Download Audio Fundamentals Part 1

Audio Fundamentals Part 1
Objectives
Upon completion of Audio Fundamentals Part 1, you will
be able to:
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Explain what audio is,
Describe Acoustic Sound Waves as well as
Rarefaction and Compression,
Define Frequency, Amplitude and dB.
Compare and Contrast Analog and Digital Audio.
What is Audio
Simply speaking, audio a very fast moving pressure wave, changing in height
(Amplitude or volume) and width (Wavelength or frequency). This wave is generated
by the vibration of an object; for
example, speakers in a sound
system or simply a pencil dropping
on the floor. You ear drum
"catches" the sound wave and
vibrates because of the pressure
waves.
The height of the sound wave is called Amplitude, represented by the letter A in the
image. Amplitude determines the volume. The further away the wave goes from the
Zero line, represented by the horizontal line in the image, the louder the sound will be.
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The width of the sound wave is called wavelength, represented by the letter B in the
image. Wavelength determines the frequency.
The audible range of human hearing is 20 to 20,000 Hz or 20 kHz. This graphic shows you
a frequency of 1Hz, or one cycle/ second. The longer the wavelength, the lower the
frequency. The shorter the wavelength, the higher the frequency.
Audio is two things.
1. A physical phenomenon and
2. A perceptual phenomenon
Physical Phenomenon
Audio has an acoustic element: which is sound waves moving through physical
medium this includes things like a solid, liquid, gas or plasma.
Audio also has an electronic or optical component. There are electrons and photons
moving through physical medium such as copper, fiber or air.
Electronic and optical audio signals can be transmitted in a range of formats such as
analog or digital.
Perceptual Phenomenon
Psycho-acoustic effects of sound waves reaching a specific listener are defined by their
human auditory system.
This is experiential and highly subjective. The perception of sound waves can vary
greatly by listener. For example, put on a piece of music that you like and see how your
grandparents or great grandparents feel about it.
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The ability of listeners to perceive sound waves is both amplitude and bandwidth
limited. You have a limited range of frequencies you can hear and a limited level that
you can hear. This varies by a lot of factors, such as age.
Acoustic Sound Waves
Vibration from a physical source causes sound waves to propagate through a medium.
For example when you drop a pebble into a pond and watch the ripples in the water. If
you were to take a cross section of the ripples in the water - they would look similar to
the graph, which is a sine wave decreasing in amplitude.
In the first part of the course we mentioned that sound was created by a vibrating
object. The vibrations of the object set particles in the medium in vibrational motion. For
a sound wave traveling through air, the vibrations of the particles are best described as
longitudinal. Longitudinal waves are waves in the displacement of the medium is the
same direction as, or opposite direction to, the direction of travel of the wave. A
longitudinal wave can be created in a slinky if the slinky is stretched out in a horizontal
direction and the first coils of the slinky are vibrated horizontally. In such a case, each
individual coil of the medium is set into vibrational motion in directions parallel to the
direction that the energy is
transported.
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A vibrating tuning fork is also capable of creating a longitudinal wave. As the tines of
the fork vibrate back and forth, they push on neighboring air particles. The forward
motion of a tine pushes air molecules
horizontally to the right and the backward
retraction of the tine creates a lowpressure area allowing the air particles to
move back to the left.
Because of the longitudinal motion of the air particles, there are regions in the air where
the air particles are compressed together and other regions where the air particles are
spread apart. These regions are known as compressions and rarefactions respectively.
The compressions are regions of high air pressure while the rarefactions are regions of
low air pressure. The diagram depicts a sound wave created by a tuning fork and
propagated through the air in an open tube. The compressions and rarefactions are
labeled.
To put this into perspective of a speaker, this image shows the rarefaction and
compression of a sound wave from a speaker.
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In other words, sound waves, propagating through a medium, create alternating bands
of high and low pressure, known as rarefaction and compression, at a given frequency
and amplitude.
Frequency
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Frequency of waves is measured in Hz (Hertz), which equals Cycles/ Second.
Typically accepted bandwidth of human auditory system is about 20Hz - 20kHz.
20Hz is a very low deep sub bass.
20kHz is a very high frequency that a lot of people can’t hear and audio systems
have a hard time to reproduce.
A high quality audio system can produce the harmonics found in the 20kHz
range. This can induce an alpha wave state in the brain and a lot of people
have an emotional response to that and recognize whether a song is, for
example recorded or live.
Frequencies below 20Hz are known as infrasonic, above this band known as
ultrasonic.
