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Using Microphones


If there is one field where the actual writing of advice, is even more secondary to actual experience, this is it! But some ideas can be imparted and the general approach indicated. There is a great deal of personal preference amongst engineers in the choice of microphones and where to place them. Perhaps, too many of us are rather fixed in our ways and if that is so, it should encourage newcomers to experiment, for in the end, it is what it all sounds like, that matters.

Figure 1. Typical mic polar patterns with response nulls.
(Click image for higher resolution version)


Newcomers will of necessity have to work with whatever mics are to hand. It's as much getting to know a mic, for what it can be used and the compromises involved in its use. But for all the effect that a given mic and its placement in a particular acoustic has, this is NOT the starting point of this article.

Monitoring Arrangements



Yes, we do start at the back end of the total system. The best possible playback set up is demanded if you are to make judgements of what you are doing. Now that sounds like a tall order and could prevent one starting off! So, rationalising my thoughts, I urge that you use a known set of loudspeakers, properly set up, off the floor, away from room corners and angled to a central listening position. None of the one speaker behind the sofa and the other on a shelf behind a vase of flowers!

Use a system that performs well on professional releases of the type of music you are imitating. Get used to the overall frequency balance obtained and any attractive aspects (or otherwise) in the sound. If possible listen to the same recordings elsewhere — spot the differences. This is all by way of making sure your monitoring is not far from an acceptable norm. For instance, it will not be the first time that a bass light monitoring system has resulted in bass heavy recordings as compensation is made when positioning, making the instrumental balance and equalising. Similarly poor judgements can be made in the very important mid range area and in the general amount of high frequency. So the plan must be to get to know your monitoring system, the effect of the room you are using, before all together believing in what you hear or think you hear.

Initial Survey



This overview on mic usage cannot be a complete instant injection of wisdom. This article is strictly about using mics, but one needs to know of the different types, the terminology associated and the generally accepted plus and minus points.

Moving Coil

Reputation for robustness, easy to get a presence peak, possibly curtailed response. Also known as dynamic mics.

Electret Capacitor

Reputation for being less robust? Nuisance of battery powering although externally powered types exist. Higher output than moving coils. Cleaner and more extended HF than moving coils. HF peak, if present, usually further up the range.

'Proper' Capacitor

The preferred mic for'quality'. Usually the most costly. Robust — as studios are full of them! Usually powered by phantom feed. Variable polar pattern either on the mic or remote possibly. Generally flat and wide in frequency response but response tailored vocal versions exist. Much higher output than other types. Low colouration lends to mixing desk equalisation for special effects.

Ribbon Mic

A largely obsolete mic design. Reputation for low level output. Mainly figure of eight. HF smoothness was a feature of several classic designs.

Carbon Granule

Used in telephones.

Tailored Response

A mic with a presence peak and probably a basic colouration which does or doesn't suit a particular application together with the monitoring and/or PA speaker characteristics. Usually moving coil type.

'Classical' Mics

Flat response types with accurate polar patterns at all frequencies. Usually proper capacitor type. Preferred usage as coincident crossed pairs. Aimed at acquiring 'natural' uncoloured pickup with coherent portrayal of the acoustic image.

Spaced Omnis

Favoured in the USA for classical music recording. Superficially gives an exciting acoustic pick up but produces very phased variable instrumental positioning and a hole in the middle, ie. two mono channels.

Cardioid

A mic with a heart shaped pickup pattern and a maximum null at 180°. There is usually 15 to 20 dB front to back ratio, sometimes varying over the frequency range.

Figure Of Eight

The classic directional pattern as it was so easily produced with the ribbon mic. Two equal lobes front and back with nulls at 90° and 270°. The nulls are particularly sharp and deep with figure of eight. Double diaphragm 'proper' capacitor mics can provide figure of eight facilities.

Figure 2. Microphone frequency curve examples.


Hypercardioid

Sometimes called Supercardioid. Sort of midway between cardioid and figure of eight — main lobe to the front and small one to the rear. Nulls around 150° and 210°.

Omni

A mic with no particular directional characteristic. In practice the high frequencies will be somewhat directional.

Rifle Mics

Highly directional at the mid and high frequencies. Many irregular frequency dependant side lobes. Tendency to omni at LF, negated by LF roll off.

Polar Pattern Frequency

For good crossed pair performance the polar pattern must be similar at all frequencies. Any HF beaming or LF looseness will mar the stereo created. Similarly, any spurious lobes or lack of tight control will make a directional mic prone to acoustic feedback in PA use.

Presence Peaks

Provided on tailored response mics (or unavoidable on cheap ones!). Intended to get vocal penetration in complex mixes.

Proximity

The rise in LF response with very close up usage of most directional mics. Some designs mitigate against it by careful acoustic design.

Colouration

The characteristic sound, particularly in the MF and upper LF range, of a particular mic. Not necessarily manifest in the mics frequency response curves. Due to various mechanical and acoustic cavity resonances and the compromises needed to achieve other 'attributes'. Particularly part of a directional moving coil mics performance. The subject is complex as the colouration can vary with different polar angle. Colouration particularly shows up in AB comparisons, but there is a danger in such instant comparisons — a flat mic can sound coloured after direct comparison with a presence peak type!

On Axis/Off Axis

Refers to a directional mic's performance in line with its main pickup lobe (on axis) and how it performs outside this area (off axis). Directionality may not be uniform over the frequency range — there may be HF beaming or off axis peaks which colour the sound or provoke feedback.

Handling Noise

Least with omni mics and the plague of any directional microphone. Some pretence of reduction made by some manufacturers by LF roll off, internal elastic suspension or special out of phase cancellation coil in the moving coil case.

'P' Blasting

Sibilant popping or 'P' blasting is due to the diaphragm being moved by actual air movement. But a diaphragm has to be coupled to the air molecules for it to be excited by the sound field! So its an inherent problem. Much less with omni mics as the rear of the diaphragm is 'sealed'. Directional mics are more prone as the rear of the diaphragm is open and in consequence susceptible to air movements. Lumps of foam are poor ways of reducing the problem. Better to create a non air movement space around the end of the mic by an acoustically transparent barrier or barriers spaced away from the business end of the mic. Singing or speaking 'across' the mic assists also.

Absolute Phase

Relates to the positive or negative parts of a waveform. A 'positive' in the sound pressure wave should similarly produce a positive, in the electrical output of a mic — on usually what is called the 'hot' pin.

Strictly the same principle should apply throughout the total record/reproduce chain, amplifiers and loudspeakers. Out of phase mics will interact when close together and colour the sound due to the cancellations at different frequencies produced.

Phasiness

Refers to a problem in stereo reproduction where, for example central images are indistinct and change their nature with small movements from the listeners central position. Can be due to poor loudspeaker unit layout, the loudspeaker room position or to multi-mic phase additions and subtractions. If a recording problem it can give poor mono compatibility.

Next issue we continue with a few more practical definitions of such things as impedances, balanced connections, phantom powering, etc. Then we will be ready to cover the practical usage of mics in a few situations.



Previous Article in this issue

Multitrack

Next article in this issue

Choosing A Cassette Tape


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

 

Home & Studio Recording - Sep 1983

Topic:

Recording


Feature by Mike Skeet

Previous article in this issue:

> Multitrack

Next article in this issue:

> Choosing A Cassette Tape


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