As with speakers, microphones can be divided into two varieties — those with a flat, uncoloured response and those which sport a judiciously contoured frequency response to emphasise the character of a particular instrument. A microphone which 'sounds good' on the original musical instrument — the human voice — usually has a series of 3 to 8dB peaks in the 2-8kHz region, where the upper harmonics lie. This type of microphone is almost universally used, possibly for no good reasons apart from tradition and effective advertising. In the past, microphones with a flat response were very expensive or unsuitable for the rigours of stage use, and so too was the equalisation needed to make them sound anything but flat and lifeless.

Flat microphones have the advantage that their 'sound' and the maximum live sound level that can be attained with them is primarily governed by the versatility of the equalisation controls and the skill of whoever adjusts the knobs. In other words, using flat microphones spells greater control, assuming of course that the speaker system is flat. In the real world, however, microphones and speakers with a flat response and versatile EQ circuits aren't cheap, and we usually have to settle for a vocals microphone with a carefully placed response 'bump'. At least with this type of microphone, a 'tasty' vocals sound is available even without equalisation, assuming reasonably uncoloured speakers. The potential drawback of the traditional (i.e. coloured) vocals microphone is not so much the generalised bump in response in the upper vocals region, but rather the existence in the cheaper models of very sharp peaks coincident with this presence region, which rise well above the overall 'bump' in the response. Being very narrow (typically only a few hundreds of Hz wide), these peaks aren't particularly audible, but they can easily and surreptitiously incite premature acoustic feedback or howlround. If for instance the microphone has a narrow peak that is 3dB higher than the overall response peak, then the maximum sound level you can achieve will be audibly halved, relative to the limits set by the general presence peak. Of course, the general presence peak is also lowering the maximum level of your vocals (and even if you were using a flat microphone, you'd probably still add a presence peak with the EQ) can attain — say by another 6dB — but at least it's contributing to the colour of your voice! By comparison, the narrow peak is stealing half the potential loudness of your vocal delivery and giving nothing audible save ear-piercing howls in return.

Equalisation won't readily repair this hindrance, because its effect cannot readily be limited to the offending response peak; instead, it will remove the whole presence peak which eliminates the premature feedback but possibly at the expense of flat, lifeless vocals! This underlines the need to seek vocal microphones which are devoid of gross and precipitous peaks (as opposed to a generalised 'bump') in their response. Such deviations aren't particularly audible, and short of studying an unbiased frequency response plot for every prospective microphone, one can only choose according to the reasonable assumption that microphones which you have seen or you know are used at high levels on stage are practical tools.

As a general guide, a vocal microphone won't unduly restrict your performance calls for an investment of at least £40 unless you're lucky enough to find a second-hand microphone in good condition. A cheaper microphone might suit, but if it doesn't, then you have little choice but to buy another! If on the other hand you invest generously in your microphone, then you can at least be sure that any lack of clarity or undue sensitivity to acoustic feedback is due primarily to shortcomings in your speaker system — or your voice!

At first glance, the directional properties of a microphone appear to be the crucial parameter that controls feedback. Ideally, we might look for a unidirectional microphone that only responds to sounds arriving head on, viz from the vocalist, whilst it ignores totally the amplified sound arriving from the rear. This would certainly obviate feedback problems, but lamentably such a microphone doesn't exist, and any affordable vocals microphone, regardless of whether it's (mis)described as 'directional', 'unidirectional' or 'hyper-directional' is at best merely biased to be some 10 to 20dB more sensitive over a limited range of frequencies to sound arriving from the front. In other words, directional microphones exhibit only a generalised tendency to be directional. Thus the 'anti-feedback' properties of the directional microphone are only a moderate aid — not a panacea, particularly when it's borne in mind that some of the amplified sound 'out-front' will bounce against the rear wall of the stage and enter the vocalist's microphone from the most sensitive position. Nonetheless, provided its limitations are heeded, the directional microphone is an invaluable tool on stage.

An important by-product of directional characteristics in microphones is their general tendency to emphasise the fundamentals of the voice — and other low frequency sounds — when the vocalist works from a short distance. This proximity effect increases exponentially from a distance of 6" to 8" and if the vocalist insists on working at zero distance, the vocals will be swamped entirely by muddy fundamentals and explosive breath sounds. A good vocals microphone will be compensated for this effect, by exhibiting bass cut beginning around 200Hz. In competition with the proximity effect, this compensation results in a roughly flat bass response at moderate distance (e.g. 8"). At closer distances, the proximity effect mildly overtakes the bass cut to provide a warm, intimate sound without undue susceptibility to pops and air blasting or outright muddiness — particularly if a windshield is fitted.

Although microphones designed to match high impedance inputs (circa 50k) are acceptable for stage vocals provided the cable lengths are short (< 4 metres), the majority of high quality microphones are designed to match low impedance inputs (200 to 1000 ohms). In addition, the sensitivity of these low impedance models is considerably lower. At the same time, your PA mixer or amplifier is likely to have inputs suited only to high impedance microphones. In an attempt to use a microphone designed for low impedance operation, the results — an abominable lack of sensitivity and excessive noise — are scarcely an encouragement to invest in a high quality mic. with its attendant low impedance!

