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Bring the Noise

If you intend to make samples of acoustic instruments or events, you're going to need a microphone - but which one? David Bradwell checks the various types and their uses.


All you wanted to know about microphones, but were too shy to ask. This guide to mics, their operation and application should help get your acoustic sampling house in order.


MICROPHONES? WHO NEEDS them? Singers do, guitarists might and drummers probably should. But synthesisers plug straight into mixing desks without any fuss, don't they? There are no amps to be miked up, and feedback? Never heard of it! The humble mic is very low on the shopping list of the average keyboardist, or at least so it would appear.

In the real world, microphones are far from humble, as anybody who has forked out £3,000 on a Calrec Soundfield will tell you. Additionally, as samplers have fallen in price and risen in popularity, more and more keyboard players have found themselves requiring a facility for natural sound recording. The result is that sales of microphones to the keyboard fraternity are on the increase, but the sheer diversity of mics available means that many people don't know exactly which one will be best for them.

Mic Basics



THE MOST COMMON type of mic in popular use is the dynamic or moving coil mic, and it works on the same principle as a loudspeaker but in reverse. In other words you have a diaphragm attached to a coil and the coil moves in a magnetic field. Then, according to Faraday's laws you get a current out of the coil proportional to the movement.

The main advantages of dynamic mics are that they are relatively inexpensive to make and therefore buy, they are very tough and reliable and they can handle high sound levels - the Sennheiser 421 can handle 170dB - proven by a test on a tank range with live ammunition. On the minus side, the moving parts are relatively heavy, meaning inertia is high. Consequently high frequency response falls off above about 15kHz - not really a problem for drums, but it may be for anything with a lot of top end, like an acoustic guitar. In addition, they're not particularly sensitive and therefore don't give a lot of output, leading to possible noise problems.

Top end microphones are normally of the condenser variety. These have a very light metallised plastic diaphragm which forms one plate of a condenser, which is polarised by a high voltage (usually from a separate power supply or a phantom power supply on the mixing desk). The main advantage of condenser mics is that they're very sensitive, so they work well on low signals. Also, because the diaphragm is very light, they have a good high frequency response. Unfortunately, all but the most recent condensers have tended to be fragile, and they're prone to going wrong in a humid atmosphere. They're expensive to buy and need a 48v power supply. Newer condenser mics can be almost as tough as the dynamics, but they still need phantom powering.

Electret microphones used to be very cheap and nasty - the sort given away free with particularly unpleasant cassette recorders. These are like condenser mics, but instead of requiring an external power supply, the charge is permanently built into the molecular structure of a plastic membrane. They still need pre-amps but this can be run by batteries of a relatively low voltage. The weight of the chargeable plastic leads to similar problems to those of dynamic mics.

Back-electret microphones are similar, except the permanently charged plastic is situated on the back plate - the non-moving part of the capsule - with a standard condenser-type diaphragm on the front. This means that the high frequency response, efficiency and sensitivity returns to that of the condenser mic, but you can get away with 1.5v or 9v batteries in the handle. Back-electret mics are generally fairly cheap, so for the same price as a Shure SM58 dynamic you can pick up a back-electret like the AKG C1000S - a directional vocal mic, sensitive enough to use for recording guitars, flutes and so on - but at a quality unavailable a couple of years ago.

The final main type of microphone is possibly the most intriguing of them all. PZM (Pressure Zone Microphone) mics are extremely cheap, and work on a principal known as the "boundary effect". When the sound gets to any boundary within a room, the air molecules can no longer move. This gives the minimum velocity and maximum pressure of a sound, and if you locate a mic capsule at a boundary on a flat plate, it translates those variations in pressure into electrical energy. One of the nice things about PZM mics are that they don't seem to discriminate between direct and reflected sound, so you don't get the "boxy" effect you normally get when somebody is a long way from a mic. Because they rely to some extent on reflected sound to work, they need to be on a large boundary to reproduce low frequencies, typically something at least a metre square. One tip to increase PZM sensitivity is to remove the 1.5v battery connections provided and solder on some 9v battery clips. Running PZMs off a 9v battery gives a further 6dB of output and, in turn, allows more headroom.



"Beyer produce 'Tour Group' mics whose non-reflective matt black appearance reduces glare on stage but adds little to their standing as sampling mics."


