Shure MUSIC EDUCATORS User manual

MUSIC EDUCATORS
By Gino Sigismondi
AUDIO
SYSTEMS
GUIDE
A Shure Educational Publication

Music
Gui
d
Educators

index
d
e
Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
Recording . . . . . . . . . . . . . . . . . . . . . . . . . . 5
The Parts of a Recording System . . . . . . . . 5
Microphones . . . . . . . . . . . . . . . . . . . . . 5
Recording Devices . . . . . . . . . . . . . . . . 9
Mixers . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Hooking it up . . . . . . . . . . . . . . . . . . . . . 11
Sound Reinforcement for Music . . . . . . . 11
A Basic Sound System . . . . . . . . . . . . . 11
Microphones . . . . . . . . . . . . . . . . . . . . . 12
Mixers, Amplifiers and Loudspeakers . . 16
Signal Processors . . . . . . . . . . . . . . . . . 18
Hooking it up . . . . . . . . . . . . . . . . . . . . . 21
Sound Reinforcement for Theater . . . . . . 22
The Realities of Theater Sound . . . . . . . . . . 22
Lavalier Microphone Techniques for Theater 24
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
About the Author . . . . . . . . . . . . . . . . . . . . 31

Introduction
An often overlooked yet vital part of modern musical performances is the sound
reinforcement (PA) system. In a perfect world, a trained professional would always
be available to purchase, setup, and operate the school’s sound system. In reality,
most educational facilities do not have the available funds for such a luxury.
The responsibility then falls to the next most likely person at school to run the sound
system, the music director. After all, you just need a few microphones and a couple
of loudspeakers, and it’s time to go on tour! And we want it recorded as well!
Unfortunately, sound system setup is not quite that simple. It doesn’t, however, need
to be overly complicated. While the extreme quantity of choices available at your
local music shop may seem daunting (Cardioid? Dynamic? Low Impedance! Help!),
with a few basic guidelines, you can learn what you need, how to connect it, and
even how to make it sound good.
This guide will help you choose equipment for a variety of applications.
For simplicity sake, we will make recommendations of the Shure microphones you
mightconsiderforeachapplication.For those interested in the more technical aspects
of audio equipment, Shure publishes several booklets on a variety of audio topics,
including specific microphone techniques and wireless microphone operation.
Typical scenarios for school music applications include:
• Sound Reinforcement for Large Ensembles
(Concert Band, Orchestra, Choir)
• Sound Reinforcement for Small Ensembles
(Jazz Combo, Vocal Jazz or "Swing" Choir, Rock bands)
• Recording small ensembles
• Recording large ensembles
• Sound for theatrical productions
Recording will be discussed first, due to the smaller number of required
components, and reduced complexity. Feedback and room acoustics play a small
(or even non-existent) role in basic ensemble recording. Additionally, many of the
microphone, and possibly mixer, choices made for recording are equally applicable
in live sound reinforcement. Remember, there are few rules in audio – if it sounds
good to you, it is good. For every application, there will be a good, better, and best
option. A little knowledge and some common sense will allow you to choose a good
system with a reasonable budget and a minimum of frustration.
4

