Article

Monday, December 12, 2011

Church Sound: Interfacing Microphones With Sound Systems

A look at phantom power and other output and impedance issues with microphones, in addition to a discussion of best mic cabling, connector and accessory practices

A key area of microphone use is the interface of the microphone with the sound system.

This primarily involves electrical considerations, so here are a few simple rules for proper interface based on the electrical characteristics of the microphone output and the sound system input, and on the requirements for cables and connectors to achieve maximum reliability.

All condenser type microphones require power for their operation. This is provided by an internal battery in some models, or by phantom power in others.

If a condenser is selected, care must be taken to assure that the appropriate power source (battery or phantom) is available. A battery-powered condenser is fine for applications such as portable recording, but phantom power should be used for any permanent micro phone installation.

Phantom power, sometimes called “simplex”, is provided through the microphone cable itself. It is a DC (direct current) voltage that may range from 9 to 48 volts, depending on the microphone requirement and the phantom power source rating.

This voltage is applied equally to the two conductors of a balanced microphone cable, that is pin 2 and pin 3 of an XLR-type connector. The voltage source may be either in the mixer itself or in a separate phantom power supply connected in line with the microphone cable.

Most recent mixers have phantom power built in, and the actual voltage will be stated on the mixer or in the operating manual.

The voltage requirement for a phantom-powered condenser microphone will also generally be stated on the microphone or in the manufacturer’s literature.

Some types, particularly those that are externally charged, may require a full 48-volt supply. Electret types, which have a permanent charge, will typically operate over the entire range from 12 to 48 volts.

Unless specifically stated otherwise by the manufacturer, these microphones will deliver their full performance at any voltage in this range, and further, they will not be damaged by a full 48-volt supply. Supplying less than the recommended voltage to either type may result in lower dynamic range, higher distortion, or increased noise, but this also will not damage the microphone.

Dynamic microphones, of course, do not require phantom power. However, many mixers have only a single switch that supplies phantom power to all microphone inputs, which may include some used by dynamic microphones.

The presence of phantom power has no effect on a balanced, low-impedance dynamic microphone. It is not possible to damage or impair the performance of a balanced microphone correctly hooked up to any standard phantom supply.

If a balanced microphone is incorrectly wired or if an unbalanced, high-impedance microphone is used, there may be a loud “pop” or other noise produced when the microphone is plugged in or switched on. In addition, the sound of the microphone may be distorted or reduced in level.

Even in these cases, the microphone will still not be damaged and will work normally when the wiring is corrected or the phantom power is turned off. If an unbalanced microphone must be used with a phantom-powered input, an isolating transformer should be inserted.

By the same token, it is also not possible to damage any standard phantom power source by improper microphone connection.

Good phantom power practices:
• Check that phantom voltage is sufficient for the selected condenser microphone(s);
• Turn system levels down when connecting or disconnecting phantom-powered microphones, when turning phantom power on or off, or when turning certain phantom-powered microphones on or off;
• Check that microphones and cables are properly wired.

Following these practices will make condenser microphone use almost as simple as that of dynamics.

Not Necessary Or Even Desirable
For the expected sound level, microphone sensitivity should be high enough to give a sufficient signal to the mixer input. In practice, most mixers are capable of handling a very wide range of microphone signal levels.

Occasionally, for extremely high sound levels, an “attenuator” may be necessary to lower the output of the microphone. These are built into some microphones and mixers. Otherwise, accessory attenuators are available that may be inserted in line with the microphone cable.

It has already been mentioned that balanced, low-impedance microphones are recommended for the majority of worship facility sound applications. This will allow the use of long microphone cables, and result in the least pickup of electrical noise.

In any case, the microphone impedance should be similar to the rated impedance of the microphone input of the mixer or other equipment. It is not necessary or even desirable to match impedances precisely. It is only necessary that the actual input impedance be greater than the microphone output impedance.

In fact, the actual impedance of a typical microphone input is normally five to ten times higher than the actual output impedance of the microphone.

The microphone input impedance of most mixers ranges from 1000 ohms to 3000 ohms, which is suitable for microphones of 150 ohms to 600 ohms.

When it is necessary to match a balanced, low-impedance microphone to an unbalanced, high-impedance input, or vice versa, transformers with the appropriate input and output connectors are readily available.

Transformers provide an impedance matching function and can also change the configuration from balanced to unbalanced as needed.

Ideally, transformers should be connected so that the bulk of the cable run is balanced, low-impedance, for maximum allowable length and minimum noise pickup. This would normally place the transformer at the connector of the unbalanced, high-impedance device.

Professional (and most semi-professional) equipment has balanced, low-impedance microphone inputs using 3-pin XLR-type connectors.

Less sophisticated musical instruments, consumer electronic products, computers and many portable recording devices typically have unbalanced, high-impedance microphone inputs using 1/4-inch phone jacks or 1/8-inch mini-phone jacks.

A few mixers offer both types of connectors for each input channel. Simple adapters may be used to mate different types of connectors if no configuration change (high/low impedance or balanced/unbalanced signal) is necessary. Use only high-quality connectors and adapters.

Phantom Power & Bias Voltage
In a condenser microphone, one function of the circuitry is to convert the very high impedance of the condenser element to a lower impedance.

For an electret condenser (the most common type), this is done by a single transistor. Some condenser designs, such as lavalier types or miniature hanging types, have their electronics separate from the microphone element.

