

Condenser examples: Shure KSM/27 (shown, courtesy Shure Inc.), AKG C1000S

Needs power? Yes

Pros: Smoothest overall frequency response

Cons: Less rugged; not suitable for close miking of loud sources

Best for: General studio recording, acoustic guitars, instruments and vocals where ruggedness is less critical


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.

You say tomato, I say to-MAH-to: Condenser mics are sometimes called capacitor mics because the electrical terms capacitor and condenser are used interchangeably depending on the country you're in. If you're in the United States, you probably call the mic a condenser, even though you call the circuitry a capacitor, whereas if you're in the United Kingdom, you'll probably call both the mic and circuit "capacitor."


Phantom Power

Phantom Power is a means of providing electricity to a condenser microphone without using additional cabling. Using one of the connections in a balanced cable, it provides a 11-52V DC power supply (often labeled simply '48V') to charge the condenser plates. Condensers can use a battery instead of phantom power ( Figure 6.7 ), but phantom power is generally preferred.

Figure 6.7. Condenser mics and other mics that need power can draw it from a battery, as in the Sennheiser K-6 power system shown here, or from phantom power supplied by a mixer or interface through the XLR cable. (Photo courtesy Sennheiser Electronic Corporation)


Phantom power can be supplied by a number of sources:

  • The balanced connection on a mixer

  • The balanced connection on a computer audio interface

  • Dedicated phantom power supply

On a mixer or computer audio interface that supports phantom power, there's usually a phantom power on/off switch.

Dynamic mics don't use phantom power, but if you do happen to apply phantom power to a balanced dynamic mic, it will function just as though phantom power were off.

Warning: You should only plug a microphone or other device into a phantom-powered jack if you're certain the device is compatible with phantom power . Check your documentation if you're unsure. Nearly all common microphones with balanced XLR jacks will work, as will many direct boxes, but some older mics or mics with unbalanced or high-impedance outputs could be damaged, and you should never plug in an unbalanced device into a mic input without first turning off phantom power.


Ribbon

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)


Ribbon examples: Royer R-121 (shown, courtesy Royer Labs), Beyerdynamic M130/160

Needs power? No

Pros: Consistent frequency response, subjectively warm sound

Cons: Expensive, sensitive to wind and air (although some modern designs are more rugged)

Best for: "Smooth" or historical sound, strings and other instrumental recordings, warm vocal recordings


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)




Real World Digital Audio
Real World Digital Audio
ISBN: 0321304608
EAN: 2147483647
Year: 2006
Pages: 96
Authors: Peter Kirn

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