Recording
What do I need?
The parts of a recording system.
To make a decent recording there are two or
three components to consider:
1. The first, and most important, is the microphone. Choosing the right micro-
phone ensures accurate pickup of the desired sounds.
2. Next, consider the recording device. Recording equipment comes in many
shapes and sizes, from simple cassette tape recorders to advanced digital
multitrack machines.
3. Lastly, depending on the capabilities of the recording device, you may need a
mixer. A mixer’s purpose is two-fold; they are used to combine (or "mix")
multiple microphones together, and to properly interface microphones to the
recording device. Most consumer-quality cassette tape recorders, for example,
do not allow a microphone to be directly connected to the record inputs.
A microphone has an extremely low output level that would result in little or no
signal actually making it to tape. A mixer provides gain, which raises the
signal level from the microphone to a level that is acceptable to recorders that
don’t have microphone inputs.
Microphones
The most important questions to ask when choosing microphones are:
a) what are you recording? and
b) what are you using to record?
First, a little about microphones. Microphones are basically simple devices designed
to do one thing: convert sound waves in the air to their electrical equivalent. One
of the first questions you may encounter is, "Do you want a dynamic or condenser
microphone?" These are the two most popular types of microphones in the world.
• Dynamic microphones are typically inexpensive and rugged, with fairly low
sensitivity. In layman’s terms, this means they are good for handheld or
"close-miked" applications. Dynamics are commonly used for solo vocalists
and on drum kits.
• Condenser microphones, on the other hand, are typically (but not always) much
more sensitive than dynamics. Make a good quality condenser microphone your
first choice for miking ensembles, or other applications where the microphone
will be placed at a distance (> 2 ft.) from the sound source. Condensers are
typically used for recording orchestras, choirs, and in other applications where
you wish to capture the sound of the ensemble, versus individual sounds.
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Large ensembles (band, orchestra, choir)
Use a stereo microphone setup to most accurately
capture the sound of a large ensemble. Stereo
recording is not as complicated as it sounds. For
simplicity sake, we’ll use the most basic type of stereo
microphone techniques, the X-Y pattern. Use two
microphones of the same model with the two mic
capsulesplacedascloseaspossible, and facingeach
other at an angle ranging from 90 – 135 degrees,
depending on the size of the sound source. (see
figure 1). For a wider coverage area, the larger angles
shouldbe used. TheX-Ypatternresults in goodstereo
separation and excellent mono compatibility.
A second, somewhat simpler way to record in stereo uses what is known as a
"single-point" stereo microphone, such as the VP88. The VP88 has a single micro-
phone housing that contains two microphone elements, electrically combined to
produce a stereo output. The advantage to this type of microphone is simplicity; put
the microphone on a stand and point it at what you want to record.
When recording a large ensemble, you may choose to use more than two microphones
to adequately cover each section. A technique known as "area" coverage uses multiple
microphones to cover small sections of the ensemble. Using a choir as an example,
use one microphone for each 6-9 foot wide section, and aim the microphone capsule
towards the last row. Microphones should be placed 2-3 feet in front of the first row of
the choir (see figure 2). The same technique can be applied to concert band or
orchestra, by using one microphone per section.
6
0.6 - 1m
(2 - 3 ft)
0.6 - 1m
(2 - 3 ft)
Figure 2: Area miking - side view
Figure 1: Choir microphone positions - stereo
top view

Recommended Equipment/
Suggested Models:
• 2 cardioid-pattern
condenser microphones
(SM81, PG81, KSM109)
• Microphone stand(s)
• Shure A27M – an accessory that allows you
to mount two microphones on one stand.
• Microphone cables with XLR connectors
• Stereo microphone mixer with at least
2 microphone inputs
• Recording device (see next section)
• Cables to connect mixer to recording device
Small ensembles (jazz combos, string quartet, vocal jazz groups)
The stereo techniques described above can also be successfully applied to smaller
ensembles, but to achieve a more "pop" sound, use multiple close microphones,
generally one per instrument. On the following page are some simple techniques for
getting good sounds for a variety of instruments or vocals.
Recommended Equipment/Suggested Models:
• Vocal microphone (Beta 58A, SM58, PG58)
• Guitar amplifier microphone (SM57, PG57)
• Drum microphones (Beta 52 or PG52, SM57,
two PG81 or SM81)
• Piano microphone (PG81, SM81, or KSM27)
• Woodwinds (SM57, PG57)
• Brass (SM57, PG57)
• Microphone stands
• Microphone cables with XLR connectors
• Microphone mixer with enough inputs to handle
the desired number of microphones
• Recording device (see next section)
• Cables to connect mixer to recording device
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Connectors commonly
used for audio
(from left to right:
3-Pin XLR female,
3-Pin XLR female – black,
3-Pin XLR male,
1/4” mono plug,
1/4” mono plug – black
PG57
Dynamic
Microphone
KSM109
Condenser
Microphone

Vocals –Try using ahandheldstyle(althoughmountedon
a stand to reduce handling noise) dynamic microphone,
such as the SM58 or Beta 58A. Place the microphone
roughly 3-4 inches away, and pointed somewhere
between the nose and mouth. If possible, try to isolate the
vocalist from any unwanted sounds – in another room
would be ideal (see figure 3).
Electric Guitar Amplifier –
Use an SM57 microphone roughly 1-4 inches
from the loudspeaker, pointed toward the center
of the speaker cone (see figure 4).
Drums (see figure 5) – While seemingly complex, you can achieve a relatively decent
drum sound with only four microphones:
– Kick Drum – Place a
Beta 52 (or PG52) inside
the drum, approximately
1-6 inches from the
beater head.
– Snare Drum – Use an
SM57 to mike the top
head, placed at a 45
degree angle.
– Toms and cymbals –
Use a pair of condenser
microphones (PG81 or
SM81) suspended over
the drum kit, either
spaced apart or using one of the stereo
microphone techniques described earlier.
If you have a limited number of microphones, use the following chart:
8
Top Side
Number of Microphones Positioning
One Use an "overhead"
Two Kick drum and overhead
Three Kick drum, snare, and overhead
Four Kick drum, snare, and two overheads
Figure 3
Figure 4
Figure 5
PG81
PG52
SM57

