Transcript Getting it From the Source

Getting it From the Source
Microphone Basics
Microphone basics
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A microphone converts sound energy into
electrical energy
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A microphone can use EITHER passive or active electronics
to do its job (aka transducer principal)
Most microphones have a diaphragm which
reacts to changes in the sound field. It is the
“eardrum” of the microphone.
 All microphones have a directional
characteristic (a.k.a. a pickup or polar
pattern)
Transducer principal:
How does it work?
Do I need to turn it on?
Dynamic Mic = passive (no power
needed): moving a wire coil through a
magnetic field induces current flow
 Condenser Mic = active (needs a
power source): changing the distance
between two electrically charged plates
(capacitor) induces current flow
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Dynamic Microphones
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Diaphragm is attached to a wire coil which is
suspended in a magnetic field
The diaphragm moves in reaction to sound waves,
setting the coil in motion within the magnetic field
inducing electron flow
The diaphragm is heavy, meaning it takes some force
to move it
This means a dynamic mic can generally withstand
high SPL’s without distorting
However, this also results in less detail in the sound
reproduction;
Dynamic microphones tend to be more rugged and
less sensitive than condenser microphones.
Condenser Microphones
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Condenser is the British word for capacitor.
 A capacitor is two electrically charged plates which
are slightly separated.
 In a condenser microphone, the diaphragm is one
side of a capacitor which moves in reaction to
changes in a sound field.
 Since the two plates are charged, the motion
changes the voltage between the two plates and
these voltage changes induce electron flow.
 Condenser mic’s require some sort of external power
source (a battery or “phantom power”)
 Because the diaphragm is very light, condenser mic’s
can have a highly detailed response and tend to be
much more sensitive than a dynamic mic
 Condenser mic’s are more delicate than dynamics,
are more easily damaged and are more prone to
distortion when exposed to extreme SPL’’s, the active
electronics will generate some noise (self-noise)
Dynamic vs. Condenser
Dynamic
Condenser
Rugged/Durable
Delicate/Sensitive
Does not need power; no
“self-noise”
Needs Phantom Power
Has Self-Noise
Less sensitive - lower output More sensitive - higher
- needs more amplification; output - needs less
amplification;
less prone to overload
more detailed sound;
distortion - can withstand
better frequency response;
higher SPL’s;
Generally cheaper
Generally more expensive
Ribbon Microphones
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Ribbon microphones are essentially a special type of
dynamic microphone. The diaphragm is usually a
thin aluminum ribbon, which has a wire attached on
either end, suspended in a magnetic field. Changes
in the sound field set the ribbon in motion in the
magnetic field inducing current
 Because the diaphragm is so light, the sound is very
detailed. There is no need for active electronics
 A ribbon mic is the only microphone where the
transducer principal dictates the directional
charateristic
 Ribbons are extremely delicate and are easily
damaged
Tube microphones
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A tube microphone is a type of condenser
microphone.
 The output of the mic is amplified by a
vacuum tube instead of transistors
 Tubes generally require a high-voltage power
source and most tube microphones will have
their own power supply rather than use
phantom power.
 Tube microphones are fairly delicate due to
the glass tube inside. They also tend to be
very costly.
Review of Polarity
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Polarity refers to positive and negative
The high and low pressure of a sound wave
is “encoded” into positive and negative
electrical voltages.
 When a speaker is given a positive voltage it
moves outward. Likewise, if a mic diaphragm
is pushed inwards, it generates a positive
voltage. When it moves outwards, it will
generate a negative voltage.
 Polarity is also important when considering
how directional patterns are achieved
Directional Characteristic:
Which way do I point this thing?
A Microphone can be designed to react to:
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absolute changes in pressure
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the difference between pressure at the front
and rear of the diaphragm
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(i.e. pressure transducer)
(i.e. pressure gradient),
or force & direction of the sound wave
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(i.e. velocity)
Ribbon mic’s fall into this category
This determines its “pickup pattern”
Omnidirectional Microphones
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Omni = all; an omni-directional mic is equally
sensitive to sounds arriving from any direction
 A true omni-directional mic is a pure pressure
transducer - it strictly measures changes in
pressure without any regard to the direction
that the wave is traveling.
 The rear of the diaphragm is sealed off so
that it can only react to pressure changes on
ONE SIDE of the diaphragm.
Bidirectional Microphones
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Bidirectional = two directions.
A true bidirectional mic can be a pressure-gradient or
velocity transducer, meaning its response to the
sound will depend on the direction the sound wave is
coming from.
The diaphragm is completely open on both sides so
that it can react to pressure changes on either side of
the diaphragm
A bidirectional mic uses polarity to encode the
direction of the sound wave.