Directions



ALL MICROPHONES COME in range of directional patterns, except PZMs. The most common type for stage use is the directional or cardioid mic, so called because it picks up sounds mainly from in front, and less from the sides and behind. This is good for excluding unwanted noise from behind and also helps to keep feedback down. All of the directional mics are also known as pressure gradient mics because they work on the difference in pressure between the front and the back of the capsule. This gives rise to a phenomenon called the proximity effect, which means that if you get closer than a few inches the bass response rises noticeably. This is a characteristic many vocalists use to obtain a bit of extra punch and to give a warmer, more intimate sound.

This doesn't happen with omnidirectional mics, which pick up from all directions. They tend to give a more natural sound with acoustic instruments because they pick up all the subtle reflections from around a room.

It's also possible to make a pattern called a figure of eight which picks up at both sides of the mic. These are often used for recording two vocalists at once or for recording bass drums. PZMs pick up in a hemispherical pattern in front of the boundary.

Sampling



IF SAMPLING IS your main priority, you ideally want a microphone with a flat frequency response, rather than one with an artificial presence peak designed for one specific type of instrument. If you've got a cheap sampler, with only around a 10kHz bandwidth it doesn't make a lot of sense to spend a lot of money on a condenser mic. If you're sampling sounds on a high quality sampler with a lot of bandwidth, a good microphone becomes essential. For sampling it seems that you can get much better results out of PZMs than you can out of dynamic mics costing four or five times the price. Upgrading from PZMs is a sizeable and costly step. Dynamic mics are well suited to sampling vocals and drums but little else if quality is of primary importance, although for breathy vocals, a condenser mic would probably yield better results. Audio Technica produce a range of good budget condenser mics, while AKG have their popular C1000S.

Mic techniques for sampling are exactly the same as for recording the same instrument in the studio. If it's a drum, close-mic it - a position about two inches from the head with the mic angled slightly towards the centre of the head is usual. For an acoustic guitar, point the mic somewhere near the body end of the neck, two or three feet away, and preferably in a room that's a little bit live so you pick up some of the room ambience. Instruments without any of the natural acoustics can sound dead, a situation which even digital reverb can't always cure.

Even in an ideal world you can get by in the studio with only two or three basic types of microphone. All you can really generalise on is that if it is a quiet acoustic instrument you need a sensitive mic. Most acoustic instruments have quite a lot of top end response, especially string instruments, and so a condenser or back electret is preferable. If it's something loud and fairly mellow like electric guitar, (which is already restricted in frequency range because it's gone through an amp that doesn't put out much above 3kHz apart from distortion), there's no point in having a mic which records above 15kHz.



"If you have a cheap sampler, with around 10kHz bandwidth it doesn't make a lot of sense to spend a lot of money on a condenser mic."


And the same applies for a miked-up bass guitar. Normally you'd use a directional mic to exclude the sound you don't want, but if you've got an instrument that sounds good in a particular room - like a triangle in a church for example - you'd have to use an omni-directional mic to pick up the rich acoustics as well as the sound.

One of the problems of sampling vocals is "pops", which are caused by the blasts of low frequency air produced when pronouncing 'B's and 'P's. A coat hanger with a pair of tights stretched over it positioned between mic and singer is an extremely effective and cheap cure for this. Foam windshields may look professional, but they're not particularly effective in a studio because they have to work very close to the mic capsule. They stop spit and lipstick clogging up the grille, but that's about it. Another trick is to move the mic slightly away from the singer's mouth, for example in line with the bridge of the nose, so that most of the air is moving slightly under it.

For sampling cymbals, a condenser mic will definitely be best, because you won't get the appropriate brightness out of a dynamic. Violins can be sampled with condensers, or again with a PZM.

If you want to sample in stereo, a good method is to use two PZMs, one on each side of a square metre of plywood, hung up so that the edge of the plywood is facing the performer.

One thing to beware of when sampling is underestimating the amount of bandwidth required, particularly in respect of percussion sounds. Listening to a tom tom, you might assume that the note it makes is only a few hundred cycles and, therefore, little bandwidth will be required to record it adequately. However, the attack portion of the waveform - the percussive part of the sound - is extremely fast, and usually contains harmonics throughout the audio frequency range. If the frequency bandwidth is restricted, the decay of the tom tom will still sound natural but the dynamics of the attack portion will be lost. There are very few natural sounds which don't actually require a full bandwidth; even bass guitar played in the slap-and-pull style generates a lot of harmonics in the attack portion.