Piano – For a grand or baby grand, place
a PG81 or KSM27 roughly 12 inches
above the middle strings, and 8 inches
from the hammers. The lid should be at full
stick to allow enough clearance for the
microphone. For an upright, place a
similar microphone just over the open top,
above the treble strings (see figure 6).
Woodwinds – Use an SM57 placed a few inches above
the bell and aimed at the sound holes (see figure 7).
Brass – Place an SM57 1 to 2 feet in front of the bell.
The closer the microphone, the brighter the sound.
Recording devices
Two things to consider when choosing a recording format are sound quality and ease
of use. While it may be tempting to buy the same gear that the local recording studio
has, keep in mind your abilities, and how much time you are willing to devote to
pouring over manuals and pushing little buttons. Let’s take at look at some of the more
popular formats.
Cassette
Everyone has operated a cassette tape recorder at some point in their life, making
this a good choice for simple, straight ahead recording. Cassettes are inexpensive
and readily available. Cassette recorders can be purchased at your local electronics
superstore for attractively low prices, or more professional units are usually available
through music and sound retailers. Cassettes, however, rate near the bottom of
the sound quality totem pole. While cassette recordings are perfectly appropriate
for rehearsal or demo tapes, this is not the format to choose if you are looking for a
more "professional" sound, especially if you intend to make the finished product
available for sale.
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Hammers
6”-12”
8”
Figure 6
Figure 7

Most consumer-level tape recorders will not accept a microphone directly. A mixer
(see the next section) will be required to properly interface microphones with a
cassette deck. Certain models of professional level cassette recorders have
microphone inputs on them, but they are often considerably more expensive.
Minidisc
Introduced in the early 1990s, the minidisc format combines the ease of cassette
recording with near-CD quality results. Sound is digitally recorded onto a disk
smaller than a computer’s floppy disk. Minidiscs also allow extensive editing of
the recorded material. Tracks can be moved, divided, combined, named, and
erased, tasks that are impossible with tape (unless you are really good with a
razor blade!) Consumer minidisc recorders are very similar to cassette recorders
in terms of their operation and audio interface, so any of the recording
techniques described above will apply to both formats.
PC-based Recording
With the price of hard disk storage continuing to plummet, PC-based recording
is quickly becoming a relatively inexpensive way to record high quality audio.
Computers are very useful for making multitrack recordings, which requires
several tracks of audio recorded and later combined into a stereo mix.
Unfortunately, a full discussion of multitrack recording techniques is beyond the
scope of this booklet. (For more information on multitrack recording, see the
Shure/Tascam publication "Microphones and Multitracks". This booklet also
introduces stand-alone multitrack recorders.)
Interfacing a microphone with a computer can be a tricky proposition. Most computer
sound cards have microphone inputs that are designed for low-cost "stick" micro-
phones that are only useful in voice recognition or internet telephony applications.
Better results are obtained with an interface that accepts professional microphones. A
computer recording interface that has microphone inputs will result in better sound
and less time spent fooling around with adapters.
Personal computers also offer an inexpensive way to produce CDs. By recording your
music into a computer, a CD recorder can take those files and "burn" them onto
a recordable CD. Even if the PC isn’t used as the main recording device, the other
formats discussed above can easily be transferred into a computer. Stand-alone CD
recorders are also available, and work on basically the same principle.
10

