SHURE Live Sound Reinforcement User Manual

A Shure Educational Publication

MICROPHONE
TECHNIQUES

LIVE SOUND
REINFORCEMENT

Microphone Techniques

Table of Contents

for

LIVE SOUND

Introduction ........................................................................... 4

Microphone Characteristics.................................................. 5

Musical Instrument Characteristics..................................... 11

Acoustic Characteristics ..................................................... 14

Microphone Placement....................................................... 22

Stereo Microphone Techniques.......................................... 32

Microphone Selection Guide ............................................. 34

Glossary ............................................................................. 35

Live Sound

3

Microphone Techniques
for

LIVE SOUND

Introduction

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.

Introduction

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Microphone Techniques
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LIVE SOUND

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

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 sound­driven electrical generator. Sound waves strike a thin
plastic membrane (diaphragm) which vibrates in response.
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.

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 electrically­charged diaphragm/backplate assembly which forms a
sound-sensitive capacitor. Here, sound waves vibrate a
very thin metal or metal-coated-plastic diaphragm.
The diaphragm is mounted just in front of a rigid metal or
metal-coated-ceramic backplate. In electrical terms this
assembly or element is known as a capacitor (historically

- The type of

transducer

inside the

called a “condenser”),
which has the ability to store
a

charge

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.

The construction of a condenser microphone must
include some provision for maintaining the electrical
charge or
microphone has a permanent charge, maintained by a
special material deposited on the backplate or on the
diaphragm. Non-electret 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

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.

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.

or voltage. When

polarizing

power (a method of supplying power to a

voltage. An

electret

condenser

5

Microphone Techniques
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LIVE SOUND

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 polariziing voltage for
the element tself. 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
souorce, 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. In general, balanced dynamic
microphones can be connected to phantom powered
mixer inputs with no problem.

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.

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.

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 Condenser/dynamic scope photo

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Microphone Techniques
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LIVE 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

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. 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

Flat response microphones typically have an extended
frequency range. They reproduce a variety of sound
sources without changing or

response - The output level or sensitivity of the

frequency response.

coloring

the original sound.

A microphone whose response has peaks or dips in certain
frequency areas exhibits a

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
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.

presence

shaped

peak or rise. A microphone may also be

response.

The Decibel

The decibel (dB) is an expression often used in electrical
and acoustic measurements. The decibel is a number
that represents a ratio of two values of a quantity such as
voltage. It is actually a logarithmic ratio whose main
purpose is to scale a large measurement range down to
a much smaller and more useable range. The form of
the decibel relationship for voltage is:

dB = 20 x log(V1/V2)

where 20 is a constant, V1 is one voltage, V2 is the
other voltage, and log is logarithm base 10.

Examples:

What is the relationship in decibels between
100 volts and 1 volt?

Flat frequency response

Shaped frequency response

dB = 20 x log(100/1)
dB = 20 x log(100)
dB = 20 x 2 (the log of 100 is 2)
dB = 40

That is, 100 volts is 40dB greater than 1 volt.

What is the relationship in decibels between

0.001 volt and 1 volt?

dB = 20 x log(0.001/1)
dB = 20 x log(0.001)
dB = 20 x (-3) (the log of .001 is -3)
dB = -60

That is, 0.001 volt is 60dB less that 1 volt.

Similarly:

if one voltage is equal to the other they are 0dB different

if one voltage is twice the other they are 6dB different

if one voltage is ten times the other they are 20dB
different

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Microphone Techniques
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LIVE SOUND

Since the decibel is a ratio of two values, there must
be an explicit or implicit reference value for any
measurement given in dB. This is usually indicated
by a suffix on the decibel value such as: dBV
(reference to 1 volt which is 0dBV) or dB SPL
(reference to 0.0002 microbar which is 0dB Sound
Pressure Level)

1. Compare

b a

b/a

One reason that the decibel is so useful in certain
audio measurements is that this scaling function
closely approximates the behavior of human hearing
sensitivity. For example, a change of 1dB SPL is
about the smallest difference in loudness that can
be perceived while a 3dB SPL change is generally
noticeable. A 6dB SPL change is quite noticeable
and finally, a 10dB SPL change is perceived as
“twice as loud.”

