SS FAll - Advanced Audio Video Technologies

SS FAll - Advanced Audio Video Technologies
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Sound Operator Manual
In This Issue
A newsletter for
anyone who
wants to learn
about sound!
P.O. Box 1793 • Sulphur, LA 70664
(337) 527-1001 • www.advancedav.biz
Microphones don’t draw-in
sound. Learn how they
actually work...
Page 1
Advanced Audio/Video Technologies exists to provide products and
services to assist in presenting a quality audio/visual experience.
Advanced A/V can provide your school with:
• Document Cameras
What can be done if you
have feedback problems...
Page 2
Discover the meaning of
Pre and Post...
Page 3
• Multimedia Projectors and
replacement lamps
The Vacuum Cleaner Myth
• Projection screens
• Portable/installed sound systems
Advanced
• And many other services
For More Information
Advanced
Technologies
Technologies
P.O. Box 1793
Sulphur, LA 70664
www.advancedav.biz
Phone:
(337) 527-1001
(337) 513-9646
Advanced Audio/Video Technologies is a
LOCAL service provider for all of your Audio/Visual needs!
Call today!
(337) 527-1001
There are many microphone mysteries. Many
users do not really understand how they work
and why some microphones work better for
certain applications. The microphone facts in
this article will help you better use your
microphones.
Website:
www.advancedav.biz
Email:
aavtinfo@yahoo.com
Myth: Many people feel that microphones
are like vacuum cleaners. The microphone is
mounted on a stand and it “sucks” in sound.
Better microphones have more suction so they
pick up sounds from farther away. The term
“reach” is often heard. One microphone is said
to have better reach than another.
Fact: However, microphones are not like
vacuum cleaners. They only sit at a point in
space and measure (pick up) what happens to
the change in air pressure at that particular
point. The air pressure changes when someone
talks, sings or plays an instrument. Of course,
the microphone will also pick up unwanted
pressure changes from airplanes flying overhead
or babies crying in the audience. The vocal
cords of the baby produce changes in the air
pressure which emanates from the baby’s
1
mouth. The sound waves are like ripples producing from a rock thrown into a smooth pond.
The ripples move out from a central point
where the rock first hit the water. As the ripples
travel outward, they gradually get weaker and
weaker until the water is smooth again.
The microphone measures the tiny changes
in air pressure. This change is converted into a
very small electrical voltage by the microphone
transducer. Therefore, one particular microphone
will not naturally “pick up” better than another
at a certain distance. There are other factors
which may make one microphone appear to
work better.
Common Types of Microphones
The dynamic microphone (Figure 1) is the
most common type of
modern microphone. It
has three basic parts:
the diaphragm, coil and
magnet. The diaphragm
is a very light domeshaped form. A coil of
a very thin wire is
Continued on page 2
Diaphragm
Ma gnet
Coil
Figure 1 Dynamic
Microphone
Continued from page 1
attached to the rear of the diaphragm. The
coil movement within the magnetic field
produces a tiny voltage in the coil. This
tiny voltage is sent down the microphone
cable to a preamplifier in the microphone
mixer.
The other major
type of microphone
transducer is the
capacitance
or
Dia phr a gm
electret microphone
(Figure 2). This type
has a diaphragm
like the dynamic
microphone.
However, it does
Back
Back Plate
Plate
not use a coil and
magnet. Instead, the
Figure 2 Electret
diaphragm is actuMicrophone
ally a capacitor (it
has the ability to store and release electricity). When air pressure moves the
diaphragm of this microphone, the distance
between the diaphragm and the back plate
changes. This produces a change in capacitance which is converted to an electrical
voltage that is sent to the microphone preamplifier. Power must be provided for the
capacitance microphone since it utilizes
some electronics. Power can be furnished
by a battery or from the mixer itself. This
type of power is usually called phantom
power since the power from the mixer uses
the same wires the tiny audio voltage travels
along.
Capacitance microphones have traditionally been more sensitive than dynamic
microphones. In other words, for the same
change in air pressure, the capacitance
microphone will produce a greater voltage.
This advantage has become less distinct as
better magnets in dynamic microphones
have produced higher outputs.
doubles. A 10 dB decrease would seem as
though the sound level dropped in half. A 3
dB drop is barely perceived. The 6 dB drop
is about a quarter drop in perceived level to
your ears.
1”
4”
4’
1’
Figure 3 Distance from the microphone
affects volume dynamics
What if the talker started at a position of
one foot from the microphone. Moving to
the two foot mark would produce only 6
dB drop in level. The talker would have to
move another 2 feet for a total of 4 feet to
get to the 12 dB level drop. In other words,
natural movement would not be very audible
if the talker started at the 1 foot position.
Maximum Gain-Before-Feedback
How is this fact used in real applications
or microphone techniques? Let’s say a
talker places a microphone 1” from his
mouth (see Figure 3). The microphone is
mounted on a gooseneck which is attached
to the podium. If the talker moves away
another 1” he will have doubled the distance
from the microphone, so the SPL reaching
L o u d s p e a ke r
What can be done if the volume is not
loud enough before that annoying feedback
occurs? There are some things which can
be adjusted before deciding to order new
microphones or change the sound system
(see Figure 4).
Fo u r w a y s t o m a x i m i z e
g a i n - b e fo r e - fe e d b a ck :
1) Move the microphone closer
t o t h e t a l k e r.
2 ) H a v e t h e t a l k e r s p e a k l o u d e r.
