Microphone techniques (the selection and placement of microphones) have a major influence on the audio quality of a sound reinforcement system.
In order to provide some background for these techniques it is useful first to understand some of the important characteristics of the microphones themselves.
The most important characteristics of microphones for live sound applications are their operating principle, frequency response and directionality.
Secondary characteristics are their electrical output and actual physical design.
Operating Principle
The type of transducer inside the microphone, that is, how the microphone picks up sound and converts it into an electrical signal.
A transducer is a device that changes energy from one form into another, in this case, acoustic energy into electrical energy. The operating principle determines some of the basic capabilities of the microphone. The two most common types are Dynamic and Condenser.
Dynamic microphones employ a diaphragm/ voice coil/magnet assembly which forms a miniature sound-driven electrical generator. Sound waves strike a thin plastic membrane (diaphragm) which vibrates in response.
A small coil of wire (voice coil) is attached to the rear of the diaphragm and vibrates with it. The voice coil itself is surrounded by a magnetic field created by a small permanent magnet. It is the motion of the voice coil in this magnetic field which generates the electrical signal corresponding to the sound picked up by a dynamic microphone.
Dynamic microphones have relatively simple construction and are therefore economical and rugged. They can provide excellent sound quality and good specifications in all areas of microphone performance. In particular, they can handle extremely high sound levels: it is almost impossible to overload a dynamic microphone. In addition, dynamic microphones are relatively unaffected by extremes of temperature or humidity. Dynamics are the type most widely used in general sound reinforcement.
Condenser microphones are based on an electrically-charged diaphragm/backplate assembly which forms a sound-sensitive capacitor. Here, sound waves vibrate a very thin metal or metalcoated- plastic diaphragm.
The diaphragm is mounted just in front of a rigid metal or metal-coated ceramic backplate. In electrical terms this assembly or element is known as a capacitor (historically called a “condenser”), which has the ability to store a charge or voltage.
When the element is charged, an electric field is created between the diaphragm and the backplate, proportional to the spacing between them. It is the variation of this spacing, due to the motion of the diaphragm relative to the backplate, that produces the electrical signal corresponding to the sound picked up by a condenser microphone.
The construction of a condenser microphone must include some provision for maintaining the electrical charge or polarizing voltage. An electret condenser microphone has a permanent charge, maintained by a special material deposited on the backplate or on the diaphragm. Nonelectret types are charged (polarized) by means of an external power source. The majority of condenser microphones for sound reinforcement are of the electret type.
All condensers contain additional active circuitry to allow the electrical output of the element to be used with typical microphone inputs. This requires that all condenser microphones be powered: either by batteries or by phantom power (a method of supplying power to a microphone through the microphone cable itself). There are two potential limitations of condenser microphones due to the additional circuitry: first, the electronics produce a small amount of noise; second, there is a limit to the maximum signal level that the electronics can handle. For this reason, condenser microphone specifications always include a noise figure and a maximum sound level. Good designs, however, have very low noise levels and are also capable of very wide dynamic range.
Condenser microphones are more complex than dynamics and tend to be somewhat more costly. Also, condensers may be adversely affected by extremes of temperature and humidity which can cause them to become noisy or fail temporarily. However, condensers can readily be made with higher sensitivity and can provide a smoother, more natural sound, particularly at high frequencies. Flat frequency response and extended frequency range are much easier to obtain in a condenser. In addition, condenser microphones can be made very small without significant loss of performance.
The decision to use a condenser or dynamic microphone depends not only on the sound source and the sound reinforcement system but on the physical setting as well. From a practical standpoint, if the microphone will be used in a severe environment such as a rock and roll club or for outdoor sound, dynamic types would be a good choice. In a more controlled environment such as a concert hall or theatrical setting, a condenser microphone might be preferred for many sound sources, especially when the highest sound quality is desired.
Frequency Response
The output level or sensitivity of the microphone over its operating range from lowest to highest frequency.
Virtually all microphone manufacturers list the frequency response of their microphones over a range, for example 50 - 15,000 Hz. This usually corresponds with a graph that indicates output level relative to frequency.
A microphone whose output is equal at all frequencies has a flat frequency response.
Flat response microphones typically have an extended frequency range. They reproduce a variety of sound sources without changing or coloring the original sound.
A microphone whose response has peaks or dips in certain frequency areas exhibits a shaped response.
A shaped response is usually designed to enhance a sound source in a particular application.
For instance, a microphone may have a peak in the 2 - 8 kHz range to increase intelligibility for live vocals. This shape is called a presence peak or rise.
A microphone may also be designed to be less sensitive to certain other frequencies. One example is reduced low frequency response (low end roll-off) to minimize unwanted “boominess” or stage rumble.
Directionality
A microphone’s sensitivity to sound relative to the direction or angle from which the sound arrives.
There are a number of different directional patterns found in microphone design. These are typically plotted in a polar pattern to graphically display the directionality of the microphone. The polar pattern shows the variation in sensitivity 360 degrees around the microphone, assuming that the microphone is in the center and that 0 degrees represents the front of the microphone.
The three basic directional types of microphones are omnidirectional, unidirectional, and bidirectional.
The omnidirectional microphone has equal output or sensitivity at all angles. Its coverage angle is a full 360 degrees.
An omnidirectional microphone will pick up the maximum amount of ambient sound. In live sound situations an omni should be placed very close to the sound source to pick up a useable balance between direct sound and ambient sound. In addition, an omni cannot be aimed away from undesired sources such as PA speakers which may cause feedback.
