Return to Zero (Part 7)
The intrepid Dave Simpson looks back over the basic types of microphone in common usage and touches briefly on the principles behind their operation. As he gets paid by the word, he's already planning to write a sequel.
Continuing the series, this month we take a look at the different types of microphone available and examine their applications.
Although some of you might have read the series of articles we ran by Wolfgang Staribacher last year detailing microphones and recording technique, so many people have written in asking for additional information (due to having missed the early issues) that we decided to pursue the subject further.
What is a Microphone? Basically, a microphone is a device that converts sound waves into electric currents, which can then be transmitted or recorded as necessary. To understand how it does this, it is helpful to look at a simple microphone that all of us use at one time or another.
The mouthpiece in your telephone consists of a diaphragm, behind which is packed carbon grains. One property of carbon grains is that their electrical resistance varies with the pressure applied to them. When you speak into the receiver your voice vibrates the diaphragm, compressing the carbon grains according to the nature of the sound. As the pressure on the grains varies, an electric current passed through them also varies and the resulting current 'copies' the pattern of the sound waves.
All microphones operate in much the same fashion - ie. the sound waves act upon a mechanism which is both sensitive to sound and able to generate electrical impulses... though carbon grains don't get used in recording microphones.
There are several different types of microphone, all of which work in different ways. A crystal microphone relies on the piezo-electric effect encountered in certain types of crystal (such as Rochelle salt). Here, a minute current is generated proportional to the rate of change of the size or shape of the crystal; in other words, if the crystal is deformed or squeezed, an electrical signal is produced.
A ribbon microphone consists of a light ribbon of aluminium foil suspended between the poles of a permanent magnet. The foil is vibrated by sound waves and its vibration between the poles of the magnet induces current to flow in coils wound round the magnets.
We will concern ourselves for the purpose of this article with five main types that you are likely to come across; dynamic, condenser, pressure zone, electret and transducer. A dynamic microphone (sometimes called 'moving coil') works on the same principle as a loudspeaker except that the process is reversed. A small coil of wire is suspended in a gap between the poles of either a permanent or an electro-magnet. The coil is joined to a diaphragm which vibrates it, and in a manner similar to the ribbon microphone, a current is induced in the coil. The condenser microphone consists of a diaphragm and a fixed metal plate that form a capacitor. The movement of the diaphragm varies the capacitance of the capacitor, and the variations are converted into audio frequency signals by an amplifier inside the mic (which is why this type of microphone requires a power supply usually known as phantom powering). The theory behind this type of mic is that if a capacitor holds a constant charge, and if the capacitor value is altered by changing the distance between the plates, then the voltage must change accordingly. The electret microphone is also a capacitor mic, but differs in that a strong electrostatic field is applied to the diaphragm during manufacture in such a way that a virtually permanent state of charge is set up. If an integrated circuit head amplifier is employed, only a small battery is required within the mic instead of external power.
The transducer type of mic usually consists of a small condenser or crystal mic which is physically applied to the object to be recorded, stuck by double-sided tape or blu-tack. No conventional diaphragm is required since the whole unit is vibrated, perhaps by a drum skin, or the box of an acoustic guitar.
Finally, the pressure zone microphone (or PZM) carries this principle a stage further, by incorporating into its design a primary boundary (the first reflecting surface a given signal encounters). This can constitute a much greater pick up area than any other type of microphone, and this results in a very clean and transparent sound, unsullied by the phase cancellation effects normally caused when a sound arrives at a microphone after bouncing off several walls or whatever is in the way.
So much for the various types of microphone you might encounter. I shall return to the subject of 'which mic for which job' later but before that, it is worth looking at the way in which a microphone picks up a sound wave and the area from which it does so. This is known as the mic's directional characteristic.
The response of a microphone to sound waves is different from that of the human ear. Whereas the ear can 'shut out' unwanted sounds (like being able to carry on a conversation in a crowded room ignoring extraneous sounds), a microphone picks up everything it is presented with. What is does not do however is to pick up sounds equally from all directions - it depends on how the microphone has been designed. There are five main patterns of response (known as 'polar' response - you can see why by looking at the diagram) - omni-directional, cardioid, hypercardioid, figure of eight and shotgun.
Response is not always the same over the whole range of frequencies, which is why polar diagrams often show plots of several patterns on one diagram. Generally, the better the microphone, the tighter the plot (the more the patterns coincide over the whole audio frequency range).
An omnidirectional mic, as the name implies picks up sound equally from all directions, and thus a polar diagram shows a circular pattern though in real life this is really spherical. Some omnidirectional mics exhibit a tendency to cardioid and even figure of eight patterns at high frequencies.
A cardioid response indicates that the mic is most sensitive at 0 degrees (directly in front) and least sensitive at 180 degrees (behind). Plotted on a polar diagram, the graph resembles a heart, hence the name cardioid. Hypercardioid microphones take this a step further, more effectively rejecting sounds from either side. This has the effect of pushing in the diagram at about 135 degrees, making the pattern look like a pea, balanced on top of a giant kidney bean (Always had a poetic streak this lad - Ed). However, rather than call it a 'pea and kidney bean' pattern, some stick-in-the-mud chose to call it hypercardioid.
The most directional of microphones is the shotgun type, with high rejection of any signal outside a narrow 30 degrees angle. It is called shotgun, because this type of microphone often looks a bit like a gun, with a long barrel housing the capsule. These are extensively used by spies, particularly those interested in ornithology.
