Transcript Fundamentals of Audio Production

Fundamentals of Audio
Production
Chapter One:
The Nature of Sound
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Production. Chapter 1
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Sound as a vibration
• Sound is created by vibrations
– Vibrating vocal chords, instrument strings,
reeds, drum heads, lips, etc.
• Sound travels as vibrations
– Air molecules vibrate, transmitting changes in
air density from one to the next
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Air Molecules in Motion
• Molecules are first compressed
– Positive (+) expenditure of
energy
• Then the molecules rebound or
rarefy
– Energy is covered (-)
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Air Molecules in Motion
As the diaphragm (left) compresses the
air molecules - the compression wave
moves through the molecules (right).
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Air Molecules in Motion
• The rate at which the molecules vibrate is
called the frequency of the sound.
• Frequency is measured in the number
cycles per second, often expressed as
hertz
• Frequency is perceive as pitch
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Air Molecules in Motion
• The intensity or magnitude of the
molecular displacement is called the
amplitude of the sound
• Amplitude is measured in decibels (db)
• Amplitude is perceived as loudness or
volume
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Air Molecules in Motion
This is often visually diagramed using a sine wave
Amplitude
+
+
+
_
_
_
Time
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Air Molecules in Motion
• Compression waves or sound pressure
waves move the air at a velocity of
approximately 1150 feet per second.
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Frequency
• The frequency response range of the
human ear is approximately 20 Hz to
20,000 Hz (20KHz)
• The ear does not perceive all frequencies
equally well
• Middle range frequencies are heard more
easily, or seem louder than high and low
frequencies – called the “principle of equal
loudness
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Frequency
The Fletcher Munson Curve illustrates principle of equal loudness. More
volume is required to hear low and high frequencies as well as midranges.
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Frequency
• Certain frequency intervals are easily
identifiable by the ear
• When frequency is doubled or halved, the
interval is called an octave
• 440 Hz is known as “concert A”
• 880 Hz is “A” one octave above concert
“A”
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Frequency
• Click on the icons below to hear various
frequencies
• Note how midranges seem louder
• Also note the relationship between
octaves
60 Hz
100 Hz
1000 Hz
2000 Hz
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10 KHz
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Amplitude
• The dynamic range of the human is
approximately 120 decibels
• The smallest change easily detected is
3db
• 120 db is the threshold of pain
• Sustained exposure to sound pressure
levels higher than 120 db can cause
permanent hearing damage
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The Nature of Sound
• The dominant frequency in a sound is
called the fundamental
• Other frequencies are also present
– Overtones at the sum and difference of
combined frequencies
• 440 Hz + 880 Hz = 1320 Hz overtone
– Harmonics at the multiples of the combined
frequencies
• 100 Hz X 60 Hz = 6000 Hz harmonic
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Phasing
These two signals are
illustrated 180° as out of
phase with one another.
Notice that when one is
positive, the other is
negative. When two signals
that are 180° out of phase
arrive at the ears, they will
cancel each other out, and be
difficult or impossible to hear.
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Acoustics
• The objective study of how sound behaves
is called acoustics
• Those who study and control sound
behavior are called acousticians
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Acoustics
• Acoustic treatments are most often
focused on two tasks:
– Isolation, or “soundproofing,” which is aimed
at keeping outside sound out, and inside
sound in
– Surface treatment, which is aimed at
controlling reverberation
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Isolation
• Isolation may be achieved by stopping the
transmission of vibrations (sound) from
one space to the next
– Isolation may be achieved by building
boundaries with great mass, that will not
vibrate
– Isolation may be achieved by mechanically
decoupling interior walls from exterior walls
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Isolation
Mechanical de-coupling
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Room resonances
• Parallel surfaces can create standing
waves, causing “room modes.”
• Modes are certain frequencies that are
may be reinforced, causing “ringing.”
• Or certain frequencies may be cancelled,
causing those frequencies to be lower in
amplitude.
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Calculating room modes
• Wavelength = Velocity ÷ Frequency
• Velocity = speed of sound = 1150 per
second
1130
= 2.56 Feet
440 Hz
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Calculating room modes
2.56 Feet
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Room modes
• Rooms which have dimensions that are
multiples of the wavelength may exhibit
modes.
– Example: 25.6 feet = 10X the wavelength
– Resonances may occur at 400 Hz in a room
whose length or width is 25.6 feet.
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Calculating room modes
• Frequency = velocity ÷ room dimension
1130 = 75.33
15
• In a room with dimensions of 15 feet
between parallel walls, resonance may
occur about 75 Hz.
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Surface Treatment
Absorbers stop the reflection of sound
waves by converting acoustic energy
into heat
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Absorbers
Commercially produced acoustic foam absorbers
convert acoustic energy to heat in the open cells of
the foam.
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Surface Treatment
Diffusers made from irregular surfaces reflect sound
waves away at various angles to discourage standing
waves. Two examples below cause the waves to be
splayed in different directions (red).
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Diffusers
Commercially produced acoustic diffusers reflect
sound waves at various angles.
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