In these models, the impedance converting transistor is built in to the microphone element itself. The main part of the circuitry is contained in a separate module or pack usually connected to the element by a thin shielded cable.

The main electronics of such designs operate on phantom power supplied through the microphone cable or by means of a battery in the pack itself.

However, the impedance-converting transistor in the microphone element also requires power in a form known as “bias” voltage. This is a DC voltage, typically between 1.5 and 5 volts. It is carried on a single conductor in the miniature connecting cable, unlike phantom power, which is carried on two conductors in the main microphone cable.

In addition, the audio signal in the miniature cable is unbalanced while the signal in the main cable is balanced.

This distinction between phantom power and bias voltage is important for two reasons. The first concerns the use of wireless transmitters. Body-pack transmitters which operate on 9 volt (or smaller) batteries cannot provide phantom power (12-48 volts DC). This prevents their use with phantom-powered condenser microphones.

However, the body-pack transmitter can provide bias voltage (1.5-5 volts DC). This allows a condenser microphone element with an integrated impedance-converting transistor to be used directly with a body-pack transmitter.

Miniature condenser lavalier types as well as other designs which have separate electronics can be operated with wireless systems in this way.

Picking Up Noise
The second reason concerns the wired installation of condenser microphones with separate electronic assemblies such as miniature hanging microphones for choir, congregation, or other “area” applications.

Since the audio signal in the cable between the microphone element and the electronics is unbalanced, it is more susceptible to pickup of electronic noise. This is particularly true for radio frequency noise because the cable itself can act as an antenna, especially for a nearby AM radio station.

For this reason it is strongly recommended to keep the length of this part of the cable as short as possible, preferably less than 35 feet. It is a much better practice to extend the length of the balanced cable between the electronics assembly and the mixer input.

Optimum microphone performance depends on the associated connectors and cables. In addition to quality connectors of the types described above, it is equally important to use high-quality cables.

Beyond the basic specification of balanced (two conductors plus shield) or unbalanced (one conductor plus shield), there are several other factors that go into the construction of good cables.

The conductors: carry the actual audio signal (and phantom voltage for condensers), usually stranded wire. They should be of sufficient size (gauge) to carry the signal and provide adequate strength and flexibility; use stranded conductors for most applications, solid conductors only for stationary connections.

The shield: protects the conductors from electrical noise, may be braided or spiral wrapped wire, or metal foil. It should provide good electrical coverage and be flexible enough for the intended use: braid or spiral for movable use, foil only for fixed use such as in conduit.

The outer jacket: protects the shield and conductors from physical damage, may be rubber or plastic. It should be flexible, durable, and abrasion resistant. Depending on the location it may need to be chemical or fire resistant.

Different color jackets are available and can be used to identify certain microphone channels or cables.

A large percentage of microphone problems are actually due to defective or improper microphone cables.

Microphone cables should be handled and maintained carefully for long life:
• Position them away from AC (electricity) lines and other sources of electrical interference to prevent hum;
• Allow them to lie flat when in use to avoid snagging;
• Use additional cable(s) if necessary to avoid stretching;
• Do not tie knots in cables;
• Coil loosely and store them when not in use;
• Periodically check cables visually and with a cable tester.

Individual, pre-assembled microphone cables are readily found in a wide variety of styles and quality. In addition, multiple cable assemblies, called “snakes”, are available for carrying many microphone signals from one location to another, such as from the sanctuary to the sound booth.

The use of only high-quality cables and their proper maintenance are absolute necessities in any successful worship facility sound application.

Range Of Accessory Options
Finally, the use of microphones for particular applications may be facilitated by microphone accessories. These are mechanical and electrical hardware items that are often used in mounting and connecting microphones.

Mechanical accessories include various kinds of acoustic devices such as windscreens and directionality modifiers. Windscreens, usually made of special foam or cloth, should be used whenever microphones are used outdoors or subjected to any air currents or rapid motion.

“Pop” filters are employed when the microphone is used close to the mouth, such as on lecterns or for handheld vocals.

These minimize noise caused by explosive consonants such as “p”, “b”, “t”, or “d”.

Although such filters are usually supplied with microphones designed for these applications, additional protection may be needed in some cases. Use only high-quality screens and filters to avoid degrading the sound of the microphone.

There are also directional or “polar” modifiers available for certain microphones that can change the pickup pattern form cardioid to supercardioid, for example, or from omnidirectional to semi-directional in the case of some boundary microphones.

Consult the manufacturer for proper use of these accessories.

Mounting accessories are of great importance in many worship facility sound applications. Stands, booms, and goosenecks should be sturdy enough to support the microphone in the intended location and to accommodate the desired range of motion.

Overhead hardware, to allow microphones to be suspended above a choir, for example, must often include a provision for preventing motion of the microphone due to air currents or temperature effects.

Stand adapters or “clips” may be designed for either permanent attachment or quick-release. “Shock mounts” are used to isolate the microphone from vibrations transmitted through the stand or the mounting surface, such as a lectern.

In addition, there are a variety of signal processors which may be used directly in line with a microphone. These can range from simple low- or high-frequency filters to complete preamp/equalizer/limiter units, though most of these functions are normally provided by the mixer and subsequent elements of the audio chain.

Creative use of these accessories can allow microphones to be placed almost anywhere, with good acoustic results and with acceptable aesthetic appearance.

(Copyright Shure Incorporated, used by permission.)

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