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Sound Advice (Part 1)

All about mikes – Part 1 of IT's unique guide.

How much do you know about microphones? Could you get a better sound if you understood how they worked? Would it assist if you knew how to use them properly? How do you choose which type to buy? Gary Cooper answers these and other questions in IT's new three-part series - SOUND ADVICE.

Generally speaking, the more you know about how something works, the better able you are to get the best from it. Perhaps more importantly, you don't run the risk of buying the wrong tool for the job if you have at least a basic understanding of what it is you're buying.

Possibly because we are (the majority of us) musicians first and singers or PA/recording engineers second, the amount of attention we pay to microphones is minute compared to the care and attention we lavish on our instruments. Fortunately, mikes are in essence quite uncomplicated devices, and so it's relatively easy to learn enough about them to make the correct choices when buying and then to use them to their best advantage. So, what do you need to know?

How They Work

For the majority of live applications, the mike type we most commonly use is the 'old faithful' dynamic. At its worst, the dynamic mike (sometimes called a moving coil type) is that tatty-looking Hong Kong effort you picked up in a jumble sale for a couple of quid; at its best it's a supremely strong and fine- sounding instrument, epitomised by definitive onstage products like Shure's near-legendary 'all-purpose' SM57 or AKG'S rapidly selling D321. In essence, a dynamic mike works rather like a loudspeaker in reverse. Without getting unpleasantly technical, sound is probably best defined as the detection by the human ear of variations in air pressure - vibrations, in other words. Just as the ear detects these changes by having a 'drum' which vibrates in response to fluctuations in air pressure, so a dynamic mike is also a pressure-operated device where a diaphragm with a coil of wire attached to it vibrates between the poles of a magnet. As sound waves move the mike's diaphragm, so an electrical current is induced into the coil; a current which varies in proportion to the diaphragm's movement. This current is then, in effect, modified and amplified by a mixer's circuitry before being powered up to a sufficient level to drive a loudspeaker. I said this was going to be kept simple, didn't I!

As you can imagine, one of the most important aspects of dynamic mike design, as a result of this operating principle, lies in getting the best frequency response to changes in sound pressure out of the coil/magnet combination. If this wasn't the case, then the mike would have a grossly disuniform response across the frequency range it needs to cover to be able to do its job properly. In other words, high frequencies might sound too loud compared with lows. On the other hand, balancing the ability of a mike to respond across the frequency range in various ways is the key to developing a new product. The individual characteristics of each mike can usually be read from its frequency response chart, showing how much bias it will have towards any particular part of the audible sound spectrum. Working from this principle, a vocal mike, for example, might (quite deliberately) exhibit a rising 'boost' effect at certain frequencies, designed to give a voice extra 'balls'. As an example of how well this can be manipulated, look at Shure's definitive SM58 vocal mike, the response graph of which is hardly flat (i,e., equal at all frequencies), but which has been specially tailored to emphasise certain frequencies, thus giving the mike its much sought-after vocal dynamism. Generally speaking, dynamic mikes have a lot of advantages which particularly lend themselves to uses on stage. They're usually extremely strong, they're reasonably sensitive, not too expensive to manufacture, and can have extremely good frequency response (in other words, they will handle from low to high frequencies fairly evenly). Probably nine bands out of ten these days will be using dynamic mikes on gigs, and you regularly see them used on 'live' television, especially Rock and Pop programmes. The other most commonly encountered kind of mikes which you will come across are called 'condenser' (or sometimes 'capacitor') types. It used to be the case that these were reserved almost exclusively for professional recording or broadcast purposes, but the ever-increasing drive for higher quality sound on stage (not to mention the improvements in PA systems) has resulted in a growing number of this variety creeping through onto professional stages.

To generalise, condenser mikes tend to be expensive, but offer the advantages of being extremely sensitive and having exceptional frequency response - closer to that of the human ear than most dynamic kinds, for example. To some extent they are more fragile than dynamic mikes; at least they have been in the past, although the efforts of the leading mike manufacturers have made them considerably more robust than they once were.

In operational terms, the basic condenser is also pressure operated, and consists of two plates; the one at the front being the diaphragm, the one at the back being fixed. A polarising voltage has to be applied to this capsule (hence the need for either battery power or electricity transmitted as 'phantom' power from the mixing desk), after which pressure fluctuations will cause a variation in the capsule's capacitance, which results, effectively, in a voltage change.

