Sound Reinforcement
Hal Harrison
hlh@clemson.edu
What is a Sound System?
A sound REINFORCEMENT system, not a
sound system.
 Reasons for having a reinforcement system:

Help people hear something better.
 To make sound louder for artistic reasons.
 To enable people to hear sound in remote
locations.
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Conceptual Model of a Sound System
Sound Waves and Propagation
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What we hear as sound is a class of kinetic energy called acoustical
energy.
Acoustical energy consists of fluctuating waves of pressure in air.
Waves have peaks and valleys.
The start and end of a wave (either peak to peak or valley to valley)
is known as a cycle.
To be classified as sound, waves must fluctuate at a rate between
20 and 20,000 cycles per second (cps).
The amount of cycles waves fluctuate per second is known as
frequency.
Hertz (Hz) is used to indicate frequency in cycles per second.
20 Hz = 20 cps
Generally speaking, the higher the frequency, the higher the
perceived pitch of a sound.
Representation of a Sound Wave
The Electrical Representation of Sound
An audio signal is an electrical
representation of sound.
 In analog audio electronics, the signal
voltage fluctuates the same way as the
acoustical energy it represents.
 The amplitude (or strength) of an audio
signal is called the signal level.
 Sound level is specified in decibels (dB).
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Input Transducers
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Air pressure or velocity microphones
Contact pickups
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Magnetic pickup
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cassette tapes
Phonograph pickups
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electric stringed instruments
Tape heads
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acoustic guitar, mandolin, violin
record players
Laser pickups
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CD, MD
Output Transducers
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Woofer loudspeakers
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Midrange loudspeakers
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mid-range frequency reproduction
500Hz-4kHz range
Tweeter loudspeakers
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low frequency reproduction >500Hz
high frequency reproduction <4kHz
Full-range loudspeakers
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Integration of 2 or more of the other loudspeaker
systems.
Output Transducers continued
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Subwoofer loudspeakers
extended low frequency
 30Hz-150Hz range
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Monitor (foldback) loudspeakers
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full range loudspeakers designed for the
performer on stage to hear themselves.
Headphones
Practical Model of a Sound System
Frequency Response
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The Frequency Response (FR) of a device
describes the relationship between the
device’s input and output with regard to
signal frequency and amplitude.
FR and Acoustical Factors
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The FR of any sound system is affected by the
environment in which it works.
Outdoor factors:
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wind – deflects sound as it travels
temperature – deflects sound as it travels
air absorption – sound (higher frequencies) are
absorbed over distance
Indoor factors:
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room reflections – walls, ceilings, floor surfaces
room resonances
The Decibel
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Decibel (dB)
Decibels describe a ratio of two quantities often
related to power.
PROBLEM: What is the ratio, in dB of 2 watts to
1 watt?
dB
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= 10 log (P1 / P0)
= 10 log ( 2 / 1)
= 10 log 2
= 10 X .301
= 3.01 or 3dB
WHICH MEANS - Whenever one power is twice
another, it is 3dB greater!
Relating the Decibel to Acoustic Levels
Volume, Level, and Gain
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Volume – defined as a power level
in audio, it means turn up the volume!
 bad term as it relates to a cubic dimension.
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Level – magnitude of a quantity in relation
to a arbitrary reference value.
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Sound Pressure Level (SPL)
Gain – assume to be transmission gain,
which is the power increase of a signal
expressed in dB.
Dynamic Range and Headroom
Dynamic Range - the difference in dB
between the loudest and the quietest
portion of a program.
 Headroom – the difference between the
normal operating level and the highest
(peak) level of program material.
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Headroom Illustration
Break Time!
Balanced Cables
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Used to connect line level and mic level
sources such as microphones (XLR) and
input/output connections (console to amp)
Unbalanced Cables
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Used to connect line-level signals such as
guitars, keyboards, bass, etc. (1/4”)
Insert Cables
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Used to connect channel or group specific
devices such as:
Compressors
 Limiters
 Gates
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Microphones: Selection
Dynamic (3.1)
 Condenser (3.2)
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Phantom power +48v needed for operation
Microphones: Pickup Pattern
A pick-up pattern refers to the area(s) from
which a microphone “picks up” its sound.