Lower frequencies have longer wavelengths, represented by the red wave in this
image. Higher frequencies have shorter wavelengths, represented by the other
colors.
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Amplitude of Acoustic Sound
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Amplitude of acoustic sound waves is measured in dB SPL (decibels Sound
Pressure Level).
One of the most misunderstood concepts in audio is the decibel. A lot people
will say something like "that concert was 130 decibels" or "that jackhammer down
the street is 120 decibels" or "Calibrate your cinema audio system at 85 decibels".
dB (decibel) is a logarithmic ratio, between two values. It is not an absolute
value, unless tied to a reference level.
dB SPL is tied to a specific reference level for sound pressure measured in Pa
(Pascals) with a typically accepted value for threshold of human hearing is: 0dB
SPL @ 1kHz = 20 µPa. This is the bench mark where we agree there is a standard,
just like we agree on what a lumen is.
Here’s the complex logarithmic formula:
Sound Pressure Level (SPL)
This graph shows SPL on the side and frequency across the bottom -> 10Hz which is
infrasonic up to 100 kHz which is ultrasonic.
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The curve varies a lot. If you have a very low frequency sound it has to be a pretty high
level for you to even know that it’s there. Our ears are very sensitive to the frequencies
of the human voice at or around 1 kHz. Through evolution we have evolved to be very
sensitive to other peoples voices as a way of communicating. This is why the center
channel is the most important channel in cinema. If dialogue doesn’t sound correct,
people will notice.
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Threshold of hearing varies by frequency and individual perception.
Typically accepted value for threshold of pain is: 120 - 130 dB SPL.
Maximum pressure (undistorted) at 1 Atmosphere = 194 dB SPL - you cannot
generate anything louder than this. The exception being a nuclear explosion or a
sonic boom.
Every increase of 3 dB SPL, equals twice the acoustic energy - For example - If
you turn up the volume by 9dB that’s 8x louder.
Amplitude Voltage
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Amplitude of electronic audio signals is measured in dB (decibels) tied to a
reference voltage
dBV (used in a lot of consumer equipment)
voltage relative to 1 volt, without consideration
of impedance
dBu (used in professional applications) “u”
stands for unloaded. AC voltage relative to
.775V at any impedance
dBm power relative to 1 millwatt, typically at
600 Ohm impedance
Standard signal reference level for analog pro
audio is +4dBu = 1.23V
Every 6 dB increase in dBV, dBu or dBm, equals
twice the electrical energy
Electronic/ Optical Audio Signals
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Acoustic sound waves can be transformed into electronic or optical audio
signals and these signals can be in turn transformed back into acoustic waves,
by using a transducer. A Transducer like a microphone or speaker.
Microphones convert sound
waves into electronic or
optical audio signals
Speakers convert audio
signals from amplifiers into
sound waves.
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Analog Audio | Digital Audio
Analog Audio
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Analog audio is an electronic audio signal analogous to patterns of acoustic
sound waves
There are five primary signal formats in widespread use:
Speaker: up to 120V, unbalanced, very low impedance
Professional: +4 dBu, balanced, low impedance
Consumer: -10dBV
unbalanced, high
impedance
Instrument: ~-20 dBV,
typically unbalanced, very
high impedance
Microphone: ~-40 dBu to -60
dBu can be balanced or
unbalanced, low or high
impedance
Digital Audio
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Digital audio is an electronic audio signal, which utilizes a series of digital
samples, typically with 16 to 24 bit resolution and sample rates between 44.1kHz
(same as a CD) and 96kHz, to very closely represent the original waveform of an
analog signal. Requires A to D (Analog to Digital) conversion for storage and/ or
transmission and D to A conversion for reception and/ or playback.
May or may not utilize audio and/ or data compression schemes
Some common signal formats currently in widespread use:
Atmos | Pulse Code Modulation or PCM, up to 128 channels - 64 is the base
number of channels, 24 bit audio, 48 or 96 kHz
DCI-AES | PCM, up to 16 channels, 20 or24 bit, 48 or 96 kHz
AES3 | PCM, 2 channels in a pair, 16-24
bit, 32 - 96 kHz
SPDIF | PCM or Dolby Digital or DTS,
bandwidth varies
HDMI | PCM or Dolby Digital or DTS,
bandwidth varies
Every effort has been made to ensure the information in this document is accurate and reliable, however in
some cases changes in the specifications may not be reflected in this document. Christie reserves the right to
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make changes to specifications at any time without notice.
training.christiedigital.com
Audio Fundamentals Part1 v1.0 Sept14