Mixers
Most likely, the recording device you
choose will not allow you to directly
connect a microphone to it. This is where
the mixer comes into play. Mixers come in
many varieties, a basic mixer will suffice for this application. Be sure that the mixer you
choose has enough inputs to handle the number of microphones you are using.
Another useful feature, common on just about all modern microphone mixers,
is something called phantom power. Not nearly as mysterious as it sounds, phantom
power is simply a voltage that travels back down the microphone cable to power
condenser type microphones. Dynamic microphones do not require phantom power,
nor will they be harmed if they are plugged into a microphone input that has phantom
power turned on. See the "Sound Reinforcement" section for a more complete
discussion of mixers.
Hooking it up
Step 1: Position the microphones as per the previous instructions.
Step 2: Using the XLR microphone cable, connect the microphones
to the mixer inputs.
Step 3: If you are using condenser microphones, be sure phantom
power is turned on at the mixer.
Step 4: Connect the outputs of the mixer to the inputs of the
recording device with the proper cables.
Step 5: Have the ensemble perform, and set levels accordingly (see
the instructions supplied with the mixer and/or recording device).
Step 6: Hit RECORD, and play!
Sound Reinforcement for Music
What do I need? A basic sound system.
A sound reinforcement system and a recording setup have two major components in
common, microphones and mixers. Microphones are used to capture the sound, and
the mixer is used to combine those signals together. The output of the mixer, instead
of feeding a recording device, is connected to a power amplifier, which boosts the
signal to "speaker" level. One or more loudspeakers are connected to the outputs of
the amplifier. At this point the signal is converted from the electrical domain back to
the variations in air pressure we recognize as sound, albeit at a much higher level.
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Signal processors are devices, usually connected between the mixer and power
amplifier, used to enhance the signal or fix problems with the sound. The most
common types of processors are equalizers, effects processors, and compressors.
An equalizer is basically an extremely selective set of tone controls that allow you to
boost or cut specific frequencies. Typical applications for equalizers are tone shaping
and feedback control (see sidebar on feedback - page 19). Effects processors are
used to create special sound effects, such as reverb (sounds like a big, reverberant
room) and delay (echo), among others. Compressors control varying signal levels.
A loud signal that passes above the compressor’s threshold is reduced (or com-
pressed) by a given amount. A compressor can also help prevent distortion or
damage to the loudspeakers. Of the processors listed above, the equalizer is the most
useful in just about any sound reinforcement application.
Microphones
Many of the same microphone rules used in recording apply equally as well for live
sound reinforcement. If you skipped the recording chapter, please back up and read the
section on microphones – the laws of physics don’t change for sound reinforcement.
A major difference between microphone placement in live sound versus recording
is proximity to the sound source. The goal in sound reinforcement is to get the
microphone as close as possible to the sound source, for two reasons:
• Primarily, placing all microphones as close as possible achieves maximum
sound level before feedback occurs.
• Secondly, close-miking reduces leakage and pickup of unwanted sounds.
Other general recommendations for live sound microphone techniques include:
• Try to get the sound source (instrument, voice, or amplifier) to sound good
acoustically before attempting to put a microphone on it.
• Use as few microphones as necessary. The more microphones you use, the
more likely you are to have feedback problems. Using fewer microphones will
allow more volume before feedback occurs.
• When multiple microphones are used, the distance between microphones
should be three times the distance from
each microphone to its intended sound
source. This prevents comb filtering, an
unpleasant, hollow sound created when
two or more microphones pick up the
same sound source. This technique is
known as the 3-to-1 Rule (see figure 8).
12
Figure 8: 3-to-1 rule

