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Using Microphones

Mic definitions and terminology.


In the first issue of Home Studio Recording we emphasised the need for a good monitoring system and started a list of the terms associated with microphones and their usage. This month we complete the glossary, taking in such areas as mic placement, impedance, and connections.

Pressure Zone Microphone

A specialist mic consisting of a small capsule (usually a small electret capacitor) arranged next to a flat plate, close enough to respond to the sound pressure at the phase coherent 'boundary'. PZM is a registered trade name of Crown/Amcron, so other manufacturers call their versions Boundary or Hemi mics, the latter name being derived from the microphone's hemispherical pickup. The microphone's LF performance can be extended by mounting them on a larger flat surface, eg. a floor, wall or piano lid.

Soundfield Microphone

Another specialist mic, the SFM is unique in having four capsules mounted in a tetrahedral array, from which different polar patterns can be derived. It is also the preferred microphone source for ambisonic surround-sound and periphonic recordings.

Dummy Head

The name given to a microphone placement system in which a pair of small omni mics are placed ear-to-ear distance apart, baffled from each other by a disc about six inches in diameter or by a similarly sized head-shaped block. Out of head sounds are easily produced this way, particularly to the sides and rear. Out front sounds, however, can be difficult to resolve.

Holophony

An 'improved' version of the above, although its inventors do not associate the term dummy head with their system. The technique utilises a more detailed model of the human head as the baffle in order to produce true out of head sounds, or 'holophonics'.

Mono Compatibility

Multimic mixes that are not mono compatible can produce cancellations of parts of the complex mix when the recording is listened to in mono. In the worst cases, an instrument or vocal can completely disappear when a stereo recording is summed into mono if the original feeds to the two stereo channels are out of phase.

Input Impedance

Resistance to AC of, for instance, an amplifier, or a microphone, when it is known as Source Impedance.

Load Impedance

The impedance presented by a load on a circuit, eg. a loudspeaker on an amplifier or mic pre-amp on a microphone.

Low Impedance Microphone

Nowaday's mics with a source impedance of between 200 and 600 ohms. This is not a critical figure and in any case is likely to vary with frequency, particularly with moving-coil mics. Long leads can be used without HF losses. Typically, low impedance mics should 'see' an input impedance of perhaps five times their source impedance.

High Impedance Microphone

Mics with source impedances between 10 and 15 kilohms, moving-coil only. These usually have a transformer inside the mic in order to raise its impedance to the higher value. Usually connected (with short leads only to avoid HF losses and hum pickup) to impedances in the 20 to 50k range.

Unbalanced Line

A method of connecting the mic to its desk using a single signal wire with the screen of the cable as the return. It's prone to 'common mode' interference noise and hum pickup on cable lengths over a few metres.

Balanced Line

In which two signal wires, twisted and screened, feed a balanced input mic preamp, the latter using either transformer or electronic balancing. Due to self-cancelling of any interference fields, there is very good rejection of common mode interference noise and hum. Balanced connections also allow long cable lengths, but can only be used with low impedance inputs.

Phantom Powering

This is necessary for quality capacitor mics and some electret types. Power is fed in parallel down each signal wire and extracted at the mic end by either a transformer or electronic circuitry. Return is along the cable screen. The power is used for the mic's internal preamp, and also to polarise the capacitive transducer of capacitor mics. While the majority are a nominal 50 volt DC feed, some phantom powered mics work over a 9V to 50V range. Balanced source moving-coil mics can safely be connected to a phantom powered input, and there can be no 'phantom' current flow through the microphone.

Mic Pre-amp Essentials

1) Low noise. The circuit should ideally add less than 1dB of noise. Good systems have a noise level of -125dB or better at a line signal level of 775mV and an input impedance of 200 ohms.

2) High Overload Margin. A low impedance moving-coil mic used at around one metre from a normal speaking voice will produce 1-2mV of signal level. On the other hand, a close-up multimic system for a drum kit can produce upwards of 200mV, and capacitor mics give even higher levels. Hence a mic pre-amp should have switchable (or continuously variable) input sensitivity. This will also increase the 'headroom' available before overload. Care should be taken to set the sensitivity according to the particular microphone usage.

Close And Distant Pickup

In classical music recording, it is normal to include in the pickup a degree of the acoustics of the recording venue. Simple crossed-pair coincident mic techniques are usually the most successful in achieving this. The greater the distance between microphones and musicians, the greater the amount of ambience in the pickup. In other musical fields, however, it is general practice to place the microphone close to the instrument. As this destroys the natural recorded ambience and also tends to exaggerate the instrument's harmonics. Stereo balance, acoustic and equalisation are all added artificially using a mixer and additional outboard effects.

DI Box

A device by which electronic instruments (guitars, synths, drum machines) can be connected to the balanced mic input of a mixer. A mic level signal is produced via a balancing transformer within the DI Box.

XLR Connectors

The type of mic and cable connector employed almost universally in the professional field. The design is both robust and reliable! There are three pins: pin 1 being earth or screen, pin 2 the 'hot' or positive going signal and pin 3 the other signal connection. Also known as 'Cannon' connectors after the original manufacturer. Note also the convention of 'pins pointing in the direction of signal' in the use of male plugs and female sockets in mic connections.



Previous Article in this issue

Tascam 32

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Interconnect


Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.

 

Home & Studio Recording - Nov 1983

Donated & scanned by: Mike Gorman

Topic:

Microphones


Feature

Previous article in this issue:

> Tascam 32

Next article in this issue:

> Interconnect


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