2. Compress 3. scale (x 20)

100=1
101=10
102=100
103=1000
104=10,000
105=100,000
106=1,000,000

Decibel scale

for dBV or dB SPL

0
20
40
60
80
100
120

Directionality

to the direction or angle from which the sound arrives.

There are a number of different directional patterns found
in microphone design. These are typically plotted in a polar
pattern to graphically display the directionality of the
microphone. The polar pattern shows the variation in
sensitivity 360 degrees around the microphone, assuming
that the microphone is in the center and that 0 degrees
represents the front of the microphone.

The three basic directional types of microphones are
omnidirectional, unidirectional, and bidirectional.

The omnidirectional microphone has equal output or
sensitivity at all angles. Its coverage angle is a full 360
degrees. An omnidirectional microphone will pick up
the maximum amount of ambient sound. In live sound
situations an omni should be placed very close to the
sound source to pick up a useable balance between direct
sound and ambient sound. In addition, an omni cannot be

aimed

which may cause feedback.

- A microphone’s sensitivity to sound relative

away from undesired sources such as PA speakers

The choice of flat or shaped response microphones again
depends on the sound source, the sound system and
the environment. Flat response microphones are usually
desirable to reproduce instruments such as acoustic
guitars or pianos, especially with high quality sound
systems. They are also common in stereo miking and

distant pickup

more than a few feet from the sound source: the absence
of response peaks minimizes feedback and contributes
to a more natural sound. On the other hand, shaped
response microphones are preferred for closeup vocal
use and for certain instruments such as drums and
guitar amplifiers which may benefit from response
enhancements for
useful for reducing pickup of unwanted sound and noise
outside the frequency range of an instrument.

applications where the microphone is

presence

or

punch

. They are also

Omnidirectional

The unidirectional microphone is most sensitive to sound
arriving from one particular direction and is less sensitive
at other directions. The most common type is a cardioid
(heart-shaped) response. This has the most sensitivity at
0 degrees (on-axis) and is least sensitive at 180 degrees
(off-axis). The effective coverage or pickup angle of a
cardioid is about 130 degrees, that is up to about 65
degrees off axis at the front of the microphone. In addition,
the cardioid mic picks up only about one-third as much
ambient sound as an omni. Unidirectional microphones
isolate the desired on-axis sound from both unwanted
off-axis sound and from ambient noise.

8

Microphone Techniques
for

LIVE SOUND

Cardioid

For example, the use of a cardioid microphone for a guitar
amplifier which is near the drum set is one way to reduce
bleed-through of drums into the reinforced guitar sound.

Unidirectional microphones have several variations on the
cardioid pattern. Two of these are the supercardioid and
hypercardioid.

Both patterns offer narrower front pickup angles than the
cardioid (115 degrees for the supercardioid and 105
degrees for the hypercardioid) and also greater rejection
of ambient sound. While the cardioid is least sensitive at
the rear (180 degrees off-axis) the least sensitive direction
is at 126 degrees off-axis for the supercardioid and 110
degrees for the hypercardioid. When placed properly they
can provide more focused pickup and less ambient noise
than the cardioid pattern, but they have some pickup
directly at the rear, called a rear lobe. The rejection at the
rear is -12 dB for the supercardioid and only -6 dB for
the hypercardioid. A good cardioid type has at least
15-20 dB of rear rejection.

Supercardioid

The bidirectional microphone has maximum sensitivity at
both 0 degrees (front) and at 180 degrees (back). It has the
least amount of output at 90 degree angles (sides). The
coverage or pickup angle is only about 90 degrees at both
the front and the rear. It has the same amount of ambient
pickup as the cardioid. This mic could be used for picking
up two opposing sound sources, such as a vocal duet.
Though rarely found in sound reinforcement they are used
in certain stereo techniques, such as M-S (mid-side).

