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Buying & Using: microphones

microphones: how to choose them and how to use them


If all microphones do is pick up sound and turn it into an electrical signal, why are there so many different ones to choose from? We guide you through the maze.


ON THE FACE of it, a microphone is a microphone is a microphone. Sound goes in at the thick end, and an electrical signal representing that sound comes out of the thin end. We don't plug in a different set of ears to listen to different things, so why isn't there just one type of microphone that can listen to everything?

The truth is that when we mike up a sound, we quite often do it in a way that wouldn't go down too well with the human ear. Imagine listening to a bass drum with your head actually inside it, or to a heavy metal vocalist screaming into your ear from half-an-inch away, and you'll get the idea.

However, choosing from the vast range of microphones available can be a nerve-wracking business, even for professional sound engineers. Many people simply plump for the models with the best reputations and then stick with them. But adopting this course of action could mean you miss out on important new developments in mike technology. And, more seriously, you could also end up using a microphone of a type that is completely unsuited to the task in hand.

SENSE OF DIRECTION



IN LIVE SOUND and recording, we generally want to keep the sounds of each musical instrument as separate as possible from each other. So we use what is known as a uni-directional microphone. These are also known as "cardioid" mikes because their pickup pattern (ie. the space around them in which sound can be picked up) is very roughly heart-shaped, but unless you happen to be a Latin heart surgeon, uni-directional will do nicely. As their name implies, these mikes are more responsive to sounds coming from one direction: the front. Sounds approaching from the rear and sides are still picked up, but at a lower level. Which is good for separation and, in a gigging situation, good for minimising feedback. Why? Well, feedback is caused when a mike "hears" too much of the sound coming from the speakers, and this sound goes round the system again, getting louder until it builds up into a scream or howl. If the mike is uni-directional and isn't pointed towards the speakers, feedback becomes less of a problem.

Should you specifically want a mike that picks up everything, then you'll need an omni-directional model (that's "all directions" for you non-Latin students). These are more often used in the studio or in broadcast work, because they give a very natural sound - but they can cause feedback in live use.

There are other microphone patterns, such as the figure-of-eight - which picks up at the two sides, but not in the middle. These were popular in the '60s because they could cover two backing vocalists at the same time; check out early Beatles film to confirm this. But in the present day, it's more usual to have a mike for each performer, so figure-of-eights are again confined to specialist studio applications.

So, to sum up what we've said so far, for nearly all live work and pop recording, you need a directional mike. But there are still hundreds of these to choose from, priced from around a tenner to over a thousand quid. Help!

In some ways, it's helpful to think of a microphone as you might think of a camera. All cameras take pictures, but some obviously take better pictures than others. (Of course, the quality of your pictures also depends on what you put in front of the camera, and the same goes for a microphone. Put rubbish in front of it, and you'll get nothing but rubbish out, no matter how good your equipment. But that, really, is another story.) In general, the wider a range of light intensities a camera can pick up, the better your picture will be. With a microphone the same principle applies - except that instead of light we're dealing with sound, and instead of intensities we're dealing with frequencies.

Now, our ears can hear a very wide range of frequencies, from the lowest bass drum right up to the subtle high overtones of cymbals. Frequency is measured in cycles per second or "Hz" for short, and the range of human hearing is from around 20Hz to 20KHz (that's 20,000Hz) depending on your age and the number of Acid House parties you've been to. To give you some idea of what these figures mean, the excruciating tone you get when you fall asleep in front of the telly late at night with an empty lager can on your knee is 1KHz.

It stands to reason that any microphone capable of faithfully reproducing what you feed into it must cover most or all of the audio range. As we're about to find out, not all microphones can do this.

DYNAMIC MIKES



MOST LIVE MIKES (and a good many studio models) are of a type called dynamic - sometimes called moving-coil on account of there being a coil inside them that, er, moves. If you've ever punched a hole in the front of your friend's hi-fi and examined the mangled remains of one of the speakers (come on now, own up), you'll have seen that the paper cone is attached to a fine copper wire coil, which normally lives in a narrow circular gap in the magnet at the rear of the speaker. In reality, the speaker works in exactly the same way as an electric motor - except the moving bits go backwards and forwards instead of round and round.

