Transcript MIC ESSAY - SAM STARK

MIC ESSAY
DYNAMIC MICS
Most of you will have used a dynamic mic at some time or another -- if it looks like a mesh ball on a stick, then it's almost certainly
a dynamic model. In live sound, nearly all the mics used are dynamics, and in the studio, instruments such as drums, electric
guitars, and basses may also be recorded using dynamic mics. Dynamic microphones have the advantages of being relatively
inexpensive and hard-wearing, and they don't need a power supply or batteries to make them operate. So, how do they work?
A lightweight diaphragm, usually made of plastic film, is attached to a very small coil of wire suspended in the field of a
permanent magnet. When a sound causes the diaphragm to vibrate, the whole assembly works as a miniature electricity
generator, and a minute electric current is produced. Because the electrical output is so very small, it has to be amplified using a
mic preamp before it is large enough to be useful.
Given the stated advantages of dynamic mics, why bother with any of the other, more expensive types? The weakness of the
dynamic mic lies in the fact that the sound energy has to move both the mic diaphragm and the wire coil attached to it. The mass
of the coil adds to the inertia of the diaphragm assembly, which in turn restricts the frequency response of the microphone. In
practical terms, the outcome is that dynamic microphones fail to reproduce very high frequencies accurately. In some
applications, this isn't too serious, but if you're working with an instrument where a lot of tonal detail is contained in the upper
harmonics, a dynamic mic is unlikely to bring out the best in that instrument.
Another side-effect of the finite mass of the diaphragm/coil assembly is that the dynamic microphone is not particularly efficient - a lot of amplification has to be used to make the signal usefully large, and the more gain you use, the more noise you add to the
signal. In the studio where the mic is used very close to the sound source, this lack of efficiency is not a major problem, but if
you're trying to capture a quiet or very distant sound, then a dynamic mic isn't likely to produce good results.
To summarise; dynamic microphones are most effective when working with relatively loud sound sources that don't contain a lot
of very high-frequency detail. They're also tough as old boots, which makes them good for live work, or for any application within
six feet of a drummer!
Another type of dynamic microphone is the ribbon microphone, but these are only used in fairly esoteric recording applications by
engineers who appreciate the subtleties of the ribbon sound. These mics are comprised of a thin metal ribbon suspended in a
magnetic field, and when sound energy is encountered, the electrical signal generated is induced in the ribbon itself rather than in
a voice coil. The main advantage of ribbon microphones is their smooth, detailed sound; the disadvantages are their higher cost
and the fact that they are more fragile than conventional dynamic mics.
ELECTRET MIC
There is another type of capacitor microphone, known as the electret mic. Despite inauspicious beginnings, these have now been
developed to the point where they can rival true capacitor quality for a much lower price. Instead of applying an electrical charge
to the microphone capsule via an external power source, electret mics use a diaphragm made from an insulating material that has
a permanent electrical charge. A preamplifier is still needed, but this can be built very cheaply, and will run from a battery in some
cases.
Electret mics made in this way don't offer any real advantage over dynamic mics, because the diaphragms have to be quite heavy
in order to carry the permanent electrical charge -- but what if the permanently-charged material was fixed not to the diaphragm,
but instead to the stationary back-plate? This way, much thinner diaphragms can be employed, made of the same metal-coated
plastic material as on a true capacitor model. What I've just described is the back-electret microphone, and the best of today's
back-electret models can rival conventional capacitor models in every aspect of performance. The best back-electret models are
actually just as expensive as top capacitor models (the famous Bruel and Kjaer mics are back-electrets), but there are some less
costly models around which deliver studio quality at a bargain price. One of the most popular low-cost back-electret models in
recent years is the AKG C1000 (yours for £281), but don't neglect the models from other manufacturers. Currently, it's possible to
buy a good back-electret mic for around the same price as one of the better dynamic models -- the new Beyer back-electret, the
MCE83, retails for just £234. Most back-electret microphones in this price range offer a choice of battery or phantom power
operation, with a fixed cardioid pickup pattern.
