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Recording Techniques (Part 2)

PART 2: Microphones. David Mellor cuts through the mythology of microphone techniques with some practical advice.


However convenient it may be to direct inject an electric guitar or bass, or plug a synthesizer straight into one of the mixing console's line inputs, there is a certain 'something' about a sound produced by the natural vibration of timber or metal, that travels through the air and finds its way on to tape via a microphone. In contrast, one of the negative aspects of today's synth and sampler based recordings is that they tend to have a 'samey' quality about them. After all, one man's 'Breathy Chiffer' (a much-used Roland D50 preset) cannot be very far removed from another's.

In an ideal world - or at least a world constructed for the benefit of creating quality music - there would be no preset sounds, and synths would be supplied like blank sheets of paper for their owners to write upon in their individual sonic handwriting. In the real world, however, manufacturers have to create presets to show off what their gear can do, and it is only human nature that most of us end up using them.

WHAT'S SO DIFFERENT ABOUT MICS?



Recording real sounds with a microphone is another matter entirely - there are so many variables that it is almost impossible for two recordings to sound exactly alike. Of course, an acoustic guitar will always sound like an acoustic guitar, but not nearly as much as Factory Preset 56 in Tartan Studio, John O'Groats, will sound like Factory Preset 56 in Laidback Studio, Los Angeles.

Let's look at just what these variables may be when you record with a microphone: First, the instrument. If you play any acoustic instrument, you will know that no two models are the same. Even if they came off the same production line on the same day, after a few month's wear and tear they will perform and sound differently. Every instrument has its own character. Assuming that we are talking about an acoustic guitar, another variable is the brand of strings used, and their age. As a new set of strings settles in, the tone changes from twangy to bright to mellow to dull - time to get a new set! The type of plectrum used also affects the sound - whether it is thick or thin, stiff or flexible.

As well as the guitar, another important factor is the guitarist. There is a world of difference between the sound of Mr Average Guitarist playing a simple E chord, and a master virtuoso. Compare that to the case of an electronic keyboard, where a note played by a master would not sound any different to a note played by an orang-utan.

From the engineer's angle, just as all instruments are different, so all microphones are different. Two mics may be very very similar when new, but like instruments they will age differently - microphone diaphragms get dirty, and different studios produce different types of dirt. However, the most important variable factors in acoustic recording are the studio and the engineer. All studios and rooms sound different, and different parts of the same room will have a different acoustic. It is one of the engineer's jobs to position the instrument and microphone suitably - not just in relation to each other, but also in relation to the room. Consider the simple situation of one instrument, one mic and one room, and imagine how many different ways they could be physically arranged during recording. When synthesizer manufacturers say that their instrument has "an infinite range of sounds", they are obviously seriously underestimating the concept of infinity.

So far, I have been talking about purely acoustic instruments, but the electric guitar can become an acoustic instrument if it is plugged into an amp and speaker so that it can be recorded via a microphone. Once again, there is a tremendous increase in the range of sounds that can be obtained in this way when compared to simple 'direct inject' techniques. Indeed, there is no reason why synths and samplers should not be amplified and miked up - it all makes for greater diversity.

MICROPHONE TYPES



Figure 1. Microphone polar patterns.

Not all microphones are suitable for making recordings, but the good news is that most modern types are. As microphone technology improves, quality mics are to be found at lower and lower prices - but obviously there is a limit beyond which you are not going to get much satisfaction, unless you are aiming for a special effect. The choice of microphone for any recording is very much an artistic choice. In other fields of sound engineering there may be other important factors, such as size, appearance and robustness, but as a recording engineer, you have a free choice of all the different types of mic you can lay your hands on.

There are several different basic microphone designs, and they all have different sound characteristics and will find different uses. The main categories are dynamic and capacitor (also known as condensor). There is also a type of capacitor mic called an 'electret' and a type of dynamic mic known as a 'ribbon'. The capacitor microphone is blessed with a very clear sound, rich in high frequencies. The dynamic mic is usually less clear, but can give 'body' to the sound when used appropriately.

