Audio rendering: from sound diffusion to sound projection
1. Audio rendering: from sound
diffusion to sound projection
Roberto Magalotti, B&C Speakers
Augusto Sarti, Politecnico di Milano
2. B&C Speakers is...
... one of the largest and most prestigious electroacoustic transducer
manufacturers in the world; designing, developing and manufacturing only for
the high-end professional audio market under its own ‘B&C Speakers’
brandname
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3. Summary
• Loudspeakers
§ A bit if history
§ Loudspeakers today
• Directivity in acoustics
§ Definition
§ Specification
§ Examples
• Line arrays in sound reinforcement
§ In the '50
§ In the '90
§ Today
• What processing can do
• Spatial audio
• Acoustic holography
• Room compensation
• Virtual acoustics
• Technological and scientific challenges
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4. A bit of History…
• 1857, Antonio Meucci invents the “Telettrofono”, which includes the first
loudspeaker
• “consiste in un diaframma vibrante e in un magnete elettrizzato da un filo a spirale che lo
avvolge. Vibrando, il diaframma altera la corrente del magnete. Queste alterazioni di
corrente, trasmesse all'altro capo del filo, imprimono analoghe vibrazioni al diaframma
ricevente e riproducono la parola”
• 1876, Alexander Graham Bell patents Meucci’s invention
The US congress finally recognizes Meucci’s paternity of the invention in 2002
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5. A bit of History…
• 1877, Ernst Siemens (Germany) patented the first loudspeaker
• 1898, Sir Oliver Lodge (England) second patent for a loudspeaker
(before music was electrified)
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6. A bit of History…
• 1924, two General Electric researchers, Chester W. Rice and Edward
Washburn Kellogg patented the Radiola speakers (moving coil, direct
radiator loudspeaker)
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7. A bit of History…
• 1926, two researchers from Bell Labs, E.C. Wente and A.L. Thuras,
designed a “phase plug” in front of the membrane of a horn-loaded
loudspeaker, creating the compression driver
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8. A bit of History...
• 1954, Edgar Villchur of Acoustic Research developed the acoustic
suspension principle for loudspeaker enclosure deisgn
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9. A bit of History...
• 1957, Quad ESL marketed as the first full-range electrostatic
loudspeaker, designed by Peter Walker and David Williamson, based
on Edward W. Kellogg's patent from 1934
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10. Loudspeakers today
• Permanent magnets have replaced the field coil
§ AlNiCo ('40)
§ Ferrites ('50)
§ Rare Earth ('90)
• Voice coils are designed to handle the large electrical power supplied
from modern amplifiers
• Finite Element Analysis is extensively used in all aspects of
loudspeaker design (acoustics, thermodynamics, electromagnetism,...)
• Digital Signal Processing is widely used in loudspeaker system set-up
and management
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11. Directivity in acoustics
• Definition: radiation pattern in the far field with respect to the source(s)
• Far field: the radiation pattern no longer changes with the distance
§ Several wavelengths form the source
§ Several times the size of the source
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41. Wavefield synthesis
Huygens’ principle (1690)
The wavefield produced by
a primary source Ψ can
be reconstructed using a
distribution of secondary
sources
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42. Wavefield Synthesis
example of focused source
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43. Limits
• Large number of speakers
• Hard to go beyond planar rendering
• Invasive installation
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44. From 2D to 3D…
• Large number of speakers
• Hard to go beyond planar rendering
• Invasive installation
HoloPlot ModuleMatrix
(Advanced Acoustic, Potsdam, Germany)
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45. DT solution to reduce invasivity…
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46. Ambisonics
fundamentals
Ø Spherical Harmonic Decomposition : Fourier-Bessel series
r ∞
p(r ) = ∑ j m jm (kr ) ∑ BmnYmn (θ , δ )
σ σ
m =0 0≤ n≤ m,σ =±1 Spherical Harmonic functions:
o Sound field represented by coefficients Bmnσ
o = Spherical Harmonic component ó “Ambisonic Signals”
o ópressure field spatial derivatives of successive orders m
o Around a reference point = listener point of view
Spherical
Bessel
Functions:
Intrinsic quality of representation
Using components Bmnσ up to a limited order (m≤M)
Angular resolution ó radial expansion % wave length
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47. Ambisonics rendering
Wavefield as a linear combination of (a basis of)
spherical harmonic functions
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48. Ambisonics rendering
Wavefield approximation as the order increases
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52. High-order speakers
Adrian Freed, Peter Kassakian, David Wessel (CNMAT)
Class-D embedded amplifiers § Embedded ethernet and DSP
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53. Soundfield sythesis: goals
• Render
a
virtual
acous0c
source
using
a
loudspeaker
array
• Render
the
acous0cs
of
a
virtual
environment
• Compensate
for
the
early
reflec0ons
of
the
environment
in
which
the
loudspeaker
array
is
opera0ng
(“room
compensa0on”)
• Exploit
the
early
reflec0ons
of
the
real
environment,
e.g.
