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Microphone techniques (the selection and placement of microphones)
have a major influence on the audio quality of a sound reinforcement
system. For reinforcement of musical instruments, there are several
main objectives of microphone techniques: to maximize pick-up of
suitable sound from the desired instrument, to minimize pick-up
of undesired sound from instruments or other sound sources, and
to provide sufficient gain-before-feedback. “Suitable”
sound from the desired instrument may mean either the natural sound
of the instrument or some particular sound quality which is appropriate
for the application. “Undesired” sound may mean the
direct or ambient sound from other nearby instruments or just stage
and background noise. “Sufficient” gain-before-feedback
means that the desired instrument is reinforced at the required
level without ringing or feedback in the sound system.
Obtaining the proper balance of these factors may involve a bit
of give-and-take with each. In this guide, Shure application and
development engineers suggest a variety of microphone techniques
for musical instruments to achieve these objectives. In order to
provide some background for these techniques it is useful to understand
some of the important characteristics of microphones, musical instruments
and acoustics.
Microphone Characteristics
The most important characteristics of microphones for live sound
applications are their operating principle, frequency response and
directionality. Secondary characteristics are their electrical output
and actual physical design.
Operating principle - The type of transducer
inside the microphone, that is, how the microphone picks up sound
and converts it into an electrical signal.
A transducer is a device that changes energy from one form into
another, in this case, acoustic energy into electrical energy. The
operating principle determines some of the basic capabilities of
the microphone. The two most common types are Dynamic and Condenser.
Dynamic microphones employ a diaphragm/ voice coil/magnet assembly
which forms a miniature sound-driven electrical generator. Sound
waves strike a thin plastic membrane (diaphragm) which vibrates
in response.
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A small coil of wire (voice coil)
is attached to the rear of the diaphragm and vibrates with
it. The voice coil itself is surrounded by a magnetic field
created by a small permanent magnet. It is the motion of the
voice coil in this magnetic field which generates the electrical
signal corresponding to the sound picked up by a dynamic microphone.
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Dynamic microphones have relatively simple construction and are
therefore economical and rugged. They can provide excellent sound
quality and good specifications in all areas of microphone performance.
In particular, they can handle extremely high sound levels: it is
almost impossible to overload a dynamic microphone. In addition,
dynamic microphones are relatively unaffected by extremes of temperature
or humidity. Dynamics are the type most widely used in general sound
reinforcement.
Condenser microphones are based on an electrically- charged diaphragm/backplate
assembly which forms a sound-sensitive capacitor. Here, sound waves
vibrate a very thin metal or metalcoated- plastic diaphragm. The
diaphragm is mounted just in front of a rigid metal or metalcoated-
ceramic backplate. In electrical terms this assembly or element
is known as a capacitor (historically called a “condenser”),
which has the ability to store a charge or voltage.
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When the element is charged,
an electric field is created between the diaphragm and the
backplate, proportional to the spacing between them. It is
the variation of this spacing, due to the motion of the diaphragm
relative to the backplate, that produces the electrical signal
corresponding to the sound picked up by a condenser microphone.
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The construction of a condenser microphone must include some provision
for maintaining the electrical charge or polarizing voltage.
An electret condenser microphone has a permanent charge,
maintained by a special material deposited on the backplate or on
the diaphragm. Nonelectret types are charged (polarized) by means
of an external power source. The majority of condenser microphones
for sound reinforcement are of the electret type.
All condensers contain additional active circuitry to allow the
electrical output of the element to be used with typical microphone
inputs. This requires that all condenser microphones be powered:
either by batteries or by phantom power (a method of supplying
power to a microphone through the microphone cable itself). There
are two potential limitations of condenser microphones due to the
additional circuitry: first, the electronics produce a small amount
of noise; second, there is a limit to the maximum signal level that
the electronics can handle. For this reason, condenser microphone
specifications always include a noise figure and a maximum sound
level. Good designs, however, have very low noise levels and are
also capable of very wide dynamic range.
PHANTOM POWER
Phantom power is a DC voltage (usually 12-48 volts) used to power
the electronics of a condenser microphone. For some (non-electret)
condensers it may also be used to provide the polarizing voltage
for the element itself. This voltage is supplied through the microphone
cable by a mixer equipped with phantom power or by some type of
in-line external source. The voltage is equal on Pin 2 and Pin 3
of a typical balanced, XLR-type connector. For a 48 volt phantom
source, for example, Pin 2 is 48 VDC and Pin 3 is 48 VDC, both with
respect to Pin 1 which is ground (shield).