Both capacitor and dynamic mics are available with different 'polar patterns'. The polar pattern of a mic describes its sensitivity in different directions, the four main types of response being Omnidirectional, Cardioid, Hypercardioid and Figure Of Eight. Figure 1 illustrates these, but bear in mind that these are two-dimensional sketches of patterns that exist in the three-dimensional real world.

FREQUENCY RESPONSE



Figure 2. Typical on-axis frequency response (the slight rise at high frequency is exaggerated for clarity).

The major factor in determining the sound quality of any microphone is its frequency response. A good quality microphone pointing directly at a sound source will have a frequency response similar to that shown in Figure 2. This is called the 'on-axis' frequency response. As you can see, the frequency response is very nearly flat - the slight rise in the high frequency range is due to reflections from the diaphragm interacting with the incoming sound, and in good quality mics this tendency is well under control.

Figure 3. The polar patterns at different frequencies of a nominally cardioid mic.

However good this response is, matters are complicated by another microphone characteristic - the 'off-axis' frequency response. The top manufacturers can make mics with ruler flat on-axis responses that will capture perfectly the particular balance of frequencies produced by any instrument. But the frequency response to sound sources which are not precisely in the mic's 'line of fire' is, for all manufacturers, a tougher problem.

Figure 3 shows the polar pattern of a typical microphone at three different frequencies. At 1 kHz, you will see that it corresponds to the cardioid pattern shown in Figure 1. But at high frequency, the response tends towards the hypercardioid - ie. it becomes more directional - and at low frequencies it tends towards omni. Shown as an off-axis frequency response plot, the result is Figure 4. This is not a flat frequency response by any standards, and even very high quality mics exhibit this problem.

Figure 4. Typical off-axis frequency response.

You may think that if you are pointing the mic directly at the sound source, then the off-axis response doesn't matter. Well, this is true if you are recording at a distance from the source in the open air, or in an anechoic chamber, but in a real room or studio, the reflections from walls, floor and ceiling make a significant contribution to the quality of the sound. If I were to define what makes a 'good' mic, then I would say that a flat on-axis frequency response, coupled with a good off-axis response, is the thing to have.

That is my view, but a good mic could also be defined as one that had the right sort of character, rather than a perfect response. By 'character', I mean an interesting combination of defects - some mics I could name have defects in abundance, but they sound great. They don't capture a sound with perfect fidelity, but the way in which they change the sound is musically useful. For example, I was told by a BBC engineer a few years ago that the World Service favoured old-fashioned ribbon mics for their broadcasts because they could cover more square miles on the other side of the world with good intelligibility than they could using more modern and more 'accurate' types.

One particular 'defect' which is worthy of mention is present in microphones which have a large diaphragm (the diaphragm is the mic's 'ear drum'). All diaphragms have a resonant frequency - a frequency at which they will naturally vibrate - which colours the sound. In small diaphragm mics, this resonant frequency will be above the audio band and thus will be to all intents and purposes inaudible, but there are many mics in popular use that have a diaphragm an inch or more across, and their resonance peak falls within the audio band. The effect that this has on frequency response is controllable, but the colouration isn't, at least not totally. But very often this colouration makes the sound more interesting and alive, which may be exactly what an engineer is looking for. For this reason, many antique microphones that should in theory have been superseded eons ago are still in common day-to-day use, while super accurate hi-tech models are lounging in their foam nests in the microphone cupboard.

TECHNIQUE



'Microphone technique' is a phrase that conjures up an image of a veteran recording engineer lecturing his young trainee about exactly where to point the microphone for every instrument ever invented, as if there are laws laid down by Act of Parliament. From my own experience as a sound engineer, and from my involvement with other engineers, I know that mics that work for one person and methods that work for one person don't necessarily work for another. I have my favourite microphones and I like to think that I can get a good sound from them in a variety of circumstances, but there are other mics that I just don't seem to be able to get a usable sound out of. On the other hand I have heard recordings using those same mics, made by other engineers, which are brilliant.