The final pattern; figure of eight looks exactly like it sounds when plotted. The mic is most sensitive at 0 degrees and 180 degrees (in front and behind) and least sensitive at each side.
So far so good. To recap, there are several different types of microphone, and there are several response patterns available. There's just one more thing...
Many people assume that the purpose of a microphone is to pick up sound waves and transmit them as accurately as possible, and at one time, microphones were designed in this way. As with polar response patterns, a diagram can also be plotted of a microphones frequency response. (See diagram). Nowadays, a top quality microphone will show a very flat graph, indicating a singularly flat response. More and more though, 'tailored' response microphones have appeared over the last few years. There are two major reasons for this. Firstly, it takes an expensive microphone to achieve a flat response, and if a microphone is 'budget', and will be coloured anyway - why not try to colour it in a particular fashion where the colouration serves some purpose? Secondly, for some applications, certain frequencies can often improve the sound if cut or boosted - for instance vocals sometimes benefit from a presence lift of 3-7kHz and a bass cut of 200-300Hz, helping to improve intelligibility. Of course, using a flat response mic in a studio, such adjustments can be made on the desk or via a graphic or parametric equaliser. In a live, or budget situation though, remedial equalisation might not be up to the job, or even absent altogether. Why not tailor a microphone response to fulfill this at source so to speak?
A further feature of some microphones - the 'proximity effect' is also used by microphone designers. This is an effect by which bass frequencies are overemphasised at close working distances. Not all mics exhibit this, but most types do (with the exception of omnidirectional and some dynamic cardioid mics). To compensate for this, some microphones incorporate a bass cut or bass roll off switch, allowing the user to have the choice. The reason that all microphones are not designed to automatically compensate for this is that firstly, it is dependent on distance, which cannot be anticipated in the design, and secondly, it is sometimes desirable - for example, the emphasis of the bass end on a vocal mic might be needed to add warmth to the sound.
So, whilst walking past your friendly local secondhand shop you spot a microphone just begging to be purchased. Further enquiries reveal that it is a dynamic cardioid mic with a frequency hump at around 100Hz. (Obviously the shop assistant reads HSR!) Having read this article, you know how a dynamic microphone works, you know roughly how it will respond to sounds in its immediate neighbourhood, and you know that it will be slightly bass heavy. Is it the microphone for you though?
If you think about it, by putting all the information I have given you together, you can probably work it out for yourself, but I'll give you a few hints anyway. Condenser microphones tend to have fairly flat frequency responses, and so are most 'transparent'. They are though, at least in the budget range, susceptible to overload, so some incorporate pad switches or else have to be located further from the sound source. Because of this, if you can only afford one microphone, it is probably not as good a bet as say, a top quality dynamic mic. If you have some dynamic mics already, a condenser microphone will prove invaluable for things like overhead and snare mics on a drum kit, or for some vocal applications - anywhere fragile over-tones or harmonics need to be captured, so they are great on acoustic instruments as well (except some of the cheap electrets).
Dynamic mics can handle greater sound pressure levels, so they are indispensable for miking up instrument cabs, close miked vocals and of course, the inside of the bass drum. Cheap dynamic mics can sometimes be used to good effect on toms where high frequency response is not needed. Due to their rugged construction, if you have to drop microphone, drop one of these!
Dynamic microphones are also usually the ones with tailored frequency response (either by accident or design!). If you can, check out the peaks and troughs, and equate them with your requirements.
Electret mics are often very cheap, but beware - although the high frequency end may sound good, they often break-up with surprisingly low signal levels, and a cheap electret microphones ability to handle bass can be suspect. They are not as robust as dynamics, so don't chuck them around too much.
The time to use a transducer, is when absolute isolation (no spillage between instruments) is a priority, such as live acoustic guitar, or perhaps toms within a drum kit. Bear in mind that a pre-amp wil be needed though, which pushes up the cost. Pressure Zone mics on the other hand, are to be used when spillage is no problem - indeed, when it is an asset, as in the case of ambience mics. I say when spillage is no problem - a PZM has a large pick up area, and it is important to ensure that no external noises (cars, planes, tanks etc) intrude into your recording. PZMs are great for dispensing with a multi-mic situation - a drum kit for instance, or as a stereo pair in front of an acoustic group. They are very transparent so make sure the drum kit sounds good before you start - you won't be able to do much afterwards, without affecting everything else.
Polar response patterns are useful when determining where the signal is that you wish to be recorded in relation to other signals. Omnidirectional microphones are useful where spillage is no problem (recording acoustic guitar, grand piano etc). Cardioid, hypercardioid and shotgun mics progressively isolate the signal you wish recorded from other instruments. Cardioids can be used in the majority of applications, giving some protection against spillage. Hyper-cardioids would be used where two closely spaced instruments need isolating from each other - snare from hi-hat for instance. Shotgun mics are designed for long distance recording, ignoring extraneous sounds, and not often used in the studio.
Finally, figure of eight microphones are normally used not in order to adapt to a situation, but rather, when a recording situation can be adapted to fit their specific qualities. Singers, or instruments can be positioned facing each other with the microphone between them, rejecting sounds from the side, but picking up the voices or instruments.
Some mics can synthesise various polar patterns by means of two diaphragms; one omni and one figure of eight for instance. If you do find one of these being sold cheaply then don't concern yourself with pros or cons, just contact me and I will buy it off you (you're probably talking about an AKG C414 or a Neumann U87 - very nice)!
Finally if in doubt, remember the old home recording homily; he with the biggest microphone doesn't always pull the best birds!
Feature by David Simpson
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