for
a
virtual
home
theater
system
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54. Rendering virtual acoustic sources
Beamshaping
Goal:
• Rendering
of
a
virtual
source
along
with
its
radia0on
paDern
in
a
listening
region,
by
means
of
a
loudspeaker
array
• Focus
on
direc0onal
sources,
modeled
as
beams
• Need
to
control:
• Source
posi0on
• Direc0on
of
emission
• Beam
aperture
loudspeakers
virtual
source
listening
region
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55. Virtual
environment
rendering
Wave
field
modeled
as
the
superposi0on
of
elementary
beams
(geometrical
acous0cs)
Use
loudspeakers
for
synthesizing
elementary
beams
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56. Virtual
environment
rendering
• Image
sources
are
suitable
only
in
simple
convex
environments
• Visibility
tests
are
very
0me
demanding
• A
convenient
solu0on
is
beam
tracing,
which
compute
visibility
very
efficiently
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58. Virtual environment rendering
in reverberant rooms
• Loudspeakers
operate
in
an
arbitrary
reverberant
room
• Image
loudspeakers
determined
through
beam-‐tracing
• Room
compensa0on
enables
a
free-‐field
behaviour
of
the
beam
shaping
• Design
of
a
virtual
environment
to
be
rendered…
• …
and
rendering
through
superposi0on
of
beams
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60. Exploi5ng
the
environment
image
array
image
array
(first
order)
(first
order)
…
is
it
possible
to
think
at
the
environment
as
an
“augmented”
rendering
system?
IDEA:
use
the
walls
(i.e.,
image
loudspeakers)
to
increase
the
array
Applica0on:
simula0on
of
a
5.1
surround
system
[Canclini2012]
image
array
image
array
(second
order)
(second
order)
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61. Exploiting the environment
Environment-‐aware
What
about
undesired
reflec0ons?
virtual
5.1
surround
Desired
response
Actual
response
L R
RL
RR
C
We
can
introduce,
again,
room
compensa0on
(slightly
modified):
G des = G roomC
ˆ
C = (GT roomGT room ) −1 GT roomG
listening
area
des
Compensated
loudspeaker
gains
ˆ
h comp = Ch ff
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62. Exploiting the environment
Example:
reproduc0on
of
the
rear-‐right
channel
Desired
Non-‐compensated
Room-‐compensated
RR
listening
area
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64. Technological challenges
• Pervasive but non-invasive audio
• Immersive experience
• Contained within the environment
• Acoustic displays should not “take over” but should seamlessly
integrate with the surroundings
• Design constraints
• slim, compact, …in the wrong place
• High quality expectations
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68. Lightness
“La seconda rivoluzione industriale non si presenta come la prima con immagini schiaccianti
quali presse di laminatoi o colate d'acciaio, ma come i bit di un flusso d'informazione che
corre sui circuiti sotto forma di impulsi elettronici. Le macchine di ferro ci sono sempre,
ma obbediscono ai bit senza peso.”
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69. Quickness
“Sogno immense cosmologie, saghe ed epopee racchiuse nelle dimensioni di un epigramma.
Nei tempi sempre più congestionati che ci attendono, il bisogno di letteratura dovrà
puntare sulla massima concentrazione della poesia e del pensiero.”
• IPAL (Integrated Powered Adaptive Loudspeaker)
§ Virtual Parameter
§ Direct Pressure Control
§ Processing Time: 10 µs
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70. Exactitude
“…il giusto uso del linguaggio per me è quello che permette di avvicinarsi alle cose (presenti
o assenti) con discrezione e attenzione e cautela, col rispetto di ciò che le cose (presenti
o assenti) comunicano senza parole.”
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71. Exactitude
“…il giusto uso del linguaggio per me è quello che permette di avvicinarsi alle cose (presenti
o assenti) con discrezione e attenzione e cautela, col rispetto di ciò che le cose (presenti
o assenti) comunicano senza parole.”
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72. Visibility
“La mente del poeta e, in qualche momento decisivo, la mente dello scienziato, funzionano
secondo un proceimento di associazioni di immagine che è il sistema più veloce di
collegare e scegliere tra le infinite forme del possibile e dell’impossibile.”
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73. Multiplicity
“Ogni vita è un’enciclopedia, una biblioteca, un inventario di oggetti, un campionario di stili,
dove tutto può essere continuamente rimescolato e riordinato in tutti i modi possibili.”
• Home systems
§ From 2-ch stereo...
§ ...to multichannel and sound bars
• Cinema Sound
§ From 5.1, 7.1, 10.1, 22.2....
§ ...to 3D Sound (Dolby Atmos)
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