The unidirectional microphone is most sensitive to sound arriving from one particular direction and is less sensitive at other directions. The most common type is a cardioid (heart-shaped) response. This has the most sensitivity at 0 degrees (on-axis) and is least sensitive at 180 degrees (off-axis).
The effective coverage or pickup angle of a cardioid is about 130 degrees, that is up to about 65 degrees off axis at the front of the microphone. In addition, the cardioid mic picks up only about one-third as much ambient sound as an omni. Unidirectional microphones isolate the desired on-axis sound from both unwanted off-axis sound and from ambient noise.
For example, the use of a cardioid microphone for a guitar amplifier which is near the drum set is one way to reduce bleed-through of drums into the reinforced guitar sound. Unidirectional microphones have several variations on the cardioid pattern. Two of these are the supercardioid and hypercardioid.
Both patterns offer narrower front pickup angles than the cardioid (115 degrees for the supercardioid and 105 degrees for the hypercardioid) and also greater rejection of ambient sound. While the cardioid is least sensitive at the rear (180 degrees off-axis) the least sensitive direction is at 126 degrees off-axis for the supercardioid and 110 degrees for the hypercardioid.
When placed properly they can provide more focused pickup and less ambient noise than the cardioid pattern, but they have some pickup directly at the rear, called a rear lobe. The rejection at the rear is -12 dB for the supercardioid and only -6 dB for the hypercardioid. A good cardioid type has at least 15-20 dB of rear rejection.
The bidirectional microphone has maximum sensitivity at both 0 degrees (front) and at 180 degrees (back). It has the least amount of output at 90 degree angles (sides). The coverage or pickup angle is only about 90 degrees at both the front and the rear. It has the same amount of ambient pickup as the cardioid. This mic could be used for picking up two opposing sound sources, such as a vocal duet. Though rarely found in sound reinforcement they are used in certain stereo techniques, such as M-S (mid-side).
Ambient Sound Rejection
Since unidirectional microphones are less sensitive to off-axis sound than omnidirectional types they pick up less overall ambient or stage sound. Unidirectional mics should be used to control ambient noise pickup to get a cleaner mix.
Distance Factor
Because directional microphones pick up less ambient sound than omnidirectional types they may be used at somewhat greater distances from a sound source and still achieve the same balance between the direct sound and background or ambient sound.
An omni should be placed closer to the sound source than a uni—about half the distance—to pick up the same balance between direct sound and ambient sound.
Off-axis Coloration
Change in a microphone’s frequency response that usually gets progressively more noticeable as the arrival angle of sound increases. High frequencies tend to be lost first, often resulting in “muddy” off-axis sound.
Proximity Effect
With unidirectional microphones, bass response increases as the mic is moved closer (within 2 feet) to the sound source. With close-up unidirectional microphones (less than 1 foot), be aware of proximity effect and roll off the bass until you obtain a more natural sound.
You can (1) roll off low frequencies on the mixer, or (2) use a microphone designed to minimize proximity effect, or (3) use a microphone with a bass rolloff switch, or (4) use an omnidirectional microphone (which does not exhibit proximity effect).
Unidirectional microphones can not only help to isolate one voice or instrument from other singers or instruments, but can also minimize feedback, allowing higher gain. For these reasons, unidirectional microphones are preferred over omnidirectional microphones in almost all sound reinforcement applications.
The electrical output of a microphone is usually specified by level, impedance and wiring configuration. Output level or sensitivity is the level of the electrical signal from the microphone for a given input sound level. In general, condenser microphones have higher sensitivity than dynamic types. For weak or distant sounds a high sensitivity microphone is desirable while loud or close-up sounds can be picked up well by lower-sensitivity models.
The output impedance of a microphone is roughly equal to the electrical resistance of its output: 150-600 ohms for low impedance (low-Z) and 10,000 ohms or more for high impedance.(high-Z).
The practical concern is that low impedance microphones can be used with cable lengths of 1000 feet or more with no loss of quality while high impedance types exhibit noticeable high frequency loss with cable lengths greater than about 20 feet.
Finally, the wiring configuration of a microphone may be balanced or unbalanced. A balanced output carries the signal on two conductors (plus shield). The signals on each conductor are the same level but opposite polarity (one signal is positive when the other is negative). A balanced microphone input amplifies only the difference between the two signals and rejects any part of the signal which is the same in each conductor.
Any electrical noise or hum picked up by a balanced (two-conductor) cable tends to be identical in the two conductors and is therefore rejected by the balanced input while the equal but opposite polarity original signals are amplified. On the other hand, an unbalanced microphone output carries its signal on a single conductor (plus shield) and an unbalanced microphone input amplifies any signal on that conductor.
Such a combination will be unable to reject any electrical noise which has been picked up by the cable. Balanced, low-impedance microphones are therefore recommended for nearly all sound reinforcement applications.
The physical design of a microphone is its mechanical and operational design. Types used in sound reinforcement include: handheld, headworn, lavaliere, overhead, stand-mounted, instrument-mounted and surface-mounted designs.
Most of these are available in a choice of operating principle, frequency response, directional pattern and electrical output. Often the physical design is the first choice made for an application. Understanding and choosing the other characteristics can assist in producing the maximum quality microphone signal and delivering it to the sound system with the highest fidelity.
Tim Vear is a veteran audio professional who works with Shure Incorporated. For more information visit www.shure.com.
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