A microphone is very similar, except it works in reverse - like a dynamo. Sound makes a small diaphragm vibrate, and these vibrations are converted into an alternating electric current. In the case of a moving-coil mike, this is achieved by fixing a very light coil of wire to the rear of the diaphragm, and this again sits in a narrow gap across which is a magnetic field. Without getting too technical, the tiny electrical current induced in the coil is an electrical representation of the vibration of the diaphragm, and this current can be amplified to drive a concert PA system or a studio mixing desk.

Every type of microphone has its good and bad points. Dynamic mikes are physically tough, so they'll stand up to a good deal of abuse on the road. And they can take very loud sounds without distorting - essential for close vocal work or drum miking. They're also relatively inexpensive when compared with condenser mikes which we'll talk about later.

The bad news is that, light though the coil of wire might be, it's still heavy enough to make the diaphragm hard to move - and the faster you try to move it, the more the inertia of the coil tries to slow it down. This has two effects on sound quality. First, the signal level you get from a dynamic mike is quite small, and so needs a lot of amplification to make it usable. No problem if the sound is loud in the first place, but wimpy vocals or acoustic guitars mean you have to turn the amplifier or mixer gain right up, which also brings up the background noise or hiss. And in the studio especially, hiss is a no-no.

The second adverse effect is that high frequencies are not reproduced as efficiently as low ones; in fact, most dynamic mikes become very inefficient above 16KHz or so. This is fine for live work, but in the studio, that fine top-end detail may be lost giving a muddier sound than you'd get from a mike that could cover the entire audio spectrum.

There are a few more points to bear in mind about dynamic microphones. For a start, all uni-directional mikes become more bassy-sounding as they are brought in close to the sound source. This is simply an effect of physics called the "proximity effect" that I won't go into here, and some vocalists use it to their advantage by moving the mike very close to their lips when they want a warmer, more intimate tone.

As it turns out, mikes designed especially for vocals quite often have specially tailored frequency characteristics. For example, the very low bass end may be artificially cut to prevent popping and spluttering on Ps and Bs. Probably the most popular mike of this type is the Shure SM58.

A mesh pop shield is fitted on the end of vocal mikes, in an attempt to minimise breath noise, but these are not 100% effective. In the studio, an external pop shield is often made by the extremely hi-tech method of stretching a pair of tights over a wire frame, which is then placed a couple of inches away from the mike, between the mike and the vocalist.

A drum mike, on the other hand, may not have a pop shield as that would make it too bulky. And drum mikes may actually have their bass end deliberately enhanced to give a punchier sound. This is especially true of bass drum mikes such as the AKG D112, which has a little bass lift at around 80Hz to give a more powerful sound.

Going back to vocal mikes, these often have a slight boost at around 3KHz. This is sometimes known as a presence boost, and is used to help speech clarity and generally give a brighter, more penetrating tone. Different mikes have slightly different "presence" characteristics, so it pays to pick a mike that complements your vocal style.

And finally, vocal mikes are often hand-held, so it's essential that the working parts (the capsule) are properly isolated from the casing to minimise handling noise.

CONDENSER MIKES



THESE ARE SIMILAR to dynamic mikes in terms of the options offered, but they use a different method of converting the diaphragm movement into electricity. They work using static electricity and use a very light, metallised plastic diaphragm. Their output current is so small that they need a preamp in the mike body to boost the signal, so they either need batteries or "phantom" powering. Power is also needed to provide the static charge on the diaphragm.

Phantom powering is simply a way of feeding the necessary power from within the mixing desk along the mike's own signal cable. This system only works with professional, balanced mikes, so you're unlikely to encounter it in a small home studio or PA system.

The great advantage of condenser mikes is that they can reproduce high frequencies very accurately and, because of the preamp in the body, they have quite a high electrical output with low background noise. The drawback is that they generally cost quite a bit more than dynamic mikes. This, coupled with the fact that they need power to run, means that they are more often used in the studio than on stage.

