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

Roberto Magalotti - Augusto Sarti

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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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 3


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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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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•  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…

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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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A bit of History...

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

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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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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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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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Specifying directivity

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-42

-36

-30

-24

-18

-12

-6

0

60°

30°

60°

90°

120°

150°

180°

210°

240°

270°

300°

330°

10FCX64

500 Hz

1 kHz

2 kHz

4 kHz

8 kHz

16 kHz

Woofer

Driver

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Piston in an infinite baffle

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Piston at the end of a long tube

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Unbaffled piston

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Curved source (horn)

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Directivity shaping with more sources: uniform linear array

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Directivity shaping with more sources: tapered linear array

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Directivity shaping with more sources: linear array with delays (beam steering)

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Line arrays in live audio systems

Grateful Dead “Wall of Sound”, 1974

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Line arrays in live audio systems

Line array system Point source system 23


Line arrays, early example: Parkin – Taylor 1952

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

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

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Contemporary line arrays: J-shape

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Contemporary line arrays: MLA by Martin Audio, 2010

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Contemporary line arrays: MLA by Martin Audio, 2010

DISPLAY 2 rendering software 31


Cardioid subwoofers: principle

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Cardioid subwoofers: in practice

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WHAT PROCESSING CAN DO


Perceptual cues for 3D audio Interaural Time/Level Difference

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Perceptual cues for 3D audio Head Related Transfer Function

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HRTF

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Multi-channel audio planar rendering (ITD only)

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Multi-channel audio vector rendering (ITD only)

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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

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Wavefield Synthesis example of focused source

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Limits

•  Large number of speakers

•  Hard to go beyond planar rendering

•  Invasive installation


From 2D to 3D…

HoloPlot ModuleMatrix (Advanced Acoustic, Potsdam, Germany)

•  Large number of speakers

•  Hard to go beyond planar rendering

•  Invasive installation


DT solution to reduce invasivity…

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Spherical Harmonic functions:

Spherical Bessel

Functions:

Ambisonics fundamentals

Intrinsic quality of representation Using components Bmn

σ up to a limited order (m≤M) Angular resolution ó radial expansion % wave length

0 0 , 1( ) ( ) ( , )m

m mn mnm n m

p r j j kr B Yσ σ

σ

θ δ∞

= ≤ ≤ =±

=∑ ∑r

Ø  Spherical Harmonic Decomposition : Fourier-Bessel series

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

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Ambisonics rendering

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

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Ambisonics rendering

Wavefield approximation as the order increases

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Ambisonics dome in PoliMI

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Commercial ambisonics installations

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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

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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

virtual source

loudspeakers

listening region

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Virtual environment rendering

Wave field modeled as the superposi0on of elementary beams (geometrical acous0cs)

Use loudspeakers for synthesizing elementary

beams

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•  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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Examples

NMSE (%): 500 Hz à 7.4 % 1000 Hz à 7.9 %

NMSE (%): 500 Hz à 5.1 % 1000 Hz à 5.3 %

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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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Environment-‐aware virtual environment rendering

Desired wave field Free-‐field wf rendering

Non-‐compensated wf rendering

Room-‐compensated wf rendering

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Exploi5ng the environment

… is it possible to think at the environment as an “augmented”

rendering system?

image array (first order)

image array (first order)

image array (second order)

image array (second order)

IDEA: use the walls (i.e., image loudspeakers) to increase the array

Applica0on: simula0on of a 5.1 surround system [Canclini2012]

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Exploiting the environment

Environment-‐aware virtual 5.1 surround

What about undesired reflec0ons?

We can introduce, again, room compensa0on (slightly modified):

Desired response Actual response

CGG roomdes =

desroomT

roomT

roomT GGGGC 1)(ˆ −=

ffcomp hCh ˆ=Compensated loudspeaker gains

listening area

CL R

RL RR

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Example: reproduc0on of the rear-‐right channel

Desired Non-‐compensated Room-‐compensated

listening area

RR

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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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Professional-grade solutions

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Consumer-oriented solutions MEMS loudspeakers

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Roberto Magalotti - Augusto Sarti 67


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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Quickness

•  IPAL (Integrated Powered Adaptive Loudspeaker) §  Virtual Parameter §  Direct Pressure Control §  Processing Time: 10 µs

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“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.”


Exactitude

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“…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.”


Exactitude

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“…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

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“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

•  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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“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.”

Technology

Audio rendering: from sound diffusion to sound projection