Microphone techniques for live sound reinforcement

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THE DECIBEL

The decibel (dB) is an expression often used in electrical and acoustic measurements. The 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 a constant, V1 is one voltage, V2 is the other voltage, and log is logarithm base 10.

Examples:

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 that 1 volt.

Similarly:
if one voltage 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 (reference to 1 volt which is 0dBV) or dB SPL (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.

Decibel scale for dBV or dB SPL

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 microphones 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 closeup 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 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.

Omnidirectional

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).

Cardiod

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.

Supercardioid

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 drums, 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 least sensitive at the rear (180 degrees off-axis) while the supercardioid and hypercardioid 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.

Monitor speaker placement for maximum rejection: cardioid and supercardioid

When using a supercardioid, however, the monitor should be positioned somewhat off to the side (55 degrees off the rear axis) for 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 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).

Proximity effect graph

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.