FREQUENCY RESPONSE
You might expect the perfect microphone to have a perfectly flat response right across the audio spectrum, but there are various
practical reasons why this isn't so. Virtually all mics have a deliberate low-frequency (or LF) 'roll-off' -- in other words, they are less
sensitive to frequencies below about 50Hz. Without this roll-off, low-frequency vibrations, knocks, breath pops and other such
problems would produce very large, low-frequency output signals, which would not only compromise the sound quality, but might
even damage loudspeaker systems.
While LF roll-off is designed to exclude unwanted sounds, mics are also often designed to accentuate specific parts of the audio
spectrum in order to create a flattering sound. For example, numerous models are designed with a 'presence' boost in the upper
mid-frequency range, to help make vocals more intelligible. Presence boost (which is usually between 3 and 6 kHz), has the effect
of making the sound more transparent or detailed.
CAPACITOR MICS
Capacitor mics have been around for several decades, and although modern capacitor mics do incorporate a
few small technical improvements, the sound character has actually changed very little -- some of the bestsounding models were designed over 20 years ago. Basically, the heart of any capacitor mic is a pair of
conducting plates, one fixed and the other in the form of a moving diaphragm. When the spacing between the
plates changes (as it does when the diaphragm vibrates) the capacitance varies, and if a fixed electrical charge
is applied to the capacitor, an electrical signal is produced which faithfully represents the diaphragm vibration.
To keep the weight down, the diaphragm is often made from gold-coated plastic film. As a result, the
diaphragm assembly is very light compared to that of a dynamic mic, so the system is much more efficient, and
is capable of capturing harmonics right up to the range of human hearing and beyond. The size of the
diaphragm also has an effect on the tonal quality of the mic -- large-diameter models are chosen for vocal work
because of their warm, flattering sound. Small-diaphragm models tend to be chosen where high accuracy is
required.
Even though they are relatively efficient, capacitor microphones still produce such a small electrical signal that
they require a special type of built-in preamplifier to bring the signal up to usable levels, and this is one factor
that contributes to the higher cost when compared to dynamic mics. Additionally, all capacitor mics need a
polarizing voltage in order to work. The most common source of polarizing voltage is the 48V 'phantom' power
source, which is why many mixing consoles have a phantom power supply built in. The term 'phantom power'
came about because the polarizing voltage is supplied via the signal leads of the microphone -- no additional
cabling is needed. Because of the way phantom power is supplied, all phantom-powered microphones must be
balanced, and must employ the same wiring configuration. Budget mixers or cassette multitrackers with
unbalanced mic inputs cannot be used with conventional capacitor microphones unless an external mic
preamp (with phantom power) is used.
Broadly speaking, capacitor microphones are more expensive than their dynamic counterparts, but they are
also much more sensitive, and can capture high-frequency detail much more accurately. Furthermore, the
capacitor principle, unlike the dynamic principle, lends itself easily to the production of mics with switchable
pickup patterns (see the box elsewhere in this article for more information on these), although the cheaper
models tend to offer just a fixed cardioid pattern. Currently, there are some real bargains to be had in the
capacitor mic market -- for example the AKG C3000 (currently retailing at £351) and the Russian-made Oktava
MK219 (£311).
PROXIMITY
• The proximity effect comes into play when vocalists sing very close
to a mic, and the effect is that the level of bass in the recorded
signal goes up enormously. The proximity effect is all down to the
laws of physics, and may be a benefit or a problem depending on
what you do with it -- experienced live performers can use the
proximity effect as a type of dynamic EQ, allowing them to alter the
tone of their voice as they sing, simply by varying the mic-to-mouth
distance.
• In the studio, mics tend to be used at a reasonable distance from
the performer, usually with a pop shield in between, so the
proximity effect doesn't affect the recording of vocals. When
recording electric guitar or bass, the proximity effect is often
deliberately brought into play, to help create a more punchy sound.