beam steering & shaping - audio engineering society s… · beam steering & shaping dr. ir....
TRANSCRIPT
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Beam Steering & Shaping
dr. ir. Evert Start
Senior Principle Engineer
Duran Audio/JBL Pro
The Netherlands
July 2015
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CONTENTS
1. INTRODUCTION
2. BASIC ARRAY PHYSICS
3. DIRECTIVITY CONTROL
4. CONTROLLING BASS
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Array myths
Arrays must be curved to shape the lobe…
FIR filtering to optimise the directivity of arrays is new…
Arrays can be put anywhere, the software will do the rest…
You cannot control bass…
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WHAT IS AN ARRAY?
A loudspeaker array is a collection of sound sources (or complete enclosures) that is assembled to achieve a coverage pattern that cannot be achieved with a single loudspeaker. The combined array is more powerful and can have a wider or narrower beam than the individual elements
BEAMFORMING
• Mechanical• Minimum interference• Beam controlled by shape of array
• Electronical• Maximum interference• Beam controlled by (digital) signal
processing of loudspeaker signals
BASIC ARRAY PHYSICSWhat?
Line array
Beam steered/shapedcolumn
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Mechanical beamforming– Line arrays: Radiation pattern dictated by shape of array.
– Minimum interference. HF horns are designed to have minimum mutual interference at higher frequencies.
– Low driver density.
– No multi-channel signal processing.
Electronical beamforming– Radiation pattern determined by (digital) filtering of output
channels (i.e., loudspeaker signals)
– High driver density.
– Maximum interference: Deliberate, controlled interference for obtaining desired radiation pattern.
ARRAY PHYSICSBeamforming Concepts
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KEYWORDS:
Sound is a wave phenomenon
• Frequency f
• Wave length l
• Speed of sound c (=340 m/s)
Waves interfere
Constructive Destructive
ARRAY PHYSICSSound Waves
f
cl
f [Hz] 20 50 100 200 500 1000 2000 5000 10000 20000
l [m] 17.0 6.8 3.4 1.7 0.68 0.34 0.17 0.068 0.034 0.017
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• A small loudspeaker (monopole) radiates sound in all directions (omni-directional sound wave).
• By combining several loudspeakers in an array, the radiation pattern becomes directional.
• In the target direction the sound waves sum, in other directions they (partially) cancel.
ARRAY PHYSICSInterference
2 monopoles spaced at l/2 4 monopoles spaced at l/21 monopole
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4 monopoles (f=1kHz, spacing=l/2)
Representation:
Space-time (yz-t)
Space-frequency (yz-f)
Angle-frequency (rq-f)
10 dB/div
Polar pattern: G(r,q)
Sound pressure level: Lp(y,z)
Pressure: p(y,z,t)
ARRAY PHYSICSInterference
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BEHAVIOUR OF A PARALLEL-DRIVEN POINT SOURCE ARRAY
Fixed driver spacing, variable array length
Fixed array length, variable driver spacing
ARRAY PHYSICSLength and Spacing
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2 LS
8 LS
16 LS
125 Hz (l=2.72 m) 250 Hz (l=1.36 m) 500 Hz (l=0.68 m) 1 kHz (l=0.34 m)Dz = 0.17 m
4 LS
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Dz=0.17 m
Dz=1.36 m
L=2.72 m
Dz=0.34 m
Dz=0.68 m
125 Hz (l=2.72 m) 250 Hz (l=1.36 m) 500 Hz (l=0.68 m) 1 kHz (l=0.34 m)
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Effect of array size and wave length:
Spatial sampling (i.e. driver spacing): (Nyquist criterion)
Note: For directional sources like waveguides this anti-aliasing criterion can be relaxed
ARRAY PHYSICSBasic Laws
LwidthBeam
l~
2
lDz
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AS SHOWN, THERE IS A NEED FOR DIRECTIVITY CONTROL
Objectives:
Consistent radiation pattern over frequency
Uniform coverage and frequency response
Minimize “spill” (e.g., avoid reflective surfaces or reduce outdoor noise pollution
Methods:
Mechanical line array optimisation Minimum interference
Signal processing Maximum interference
– “constant-l” design, i.e. Leff = C ∙ l
– Beam steering
– Beam shaping
DIRECTIVITY CONTROLBeamforming technology
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DIRECTIVITY CONTROLBeam Steering