Large ensembles
(band, orchestra, choir)
Large ensembles generally do not need
much sound reinforcement, unless
the performance area is unusually
spacious. For choirs, use a technique
known as area miking (see figure 2, page 6). To determine the right number of
microphones, use one for every 10-15 voices. Remember not to use more micro-
phones than absolutely necessary and follow the 3-to-1 Rule (see page 12). As
in recording, use a flat frequency response condenser, preferably in a cardioid-
pattern. If the microphones will be suspended from the ceiling, make certain they
are aimed towards the singers’ mouths, not at tops of their heads. Handheld,
dynamic microphones are designed for up-close use only, and are not appropri-
ate for choir-miking under any circumstance.
Similar techniques apply to orchestras and concert bands. Depending on the
size of the ensemble, use at least one microphone per section. Remember to
keep the microphones as close in as possible and as far from the loudspeakers
as possible. For soloists or lead instruments, consider using a microphone
attached directly to the instrument. Miniature condenser microphones can be
attached or clipped onto the bells of brass and woodwind instruments or the chin
rest of string instruments.
Recommended Equipment/Suggested Models:
• One condenser microphone (PG81 or SM81) per section
• Soloist microphones – SM57 (horns, stand-mounted) or Beta 98H/C
(horns or strings, clipped-on)
• Choir microphones – MX202 (permanent install, hung from ceiling or
stand-mounted) or PG81(stand-mounted only)
• Microphone stands
• An assortment of XLR microphone cables in varying lengths
• A mixer with enough input channels
• Power amplifier
• Loudspeakers
• Interconnect cables
• Signal processors (if desired)
Small ensembles (jazz combos, string quartet, vocal jazz groups)
For smaller groups, close-miking becomes essential for reducing leakage between
the instruments. Let’s examine each instrument type individually.
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Lead vocal - Use a handheld, directional microphone (PG58, SM58 or Beta 58A).
The microphone should be touching the lips or just a few inches away. The same
technique applies to background vocalists or vocal ensembles (see figure 3).
Electric Guitar Amplifier – Use an SM57 (or PG57) microphone
roughly 1-4 inches from the loudspeaker, pointed toward the
center of the speaker cone (see figure 4).
Drums – Jazz Band – Generally, three microphones are sufficient
to achieve an authentic jazz drum sound. Place one microphone
in front of the kick drum (Beta 52 or PG52), and use a pair of decent
condenser microphones (PG81 or SM81) above the kit for
cymbals, toms, and snare drum.
Drums (see figure 9) – Rock Band – To get a "big" rock sound, try
toplacea microphone on every drum, withthefollowingguidelines:
Snare drum - Use a dynamic microphone (SM57) on a short boom stand. Aim the
microphone at the top head, just above the top edge of the drum.
Kick (bass) drum - Use a front drum head that has a hole in it, or remove the front
head altogether. Mount a microphone (Beta 52 or PG52) on a short boom and
position it inside the drum a few inches from the beater head, on axis with the beater.
Use a wood beater to get more "attack", or stuff the drum with a blanket or pillows to
reduce boominess.
14
PG81
SM57
SM81
PG56
PG52
Figure 9
PG81
Condenser
Microphone

Tom-toms – Use one microphone (SM57
or PG56) for every two toms. Position them
close to the heads, in a similar method as
used for the snare drum. If enough
microphones are available put one on
every tom-tom.
Cymbals – Place a condenser microphone (PG81 or SM81) above the hi-hats,
pointed down and slightly away from the drummer. Or, angle the snare mic slightly
towards the hi-hats. To pick up the rest of cymbals, place another condenser near the
ride cymbal, a foot or two above.
Piano – Open the lid and aim a condenser microphone (PG81) just over the top,
above the treble strings (see figure 10).
Grand piano – Use two flat-response condenser microphones (PG81 or SM81), one
positioned 12 inches above the treble strings and the other above the lower strings.
Both mics should be about 8 inches from the hammers.
Upright bass – For the most natural sound, place a condenser (PG81) about
6 inches in front of the bass, just above the bridge.
String quartet: For violins and violas, use a miniature condenser microphone
(Beta 98H/C) clipped to the instrument. The best place to attach a microphone
without altering the sound of the instrument is the chin rest. For cellos, use a stand-
mounted microphone (PG81) aimed at the bridge, about a foot away.
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Open
Mic
Open
Figure 10

16
Saxophone (see figure 11) – Again, a miniature
condenser microphone (Beta 98H/C) clipped to the
instrument and aimed into the bell yields good, up-front
sound quality with great isolation. Alternatively, a
dynamic microphone on a stand (SM57) provides
similar results, but is slightly more cumbersome.
Brass (Trumpets, trombone, tuba)– Similar to the sax-
ophone, clip a Beta 98H/C to the bell of the instrument.
If the sound seems excessively bright, especially the
trumpet, try slightly off-axis to get a more mellow tone.
Recommended Equipment/Suggested Models:
• Vocal microphone (PG58, SM58 or Beta 58A)
• Guitar amplifier microphone (PG57, SM57)
• Drum microphones (Beta 52 or PG52, SM57s, two SM81 or PG81)
• Piano microphone (PG81, SM81, or KSM27)
• Woodwinds (SM57 or Beta 98H/C)
• Brass (SM57 or Beta 98H/C)
• Stringed instruments (PG81 or Beta 98H/C)
• Microphone stands
• An assortment of XLR microphone cables in varying lengths
• A mixer with enough input channels
• Power amplifier
• Loudspeakers
• Interconnect cables
• Signal processors (if desired)
Mixers, Amplifiers and Loudspeakers
As in recording, mixers are used to combine microphone signals together. Mixers
designed for sound reinforcement of musical instruments usually feature a set of extra
controls for each microphone beyond just a volume control. These typically include a
gain, or "trim" control, an EQ section, a set of auxiliary send volume controls, a bus
section, a pan control, and a fader for volume control (see figure 12).
The gain control boosts the signal from microphone up to line level. The next stage
is the EQ, used for tone shaping. A typical mixer will have knobs to boost or cut the
highs and lows. Also common is a "sweepable" midrange, which utilizes two controls.
One knob selects the frequency, and the other one boosts or cuts that frequency.
Figure 11
SM58
Dynamic Vocal
Microphone