If a sound approaches directly from the side, it will
cancel itself out completely. There is a “null” in the
pickup pattern of the mic at 90-degrees
This results in a “figure-8” pattern - it is sensitive only
to sounds arriving from directly in front or directly
behind
Unidirectional/Cardioid
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Unidirectional = one direction. The microphone is
most sensitive to sounds approaching directly from
the front (a.k.a. on-axis). As the direction of the
sound source moves around to the rear, there is less
and less sensitivity.
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Generally a pressure-gradient transducer
The microphone has a null at 180-degrees - it will not
respond to sound approaching directly from the rear.
This results in a “heart-shaped” pattern
(cardio=heart)
A cardioid is a marriage of an omni and a
bidirectional pattern. It has specially designed vents
to allow some sound pressure behind the diaphragm.
If you cover up these vents, the mic will become
omnidirectional.
Other patterns
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Super- and Hyper-cardioid mic’s are cardioids that
use more bidirectional in the “recipe.” This results in
a more narrow pickup in the front and a small pickup
lobe in the rear. The nulls are moved to 120 or 110
degrees.
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Shotgun - uses an interference tube to get a very
narrow forward pickup. The longer the tube, the
more narrow the pickup.
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Multi-pattern mic’s - often use two cardioid capsules
back-to-back. For uni-directional the rear capsule is
off; for omni-directional pickup it is turned on; the
polarity on the rear capsule can also be reversed to
generate a bi-directional pattern.
Frequency response
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Microphones all sound different! One reason is difference in
frequency response
Few microphones react to all frequencies equally (“flat
response”).
A mic’s frequency response is one way of describing how it will
react to or “color” the frequency spectrum generated by
instrument or voice
Some frequencies may be favored over others; some may not
be reproduced at all!
Polar response and Off-axis coloration:
 A microphone’s frequency response changes as you move offaxis (i.e. to the side or rear of). Moving a sound source to the
side of a mic will affect how the mic “colors” that sound source.
Proximity effect:
 As a sound source gets closer to a directional microphone, the
bass frequencies are emphasized, resulting in a “boomy” sound
(the Brits call it “Bass tip-up”)
Other microphone specifications
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Max SPL
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Sensitivity
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How large a signal the mic puts out for a given SPL;
the higher the number, the more sensitive the mic is.
Measured in mV/Pa
Self or Equivalent Noise 
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How much sound pressure the mic can handle before it
will distort.
Measured in Pa* and/or dB SPL
How much distortion is acceptable is up to the
manufacturer; it is expressed as % THD (Total Harmonic
Distoriton)
condenser mic’s only
How much noise the electronics generate
Measured in dB SPL
Pa - Pascal - measures pressure; 0 dB SPL = 2 x 10-5 Pa
1 Pa = 94dB SPL
Other stuff to know…
Amplifiers & Pre-Amplifiers
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An amplifier increases the amount of signal. The
ability of the amplifier to boost the level of a signal is
called gain.
 Each point where a signal passes through an
amplifier is called a gain stage. This simply means
that this is a stage where gain made be added to the
signal.
 A microphone puts out a very low-level signal.
Special ultra-quiet microphone amplifiers are needed
to boost this signal to a level that the other equipment
in the signal chain can use.
 These microphone amplifiers come before the power
amplifier (which powers the speakers) in the signal
chain, and are sometimes called preamplifiers. This
is often the first gain stage in any signal chain.
DI or Direct Box
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Studio microphones generally have balanced
low-impedance outputs.
 Instruments generally have unbalanced highimpedance outputs.
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This signal can overload an input designed for a
microphone-level signal, and could have noise
problems on a long cable run
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A DI converts the high-impedance to a lowimpedance and creates a balanced signal out
of the unbalanced signal.
 DI’s can be passive or active
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most active DI’s can use phantom power or
battery
Care and feeding of microphones
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Never blow into a microphone to test it gently tap the mic or snap your fingers in front
of it
Storing condenser mic’s in open plastic bags
inside their cases is a good thing to do
Do not use phantom power on a ribbon mic or
on tube mic’s that have their own power
supply
Switch off phantom power to microphones
before plugging in or un-plugging
Do not drop microphones
treat them gently and they will last a good
long time
Summary
The two most common types of microphone:
Dynamic and Condenser
 Condenser mic’s are generally more
sensitive
 Dynamic mic’s are generally more rugged
 Condenser mic’s need phantom power;
 There are three basic pickup patterns: omnidirectional (all around), bi-directional
(figure-8), and uni-directional (cardioid)
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