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How It Works - The Microphone (Part 5)

Continuing our 'How It Works' series, David Mellor lays bare the facts about the applications and workings of the commonest types of microphone.


There is no greater repository of folklore and myth in the audio industry than the microphone cupboard. Many a tale has been told about "the man who put a U47 on the dulcimer!" What became of him? Was he ever seen in the Lamb & Flag again? For the answers to questions other than these read the true story of the microphone, by David Mellor.

"Pass me the MC 740N (C/5) P48, and make it snappy!"

Yes, it's a real microphone made in Germany by Beyer Dynamic - and very good it is too. Sometimes I wonder why you don't get names like the 'Neumann Golf or the 'AKG Sierra' in the microphone world. I can't cope with all these numbers and not all microphone sales reps can either.

Some people would have you believe that there is something called 'microphone technique' that you have to learn to get anywhere in this business. Of course, there is - the technique they learned over the years by much trial and error. But there is no one microphone technique that you have to subscribe to. The more microphones you are able to use and the more different ways you use them, the better. A few simple tricks can form the basis of a repertoire of techniques which can help you cope with anything. I do not intend to delve too deeply into how microphones are used, that would be a whole series of articles in itself, but some explanation is necessary to appreciate why microphones are the way they are. So why are some mics long and thin, some short and fat? It's a good question which I shall answer in the proverbial fullness of time.

MIC TYPES



Microphones can be differentiated in two ways, by their mode of operation and by their polar pattern. The two modes of operation in common use today are dynamic (moving coil) and capacitor (sometimes called 'condenser'). Microphones can be made in other ways, which are probably of more use to British Telecom than to a recording engineer, so I shall concentrate on the meat of the argument rather than the vegetables.

A dynamic microphone works in much the same way as the dynamo that used to power your bicycle lights before Duracells were invented. They may not look too similar but it's amazing how versatile those good old laws of electricity can be. Named after Thomas Dynamo (only kidding !), a magnetic field surrounding a moving coil of wire causes a current to flow.

It's the same in a microphone. Sound vibrations in the air strike a diaphragm (imagine cling-film stretched over a jam jar, but about half an inch across) to which a coil is attached. The coil is in a magnetic field, so a current proportional to those vibrations is generated which flows all the way down your mic cable to the mixer. Now you can go and build the Mark II version of the 'baked bean tin' telephone that you use for studio talkback!

It may surprise you to learn that you can send audio into a microphone and hear sound from the diaphragm. DON'T TRY IT. I did hear the tale (more folklore) of the theatre sound engineer who got annoyed with someone blowing down an orchestra foldback mic during an interval. Quick as a flash, he re-routed it to the mixer talkback output and sent an expletive down the line to the offending microphone abuser. Apparently it did the trick! Seriously, although I do not recommend that you try it with anything from your own collection, some mics are designed to be used in this way for communication purposes. An interesting aside.

The good thing about dynamic microphones is that they are cheap. Don't pay less than a hundred pounds and expect to get quality though, I meant cheap for a microphone. Some cost over £1000.

Dynamic mics are also generally fairly robust. Roger Daltrey may have given up swinging his Shure around his head but I'll bet the mic is still going strong! Sound quality can be a bit dubious with dynamics. Clarity is not their strongest suit but if we're talking rock-n-roll, who needs it? I heard a rumour that Status Quo are buying up any examples of a particular mic, that isn't being made any more, that come on to the market. I won't mention what it is 'cos I want a couple myself and it may push the price up.

I can't leave the topic of dynamic microphones without a word or two about ribbon mics. They work in a similar way to ordinary dynamics but use a narrow metallic - or metallised - ribbon to perform the function of both diaphragm and coil. Sound quality can be very good but they can be delicate. Beyer Dynamic make some tough ribbon mics (which I have given the drop test to!) which are worth investigating.

An early AKG C12A capacitor mic with external power supply box.


If dynamic mics are the Ford Transits of microphonekind, then capacitor mics range from the previously mentioned Sierra, all the way up to the Porsche and Rolls Royce bracket. These will cost you a few pennies more but for anyone serious about recording it is essential to have at least one available. Prices start at below £200 for an AKG C451/CK1 which I can recommend as a good everyday capacitor mic. (See what I meant about the numbers?)

The great advantage of the capacitor mic is that the diaphragm does not have a dirty great coil on its back to lug around, backwards and forwards. The diaphragm is charged up and it works by the principle of voltage changing with changing capacitance. A bit of algebra would make it clear but you can find that in a textbook - you know, those books where they give you the equations and miss out all the interesting bits! Having a light diaphragm means that the capacitor mic is more responsive to incoming sound, which accounts for its clarity.

The disadvantage is that it needs to be powered. Not only does the diaphragm need to be kept charged up, but the mic is only capable of producing a feeble electric current which couldn't find its way to the end of the cable without assistance. There has to be an amplifier built into the body of the microphone, which needs to be powered in some way of course. Batteries are a possible solution but you wouldn't want them to run down at an inopportune moment, would you?