Auxiliary sends are additional outputs
from the mixer that can be used to route
the microphone signal to
other devices without
affecting the main output
of the mixer. Common
applications for auxiliary sends include stage monitors or effects
processors. The bus section is used to assign the microphone sig-
nal to the main outputs of the mixer. The pan control typically serves
two functions, either working with the bus section to choose which
output the signal is routed to, or to adjust the left-to-right balance if
you are using a stereo sound system. At the bottom of the channel
strip is the fader, which gives you precise control of the volume of
each microphone that is assigned to the master outputs of the mixer.
Amplifiers and loudspeakers work together to reproduce the com-
bined microphone signals from the mixer at a much higher level. A
typical power amplifier has two inputs (left and right) and two outputs
to connect to loudspeakers. Note that amplifier outputs are designed
for loudspeakers only! The signal at this point is much too high
to connect to any device besides a loudspeaker. Most amplifiers also
have input sensitivity controls as well.
Loudspeakers (see figure 13) come in many shapes and sizes,
choosing the right one depends largely on the individual application.
Some things to consider: the size of the auditorium, portability
requirements, and budget. Most sound
reinforcement loudspeakers are multi-way
devices, which means each speaker cabinet includes two (or
more) individual loudspeakers, each optimized to reproduce a
specific frequency range. Two-way boxes with a woofer for low
frequencies and a horn for highs are the most common variety.
This type of loudspeaker is appropriate for applications involving
voice reproduction and most musical instruments that do not
require a good deal of low-end response. Subwoofers can
complement the two-way boxes for rock bands or music with a
considerable amount of low frequency content. Large touring
sound systems occasionally employ up to a five-way system.
For ease of setup and portability, these devices are occasionally combined into
one unit, either a powered mixer (mixer/amplifier) or powered loudspeaker
(amplifier/loudspeaker).
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Figure 13:
Loudspeaker
Figure 12:
Mixer Input
Channel

Signal Processors
A signal processor enhances the audio signal, or can assist in correcting
imperfections in the sound system. While not usually essential to the operation of a
sound system, they offer some significant advantages. Signal processing equipment
includes equalizers, special effects processors, and dynamic processors.
Equalizers come in two basic varieties: graphic and parametric. The graphic equalizer
(see figure 14) is the most common type. The controls on a graphic equalizer consist
of a row of faders that are used to cut or boost specific frequencies. The more faders
there are, the more precise the level of control. Each fader represents a "band";
common graphic equalizers are 5-band, 10-band, 15-band, and 31-band. Graphic
equalizers give the user visual "feedback" (no pun intended) by graphically representing
which frequencies have been cut or boosted. While easy to use, graphic equalizers
only allow alteration of the frequencies shown on the front panel. For example, a 31-
band equalizer allows you to cut at 400 Hz (Hertz) and 500 Hz, but not anywhere in
between. A cut at 450 Hz could be simulated by reducing both 400 and 500 Hz,
but this results in cutting more frequencies than is necessary. A parametric equalizer,
on the other hand, offers more precise control. Fully parametric equalizers consist of
three controls; frequency, boost/cut, and bandwidth. The frequency control permits
the user to select a specific frequency, the boost/cut control selects how much that
frequency is raised or lowered, and the bandwidth control selects how many adjacent
frequencies are affected by the adjustment.
Equalizers are often employed in an attempt to control feedback. While not the
ultimate solution (see sidebar on feedback), they can be a useful tool in this applica-
tion. To begin, turn on all microphones, and raise their individual levels to the point
where they will most likely be used during the performance. Slowly bring up the
master output level of the mixer until feedback occurs. Now go to the equalizer and
pull down the offending frequency roughly 3dB (decibel). If the feedback is a "hoot"
or "howl" try cutting in the 250 to 500 Hz range. A "singing" tone may be around 1 kHz.
"Whistles" and "screeches" tend to occur above 2 kHz. Very rarely does feedback
occur below 80 Hz or above 8 kHz. It takes practice to develop an ear for equalizing
a sound system, so be patient. After locating the first feedback frequency, begin
turning up the system again until the next frequency begins ringing. Repeat the above
18
Figure 14: Graphic Equalizer