This setback can be neatly overcome by investing in a microphone matching transformer, which will enable you to use your low impedance microphone as if it were a high impedance model, with the advantage of higher quality and the absence of cable length restrictions. And as most high quality microphones feature balanced terminations (3 wire), which don't suit the average musician's amplifier, the microphone transformer can also usefully feature balanced-to-unbalanced conversion; that is, to convert to conventional two wire operation (Figure 2). Best of all, when you graduate to using professional quality sound equipment, your microphone's performance will remain commensurate with such gear, and by unplugging the matching transformer your microphone will revert to its balanced, low impedance configuration and thus immediately suit your new requirements.

The above considerations apply principally to dynamic microphones. Capacitor microphones feature internal amplifiers, and whilst their output (or source) impedance may be low, their output voltage is generally 10 to 20dB (3 to 10 times) higher than the average low impedance dynamic microphone. Indeed, it's often equal to that of the cheap high impedance varieties. It's important to note that it's not an inherently abusive practice to mate a low impedance microphone to a high impedance input; it's simply a noisy and unproductive practice in the context of the minute output of a low impedance dynamic microphone. By contrast, whilst the low source impedance of capacitor microphones is best suited to low impedance inputs for the sake of nth degree low noise, they often have sufficient output to mate happily and hisslessly with high impedance inputs. The rugged and affordable Calrec 654/656 vocally compensated capacitor microphones are perhaps the best examples of this species as regards vocals on stage; their high output level readily matches both high and low impedance inputs, and they're widely regarded as exhibiting sound quality that's equal to dynamic microphones costing many times the price. This is hardly surprising considering that the capacitor microphone family encompasses the pinnacle of finesse in microphone technology! The only drawback of capacitor microphones is the need to provide a supply to power the internal electronics; Figure 3 shows a DIY power supply designed to power a number of Calrec CM600 series microphones from a common stage box. A less arduous idiosyncrasy of capacitor microphones is their refreshingly 'crisp' sound, which stems from the presence peak being displaced upwards to the 10-15kHz region; a characteristic that particularly enhances the overtones of certain female vocalists!

Microphones in practice

Unless you aim to emulate Sid Vicious, it's unwise to copy willy-nilly the microphone technique displayed by many well known Rock vocalists. Too often they try to eat the microphone, with the simultaneous emission of blood-curdling 120 decibel screams! Few vocal microphones or PA systems can be expected to give sympathetic rendition under these circumstances. Yet vocalists often have good reason to shout into their microphone at zero distance: throughout their career, the acoustic feedback threshold has either been so low or the guitarist(s) have been so loud that an audible vocal level has been impossible without recourse to gross countermeasures. (Possible countermeasures against noisy guitarists and feedback prone PA's will be studied later). When, at the peak of their career they meet a well designed sound system, the bad microphone technique suddenly becomes unnecessary but nonetheless, hard to relinquish. Ideally, loud close-miking should be reserved for the very passionate and aggressive effect its distorted texture conveys (e.g. Paul Weller, Paul D'Anno), whilst the more subtle varieties of close microphone technique can be used to convey laid-back sensuality or intimacy through the medium of the proximity effect (e.g. reggae, soul, funk and cabaret). These special requirements apart, a quality vocals microphone will give of its best if you maintain a reasonable distance — say 4" to 12". And bear in mind that your distance from the microphone can profoundly change the nature of your vocal sound through the PA, and the closer you approach the microphone, the more critical your positioning needs to be to obtain a consistent sound.

A common feature of many microphones is a series of slots along each side. In cheaper models, these are essentially decorative, but in high quality mics, these slots are crucial to the microphone's directional properties. Thus if they're obstructed by a carelessly placed hand, the resulting distortion of the directional pattern can readily precipitate howlround, so beware of clasping microphones!

For vocalists who leap around, considerable skill is needed not only to maintain the microphone at a reasonably constant distance, but also to avoid tripping over the mic lead — or worse still, boa-constrictor-esque entanglement! With this style of performance in mind, together with the tendency of microphone leads to suffer continual entramplement, a tough rubber microphone cable is a sensible investment. In the context of raucous stage acts, connectors which feature effective strain relief are also worthy of consideration; if the cable is overextended (e.g. vocalist falls off stage!), it's much better for the plug to be torn from the microphone or amplifier rather than the lead from the plug. In general, this means jack or XLR connectors — the latter preferably without latches for obvious reasons. Manufacturers who customarily fit DIN sockets to their microphones will usually provide the superior XLR termination for a nominal surcharge.

Finally, the false economy of buying cheap, feedback-prone microphones cannot be overemphasised. If you're short of money, to spend £25 on a scratched and battered ex-Rock concert PA or studio microphone in working condition is preferable to being enticed by a new and shining chrome-plated Oriental model. And bearing in mind the ease with which microphones disappear without trace, this item above everything else on stage benefits from insurance against theft.