Microphone Polar Patterns Compared

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Microphone Techniques
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LIVE SOUND

Using Directional Patterns to
Reject Unwanted Sources

In sound reinforcement, microphones must often
be located in positions where they may pick up
unintended instrument or other sounds. Some
examples are: individual drum mics picking up
adjacent drums, vocal mics picking up overall
stage noise, and vocal mics picking up monitor
speakers. In each case there is a desired sound
source and one or more undesired sound sources.
Choosing the appropriate directional pattern can
help to maximize the desired sound and minimize
the undesired sound.

Although the direction for maximum pickup is
usually obvious (on-axis) the direction for least
pickup varies with microphone type. In particular,
the cardioid is least sensitive at the rear (180 de­grees off-axis) while the supercardioid and hyper­cardioid types actually have some rear pickup.
They are least sensitive at 125 degrees off-axis and
110 degrees off axis respectively.

For example, when using floor monitors with vocal
mics, the monitor should be aimed directly at the
rear axis of a cardioid microphone for maximum
gain-before-feedback. When using a supercardioid,
however, the monitor should be positioned
somewhat off to the side (55 degrees off the rear
axis) for best results. Likewise, when using
supercardioid or hypercardioid types on drum kits
be aware of the rear pickup of these mics and
angle them accordingly to avoid pickup of other
drums or cymbals.

Other directional related microphone characteristics:

Ambient sound rejection

microphones are less sensitive to off-axis sound than
omnidirectional types they pick up less overall ambient or
stage sound. Unidirectional mics should be used to control
ambient noise pickup to get a cleaner mix.

Distance factor

up less ambient sound than omnidirectional types they
may be used at somewhat greater distances from a sound
source and still achieve the same balance between the
direct sound and background or ambient sound. An omni
should be placed closer to the sound source than a
uni—about half the distance—to pick up the same
balance between direct sound and ambient sound.

Off-axis coloration

response that usually gets progressively more noticeable
as the arrival angle of sound increases. High frequencies
tend to be lost first, often resulting in “muddy” off-axis
sound.

Proximity effect

response increases as the mic is moved closer (within 2
feet) to the sound source. With close-up unidirectional
microphones (less than 1 foot), be aware of proximity effect
and roll off the bass until you obtain a more natural sound.
You can (1) roll off low frequencies on the mixer, or (2) use
a microphone designed to minimize proximity effect, or (3)
use a microphone with a bass rolloff switch, or (4) use
an omnidirectional microphone (which does not exhibit
proximity effect).

- Because directional microphones pick

- Change in a microphone’s frequency

- With unidirectional microphones, bass

- Since unidirectional

10

Proximity effect graph

Monitor speaker placement for

maximum rejection:

cardioid and supercardioid

Unidirectional microphones can not only help to isolate one
voice or instrument from other singers or instruments, but
can also minimize feedback, allowing higher gain. For
these reasons, unidirectional microphones are preferred
over omnidirectional microphones in almost all sound re­inforcement applications.

The electrical output of a microphone is usually specified
by level, impedance and wiring configuration. Output level
or sensitivity is the level of the electrical signal from the
microphone for a given input sound level. In general,
condenser microphones have higher sensitivity than
dynamic types. For weak or distant sounds a high sensitivity
microphone is desirable while loud or close-up sounds can
be picked up well by lower-sensitivity models.

The output impedance of a microphone is roughly equal to
the electrical resistance of its output: 150-600 ohms for
low impedance (low-Z) and 10,000 ohms or more for high
impedance.(high-Z). The practical concern is that low
impedance microphones can be used with cable lengths
of 1000 feet or more with no loss of quality while high
impedance types exhibit noticeable high frequency loss
with cable lengths greater than about 20 feet.

