Basic Acoustics - Loyola University New Orleans
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Transcript Basic Acoustics - Loyola University New Orleans
Digital Audio —
The Nuts and Bolts
A digital audio overview ranging from
bit rate, sample rate, and compression
types to room acoustics, microphones,
and digital effects
Sound Waves/Analog Audio
Sound waves are continuous
Infinite number of amplitude points
can be identified between any two
points in time
Digital Audio
Computers don’t deal with continuous
concepts (infinity)
Digital technology converts analog
audio to computer values
Digital Conversion
Digitizing a continuous wave = sampling
Amplitude measurements of a sound
signal are regularly sampled
ADC and DAC
ADC – Analog to Digital Converter
Converts analog signal to digital
samples
DAC – Digital to Analog Converter
Converts digital samples to analog
signal
Characteristics of
Digital Audio
Sampling Rate
– How often signal is sampled
– Number of samples per second
Bit Depth
– Size of number used to store samples
– larger number gives more degrees of
value
Sampling Rate
Harry Nyquist (Bell Labs – 1925)
Nyquist Theorem: To represent
digitally a signal containing frequency
components up to X Hz, it is necessary
to use a sampling rate of at least 2X.
Humans hear to 20 kHz, requiring
sample rate of at least 40k
Aliasing
In movies, car wheels appear to move
backwards if between ½ and 1
revolution per frame
In sound, this is not acceptable
Filters are used to remove any
frequencies above Nyquist frequency
Undersampling
Undersampling = Aliases
Critical Sampling
Lowpass Filter
Reduces or eliminates higher frequencies
Used to remove any frequencies above
Nyquist frequency
Bit Depth (Quantization)
Amplitude values are stored as binary
numbers
Accuracy depends on how many bits
are available to represent these values
For CD Audio we use 16 bits
Quantization
No matter how many bits are used,
there is always a margin of error
Low-level signals do not use all
available bits, so signal-to-error ratio is
greater
Quantization
Quantization error creates a kind of
distortion
Dither adds low-level noise to audio
signal before sampling
Dither turns distortion (bad) into noise
(less bad) – still less noise than analog
Digital Recording Process
Dither – Low-level noise added (prior
to sampling) to reduce quantization
error distortion
Digital Recording Process
Lowpass Filter – Removes frequencies
above Nyquist Frequency; cutoff starts
a few thousand hertz lower
Digital Recording Process
Sample and Hold – Analog voltages
are measured and held long enough to
be read by ADC
Digital Recording Process
Analog-to-Digital Converter – Converts
analog voltages into binary numbers
Digital Recording Process
Multiplexer – Combines the parallel
data streams (stereo) into a single
serial bit stream
Digital Recording Process
Error Correction – Variety of measures
to eliminate, reduce, or compensate
for errors
Digital Recording Process
Encoding – Encoded for playback
Digital Recording Process
Storage
Digital Playback Process
Buffer – To ensure that samples are
processed at a constant rate
Digital Playback Process
Error Correction – Attempt to
eliminate, reduce, or conceal data
errors
Digital Playback Process
Demultiplexer – Splits the serial
bitstream into parallel data streams
(stereo)
Digital Playback Process
DAC – Digital-to-Analog converter
translates binary numbers to voltage
values
Digital Playback Process
Sample and Hold – Reads the value
from the DAC and holds it until the
DAC’s next stable state
Digital Playback Process
Lowpass Filter – Smooths the output
from the sample and hold circuit
Digital Playback Process
Audio – The finished product
Room Acoustics
Characteristic room sound is
determined by the relationship
between direct and reflected sound
Virtually all sound reaching listeners is
a combination of direct & reflected
At greater distances, most sound is
reflected sound
Room Acoustics
Direct Sound
– Directly from the source to the listener
– Direct sound arrives before reflected
sound; even if reflected sound is louder,
we hear direct sound first and determine
direction of the source
Room Acoustics
Early Reflections
– First-order reflections that reach the
listener after reflecting once from the
floor, ceiling, or walls
– If arriving in the first 35ms after the
direct sound, reinforces with clarity &
intelligibility
– “Intimate” halls have first-order
reflections of less than 20ms
Room Acoustics
Diffuse Reverberations
– Second- (and higher) order reflections
– Reverberation time is the time required for
the SPL to drop 60dB
– Larger room is likely to have longer
reverberation time than a smaller room
– Reverberation time is frequency dependent;
lower frequencies reverberate longer
Types of Reflections
Specular
– Reflections off smooth and regular
surfaces
– reflection in one direction
Diffuse
– Reflections off irregular surfaces
– Reflections scattered in many directions
– Contribute to sound of older concert halls
Absorption
Small Room
Space has potential to act as closed
tube, producing standing wave
Result is amplification of certain
frequencies based on room’s
dimensions
Not a factor in large rooms because air
temperature varies more
Microphones
Receptor type
– Diaphragm acts as receptor
– Diaphragm vibrates
Transducer type
– Transducer converts vibrations to electricity
Directionality
– Determines strength of signal produced by
sounds arriving from different directions
Receptor Types
Pressure
– Diaphragm responds to sound pressure
changes on only one side of diaphragm
Pressure Gradient
– Diaphragm responds to sound pressure
changes from the front or rear
– Signal is determined by difference
(gradient) of pressures from either side
Transducer Types
Dynamic
(Electrodynamic, Electromagnetic, Ribbon, Moving Coil)
– Principle of magnetic induction – wire moves
within a magnetic field, producing a current
– Inexpensive and sturdy
Condenser (Capacitor)
– Two oppositely-charged metal plates
– Current moves from one to the other
– Sharper transients
– Expensive
Directionality
Determines the strength of signal
produced by sounds arriving from
different directions
Directionality varies with frequency
Specs often include polar plot with
patterns for different frequencies
Omnidirectional
Responds equally to sound from all
directions
Pressure mics are omnidirectional
Bidirectional
Figure-eight response
Responds equally to sounds from front &
back; none from sides
Pressure gradient mics are bidirectional
First-Order Cardioid
Most common directional microphones
Cardioid refers to heart-shaped pattern
Directional patterns are obtained by
combining pressure and pressure
gradient elements in varying proportions
Cardioid Variations
50% Pressure/50% Pres. Gradient
75% Pressure/25% Pres. Gradient
37% Pressure/63% Pres. Gradient
25% Pressure/75% Pres. Gradient
Effects
All music that is recorded or amplified
relies on effects to enhance the sound.
Effects are necessary to make
electronic audio signals sound like
natural sound.
Effects = Filters
Effects are created by filter
combinations
Filtering involves combining original
signal with delayed version
Higher internal processing bit rate
means more accurate arithmetic
Simple Delay
Signal combined with delayed version of
itself.
Multitap Delay
Series of Simple Delays; output is
combines with a succession of delays.
Feedback Delay
Combines delayed output with input,
then sends through delay again.
Delay-Based Effects
Flanging
Chorusing
Phase Shifting
Reverberation
Non-Delay-Based Effects
Ring Modulation
Amplitude Modulation
Compression/Limiting
Expansion/Noise Gating