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Looking at Microphones

A microphone converts sound energy into an electrical signal, and as such is the first device in the audio reproduction chain. Unlike the other parts of the reproduction chain (tape recorders, amplifiers, speakers) microphones are not often seen in the High Street shops, thus leaving a large number of people unaware of their importance. Indeed, a microphone will probably place more characteristics on the final sound than any other item in the chain. This implies that the choice of microphone is critical and that it is well worth while taking some trouble to establish which microphone(s) is best for the job in hand, particularly as one or two microphones will really stand out in any one application.

The array of microphones available is considerable, and while the products of three manufacturers (AKG, Beyer Dynamic and Shure) dominate the scene, several other manufacturers contribute on a smaller scale. As a rule the characteristic of the microphone is dependent on the 'cartridge' employed and so little is gained by putting a cartridge in another mounting; unlike the domestic loudspeaker market where small companies all over the place are designing their own boxes around standard products.

Figure 1. Cutaway view of a typical microphone (Shure Unidyne B)

A microphone consists of several parts (Figure 1). The actual work is done by the cartridge which is generally mounted on shock absorbing rubber supports. The cartridge is protected by the grille and case, the cable usually leaving the rear end of the case. Additional parts may be included depending on the end product such as switches and transformers. The cartridge will be one of three types, ignoring carbon types which ceased to be seriously used many years ago.

These types are dynamic, ribbon and capacitor, each offering different characteristics, although some overlap does occur.

Dynamic Microphones

The dynamic microphone (sometimes called moving coil) is the converse of a normal loudspeaker. A diaphragm is fixed to a set of coils suspended between the poles of a magnet, and as the sound causes the diaphragm to move current is induced in the coils by the magnet (Figure 2). The coil and diaphragm must be very light to allow the microphone to respond quickly to sound (ensuring a wide frequency response) and yet be strong enough to withstand jolts during its life. This is why high quality dynamics are expensive, the cost purely related to the difficulty of manufacture of the diaphragm/coil assembly. A small transformer is often used to adjust the output voltage and impedance to make the microphone easier to interface to other equipment.

Figure 2. Dynamic Microphone Cartridge

Figure 3. Humbucking coil connection

One problem with dynamic microphones is that they are prone to pickup hum in the coil, and in view of the very low signal levels present this can be quite serious. Many microphones overcome this problem by mounting another coil next to the moving coil, but wound in the opposite direction. The outputs of the two coils are added, either directly or by means of a transformer, and any hum induced into both coils cancels out since the induced hum will be of opposing phase. The fixed coil is often called a 'humbucker' (Figure 3).

Ribbon Microphones

A ribbon microphone still employs a magnet to induce an electric field, but this time into a single fine metal strip. The strip is very light and carefully shaped, making ribbon microphones expensive and delicate. They do offer tremendous clarity of sound because of the lightness of the ribbon and are thus very popular professional microphones.

Capacitor Microphones

It is important to distinguish between true capacitor (previously called condenser) microphones and the cheap electret capacitor microphones (see Figure 4). The true capacitor microphones work on a totally different principle to the magnetic microphones, and depend on the charge stored on a capacitor. The amount of charge that can be stored on a two plate capacitor is related to the area of the plates, and the distance between them, implying that if the distance is altered the charge stored must change resulting in current flow.

Figure 4. Capacitor microphone cartridge

In a capacitor microphone the base of the cartridge forms one plate, and the diaphragm forms the other. As sound impinges on the diaphragm it moves closer to the base, resulting in a capacitance change (Figure 4), causing a flow of current. However, the plates must be charged for this to occur, resulting in the major problems with capacitor microphones because an external power supply is required, capable giving up to 50 Volts or so. This supply can be arranged to use the microphone signal cables, a technique called phantom powering (Figure 5). A D.C. path is maintained to the microphone via the coil which prevent any A.C. interfering with the power supply. The A.C. signal is superimposed on the D.C. and then retrieved by means of the capacitor which removes the D.C. component at the receiver end. This sort of capacitor microphone is in great demand in spite of the power supply, because they are very versatile and tough.

