In last month's Sound Advice we saw just how many different kinds of mikes there are, how each type works, what a 'response pattern' is and more. This month, Gary Cooper continues our unique 3-part All About Mikes series with an explanation of the mysterious Proximity Effect.

In addition to the various kinds of microphones, and features of different individuals, which we looked at in part one of this short series, there's another and very important factor to be aware of when choosing and using mikes — the mysteriously titled 'proximity effect'. This is a feature of virtually all dynamic unidirectional mikes (hence almost all the mikes you're likely to find being used by the average band), and is something that a lot of singers are probably aware of and make deliberate use of — even if they can't put a name to it. In principle, the proximity effect means that the closer you place a dynamic unidirectional mike to the sound source, the more the bass frequencies are emphasised. As you can imagine, a lead vocalist using a hand held mike and varying the distance from which they sing into it can make quite a feature of this effect to add more 'body' to their voice; but (and perhaps more importantly) it also has a great bearing on where you position a mike when you're either trying to reproduce an acoustic instrument's sound on stage, or when miking up a speaker cab.

There's certainly benefit to be gained from making use of this effect when miking up instruments and cabs. Placing your mike close to the sound source will obviously give a louder sound, and this is aided by the proximity effect's emphasis on bass frequencies, which allows for lower gain levels being used on the mixer and, with luck and a bit of skill, less feedback. Where you place the mike can also, of course, 'tune' the sound to a certain degree.

On the other hand, too pronounced a bass boost isn't what you'd want for a vocalist, and nearly all mike makers design most of their unidirectional vocal mikes with an degree of bass roll-off present. Consequently, a major part of the knack of designing a good vocal mike lies in balancing qualities like bass roll-off, proximity effect and presence boosting to give the mike a desirable 'voice'. As ideas differ from designer to designer, this means that a fair amount of auditioning has to be undertaken to find the mike you like best — they're mostly quite individual in their sounds, and a lot of finding the perfect mike is down to personal taste.

Assuming that you're going to be using a directional mike for vocals (probably a cardioid responding one), your singer can probably handle any unwanted bass boom by keeping a reasonable distance from the mike. On the other hand, you might still have to make some adjustment to the Eq setting on the mike channel to control it. The problem can be greater when close-miking instruments and speaker cabs with this kind of mike, however, and here you might be better off either using a mike with a built-in filter switch (such as AKG's D330) to roll off some unwanted bass, or using an omnidirectional mike. Omnis don't suffer from the proximity effect syndrome. Alternatively, there are a few mikes which are unidirectional, dynamic, and feature a very reduced proximity effect. These latter are those Electrovoice mikes described as having 'Variable D' (as opposed to their 'Single D') characteristics. Although some bass boost is present when working close with them, it's greatly reduced. E-Vs featuring the 'Variable D' response include their PL5, PL6, PL91A, PL11 and PL20.

Response

It might be tempting to believe the spec sheet accompanying a cheap Far Eastern mike, which proudly boasts an ability to pick up every frequency from 20Hz-20,000Hz — and all for the amazing price of £10! — but life's just not like that, I'm afraid. Obviously it's best to use your own common sense (and a healthy amount of scepticism) when looking at the frequency response graphs of cheap mikes. With a mike maker you can trust (and that includes just about anyone from the UK, Western Europe and the USA), the frequency response charts provided in brochures are reliable as a guide (but a guide only) to the highest and lowest frequencies it will reproduce. Mind you, saying that a mike will reproduce from, say, 20Hz-1.6kHz doesn't tell you very much at all about how it will reproduce those frequencies. Can you imagine what the result would sound like if your vocal mike reproduced the bass end frequencies of your voice three times louder than the top end?! Equally to the point, can you (or your audiences) still hear frequencies of 20,000Hz, and can even the huge hired PA rig you're using reproduce frequencies that high? Does your voice or instrument even produce frequencies as high as 20,000Hz? Bear in mind that the fundamentals of a top soprano's voice cut off at around 1.5 kHz and even the harmonics are gone by 9kHz or so — do you really need a mike capable of reproducing 20kHz!?.

What you really need to know is not only how wide a band of frequencies a mike will pickup and reproduce, but how each frequency is treated in terms of its level against the other frequencies. For this reason, mikes are supplied with a frequency response chart which plots the response level (in decibels — dBs) against the frequencies in Hertz (Hz). A mike which reproduced every frequency equally would show up as having in effect, a straight line on such a graph, as it handled each frequency equally, and a mike responding in this way is usually described as having a 'flat response', for reasons which should be self-evident. Although this might seem ideal, it often isn't anything like as desirable as you might suppose. In fact a completely 'flat response' mike can sound rather dull, especially in live applications. For recording it could be useful, because there you then have the vast equalisation facilities of the mixer to shape and 'colour' the recorded sound during both the initial recording and subsequent mixdown. On stage, however, one is usually looking for an 'improved' sound, which is why so few stage vocal mike charts show the response curve as being flat.

Generally speaking, most vocalists' dynamic mikes have, as I said earlier, a pronounced bass roll-off effect, largely to aid the intelligibility which would be ruined if too many bass frequencies were reproduced — especially taking into account the proximity effect. Equally, it's generally felt that a little extra boost in the upper-mid (often called 'presence") frequency range will give a little extra oomph and sparkle to a vocal sound, which is why none of the best regarded stage vocal mikes have an absolutely flat frequency response graph.

Typical examples of good quality vocal mikes featuring a bass roll-off and presence boost would include the Shure PE86, Audio-Technica's ATM91 or E-Vs PL88.