Early capacitor microphones used external power supply boxes which plugged into the mains, like the AKG C12A in the photo. The earlier C12 model had to be plugged into two separate boxes and the mains before you could get any sound out of it. Obviously, this was not too convenient. Help comes in the form of 'phantom powering'.

Phantom powering is so-called because you can't see it. Eh? Take microphone in left hand, plug in cable. Plug other end of cable into mixer. Mic works. So where's the power? Actually, it's in the mic cable. Phantom powering uses the microphone's ordinary audio cable to carry the necessary 48 volts. Figure 1 not only shows you how it works, if your mixer doesn't provide a phantom supply just build one of these and away you go.

Figure 1. A 48V phantom power supply for a capacitor mic.


Don't worry about plugging your dynamic mic into a phantom powered line, by the way. Unless it is unbalanced, which is only the case with cheap mics these days, it will come to no harm.

There are other microphone powering schemes besides the phantom type. If the secondhand mic you fancy says on it anything like 'Tonader' or 'AB', give it a miss. These are powering methods used in Europe and are not at all common in Britain.

A form of capacitor mic which needs power only for the internal amplifier and not to charge up the diaphragm is the electret. An electret is like a magnet but instead of having a permanent magnetic charge, it has a permanent electric charge. Imagine a battery that never ran down. The charge is sealed in though, so it cannot supply any current. In a microphone of this type, the diaphragm is made of an electret material and needs no voltage supply. A tiny battery can power the amplifier.

A relatively new kind of microphone, which could in theory be dynamic, capacitor or electret, is the boundary effect microphone - often known as the 'pressure zone' microphone (PZM) after the first example of its type. The theory is that if you put a mic in a room, then it will be subject to interfering sound reflections from nearby walls, which will have an undesirable filtering effect on the sound. If, however, you place the mic within a distance of seven or so millimetres from a wall (or boundary) then all reflections will be supportive rather than interfering. (The figure of seven millimetres or so comes from half a wavelength at a frequency of 20kHz.) Whatever the technical arguments, the fact is that boundary effect microphones have a very distinctive sound which makes them a useful tool in one's armoury. The only problem is that they are not happy mounted on a stand. If you don't have a convenient boundary to Gaffa tape your mic to, you will have to make one. Try something around one metre square to start with and work up from that. The bigger the boundary, the more realistic the bass.

POLAR PATTERNS



Nothing to do with the Arctic or Antarctic. The polar pattern of a microphone is all about how sensitive it is to sounds from different directions. It is instinctive to point microphones at whatever sound you want to pick up but, as we shall see, there may not be as much benefit to be gained as you might think. Let's run through the commonest patterns...

OMNIDIRECTIONAL
There is currently a school of thought that a directional microphone is better than a non-directional microphone. Twenty years or so ago, the cognoscenti theorised in exactly the opposite way. The fact is that it is easier to make an omnidirectional microphone. All you have to do is make sure that the diaphragm is open to the outside world on one side only. The other side is sealed. To make a microphone sensitive to one direction only (unidirectional), access to the back of the diaphragm has to be carefully controlled. Hence the fact that a couple of decades ago, when mics were not so highly developed, omnis were considered better.

Boundary effect microphones are omnidirectional, although the race is on to produce other types.


FIGURE OF EIGHT
This is the characteristic pickup pattern of the ribbon microphone. It can be produced by other means though. A popular theory says that you can make a perfect stereo recording using two 'figure of eight' microphones. It works out on paper, which is enough for some people to insist on using this technique. It's just a pity that ears can't read.

CARDIOID
Popular fallacy number one: cardioid microphones (ie. those with a heart-shaped pickup pattern) are sensitive in one direction only. Wrong !

If you look at the cardioid polar diagram you will see that, far from only being sensitive in one direction, it is only not sensitive in one direction. This is something you learn very quickly when miking up an orchestra; those bassoons get everywhere.

Popular fallacy number two: omnidirectional mics are always omnidirectional and cardioids are always cardioid. Wrong again! Fact is that at high frequencies omnis tend towards directionality and at low frequencies cardioids tend towards omni. It's a good manufacturer who can keep these tendencies at bay to any extent.

HYPERCARDIOID
Like cardioid but more directional (see diagram). It's a pity this useful design is not more common. Not to be confused with 'rifle' mics which you see film crews using - these are optimised for directionality and other factors, such as off-axis frequency response, are liable to be compromised.

The types of microphone I have listed so far are nearly always the long, thin ones. Short, fat ones may not look as, er... whatever you want them to look like, but they are often multi-pattern designs.

It's a clever trick of science that if you put two cardioid microphone capsules (that's the bit that contains the diaphragm) back-to-back, you can combine the outputs in various ways to produce different overall polar patterns. Normally this will be done with a switch to give a range like the following: omni - cardioid - hypercardioid - figure of eight. The advantages are obvious and have to be paid for. Think about £500 or so upwards.

The disadvantage is not so obvious but you end up with a mic that is not optimised for any one pattern. One way round this is available relatively cheaply in the AKG 451 series and more expensively in the Schoeps Collette series (Aha! A microphone with a name!) - interchangeable capsules. These are usually available for less than half the cost of the equivalent cardioid or figure of eight microphone.