Because the voltage is exactly the same on Pin 2 and Pin 3, phantom
power will have no effect on balanced dynamic microphones: no current
will flow since there is no voltage difference across the output.
In fact, phantom power supplies have current limiting which will
prevent damage to a dynamic microphone even if it is shorted or
miswired.
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In general, balanced dynamic
microphones can be connected to phantom powered mixer inputs
with no problem. |
Condenser microphones are more complex than dynamics and tend to
be somewhat more costly. Also, condensers may be adversely affected
by extremes of temperature and humidity which can cause them to
become noisy or fail temporarily. However, condensers can readily
be made with higher sensitivity and can provide a smoother, more
natural sound, particularly at high frequencies. Flat frequency
response and extended frequency range are much easier to obtain
in a condenser. In addition, condenser microphones can be made very
small without significant loss of performance.
TRANSIENT RESPONSE
Transient response refers to the ability of a microphone to respond
to a rapidly changing sound wave. A good way to understand why dynamic
and condenser mics sound different is to understand the differences
in their transient response.
In order for a microphone to convert sound energy into electrical
energy, the sound wave must physically move the diaphragm of the
microphone. The amount of time it takes for this movement to occur
depends on the weight (or mass) of the diaphragm. For instance,
the diaphragm and voice coil assembly of a dynamic microphone may
weigh up to 1000 times more than the diaphragm of a condenser microphone.
It takes longer for the heavy dynamic diaphragm to begin moving
than for the lightweight condenser diaphragm. It also takes longer
for the dynamic diaphragm to stop moving in comparison to the condenser
diaphragm. Thus, the dynamic transient response is not as good as
the condenser transient response. This is similar to two vehicles
in traffic: a truck and a sports car. They may have equal power
engines but the truck weighs much more than the car. As traffic
flow changes, the sports car can accelerate and brake very quickly,
while the semi accelerates and brakes very slowly due to its greater
weight. Both vehicles follow the overall traffic flow but the sports
car responds better to sudden changes.
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Condenser/dynamic scope photo
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Pictured here are two studio
microphones responding to the sound impulse produced by an
electric spark: condenser mic on top, dynamic mic on bottom.
It is evident that it takes almost twice as long for the dynamic
microphone to respond to the sound. It also takes longer for
the dynamic to stop moving after the impulse has passed (notice
the ripple on the second half of the graph). |
Since condenser microphones generally have better transient response
then dynamics, they are better suited for instruments that have
very sharp attack or extended high frequency output such as cymbals.
It is this transient response difference that causes condenser mics
to have a more crisp, detailed sound and dynamic mics to have a
more mellow, rounded sound.
The decision to use a condenser or dynamic microphone depends not
only on the sound source and the sound reinforcement system but
on the physical setting as well. From a practical standpoint, if
the microphone will be used in a severe environment such as a rock
and roll club or for outdoor sound, dynamic types would be a good
choice. In a more controlled environment such as a concert hall
or theatrical setting, a condenser microphone might be preferred
for many sound sources, especially when the highest sound quality
is desired.
Frequency response - The output level or sensitivity
of the microphone over its operating range from lowest to highest
frequency.
Virtually all microphone manufacturers list the frequency response
of their microphones over a range, for example 50 - 15,000 Hz. This
usually corresponds with a graph that indicates output level relative
to frequency.
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Flat frequency response
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The graph has frequency in Hertz
(Hz) on the x-axis and relative response in decibels (dB)
on the y-axis.
A microphone whose output is equal at all frequencies has
a flat frequency response. |
Flat response microphones typically have an extended frequency
range. They reproduce a variety of sound sources without changing
or coloring the original sound.
A microphone whose response has peaks or dips in certain frequency
areas exhibits a shaped response.
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Shaped frequency response
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A shaped response is usually
designed to enhance a sound source in a particular application.
For instance, a microphone may have a peak in the 2 - 8 kHz
range to increase intelligibility for live vocals. This shape
is called a presence peak or rise. |
A microphone may also be designed to be less sensitive to certain
other frequencies. One example is reduced low frequency response
(low end roll-off) to minimize unwanted “boominess”
or stage rumble.
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