Audio rendering: from sound diffusion to sound projection

Audio rendering: from sound
diffusion to sound projection

Roberto Magalotti, B&C Speakers
Augusto Sarti, Politecnico di Milano

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

2 Roberto Magalotti - Augusto Sarti

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

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


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)


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)


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


A bit of History...

•  1954, Edgar Villchur of Acoustic Research developed the acoustic
suspension principle for loudspeaker enclosure deisgn


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


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


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


Specifying directivity

0°
6

330° 0 30°

-6

-12

-18 10FCX64
300° 60°
-24

-30
500 Hz
-36 Woofer
1 kHz
-42

270° -48 90° 2 kHz

4 kHz
Driver
8 kHz

240° 120° 16 kHz

210° 150°

180°

Piston in an infinite baffle


Piston at the end of a long tube


Unbaffled piston


Curved source (horn)


Directivity shaping with more sources:
uniform linear array


tapered linear array


linear array with delays (beam steering)


Line arrays in live audio systems

Grateful Dead “Wall of Sound”, 1974


Line arrays in live audio systems

Point source system Line array system

Line arrays, early example:
Parkin – Taylor 1952


Line arrays:
V-DOSC by L-Acoustics, 1992


Contemporary line arrays:
J-shape


MLA by Martin Audio, 2010


MLA by Martin Audio, 2010

DISPLAY 2 rendering software

Cardioid subwoofers:
principle


Cardioid subwoofers:
in practice


WHAT PROCESSING CAN DO

Roberto Magalotti - Augusto Sarti

Perceptual cues for 3D audio
Interaural Time/Level Difference


Perceptual cues for 3D audio
Head Related Transfer Function


HRTF


Multi-channel audio
planar rendering (ITD only)


Multi-channel audio
vector rendering (ITD only)


Beyond ITD

Binaural Rendering

Wavefield Rendering


Wavefield synthesis

Huygens’ principle (1690)
The wavefield produced by
a primary source Ψ can
be reconstructed using a
distribution of secondary
sources


Wavefield Synthesis
example of focused source


Limits

•  Large number of speakers
•  Hard to go beyond planar rendering
•  Invasive installation


From 2D to 3D…

•  Large number of speakers
•  Hard to go beyond planar rendering
•  Invasive installation

HoloPlot ModuleMatrix
(Advanced Acoustic, Potsdam, Germany)

DT solution to reduce invasivity…


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  ópressure 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


Ambisonics rendering

Wavefield as a linear combination of (a basis of)
spherical harmonic functions


Ambisonics rendering

Wavefield approximation as the order increases


Ambisonics dome in PoliMI


Commercial ambisonics installations


High-order speakers
Adrian Freed, Peter Kassakian, David Wessel (CNMAT)

Class-D embedded amplifiers §  Embedded ethernet and DSP


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


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


Virtual
environment
rendering

Wave
ﬁeld
modeled
as
the
superposi0on
of

elementary
beams
(geometrical
acous0cs)

Use
loudspeakers
for

synthesizing
elementary

beams


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


Virtual
environment
rendering

Examples

NMSE
(%):

500
Hz

à
7.4
%

1000
Hz
à
7.9
%

NMSE
(%):

500
Hz

à
5.1
%

1000
Hz
à
5.3
%


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


Environment-‐aware
virtual
environment
rendering

Free-‐ﬁeld
wf
Non-‐compensated
Room-‐compensated

Desired
wave
ﬁeld
rendering
wf
rendering
wf
rendering


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)


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


Example:
reproduc0on
of
the
rear-‐right
channel

Desired
Non-‐compensated
Room-‐compensated

RR

listening
area


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


Professional-grade solutions


Consumer-oriented solutions
MEMS loudspeakers


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.”


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


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.”


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.”


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)


Audio rendering: from sound diffusion to sound projection

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