Although there are no rules on microphone technique, I can give some hints on how to go about miking up instruments in general. The first thing to do, when faced with an acoustic instrument to record, is to engage the brain in first gear and release the clutch - a little thought can go a long way towards getting the sound you want to hear. Instruments, as you know, come in a variety of shapes and sizes, from the tiny piccolo to the six feet tall double bass, and each instrument radiates a sound field in its own particular way. For example, an acoustic guitar radiates a thin metallic sound from the strings, a middle frequency sound from the body, and a mellow tone from the sound hole. The accompanying photographs show an AKG C451 microphone in two positions. The first picks up a very full, bassy sound, while the second picks up the brightness of the strings in a larger proportion. Either mic position can be useful. I took the photographs during a session where the commonly-used sound hole position was just too bassy to fit in with the rest of the track. I could have cut the bass with EQ, but I tried this and it didn't sound nearly as good as the alternative mic position for this particular track.


So, Point To Remember Number 1 is that all instruments emit different types of sound from different parts of their bodies. Another example of this is the flute, which has a breathy sound when miked close to the player's lips and a more mellow sound from other parts of the instrument. Any mic position may be suitable for the music you are recording, it is just a matter of experimenting.



"Microphone technique is not a set of hard and fast rules to be followed. It is something that you learn as you progress as an engineer."


You will notice from the photographs that the microphone is very close to the guitar. All instruments sound different according to the distance between mic and instrument, for two reasons. The first is that the further away the mic is from the instrument, the more reverberation or ambience will be picked up from the room. That is fairly obvious. The less obvious point is that since all points of the instrument radiate with a different sound quality, the further away you get from the instrument, the more you pick up the sound from its entire body, and integrate these different sounds. Figure 5 shows how it works with a saxophone. The close mic picks up just the sound from the sax bell because, in comparison, the sound from other parts of the instrument is too soft - the parts that radiate it are too far away. The distant mic, on the other hand, is more or less equidistant from all parts of the instrument and picks up the whole sound.

Figure 5. Close and distant instrument miking.


To put this in as neat a nutshell as I can, if you want a natural sound, use the mic at a distance of at least 1½ times the maximum dimension of the instrument. This will pick up the instrument sound as the ear normally would. If you want a sound with 'character', place the microphone close to the instrument, and remember that different parts of the instrument will have a different sound quality.

There is one more aspect to the distance between instrument and microphone that deserves consideration, which is that the amount of room ambience that will be picked up at any given distance depends on the polar pattern of the mic. An omnidirectional microphone will pick up much more room sound than a hypercardioid at the same distance. This is a simple point, and is explained very quickly, but it is very important to be aware of this when positioning the mic.

To summarise, microphone technique is not a set of hard and fast rules to be followed. It is something that you learn as you progress as an engineer, and will probably develop differently to other engineers. The only rule is to experiment. That way you will find the techniques that suit you, and eventually you will develop a mental database of techniques which will enable you to very quickly obtain the precise sound you want from any instrument.

MULTIPLE MICROPHONES



Put two people together in the same room for any length of time and, sooner or later, an argument will develop. Like people, microphones argue too. Two identical microphones picking up one sound source will hear the sound differently, simply because they are located at different points in space, and mixing together the sounds from the mics often produces a worse sound than using just one mic. On the other hand, there are situations where the use of multiple mics can be beneficial, or even essential.

A favourite technique of many engineers is to use a close mic and a distant mic at the same time, to combine the clarity of the former with the warmth of the latter. This doesn't cause too many problems because the two mics are picking up sounds which are very different, and which can therefore be easily mixed - just experiment with the mic positions and mix to taste. The classic source of multi-mic problems is recording a drum kit. Recording drums is probably the most difficult situation an engineer ever has to face, simply because of the number of mics active at the same time. But why use so many mics on a drum kit in the first place?