Finally, the wiring configuration of a microphone may be
balanced or unbalanced. A balanced output carries the
signal on two conductors (plus shield). The signals on each
conductor are the same level but opposite polarity (one
signal is positive when the other is negative). A balanced
microphone input amplifies only the difference between
the two signals and rejects any part of the signal which is
the same in each conductor. Any electrical noise or hum
picked up by a balanced (two-conductor) cable tends to be
identical in the two conductors and is therefore rejected by
the balanced input while the equal but opposite polarity
original signals are amplified. On the other hand, an
unbalanced microphone output carries its signal on a
single conductor (plus shield) and an unbalanced
microphone input amplifies any signal on that conductor.
Such a combination will be unable to reject any electrical
noise which has been picked up by the cable. Balanced,
low-impedance microphones are therefore recommended
for nearly all sound reinforcement applications.

The physical design of a microphone is its mechanical
and operational design. Types used in sound
reinforcement include: handheld, headworn, lavaliere,
overhead, stand-mounted, instrument-mounted and
surface-mounted designs. Most of these are available in

Microphone Techniques
for

LIVE SOUND

a choice of operating principle, frequency response,
directional pattern and electrical output. Often the
physical design is the first choice made for an application.
Understanding and choosing the other characteristics
can assist in producing the maximum quality microphone
signal and delivering it to the sound system with the
highest fidelity.

Musical Instrument Characteristics

Some background information on characteristics of musical
instruments may be helpful. Instruments and other sound
sources are characterized by their frequency output, by
their directional output and by their dynamic range.

Frequency output - the span of fundamental and
harmonic frequencies produced by an instrument, and the
balance or relative level of those frequencies.

Musical instruments have overall frequency ranges as
found in the chart below. The dark section of each line
indicates the range of fundamental frequencies and the
shaded section represents the range of the highest
harmonics or overtones of the instrument. The
fundamental frequency establishes the basic pitch of a
note played by an instrument while the harmonics
produce the

timbre

or characteristic tone.

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Microphone Techniques
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LIVE SOUND

Instrument frequency ranges

It is this timbre that distinguishes the sound of one
instrument from another. In this manner, we can tell
whether a piano or a trumpet just played that C note.
The following graphs show the levels of the fundamental
and harmonics associated with a trumpet and an oboe
each playing the same note.

trumpet in B

200 500 50004000300020001000

frequency

Instrument spectra comparison

The number of harmonics along with the relative level of
the harmonics is noticeably different between these two
instruments and provides each instrument with its own
unique sound.

A microphone which responds evenly to the full range
of an instrument will reproduce the most natural sound
from an instrument. A microphone which responds
unevenly or to less than the full range will alter the
sound of the instrument, though this effect may be
desirable in some cases.

Directional output - the three-dimensional pattern of sound
waves radiated by an instrument.

A musical instrument radiates a different tone quality
(timbre) in every direction, and each part of the instrument
produces a different timbre. Most musical instruments are
designed to sound best at a distance, typically two or more
feet away. At this distance, the sounds of the various parts
of the instrument combine into a pleasing composite.
In addition, many instruments produce this balanced
sound only in a particular direction. A microphone placed
at such distance and direction tends to pick up a natural
or well-balanced tone quality.

On the other hand, a microphone placed close to the
instrument tends to emphasize the part of the instrument
that the microphone is near. The resulting sound may not
be representative of the instrument as a whole. Thus, the
reinforced tonal balance of an instrument is strongly
affected by the microphone position relative to the
instrument.

oboe

Unfortunately, it is difficult, if not impossible, to place a
microphone at the “natural sounding” distance from an

b

instrument in a sound reinforcement situation without
picking up other (undesired) sounds and/or acoustic
feedback. Close microphone placement is usually the
only practical way to achieve sufficient isolation and
gain-before-feedback. But since the sound picked up
close to a source can vary significantly with small
changes in microphone position, it is very useful to
experiment with microphone location and orientation.
In some cases more than one microphone may be
required to get a good sound from a large instrument
such as a piano.

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