Microphone techniques have a major influence on the overall audio quality of a sound reinforcement system. For reinforcement of musical instruments, there are several main objectives of microphone techniques: to maximize pick-up of suitable sound from the desired instrument, to minimize pickup of undesired sound from instruments or other sound sources and to provide sufficient gain-before-feedback. "Suitable" sound from the desired instrument may mean either the natural sound of the instrument of some particular sound quality which is appropriate for the application. "Undesired" sound may mean the direct or ambient sound from other nearby instruments or just stage and background noise. "Sufficient" gain-before feedback means that the desired instrument is reinforced at the required level without ringing or feedback in the sound system.
Obtaining the proper balance of these factors may involve a bit of give-and-take with each. In this guide, Shure application and development engineers suggest a variety of microphone techniques for musical instruments to achieve these objectives. In order to provide some background for these techniques, it is useful to understand some of the important characteristics of microphones, musical instruments, and acoustics.
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, or 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 metal-coated-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 charge, 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. Non-electret 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 microphone 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 most 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 high 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.
Phantom power is a DC voltage (usually 12-48 volts) used to power the electronics of a condenser microphone. For some (non-electret) condensers it may also be used to provide the polarizing voltage for the element itself. This voltage is supplied through the microphone cable by a mixer equipped with phantom power or by some type of in-line external source. The voltage is equal on Pin 2 and Pin 3 of a typical balanced, XLR-type connector. For a 48 volt phantom source, for example, Pin 2 is 48 VDC and Pin 3 is VDC, both with respect to Pin 1 which is ground (shield).
Because the voltage is exactly the same on pin 2 and Pin 3, phantom power will have no effect on balanced dynamic microphones: no current will flow since there is no voltage difference across the output. In fact, phantom power supplies have current limiting which will prevent damage to a dynamic microphone even if it is shorted or miswired. In general, balance dynamic microphones can be connected to phantom powered mixer inputs with no problem.
Transient response refers to the ability of a microphone to respond to a rapidly changing sound wave. A good way to understand why dynamic and condenser mics sound different is to understand the differences in their transient response.
In order for microphone to convert sound energy into electrical energy, the sound wave must physically move the diaphragm of the microphone. The amount of time it takes for this movement to occur depends on the weight (or mass) of the diaphragm. For instance, the diaphragm and voice coil assembly of a dynamic microphone may weigh up to 1000 times more than the diaphragm of a condenser microphone. It takes longer for the heavy dynamic diaphragm to begin moving than for the lightweight condenser diaphragm. It also takes longer for the dynamic diaphragm to stop moving in comparison to the condenser diaphragm. Thus, the dynamic transient response is not as good as the condenser transient response. This is similar to two vehicles in traffic: a truck and a sports car. They may have equal power engines but the truck weighs much more than the car. As traffic flow changes, the sports car can accelerate and brake very quickly while the semi accelerates and brakes very slowly due to its greater weight. Both vehicles follow the overall traffic flow but the sports car responds better to sudden changes.
Pictured below are two studio microphones responding to the sound impulse produced by an electric spark: condenser mic on top, dynamic mic on the bottom. It is evident that it takes almost twice as long for the dynamic microphone to respond to the sound. It also takes longer for the dynamic to stop moving after the impulse has passed (notice the ripple on the second half of the graph). Since condenser microphones generally have a better transient response then dynamics, they are better suited for instruments that have a very sharp attack or extended high-frequency output such as cymbals. It is this transient response difference that causes condenser mics to have a more crisp, detailed sound and dynamic mics to have a more mellow, rounded sound.
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. The graph has a frequency in Hertz (Hz) on the x-axis and relative response in decibels (dB) on the y-axis.
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.
The decibel (dB) is an expression often used in electrical and acoustic measurements. A decibel is a number that represents a ratio of two values of a quantity such as voltage. It is actually a logarithmic ratio whose main purpose is to scale a large measurement range down to a much smaller and more useable range. The form of the decibel relationship for voltage is:
dB = 20 x log(V1/V2)
where 20 is constant, V1 is one voltage, V2 is the other voltage, and log is logarithm base 10.
What is the relationship in decibels between 100 volts and 1 volt?
dB = 20 x log (100/1)
dB = 20 x log(100)
dB = 20 x 2 (the log of 100 is 2)
dB = 40
That is, 100 volts is 40dB greater than 1 volt.
What is the relationship in decibels between 0.001 volt and 1 volt?
dB = 20 x log (0.001/1)
dB = 20 x log (0.001)
dB = 20 x (-3) (the log of .001 is -3)
dB = -60
That is, 0.001 volt is 60dB less than 1 volt.
If one volt is equal to the other they are 0dB different
If one voltage is twice the other they are 6dB different
If one voltage is ten times the other they are 20dB different
Since the decibel is a ratio of two values, there must be an explicit or implicit reference value for any measurement given in dB. This is usually indicated by a suffix on the decibel value such as dBV (a reference to 1 volt which is 0dBV) or dB SPL (a reference to 0.0002 microbar which is 0dB Sound Pressure Level)
One reason that the decibel is so useful in certain audio measurements is that this scaling function closely approximates the behavior of human hearing sensitivity. For example, a change of 1dB SPL is about the smallest difference in loudness that can be perceived while a 3dB SPL change is generally noticeable. A 6dB SPL change is quite noticeable and finally, a 10dB SPL change is perceived as "twice as loud".
The choice of flat or shaped response microphone again depends on the sound source, the sound system, and the environment. Flat response microphones are usually desirable to reproduce instruments such as acoustic guitars or pianos, especially with high-quality sound systems. They are also common in stereo miking and distant pickup applications where the microphone is more than a few feet from the sound source: the absence of response peaks minimizes feedback and contributes to a more natural sound. On the other hand, shaped response microphones are preferred for a close-up vocal use and for certain instruments such as drums and guitar amplifiers which may benefit from response enhancements for presence or punch. They are also useful for reducing pickup of unwanted sound and noise outside the frequency range of an instrument.
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 260 degrees around the microphone, assuming that the microphone is in the center and that 0 degrees represent 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 a 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 -12dB 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).
USING DIRECTIONAL PATTERNS TO REJECT UNWANTED SOURCES
In sound reinforcement, microphones must often be located in positions where they may pick up unintended instrument or other sounds. Some examples are: individual drum mics picking up adjacent drum, vocal mics picking up overall stage noise, and vocal mics picking up monitor speakers. In each case there is a desired sound source and one or more undesired sound sources. Choosing the appropriate directional pattern can help to maximize the desired sound and minimize the undesired sound.
Although the direction for maximum pickup is usually obvious (on-axis) the direction for least pickup varies with microphone type. In particular, the cardioid is at least sensitive at the rear (180 degrees off-axis) while the supercardioid and hyper-cardioid types actually have some rear pickup. They are least sensitive at 125 degrees off-axis and 110 degrees off axis respectively.
For example, when using floor monitors with vocal mics, the monitor should be aimed directly at the rear axis of a cardioid microphone for maximum gain-before-feedback. When using a supercardioid, however, the monitor should be positioned somewhat off to the side (55 degrees off the rear axis) for the best results. Likewise, when using supercardioid or hypercardioid types on drum kits be aware of the rear pickup of these mics and angle them accordingly to avoid pickup of other drums or cymbals.
Other directional related microphone characteristics:
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 clear mix.
Distance factor - Because directional microphones pick up pless ambient sound than omnidirectional types they may be used at somewhat greater distances from a sound source and still achive 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 omni directional 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 in 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.