In the early days of recording, one microphone might have been considered adequate for recording the entire kit. In those days it was adequate, because that particular technique suited the sounds and styles of the period. One mic, or a pair for stereo, on a drum kit can produce a very good representation of what the kit sounds like in real life, but this is not what modern music often calls for. Today's recordings demand extreme clarity from each drum and cymbal, and to achieve this clarity we need to use one mic on each drum, a mic on the hi-hat, and two overhead mics to pick up the cymbals. Even on a small kit, that adds up to eight or so mics. Unfortunately, clarity is reduced by the fact that each mic picks up a little of the sound from every drum and cymbal, and when all the mics are mixed together the result can be less clear and distinct than using just one mic!

The solution is to use directional mics (not omni) and place them close to the drums - there is a simple rule of thumb that there must be a 10-to-1 ratio between the distance of the mic that is intended to pick up a particular drum and the distance of the next closest mic. Some negotiation with the drummer may make this possible. Also, if the mixing console has a phase reverse facility, it is often worth experimenting with the phase of the various mics. Sometimes the complex interaction between the distances of the mics and the frequencies produced by the drums may cause phase cancellations that may be at least partially cured by a push of the appropriate phase reverse button on your console.

Of course, there is a lot more to recording drums than this, but these are the basics, and everything else builds on this. For the moment, I shall ignore problems that might occur with the drum kit itself and leave them, along with some more sophisticated drum recording techniques, for another installment.

There is a whole world of experience waiting for the engineer who wishes to get his hands dirty and abandon the nice clean world of direct injection synths and samplers. So far, I have covered the fundamentals of the subject, but there is much more to tell and as this series progresses I shall be passing on further microphone-related hints. But remember that, above all else, the best thing to do is to experiment and discover the creative possibilities of the acoustic world.

BASS 'TIP UP'

Any microphone, other than an omnidirectional type, will exhibit the phenomenon sometimes called 'bass tip-up' (or 'proximity effect') when placed close to the sound source. This is due to the curvature of the sound wave front, which is more marked at a close distance from the source, and the way in which it interacts with the diaphragm. Often, the more bassy sound offered by a close non-omni microphone (ie. cardioid, hypercardioid or figure of eight) is 'warmer' and preferable to a strictly flat response. The engineer will take this into account when choosing a mic.


STEREO RECORDING

Figure 6. Crossed pair of directional microphones. Figure of eight mics should cross at 90 degrees, cardioids at approximately 120 degrees.


There are several well-established techniques for making simple stereo recordings. Figure 6 shows the 'Crossed Pair', which can be used with all types of mic except omnidirectional. Since both mics are in the same place, the stereo effect depends on the mic pointing to the left picking up more of the sounds coming from the left-hand side of the sound stage than the mic on the right. This configuration produces a clear stereo image which is mono compatible, although it doesn't give the feeling of spaciousness that other techniques offer.

Figure 7. Spaced omnidirectional microphones. Separation is from about one metre up to about half the width of the 'sound stage'.


Figure 7 shows the 'Spaced Omni' technique, which is sometimes supplemented with a centre mic. Despite having 'omni' in the name, it can be used with all types of mic. This technique does not give precisely localised images, but does give a full spacious sound. Usually, it doesn't combine as well into mono as the Crossed Pair.

Figure 8. Spaced crossed pair of microphones.


Figure 8 shows a combination of the two - the 'Spaced Crossed Pair' to give it an unofficial title. With a separation between the mics of a few centimetres, it has all the advantages of the Crossed Pair with added spaciousness. It can be used successfully with a spacing of up to one foot or so - any more than that and it approximates to the Spaced Omni technique but risks losing instruments placed centre stage.

In Figure 8 you may notice that the mic on the left is actually pointing right and that the mic on the right is pointing left. Some engineers would say that this is incorrect and that it ought to be arranged so that the left-hand mic points left and the right-hand mic points right. I happen to prefer this arrangement and make no apology!


Series

Read the next part in this series:
Recording Techniques (Part 3)



Previous Article in this issue

The Big One

Next article in this issue

Kawai K4


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 - Jan 1990

Donated & scanned by: Mike Gorman

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Topic:

Microphones

Recording


Series:

Recording Techniques

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7


Feature by David Mellor

Previous article in this issue:

> The Big One

Next article in this issue:

> Kawai K4


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