As recording applications are inevitably demanding, a vast amount of work has gone into the design of different condenser mikes, and although the above description of how one operates is accurate in essence, there are very many possible variations which - I'm delighted to say - we don't need to go into here!

Just for the record, you will often come across a mike being described as an 'electret' condenser. This breed operates in an almost identical manner to the basic condenser I've already described, except that it's permanently charged - in other words, it doesn't need either battery or desk-derived power to work. Usually they're rather non-descript beasts beloved of Japanese Hi-Fi makers, but they don't require an outside polarising current and hence can be very useful, albeit in some rather limited applications. You won't come across very many of these either on stage or in studios, however, it generally being true that the phantom or battery powered condenser is by far the better of the two types.

Good examples of condenser mikes (both of which are suitable for stage uses) would be Beyer's superb flat response (vocal) MC734 or AKG's switchable response pattern C414 (just about perfect for live piano applications). An important vocal quality condenser with unusual toughness and a surprisingly low price for such ability is to be found in E-V's BK1.

Polar Whethers

So, now you know that there are really only two basic mike types which you're likely to be concerned with, the dynamic and the condenser. Does this mean that we only have two choices to make? Oh that it did! Before we even begin to start considering such questions as ruggedness and sound quality, we need to understand a fair bit about what the engineers call 'polar response'. Polar, in this sense, doesn't imply cold weather and furry white bears preying on Eskimos - what it means is the response pattern around a fixed pole. It's here that we begin to find those often confused terms 'unidirectional', 'cardioid' and 'omnidirectional' cropping up. Fortunately, it's easy to understand which is which - I say fortunately because it's vital that we know what's what.

Without delving too far into why, individual mikes exhibit very definite pickup patterns of their own. One will reproduce all the sounds arriving at it from whatever angle, another might just pick up those which occur directly in front of its head. There, with no apparent effort, we've just understood the terms omnidirectional and unidirectional - the omnidirectional accepts, with no discretion, any sound occurring around it regardless of its source, while the unidirectional, taking a partial approach, accepts only sounds coming from one direction. Having grasped that, we can now begin to see why we need to know which type we need. Obviously, a singer who hand-holds a vocal mike won't want it to pick up the sound pouring out of front-of-stage monitors. Equally, they won't want their mike to gather in not only their own vocal sound but also that of the drummer behind them. What's called for here is a unidirectional type. In fact, as most modern stage miking calls for getting sounds to a mixer or mixer amp in as separate sections as possible, unidirectional mikes are far and away the most common types we encounter.

But (no doubt just to create a little more confusion for us poor bewildered musicians!) there are several different variations of pickup response pattern which all come under this general unidirectional description. By far the most common pickup pattern encountered today is that termed 'cardioid', or heart-shaped. If described thus, a mike will pick up those sounds occurring to it within a heart shaped-field. Sounds hitting the mike head-on are picked up with the greatest sensitivity, while those striking it from the sides will have a much reduced response, and those from the back are almost ignored. Some manufacturers also use terms like 'supercardioid' or 'hypercardioid' to describe their mikes' response patterns, and this implies an even more restricted pickup area. To be absolutely truthful, though, I'd have to say that the practical differences between what one maker calls cardioid and another terms hypercardioid seem to be minimal in practice.

Typical high quality cardioid (or related) responding dynamics would include Audio-Technica's ATM41a or Beyer's remarkably accurate 'hypercardioid' M88.

Having said that most stage mikes are unidirectional, and having shown why, it might seem unlikely that there would be any need for any other type of response pattern. In fact (at least where nearly all Rock, Jazz and Pop stage work are concerned) this is virtually true. Omnidirectional mikes are most often creatures of the recording studio, where their abilities to pick up frequencies from all around can make them very useful when ambient sounds are required, those where a live feel is to be captured, or where several widely spaced instruments or voices are to be recorded with one mike. These types do, however, serve on stage for some drum miking applications, especially when placed inside shells. Examples of omnidirectional mikes are pretty rare, but one of the most affordable (and one that is superb for home and lower cost recording uses) is Shure's PE25. Alternatively - and especially useful placed inside drums - is Electro-Voice's PL9.

Even then, it can be that some other pickup response patterns are very effective, as we found when testing Beyer's HM10 headset mike (see Issue 7) where its 'figure of eight' response worked remarkably well. For the most part, though, we're nearly always choosing between unidirectional and omnidirectional response pattern mikes, and which we go for will depend on whether we want to pick up and reproduce a very tightly restricted sound angle, or a more broadly spread pattern.


Series - "All About Mics"

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