Studio Secrets



IF YOU HAVE access to a compressor it's worth using it to compress a signal as you sample it. This keeps peaks down, so you can get a higher average signal level, and that, in turn, means less noise. The other thing to think about is the movement within a waveform. Even when you have a constant sound, like a violin, the waveform has a lot of peaks and troughs as the harmonics beat with each other. When you try to loop a sample, you may end up trying to join a point of high amplitude to a point of low amplitude and you get a glitch. If you compress the signal, everything becomes much nearer to the same level, and a successful loop is a lot easier to achieve.



"For sampling it seems that you can get much better results out of PZMs than you can out of dynamic mics costing four or five times the price."


Noise gates are useful for reducing background noise. Set the gate at the same level as the sample threshold, and nothing will go into the sampler until the sound you actually want occurs.

Another problem is that of a sound with a very slow attack - you might find that the automatic trigger on the sampler doesn't realise sound is coming in until you've lost a lot of the sound's character. In this case, put an artificial click into the sampler just before the sound starts - to activate the trigger - and then use the edit facility to trim it off again afterwards.

Pick of the Crop



SO MUCH FOR the basic types of microphones and their application in sampling. When you're looking to buy a mic, you have to decide whether you want to buy new or secondhand. Unlike synthesisers, most of the best mics of the last 20 years are still being made and so you have a fairly free choice. Here are a few names and numbers to look out for.

Probably the most famous of all mics are the Shure SM57, SM58 and Unidyne B. The SM58 is dynamic and a favourite for rock vocals. It gives a warm vocal sound, with a peak at around 3kHz, and is often used to give a mellow quality to "edgy" voices. The Sennheiser 421 is also a good dynamic vocal mic. It's been around for 25 years without any improvements at all, adding weight to the argument that designers got most things right with dynamic mics 25 years ago. Bass drum mics still tend to be dynamic and the AKG D12 is a particular favourite. It has a figure-of-eight capsule, a large-diameter membrane (for a good low frequency response), and a peak at around 80Hz which gives a lot of "kick" to bass instruments. It gives the sort of '60s/'70s bass drum sound that's currently fashionable - plenty of punch but no attack. AKG have recently brought out the D112 which is an egg-shape mic, giving a much more percussive bass drum sound. Beyer also produce a range of "Tour Group" mics, so called because of their non-reflective matt black appearance which reduces glare on stage but adds little to their standing as sampling mics.

In most recording studios, vocals are recorded with Neumann U87s which cost around £900. Also in widespread use is the AKG C414, which is a little cheaper and renowned for giving a very transparent, bright sound. Alternatively, there is the new Sennheiser MKH40, which is a cardioid RF condenser mic.

The new generation of back electrets give almost the same quality performance as a condenser for about the price of a dynamic, so they would seem to be the mics to aspire to if you're in the medium budget range. Prices start around £100.

Secondhand bargains weigh in at around half to two-thirds of full retail prices. It's possible to pick up used SM58s quite cheaply and Sennheiser 421s are also available. You're less likely to see back electrets secondhand because they're a more recent innovation. Upmarket condenser mics are something of a rarity, but at a price between £500 and £1,000, they're not the sort of thing that many people need to buy for sampling.

Calrec have some fairly cheap condenser mics out which are fairly well worth looking at. At the other end of the scale, the Soundfield costs £3,000 and has four capsules in it, so it picks up ambi-sonically (something like quadrophonic but more subtle). This means that, by recording all four capsules on separate tape tracks, you can change its apparent direction characteristics after recording. Depending on how they're reconstituted, you can focus the sound retrospectively. In case you wondered why Rick Astley and Kylie Minogue sound the same, the Calrec Soundfield is a big favourite down at PWL.



Previous Article in this issue

Competition

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Toa MR8T


Music Technology - Copyright: Music Maker Publications (UK), Future Publishing.

 

Music Technology - Feb 1989

Donated & scanned by: Mike Gorman

Feature by David Bradwell

Previous article in this issue:

> Competition

Next article in this issue:

> Toa MR8T


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