The Story So Far

So, how does all this information help us? Well, from the data given by the manufacturer you now know what it means when you're being offered a choice between a dynamic or condenser mike. You have the basics of how to understand a frequency response chart, and you know what the difference in pickup pattern is between a unidirectional cardioid mike and an omnidirectional. From these fundamentals (especially now we also know a bit more about which mike suits which role by having a grasp of what proximity effect is and how it works), it's possible to start making some educated assumptions about which mike we ideally want to use or buy for a given task.

It's fairly easy to deduce from what we now know, for example, that if we want a robust, relatively inexpensive, hand-held vocal mike with good frequency response is required, then a dynamic mike will probably be our logical choice. Mind you, if really high quality was our goal then we certainly shouldn't rule out a condenser. We can go further in narrowing down the field if we assume that we'll want a fairly tight pickup pattern, so as not to get too much backline noise coming into the vocal mike, and also to minimise feedback problems from the monitors. This calls for a unidirectional response (preferably of the cardioid type). Finally, we can decide whether or not we'd like the proximity (bass-boost) effect when the mike's used close up, or not. We know the general range we require in terms of frequency response (human voice, different instrument types, etc.) and that, possibly, a degree of bass roll-off and presence boost could be nice — especially for the Rock vocalist. Equally, we might want something really 'flat' if we desire a 100% accurate reproduction of a truly rare voice or a pure instrument sound. If nothing else, we at least know enough to have a pretty good stab at what we're after!

Perhaps more to the point, we're also becoming aware that every mike maker has their own ideas about how a mike should sound — which is, of course, both why there are so many mike makers, and why so many different models are offered in each maker's range. We can also surmise (correctly, as it happens) that one mike may be good for vocals, whilst another could be more suited to, say, bass drum reproduction.

On the other hand, we may want a mike for reproducing the sound of a 100 watt guitar combo. Here, an ability to handle mighty dB levels without having the sound break up will be important, as will having a good, 'punchy' sound, possibly with a presence boost. A huge frequency range isn't crucial, and we might well be limited in price, having decided to give our vocal mikes greater priority. Looking through the various makers' catalogues, we should soon be able to see for ourselves why some of the major makers recommended the following of their models: AKG D125, E-V PL11, Shure PE66, Audio-Technica ATM21, Sennheiser MD-427, Beyer M300 and so on.

Awareness of the different factors now enables us to begin to draw up a shortlist of possible options, from which point the only way to make a choice is by a trial of each contender. We know the price range we have in mind, we know what we want to use the mike for, and from each maker's offerings we are beginning to be able to make some intelligent guesses as to which particular model could suit our purpose.

Thankfully, most manufacturers help us a good deal further, by offering 'applications' literature, suggesting which of their range suits which purpose. It's never quite as simple as they suggest — but these leaflets can help us narrow down the choices.

(Almost) the Final Straw

Having digested the information so far (assuming you've a strong enough stomach!), I bet you're now thinking that there's really no more to be said. Are you sure you'd put money on that? Well, it's pay-up time. I'm afraid, because there's one final point that you must consider, and that's the question of matching your mikes to your PA desk, mixer amp or whatever.

Again, trying to avoid blinding you with science, mikes come in two types: low and high impedance. High impedance mikes have higher output levels, but for several reasons they are subject to being used with relatively short cables — probably 15-20' being the limit before high frequencies become degraded, and induced interference from lights, amps etc. becomes a problem.

Until relatively recently, most mixer amps were designed to take high impedance mikes, which means that you must be certain which type of mike you need before buying, especially if you intend to use it with an old-fashioned amp. Fortunately, all is not lost if you do happen to have the wrong kind (or find yourself stuck with using a high impedance amp and a low impedance mike one day) because you can buy line matching transformers which will convert one type to another; and they're not hard to come by. Having said that, nearly all modern equipment caters for low impedance mikes, and these are the types you're best advised to opt for.

Regrettably, yet another hazard lies in the happy mike user's path — that of how to wire-up the mike in question. You've probably encountered the term 'balanced line' in ∫'s pages from time to time, and might well have wondered what it meant. Well, here goes!

By and large, the bog-standard high impedance mike will be found connected to a conventional ¼" jack plug. Unscrew the plug's cap and you'll find that a two element wire is being used to connect it. One of these wires will be a braided wrap-around 'shield' (probably connected to the 'outside' terminal on the plug); the other will probably be plastic covered and soldered on to the plug's central tag. This arrangement is technically called an 'unbalanced' connection. The outer shield, in this case, functions both as a protector against outside interference being picked up and as one of the signal carriers.

Low impedance mikes are usually wired for 'balanced' operation which, simply, can be inferred from the presence of three elements in the cable. As with the cables used to connect high impedance mikes in the 'unbalanced' format, one of these will be a braided shield, but there will be two plastic sheathed inner wires, too. The shield, in this case, just does what it says — it protects the inner two signal-carrying wires from interference.

For those who enjoy the technical explanations behind such facts, the reason for the term 'balanced' is because, in this system, any unwanted current picked up (despite the presence of the screen) will be equal in both conductor wires. As a transformer is used at the terminating end of system, these two noise currents effectively flow in opposite directions, and hence cancel one another out. Ever heard the word 'humcker'?

Of perhaps more practical use is the knowledge that a low impedance mike, connected to a suitable mixer with a balanced wired system, will enable quite enormous cable runs to be used with little danger of either HF loss or unwanted interference being picked up by the lead. With this final hurdle overcome (whether we require high or low impedance mikes, wired balanced or unbalanced), we now know, more or less, how to put together our shopping list. Assuming that we do, how should we start making a final choice, and how do we position our mikes once we've got them? We'll be going into that next month!