steps until the desired level is reached, but
do not over equalize. Keep in mind the
equalizers can only provide a maximum
level increase of 3 to 9 dB. Parametric
equalizers, though more confusing to the
novice user, allow for more precise control
of feedback frequencies.
Automatic feedback reducers will accomplish the same results as above. They find and
cut the frequencies that are feeding back automatically. The same precautions listed
above apply to feedback reducers as well as equalizers. Automatic feedback reducers
are very helpful in wireless microphone applications. Remember that microphone
placement is crucial to eliminating feedback, and the temptation to wander away from
the ideal microphone position when using a wireless is great. If the performer gets too
close to a loudspeaker, feedback will result; a good feedback reducer will be able to
catch and eliminate the feedback faster than a human operator.
Feedback
One of the most commonly asked questions in professional audio is "What
microphone can I use that doesn’t cause feedback?" The answer to the question
is, of course, that no such microphone exists. Feedback results from a combina-
tion of many factors, including loudspeaker placement, microphone placement,
the frequency response of both devices, and room acoustics.
What is feedback?
Feedback is characterized by a sustained, ringing tone, which can vary from a
low rumble to a piercing screech. Echoes and reverberation caused by room
acoustics, as well as ground buzz and other extraneous noises, are not the same
thing as feedback, and cannot be cured in the same manner.
What causes feedback?
Feedback occurs whenever the sound entering a microphone is reproduced by a
loudspeaker, picked up by the microphone, and re-amplified again and again. The
familiar howl of feedback is an oscillation that is triggered by sound entering the
microphone. The easiest way to create feedback is to point a microphone directly
into a loudspeaker. (We don’t recommend you try this!) Placing the microphone
too close to the loudspeaker, too far from the sound source, or simply turning
the microphone up too loud exacerbates feedback problems. Other contributing
factors are too many open microphones, poor room acoustics, and uneven
frequency response in either the microphones or loudspeakers.
Music
Guide
Educators
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What can I do about feedback?
The single easiest way to reduce feedback is to move the microphone closer
to the desired sound source. Additionally, using a directional microphone
(cardioid, supercardioid, etc.) will typically increase the amount of gain
before feedback. Reducing the number of open microphones with an
automatic mixer will also improve the situation. Try to keep microphones and
loudspeakers as far away from each other as possible. Lastly, acoustically treat
the room to eliminate hard, reflective surfaces such as glass, marble, and wood.
When all of the above solutions have been exhausted, the next step is to look
towards equalizers and automatic feedback reducers.
Compressors, as mentioned above, are used to control varying sound levels. When
a singer is performing, certain notes or phrases tend to be louder than
others. Typically, that singer’s volume is set to the maximum possible level
without causing distortion further downstream in the sound system. In a loud
environment, quieter passages will get lost unless someone "rides" the vocal fader
to boost the singer during these quiet passages. The volume then needs to be
reduced to prevent distortion when the singer gets loud again. A compressor allows
the volume to be left at the "boosted" level by reducing the peaks (loud parts) by a
preset amount so they don’t cause distortion. An unfortunate myth that arose from
this practice promotes the idea that the compressor actually makes quiet sounds
louder. Two controls common to most compressors are threshold and ratio.
The threshold determines at what point the compressor function activates, and
ratio controls how much the signal is compressed. A lower threshold causes the
compressor to activate on quieter sounds, while a higher threshold requires louder
sound to activate. Similarly, a lower ratio control reduces the level by a small amount
(e.g. a ratio of 2:1 results in a level half of what it would normally be.) A higher ratio
(such as 10:1) would reduce the signal to a much greater degree.
A specialized form of compressor, called a limiter, uses a very high ratio (often ∞:1)
that prevents any signal from surpassing the level set by the threshold. Limiters are
primarily used as overall system protection at the output of the mixer, while
compressors are typically employed on an individual channel basis.
Finally, special effects processors allow the user to add ambience (reverb) or create
unique sounds (chorus, flange, delay).
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