TRIAL & ERROR



Figure 3. Classical recording techniques.

So much for microphone types but how do you use them? Well I said earlier that trial and error is the best teacher but there are a few tried-and-tested tricks of the trade. Let's start off with a few classical tricks which are good for recording choirs and orchestras.

If you are American you will swear by this technique, we British traditionally swear at it but if you listen to any recordings on the Telarc label you'll hear something like it - and it will probably blow you away! Take two omnis...

Actually, take three omnidirectional mics (as in Figure 3) because if you move two omnis too far apart then you get a sort of 'hole in the middle' effect between the speakers, which is difficult to describe. The third (centre) mic is brought up in level just enough to fill the hole.

On this side of the Atlantic it is more common to use a crossed pair of directional microphones. I think hypercardioids are best for this application, though good quality cardioids can also work well. A figure of eight response produces a good sound from in front of the microphones but reverberation hitting them from the sides causes effects which to me are unpleasant. Spacing the mics apart by a few centimetres can improve the stereo quality of any crossed pair recording.

If you think I am being a bit stingy with the quantity of microphones I am using, I am talking about situations with very good acoustics where you have lots of time to find the exact spot for the mics. My principle is only to use extra mics if there is a problem with the basic set-up - not enough harp, for example. More mics often means more 'mush'.

Recording popular music is different - the aim is to find new sounds, so there is little point in apeing someone else's methods. One point deserves a mention, however.

I avoided saying earlier that a directional microphone will give a level boost to the bass frequencies when used very close to the sound source. This does not imply that there is anything wrong with its frequency response, it is a result of the curvature of the sound field hitting the microphone diaphragm unevenly. This might seem like a drawback but actually, it's very good for vocals. Whereas an omnidirectional mic will give a thin (some would say 'realistic') vocal sound, a cardioid or figure of eight will give a wonderfully rounded tonal quality (wonderfully rounded bottom end, if you prefer). An unfortunate side-effect of this bass boost is an increased tendency to make a 'popping' noise on 'P' and 'B' sounds - a foam windshield will help prevent this. Using a cardioid mic, some of the natural reverberation from the studio will also be rejected, giving a more intimate and more easily processable sound.

Some studio engineers and producers have favourite mics for different jobs. The AKG D12, for example, has often been quoted as the best bass drum microphone. I like using the Neumann U47 for vocals - I just wish I could afford one!

EVALUATION



So you want a new microphone. How do you choose? If you are running a high-class studio then it's easy - you buy what the producers keep asking for. No brain power involved in a decision like that, but if you are running on a tighter budget how do you know what to buy?

I can think of three ways of evaluating microphones, one of which is often done but doesn't give meaningful results.

The easy way, if you are into natural recordings of real instruments, is to set up a pair of the microphones you fancy in front of a willing musical volunteer. A little experimentation with positioning will soon tell you whether you can get a natural sound from them.

It is harder choosing a mic for electric instruments because you are unlikely to be going for a natural sound. The only real way to test a mic is to use it over a period of time for different purposes, then you will find out its good and not-so-good qualities. (Why don't dealers offer a try before you buy policy?) The microphone test that doesn't work is the comparative test. This is where several microphones are set up and pointed at the same sound source. The object is to see which one sounds best and, of course, there will always be a winner. It is not a good idea to make a judgement based on information like this because you are not comparing like with like. A microphone responds to where it is with respect to the instrument and in relation to the room. By comparing microphones in one place, the best-sounding mic would always be the one that happened to 'be in the right place at the right time' as they say. I have tried this method, moving the mics around, but there are so many variables that the ear quickly becomes tired and the exercise pointless.

Setting up several mics is a good way of getting an interesting sound to record - and only that. There is no easy way to choose a good mic, you have to use them in varied situations to find out their capabilities.

CLASSICS



Before I leave the topic of microphones I must mention the classics. Like the '57 Stratocaster and the Minimoog, there will always be the excellent model that the manufacturer has seen fit to 'improve' or withdraw from production. The Holy Grail of microphones varies according to who you talk to but chances are it's going to be an old valve job.

Mics such as the AKG C12 and Neumann M50 are like gold dust. Pigs to use because they are old and temperamental, and usually need separate power supplies. The reason they are sought after is because there is something so appealing about the sound they make which doesn't show up in the specifications. Modern microphones have it all on paper but they very often don't have it where it counts.

One manufacturer even went as far as to bring out a new 'old' valve microphone - AKG's 'The Tube'. Good luck to them. Join the 'bring back the bottle' campaign, that's what I say. As long as they don't bring back the baked bean can!


Series

Read the next part in this series:
How It Works - Loudspeakers (Part 6)



Previous Article in this issue

Digital Recordings

Next article in this issue

Fix it in the Mix


Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.

 

Sound On Sound - Jul 1987

Topic:

Microphones

Sound Fundamentals


Series:

How It Works

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 (Viewing) | Part 6 | Part 7 | Part 8 | Part 9 | Part 10 | Part 11 | Part 12


Feature by David Mellor

Previous article in this issue:

> Digital Recordings

Next article in this issue:

> Fix it in the Mix


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