 Types:
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Omni (3.3)
 Cardioid (3.4)
 Super-cardioid (3.5)
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Typical Console
Signal Flow
Mono Input
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A – XLR microphone level input
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B – ¼” line level input
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balanced/unbalanced
C – ¼” line level insert (1.1)
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balanced
Inserts are both sends and returns
Unbalanced
D – ¼” line level direct output
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balanced/unbalanced
Mono Input
E – Gain / trim control
 F – High Pass Filter (HPF)
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aka Low-cut filter
G – Equalization section
Mono Input
H – Auxiliary send section
 I – Pan control
 J – Pre-Fade Listen (PFL) switch
 K – Channel “On” switch
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aka mute button
L - Fader
 M – Assignment switch
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aka routing button
Subgroups
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Allow logical
assignment of groups of
instruments or vocalists
so that they may be
controlled by just one
pair of faders, or even a
single fader.
Console Master Section
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N – line level master outputs
(¼” or XLR)
O – line level control
room/monitor output
P – line level 2-track tape return
(RCA)
S – meters
U – 48v global Phantom power
switch
V – headphone output (1/4”)
headphone level
Console Master Section
Q – auxiliary send master
section
 R – After-fade listen (AFL)
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Typical Console Input/Output Setup
Input Gain
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Input gain controls are designed to take a
signal and adjust it till it is usable by the
mixing console.
Setting Gain Structure
1.
2.
3.
4.
Press the PFL/Solo switch on the
relevant input.
Adjust gain/input sensitivity until meters
read within the yellow (‘3’ to ‘6’ on
meter scale). This allows for the extra
10dB of gain that is available on
Soundcraft input faders.
Release PFL/Solo.
Repeat for all other inputs.
Faders
Faders allow for fine adjustments to your
inputs.
 Normal operating levels for faders should
be at “0” or unity.
 Faders are logarithmic.
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Equalization
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Used to make corrective and creative sound
changes.
BASIC RULE OF EQ:
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Console EQ types:
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If it sounds good, it is good!
fixed band, semi-parametric, fully parametric
Center position has no effect on sound.
Counterclockwise – decrease in EQ frequency
level.
Clockwise – increase in EQ frequency level.
Fixed Equalization
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Same as treble/bass controls on home
stereo systems.
Semi-parametric Equalization
Similar to fixed EQ, but has more control as
the midrange EQ is usually sweepable.
 Sweep – console EQ knob which is
selectable between a range of frequencies.
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Removing Unwanted Sound with EQ
1.
2.
3.
4.
Increase sweep-EQ gain.
Sweep the frequency pot until the aspect of the sound
you wish to modify becomes as pronounced as
possible. This should only take a few seconds.
The cut/boost control is now changed from its full
boost position to cut. The exact amount of cut required
can be decided by listening to the sound while making
adjustments.
Even a small amount of cut at the right frequency will
clean up the sound to a surprising degree.
Other sounds may benefit from a little boost, one example being the electric guitar which often
needs a little extra bite to help it cut through the mix. Again, turn to full boost and use the
frequency control to pick out the area where the sound needs help. Then it’s a simple matter
of turning the boost down to a more modest level and assessing the results by ear.
Caution: when adjusting EQ, there is a danger of feedback which can cause damage to your
speakers. You may need to reduce your levels to compensate.
Fully Parametric Equalization
Similar to a semi-parametric EQ, but has
more control.
 Fully parametric EQs have sweeps on all
bands of EQ and most have bandwidth
control as well.
 Bandwidth aka (Q) – the broadness or the
steepness of a frequency curve.
 The higher the bandwidth, the steeper the
curve.
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Fully Parametric Equalization Curve
Auxiliary Sends
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Two types:
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Pre-fade auxes – monitors
Post-fade auxes – reverbs, delays, recording feeds, etc
Pre-fade auxiliaries are independent of the fader so that
the amount of effect will not change with new fader
levels. This means you will still hear the effect even
when the fader is at the bottom of its travel
It is important to use post fade auxiliary sends for effects
units. This is because post fade auxiliaries ‘follow’ the
input fader so that when input level changes the amount
of effect remains proportional to the new input level.
Gates
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A gate is designed to shut down the audio
signal path when the input signal falls
below a threshold set by the user. It may
be used to clean-up any signal that has
pauses in it.
Compressors
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A compressor reduces the difference between the
loudest and quietest parts of a performance.
It works on a threshold system where signals
exceeding the threshold are processed and those
falling below it pass through unchanged. When a
signal exceeds the threshold the compressor
automatically reduces the gain.