3) Move the loudspeaker closer
t o t h e l i s t e n e r.
Ta l ke r
4) Move the microphone further
f r o m t h e l o u d s p e a k e r.
Microphone Placement
Let’s go back to the analogy of the rock
and ripples. The ripples are the strongest or
highest at the point of impact. As they radiate
from the center, they become weaker. This
is the same way sound waves travel. The
sound pressure level is called the SPL for
short. The SPL will decrease by 6 dB every
time the distance from the sound source
the microphone will have dropped 6 dB. If
he now moves back another 2 inches, the
distance will have doubled again, so the
SPL will be down a total of 12 dB. The
sound level from the sound system will
have apparently dropped in half. If the
listener could hear the talker just right
when the talker was at the 1” position, he
will probably not hear very well after the
talker moved back only 3”. It would be
very easy for a talker to move this much
while speaking. The result would be wildly
varying volumes for the listener.
Microphone
Listeners
Figure 4
2
The absolute gain of your sound system
is primarily set by the physical distance
between the loudspeaker system and the
microphone. The room’s acoustics are also
an important factor. The absolute gain is
the maximum gain in volume you can
achieve before feedback occurs. The actual
absolute gain can be influenced by high
quality equipment and proper design,
installation, and positioning of the equipment.
If the sound system is operated at a level
below the absolute gain, feedback will not
occur whether the talker whispers or
shouts. The sound system is said to be stable.
Increasing the gain by turning up the volume
control on the mixer will eventually
exceed the absolute available gain and
feedback will be heard. Since we do not
want to hear feedback, the only option is to
turn down the volume control, which
decreases the system gain, until the feedback
stops. However, the talker may not be loud
enough for easy listening. There are two
methods which can increase the volume.
The first method is to have the talker
speaker louder. When the talker speaks
louder, the microphone will pick up a louder
signal and that will be amplified louder.
Too often the natural reaction of the talker
to hearing feedback is to talk softer. This is
exactly opposite of what should happen.
The talker needs to talk louder so the
sound system gain can be turned down.
If the talker can’t speak louder, they
then need to get closer to the microphone.
Remember the ripple example? The sound
level reaching the microphone will be
louder if the talker moves closer. The
sound system gain or volume has not been
adjusted, but the signal entering it has
increased in volume. The result is louder
volume from the loudspeaker system and
the listener can hear better.
TECH TALK
Pre or Post . . . What language is that?
Like most technical crafts, sound system
operators have a language of their own.
The key to understanding other operators
and technical manuals is understanding the
language. However, it is not enough to
know the words. The key to expert sound
system operation is knowing the meaning
of the words, too. Pre or post are terms that
are often used in relation to operating a
sound system mixer. Let’s explore the
meaning of these terms. Pre and post are
usually heard when someone is talking or
reading about the Auxiliary (Aux) Sends
of the mixer. These sends could also be
called Monitor or Effects Sends. Whatever
the name on your mixer, they are used to
send a signal to another separate loudspeaker or device besides the main loudspeakers. We will use the term Aux Send in
this article.
Pre or post tells the operator what path
the microphone signal is traveling. But
pre- or post-what? The “what” is the
Channel Fader or Volume Control. In other
words, the Aux Send is pre (before) or post
(after) the Channel Fader (Figure 5).
If the Aux Send is pre-fader, the Aux
signal is independent of the Channel Fader.
Any change in the Channel Fader will not
affect the Aux Send. This is commonly
used for sending a signal to a monitor
loudspeaker. Because the Aux is pre-fader,
the sound operator can adjust the sound
level in the main system without changing
the level of the monitor loudspeaker. In
other words, the signal will route through
the Aux Send before the Channel Fader
adjustment. This is very important when
Summary
accompaniment CD’s (trax) are used. The
operator may turn the level down in the
main system, but the singers still need the
trax at a level to hear the sound for proper
pitch and tempo.
Post-fader means the Aux Send gets its
signal after the Channel Fader. Now a
change in the Channel Fader will affect the
Aux Send. The Aux “follows” the Channel
Fader. This would be used when the operator
wants to add artificial reverb to the vocal.
The signal to the reverb device would need to
“follow” the volume of the singer. If the
operator turned down the vocalist with the
Channel Fader, the signal going to the reverb
device should also decrease proportionally.
Knowing whether the Aux Sends are
pre or post lets the operator use the mixer
wisely and properly. Many mixers have 3
or 4 Aux Sends. More recent mixers may
have 6 or 8 Aux Sends. The manufacturer
will designate some of the Aux Sends to be
pre and some post. Often there may be a
switch to allow some Aux Sends to be
switched to either pre or post. That feature
provides a lot of flexibility to the operator.
Some situations may require the use of the
pre-fader control on some channels and
post-fader on others.
If you have a mixer with Aux Sends,
take the time to learn which Aux Sends are
pre-fader and post-fader. Your mixer manual
will help you or you could find out by
experimenting. It will pay off with better
sound and happier musicians. If you have
trouble understanding your mixer give us a
call. We would be glad to help solve the
mystery.
■ Ron Huisinga
Using these basic microphone facts will
help you better understand how to get the
most out of your present equipment. If you
still have problems, please call us for help.
We desire to serve you by helping you
remove your sound system frustrations.
■ Ron Huisinga
Maximum Gain-Before-Feedback
Figure 5
3
The path of a signal through a mixer.
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