Mechanical aiming
Electronic steering
Mechanical Aimingversus
Electronic Steering
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Mechanical aiming versus electronic steering
DIRECTIVITY CONTROLBeam Steering
Mechanical aiming
Electronic steering
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DIRECTIVITY CONTROLBeam Steering
direct
reflected
Direct+
reflected
Mechanical aiming
Electronic steering
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Early attempts to control the opening angle (“constant-l”):
Electrical Low-pass filter circuit
Mid/wide band loudspeaker arrangement
Barber pole
Acoustic low-pass filtering
DIRECTIVITY CONTROLSome History
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Electro-Voice LR-4S (1950s)
DIRECTIVITY CONTROLSome History
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UL (1950s)
DIRECTIVITY CONTROLSome History
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“Barber pole” (Philips 1958)
DIRECTIVITY CONTROLSome History
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DIRECTIVITY CONTROLSome History
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DIRECTIVITY CONTROLAdvanced techniques
1. DDC – BEAM STEERING (Developed and introduced in the early 90-ies by Duran Audio)
2. DDS – BEAM SHAPING (Developed and introduced in 1999)
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Digital Directivity Control (DDC)
– “Beam Steering”
– Parametric beam control
– Applied in:
• Intellivox-DC range
DDC
BEAM STEERING
Aiming angle
Opening angle
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2 LS
8 LS
16 LS
125 Hz (l=2.72 m) 250 Hz (l=1.36 m) 500 Hz (l=0.68 m) 1 kHz (l=0.34 m)L=1l
Dz = 0.17 m
4 LS
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2 LS
8 LS
16 LS
125 Hz (l=2.72 m) 250 Hz (l=1.36 m) 500 Hz (l=0.68 m) L=1l
Dz = l/2
4 LS
1 kHz (l=0.34 m)
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Frequency independent:
+
Logarithmic positioning:
Reduction of the number of loudspeakers and signal processing for a given array length
DDC - BEAM STEERINGTransducer spacing
Patented positioning scheme
z
1zzD
ll constLeff )(
2
minlD z
LF
HF
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DDC - BEAM STEERINGBeam parameters
WinControl
RS485
DDC beam parameters
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A/D
d(t-t1) g1
d(t-t2) g2
g3d(t-t3)
gNd(t-tN)
IIR
DDC - BEAM STEERINGBlock diagram
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3.8
m
55 m
zc=2.5 m
zli=1.7 m
DDC - BEAM STEERINGExample
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Opening Angle: 6°
Elevation: -1°
Focus distance: 50 m
Opening Angle: 14°
Elevation: -1°
Focus distance: 50 m
Opening Angle: 6°
Elevation: -5°
Focus distance: 50 m
Opening Angle: 6°
Elevation: -1°
Focus distance: 10 m
Plan view Side view
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DDC - BEAM STEERINGTypical use
Single lobe,
horizontal plane
Single lobe,
tilted plane
Dual lobe,
horizontal + tilted plane
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• Simple and intuitive parametric control• Opening angle
• Aiming angle
• Focus distance
• Constant SPL over distance (up to 70m)
• Large direct-to-reverberant ratio
• High speech intelligibility
• Most suitable for flat audience areas
• Mounting height restrictions:• Offset between acoustic center and audience plane
0.3-0.6 m (~ 1-2 ft.)
DDC - BEAM STEERINGFeatures
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What if: we could not only steer but also shape the beam?
we could extend the frequency response?
we could control bass?
BEYOND BEAM STEERING…
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Digital Directivity Synthesis (DDS)
Invert the desired “illumination” of the room to the array.
Boundary conditions:
– Minimum sound power
– minimum “spill”
– Robustness & Stability
Desired “illumination” DSP
DDS - BEAM SHAPINGThe Inverse Approach
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Digital Directivity Synthesis (DDS)
– “Beam Shaping”
– Beam can be adapted to geometryof the room
– Applied in:
• Intellivox-DS(X) range
DDS
BEAM SHAPING
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Digital Directivity Synthesis (DDS)
Invert the desired “illumination” of the room to the array.