You may have seen the term "electret" printed on the side of microphone packaging or on spec sheets, and the electret is, in fact, a very close relative of the condenser mike. The main difference is that instead of a power supply to provide the static charge on the diaphragm, an electret uses a special material which has the charge permanently built into it. The drawback here is that this material is much heavier than that used in a true condenser mike, so we're back to low output and limited treble response again - and the preamp still needs batteries.

Electrets were pretty much a non-starter until some bright R&D engineer decided to attach the charged material to the part of the capsule that didn't move, and use a regular condenser-style metallised plastic diaphragm for the bit that did. The result was the "back-electret" - a variety of microphone that can turn in virtually the same performance as a true condenser mike, at a fraction of the cost. Back-electrets still need batteries, but battery life can be hundreds of hours, so no real problem there. One of the best examples of the type is the AKG C1000S, which uses a standard 9V battery, is tough enough to take on the road, yet is also sufficiently sensitive to mike up even quiet instruments - such as acoustic guitars - in the studio. And the cost is about the same as you'd pay for an SM58.

ELECTRICAL MATTERS



HAVING COVERED THE main types of mike you're likely to encounter, there remains only the mysterious subject of "impedance" to cover. Mikes come in either high-impedance or low-impedance varieties. If you don't know what impedance means, just think of it as the electrical equivalent of gears in a car. Just as the right speed needs the right gear, a low-impedance mixing console needs a low-impedance mike, while a high-impedance device (such as a budget mixer-amp) needs a high-impedance mike to work properly. Always check the specification on your equipment, and if in doubt, ask your dealer. In very general terms, anything rated at between 50 ohms and 500 ohms is classed as being low-impedance, ohms being the measure of impedance (often abbreviated using the symbol Ω). An impedance of between 10KΩ (or 10,000 ohms) and 100KΩ or more is considered to be high. The advantage of low-impedance mikes is that they are less prone to interference or treble loss when long leads are used to attach them, but then, the low-impedance equipment they are plugged into is more expensive to design. Swings and roundabouts, I'm afraid.

Other terms you may come across in connection with mike specifications include "balanced" and "unbalanced". In practice, most mikes which have a three-pin connector in the handle are wired for balanced use and so will work with either balanced or unbalanced systems. A balanced system uses two cores in the mike lead rather than just the one, and special circuitry at the mixer end is used to make any interference picked up on the lead cancel itself out. So long as you use the right lead, you can use balanced mikes with unbalanced mixers or vice versa, but you'll only get the benefit of balancing if both the mike and mixer are wired for balanced operation and you use the correct twin-cored cable.

MONEY MATTERS



ITS VERY RARE that a cheap mike will give good results. And by cheap, I mean anything under £60. The main compromise with budget mikes is that the frequency response has all kinds of undesirable peaks and dips that make the sound seem boxy or unnatural, and also increase the risk of feedback. On the other hand, it's no good spending £1000 on a studio condenser mike if all you're using is a Portastudio or you're doing gigs at the local pub for a tenner a time. Be sensible, and pick a mike to complement the rest of your gear.

For vocals, try to get a mike that suits your voice rather than one that just looks good on paper, and check it out for handling noise and popping before you buy. Most vocal mikes will happily double up as guitar amp mikes or even drum mikes, but for bass guitar or bass drum, choose a model that has a bass response extending down to at least 40Hz and preferably lower. If you have a really good PA system that can reproduce the high end properly, you might benefit from a back-electret model. But bear in mind that performers are so used to the sound of dynamic mikes that an "honest" mike may sound thin by comparison.

And finally, it's often difficult (if not impossible) to check out a mike in a music shop. So arrange to hire one first, or get a sale-or-return deal, or try all your friends' mikes to see if they have one you feel comfortable with. Then, and only then, will it be time to get the wallet out.


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Jesus Jones

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Your First Gig


Phaze 1 - Copyright: Phaze 1 Publishing

 

Phaze 1 - Apr 1989

Feature by Paul White

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