Figure 5. Phantom powering

The electret capacitor microphone was developed a few years ago in an attempt to gain the advantages of capacitor techniques without the need to provide an external power supply. In an electret, one of the plates is charged at the time of manufacture (a process involving heating and cooling — analogous to magnetisation) and so the polarizing voltage is not required. The output of these cartridges is very low, and a small field effect transistor is usually used to amplify it. This amplifier is usually mounted in the cartridge itself and is powered by a 1.5 Volt battery held in the handle of the microphone.

The major problem with these microphones is that the charged plate is necessarily the diaphragm and needs to be slightly thicker than a normal diaphragm to hold sufficient charge. As a result these electret microphones generally exhibit poor frequency response and dynamic range, and should be avoided at all costs, although there are a few notable exceptions, by Sennheiser in particular.

Technical Specifications

The most important specifications of any microphone are the polar response pattern and the frequency response. It is essential that these characteristics be decided upon before any microphones are actually considered for purchase. Other factors such as sensitivity and impedance can often be considered later because many microphones are offered in different configurations.

Polar Pattern

Figure 6. Polar patterns

The polar pattern defines where the sound source has to be placed with respect to the microphone. Three main patterns exist (Figure 6), although a development of the cardioid gives steeper sides to the pattern, and is not surprisingly called 'hyper-cardioid'. In a meeting or open interview an omni-directional type is desirable because it does not matter where the person speaking is with respect to the microphone. The bidirectional ('figure of eight') type is generally used in a pair at 90° to each other to cover the entire area, but in stereo. By far the most common is the cardioid, since sound in front of it is picked up, but sound from behind (audience noise, etc.) is suppressed. This pattern is also much used by the PA industry because sound emerging from the PA, and then reflecting from walls causing feedback is largely rejected. It is interesting to note that the polar pattern is likely to change with frequency, and account of this should be taken.

One very important property of the true capacitor microphone is that its polar pattern may be varied by altering the supply voltage. This is because the plates tend to pull further together in the presence of high voltages, resulting in different stress points, hence a different polar pattern. Some capacitor manufacturers supply remote control boxes for this purpose. An additional trick which is very popular is to design the microphone as a standard body, upon which is screwed a capsule offering the desired response. This means that an entire range can be obtained more cheaply than buying several different microphones.

Frequency Response

The table in Figure 7 shows the ranges of typical instruments and voices. Obviously the microphone should be capable of picking up all these frequencies if a natural sound is required. The response of a typical vocal microphone, the Shure SM58, is also shown. It can be seen that the response is flat for the most part, rising above 2 kHz. The rise will give the voice 'presence' and generally make it more pleasant to listen to, although it would be disasterous for orchestral recording.

This peak leads on to feedback, which occurs when sound emerging through a PA travels back into the microphone and is preamplified causing the characteristic 'ringing' or 'howl-round' so often heard from poor PA systems. Many people believe that a wide frequency response will give rise to more feedback problems, but it is in fact the peaks in the response which cause the trouble. For example the SM58 peaks at 5kHz and 9kHz, and thus feedback at these frequencies is much more likely due to the increased 'gain' of the system. In spite of this the SM58 is one of the most popular vocal microphones ever made because it not only sounds good, but is easy to use and very robust. As a rule capacitor microphones exhibit very flat responses, and for comparison purposes the response of the AKG C414 is shown next to the SM58.

Figure 7. Frequency ranges of various instruments and responses of two microphones

Sensitivity and Impedance

Microphones are also specified in terms of sensitivity, which simply describes how much output voltage a given sound level will produce. As a rule the sensitivity is not all that relevant, but regard should be paid to overload, either electrical or physical when dealing with loud sound. Electrical overload is more likely to occur at the tape recorder or mixer than in the microphone, and happens when a very loud sound is picked up, resulting in a very large electrical signal which is clipped at the following electrical stage. Some microphones, particularly capacitor ones with built-in amplifiers, overcome this by providing a built-in attenuator to reduce the output signal level. Physical overload occurs when the diaphragm cannot move far enough and hits the end stops. This can quite often damage the microphone, and must be avoided. A bass drum is a very severe test of a microphone's capabilities, and only one microphone is universally accepted as being tolerant enough for this, the AKG D12, which was designed in 1954, and generates only 0.5% t.h.d. for a sound level of 128 dBA (Comparison — about the threshold of pain).

