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In
the beginning there was sound...!
This week
we start a meandering series, following the signals
long journey from the stage to the speakers. During
our stroll down the signal path we will be looking at
what all the different bits of gear do, why they are
patched-in in that order and how to get the max out
of them...easy!!
For
the next few weeks we'll have a look at how mics work
to change sounds into electrical signals, how they respond
to sound coming from different directions, and how to
keep them in tip-top condition.
Along
the way we will take detours and stops as we need them,
to have a wee bit more detailed look at some of the
components and the physics/science behind the bits of
gear...so let's get walking!!
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Inside
Dynamic Mics.
All microphones are transducers,
that is they change one form of energy into another.
In a PA system, acoustic or sound energy is turned into
electrical energy by the microphone for use by the mixing
desk and other parts of the system, before being turned
back into sound at the speaker boxes. [Think of mics
as the opposite of speaker systems.]
The names
used to describe the various types of microphone come
from the different methods used to convert acoustic
energy into electrical energy. The most common mics
used in the live arena are Dynamic or Moving Coil Microphones,
and to a lesser degree, Condenser / Electret mics.
Dynamic
Mics are the most commonly used mics in the live arena
and use the basic principles of Electrical Generation
Theory to convert sound energy into electrical energy.
Let's think back to our Junior Science classes...In
simple terms, an EMF [electro-motive force] is generated
when a conductor - copper wires - moves through
the 'lines of flux' of a magnetic field. The EMF induced
in the windings is proportional to the relative rate
of movement / flux lines cut. (Phew, easy!)
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The conductors of a dynamic mike
are attached to the Diaphragm - the bit we say 'Test 1 -2
!!' into - and sit suspended between the poles of a permanent
magnet. The diaphragm moves in response to sound waves, which
in turn moves the windings relative to the magnets. This generates
a voltage that varies in direct proportion to the acoustic
energy - both in amplitude or level, and in frequency.
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The ability of a diaphragm to
move freely and instantly in response to sound pressure variations
is necessary for accurate sound reproduction. As the diaphragm
is attached to windings, weight and resistance to movement
from inertia will affect the mics performance. In particular,
many dynamic mics suffer poor Transient and High Frequency
Response characteristics, with the high frequency falling
off after about 15 kHz. This can result in a slightly less
detailed sound than more expensive types of mic, often requiring
some serious EQing for high-frequency rich content and complex
upper-harmonics. [A Transient is a relatively high level
signal with a sharp Attack and Decay, for example the 'crack'
of a Snare Drum. Chill, we will be looking at 'Sound Envelopes'
later in the series when we look at using compressors and
gates !!]
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Anyone who
has been to a live gig has seen the industry standard
vocal Mic, the Shure SM 58, - the ones that look like
an ice-cream cone - which has inspired many copy-cat designs.
These became the industry favourite because their characteristic
sound flatters vocalists by giving a bigbass-boost
up close, a peak in the high-mid frequencies - the 'Presence
Range' - and have excellent feedback rejection. They are
also very popular among touring bands and hire companies
as they are rugged and reliable, well up to the abuses
of touring life. [Within reason!!] |
Mic Applications.
As a rule, dynamic mics are good
for miking drums and electric instruments, whereas capacitor
mics - which need a 48V 'Phantom power' to charge the capacitor
used in the capsule - are better for high-quality vocal and
acoustic instrument work. (The Shure SM-87 is a popular high-end
live vocal condenser mic.) Large-diaphragm capacitor mics
tend to be more flattering than small-diaphragm models, especially
on vocals, though they are usually kept in
the studio as they are more fragile and expensive than the
classic dynamic mic.
Well that's about
it for this week. Next time we'll take a look at 'microphone
response patterns' to see how they are made to be Directional
, so that they only pick-up what they are pointed at, while
rejecting other sounds.
Until then....
Have a good one !!
Willy T.
This is extracted from the book:
“LIVE-AUDIO WORKSHOP: A Hands-on Guide to Rigging and Operating PA Systems.”
ISBN 0 646 36408 1.
© Giraffe Communications 1998.
Order via e-mail or at all good Music & Bookshops.
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