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Fundamentals of Audio Production. Chapter 1
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Fundamentals of Audio Fundamentals of Audio ProductionProduction
Chapter One:Chapter One:
The Nature of SoundThe Nature of Sound
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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
Time
Am
plitu
de
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Air Molecules in Motion
• Compression waves or sound pressure waves move through 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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FrequencyThe 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 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 as 180° 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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IsolationMechanical 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 440 Hz
= 2.56 Feet
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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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• Frequency = velocity ÷ room dimension
• In a room with dimensions of 15 feet between parallel walls, resonance may occur about 75 Hz.
Calculating room modes
1130 15
= 75.33
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Surface Treatment Absorbers stop the reflection of sound
waves by converting acoustic energy into heat
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AbsorbersCommercially produced acoustic foam absorbers convert acoustic energy to heat in the open cells of the foam.
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Surface TreatmentDiffusers 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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DiffusersCommercially produced acoustic diffusers reflect sound waves at various angles.