Impedance ratings define the load (usually a resistance) that the microphone is designed to operate into. This is important because a wrong impedance load will not only alter the levels, but also affect the frequency response. Most professional microphones have quoted impedances of 200 ohms or 600 ohms. This means thatthey should be plugged into a tape recorder or mixer with an input 'resistance' of about 600 ohms, the difference between 600 and 200 not being critical. Most tape recorders are not satisfactory as they stand having input impedances of about 50k ohm and a matching transformer should really be purchased, although the trick of connecting a 680 ohm resistor across each input jack often works.

Wiring Up

Some care is necessary in order to obtain the best results. Most professional microphones employ balanced line outputs (a full description of which appears elsewhere in this issue) and use cannon connectors, although ¼ inch jacks are sometimes seen. It is essential that all microphones are connected up in the same phase so that a positive sound pressure produces a positive voltage on the same input connector pins irrespective of the microphone used. If this is not observed the final sound will be very hollow, and lack bass, due to phase cancellation of signals from oppositely wired mics. If all the microphones are from one manufacturer no problem will exist, but it is worth the time to check if a mixture of makes are used.

Figure 8. Construction of high quality cable

The cable used to connect the microphones up should be of a high quality, preferably with a proper braided screen, and of the low noise type designed specially for microphones. This sort of cable employs a semi-conducting screen between the braid and the core insulator (Figure 8) this must be stripped well back out of the way since it exhibits a fairly low resistance and will affect the operation of the microphone. It is false economy to use plastic jack plugs since they are very unreliable - buy decent metal ones, preferably the sort with integral cable clamp.

1 A Beyer Dynamic Ribbon unit (M500).
2 & 7 Two extremes of dynamic microphones, the Shure Unidyne B and the AKG D222. The D222 uses two separate pickups for bass and treble giving a very extended response.
3 AKG D190 - all is not what it seems. Good for snare drums and roto-toms in spite of what AKG say.
4 Shure SM58. Often used vocal microphone although frequency response is hardly flat.
5 AKG D12. Bass drummers idol.
6 Top range capacitor - the fairly recently introduced Shure SM81, but at £150 it had better be good.

One from the Hundreds?

It can be very hard to choose the correct microphone, much advice given is misleading, and some of it simply wrong. In general it is best to go to a good dealer, who will let you test the microphone in the environment for which it is intended. Under no circumstances buy a microphone without listening to it first.

Firstly it is necessary to decide on the polar pattern required, followed by the decision about the frequency response. For example, recording a chamber orchestra needs an extended top with a flat response, an obvious candidate for capacitor types, as would be a hi-hat cymbal. However vocal work through such a microphone would probably sound rather flat, indicating a 'presence' peak may be required.

Other problems emerge as the microphone is tested, most particularly susceptability to stray noise such as wind, pops and handling bangs. Pops are caused by close miking vocals when like letters 'P' or 'B' start the word. The rush of air causes a nasty popping sound to occur. Wind noise is only a problem outside (or near big air conditioners?) and simply results from the noise created by the airflow around the microphone. Both these may be reduced by fitting a pop shield - a sleeve of foam over the top of the microphone. Handling noise is only relevant if the microphone will not remain in its stand all night. A solid 'clunk' as the artist removes or replaces the microphone in its stand is not a very desirable effect. All these factors should be taken into account, and most of them can only be explored by using the microphone in question.

Most manufacturers quote applications for much of their range. Inevitably these applications are a little stretched since the manufacturer wants to convince the buyer that his microphones are more universal than those made by other people. Exceptions do exist - AKG modestly refer to the D190 as for 'General semi-professional use, movie sound and announcer studios'. In fact, the 190 is not a particularly good vocal microphone being very susceptible to popping, but it is blessed with a considerable transient response and is used in several rigs and studios for snare drums and roto-toms, and sometimes on cymbals, although it has a tendency to sound a bit 'splashy' at times.

This section can be simply summarised however with two words, 'try it', but you may need to find an understanding dealer first.

One final note: It is not worth buying cheap microphones and an expensive tape recorder, better results will be obtained by putting half the available funds on a tape machine and splitting the rest between two microphones. On no account buy several cheaper ones, since in general two good microphones will last a lifetime, as well as sounding a great deal better.

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


Electronics & Music Maker - Mar 1981

Feature by Chris Lare

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