Transcript Acoustics - AIS :: Audio Post for Film and Video

Acoustics
Reverberation
What is Reverberation?
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Reverberation is multiple,
random, blended repetitions of
a sound.
Three parts: Direct Sound,
Early Reflections, & Later
Reflections.
Reverberation Time (Decay
Time) is the time required for
the sound in a room to decay
60 dB (also known as RT60).
This represents a change in
sound intensity or power of 1
million (10 log 1,000,000 = 60
dB, or a change in sound
pressure level of 1,000 (20 log
1,000 = 60 dB).
Growth & Decay of Sound
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W. C. Sabine, the Harvard
pioneer in acoustics
introduced the concept of
RT60.
Measuring Reverberation Time
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A common approach to measuring reverberation time.
Figure B is a more common occurrence than figure A.
Measuring Reverberation Time
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The sound sources used to excite the room
must have enough energy throughout the
spectrum to ensure decays sufficiently above
the noise to give the required accuracy.
Both impulse sources and those giving a
steady-state output are used.
Impulse Sources
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Common
impulse sources
are balloon
pops and starter
pistols. The
diagram shows
the reverb
decays at
several different
octave ranges
using a starter
pistol.
Steady-State Sources
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Bands of random
noise give a steady
and dependable
indication of the
average acoustical
effects taking
place.
Octave and 1/3
octave bands of
random noise
(white or pink) are
most commonly
used.
Mode Decay Variations
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The fluctuations in
the decays result
from beats between
closely spaced
modes.
The differences in
the four decays is
due to the random
nature of the noise.
It is good practice to
record several decays
for each octave for
each mic position of
a room.
Acoustical flaws can
often be identified
from aberrant decay
shapes.
Room Modes
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When sound is emitted in a
room with parallel opposing
walls, the room exhibits a
resonance at a specific
frequency determined by the
equation f0 = 1,130/2L (or
565/L), where L is the length
(in feet) of space between the
two walls.
A similar resonance occurs at
2f0, 3f0, 4f0, etc.
These resonances are called
modes; specifically, axial
modes.
Frequency Beats
500 Hz
500 & 505 Hz
505 Hz
Modal Interaction with Decay
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The diagram
shows four
different axial
mode
frequencies in
the octave
centered on
63 Hz.
The lower the
frequency, the
less axial
modes there
are, so the
more
noticeable the
beats become.
Types of Room Modes
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Axial modes are derived from two walls, tangential
modes are derived from four walls, and oblique modes
are derived from all six surfaces.
Frequency Effect
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This diagram shows
typical fluctuation
due to modal
interference.
Variation with Mic Position
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There is enough variation of reverb time from
one position to another in most rooms to justify
taking measurements at several positions.
The average gives a better statistical picture of
the behavior of the sound field in the room.
If the room is symmetrical, measure only one
side to minimize time and effort.
Acoustical Coupling
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Acoustically coupled
spaces are quite
common in large public
gathering spaces, but
are also found in offices,
homes, and other
smaller spaces.
Assuming that slope A is
correct for the main
room, persons subjected
to slope B would hear
inferior sound.
Electroacoustical Coupling
What is the overall effect when sound picked up
from a studio having one reverberation time is
reproduced in a listening room having a different
reverberation time?
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The combined reverb time is greater than either
alone
If the reverb time of each room alone is the
same, the combined reverb time is 20.8% longer
than one of them.
Optimum Reverberation Time
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The best reverb
time for a space
in which music is
played depends
on the size of the
space and the
type of music.
Spaces for
speech require
shorter reverb
times than for
music.
Optimum Reverb Time Examples
Optimum Reverb Time Examples
Bass Rise
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Taking the 1 kHz value as a reference, rises of 80% at 63
Hz and 20% at 125 Hz were found to be acceptable in
studios designed for voice recording.
Living Room Reverb Times
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The average
reverb time
decreases
from 0.69
seconds at
125 Hz to
0.4 seconds
at 8 kHz.
The Sabine Equation
The absorption coefficients published by materials manufacturers
are typically Sabine coefficients and can be applied directly in the
Sabine equation.
Absorption and Absorption Coefficients
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Absorption: in acoustics, the conversion of
sound energy to heat.
Absorption Coefficient: the fraction of sound
energy that is absorbed at any surface. It has a
value between 0 and 1 and varies with the
frequency and angle of incidence of the sound.
Multiplying the surface area (in sq. ft.) by the
absorption coefficient results in absorption units
(sabins).
Reverberation Calculations
The
diagram
shows an
example of
the RT60
calculations
using the
Sabine
equation.
Reverb Time (RT60) Calculations
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1) Calculate the total areas of each type of
surface
2) Find the absorption coefficients for each type
of surface for the six frequencies: 125 Hz, 250
Hz, 500 Hz, 1 kHz, 2 kHz, & 4 kHz
3) Multiply the area by the coefficient to
determine the absorption units (sabins)
4) Add all sabins to find total sabins for each
frequency
5) Plug all info into the Sabine equation to find
the reverb time (RT60) for the room.
Determining Room Treatments
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The result of the
RT60 calculations
show a short reverb
time at low
frequencies, long
reverb time in the
midrange, and
medium reverb time
in the high
frequencies.
Determining Room Treatments
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1) Find treatments that will achieve the desired
response
2) Determine how much treatment (in sq. ft.)
would be necessary to add the desired amount
of absorption (sabins) by dividing the sabins by
the absorption coefficient. The result will be the
amount of treatment in sq. ft.