Boundary conditions:
– Minimum sound power
– minimum “spill”
– Robustness & Stability
Desired “illumination” DSP
DDS - BEAM SHAPINGThe Inverse Approach
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DDS - BEAM SHAPING
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DDA (User Edition)
DDS - BEAM SHAPINGUpload Process
DDA exchange file
WinControl
RS485
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FIR 1
FIR 2
FIR 3
FIR N
d(t-t1) g1
d(t-t2) g2
g3d(t-t3)
gNd(t-tN)
DDS BEAMFORMINGBlock Diagram
A/D IIR
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SWEDISH PARLIAMENT Fan-shaped hall
Reflective curved back wall
2x Intellivox-4c-XL (predecessor of Intellivox-DS430)
DDS - BEAM SHAPINGIntellivox Application Example
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• Swedish Parliament– Fan-shaped hall
– Reflective curved back wall
DDS INTELLIVOXIntellivox Application Example
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Weights (priority factors)
DDS - BEAM SHAPINGIntellivox Application Example
Desired direct SPL distribution
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Realized direct SPL distribution
DDS - BEAM SHAPINGIntellivox Application Example
Far field polar pattern
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• Flexible array set-up
• Tailor-made directivity pattern
– Requires (basic) 3D geometric model of space SketchUp ® + Plugin
• Constant spectral balance for all listening positions
• Optimum direct-to-reverberant energy ratio
• Both far field and near field control
• Directivity pattern can be changed by software, i.e., without re-angling the boxes
DDS - BEAM SHAPINGFeatures
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Intellivox-DS430
H=2.5 m
Dz = 0.8 m
H=4.5 m
Dz = 2.8 m
DDS - BEAM SHAPINGMounting height vs. Coverage
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Intellivox-DS430
H=2.5 m
Dz = 0.8 m
H=4.5 m
Dz = 2.8 m
DDS - BEAM SHAPINGMounting height vs. Dispersion
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<D/R>= -4.6 dB
<D/R>= -7.0 dB
V=6,400 m3
RT=3 s
DDS - BEAM SHAPINGMounting height vs. D/R ratio
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<STI>=0.50
<STI>=0.45
V=6,400 m3
RT=3 s
DDS - BEAM SHAPINGMounting height vs. Intelligibility
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DDS - BEAM SHAPINGMounting height
Conclusion:• Larger mounting height
Larger steering angle & wider dispersion
Lower D/R ratio
Poorer speech intelligibility and musical clarity
Extremely large steering angles don’t make sense!
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CONTROLLING BASS
1. WHAT ARE BEAM-SHAPED DIFFERENTIAL SUBWOOFER ARRAYS?
2. ACOUSTIC MODELLING BY PSM-BEM
3. VALIDATION OF PSM-BEM BY MEASUREMENTS
4. SUMMARY AND CONCLUSIONS
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Normal versus cardioid bass arrays
Cardioidbass array
Normalbass array
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SUBWOOFER ARRAYS“Summing”
Directivity:
Gain and robustness:
l
LQ
L
l
l
array
array
fP
fPG
1
2
2
)(
)(log10
80 Hz
)log(10 QDI
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SUBWOOFER ARRAYS“Differential”
80 Hz
+ “Superdirectional”, i.e., high Q for small L/l
- Less robust than delay-and-sum arrays
1x B-215DIFF
3x B-07
Front Back
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Combination of delay-and-sumand differential array
DDS-optimised
– Requires an accurate model of each box
BEAM-SHAPED DIFFERENTIAL
SUBWOOFER ARRAYS
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Point Source Model (PSM)
Each loudspeaker in the array is represented by a point source with a certain directivity
Radiation into free space (free field conditions)
ACOUSTIC MODELLING OF SUB
ARRAYS
,mr
m
fG
,
,,,
,
,,m
rkj
mmmr
efGfH
m
q
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Benefits:
Computationally efficient
Only one directivity function for each loudspeaker type
Shortcomings:
No LF 'coupling' between stacked subwoofers
– In reality, sensitivity of each box depends on stack size
No modelling of LF diffraction around array
– In reality, directivity and F/B ratio of each box depends on stack size
No accurate ground plane modelling (i.e., half-space) possible with simple mirror image source model
POINT SOURCE MODEL (PSM)
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COUPLING EFFECTSARRAY SIZE AND LOUDSPEAKER POSITION
Free field
5U1
5U3
5U2
Magnitude (6 dB/div)
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COUPLING EFFECTSBoundary plane
Full-
space
Half-
space
5U1
FS5U1
HS
Magnitude (6 dB/div)
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Idea:
Each loudspeaker in the array is modelled as a directional point source
BEM is applied to calculate directivity functions of loudspeaker facing the actual Acoustic Boundary Conditions (ABC), including half space conditions
Benefits:
One-time only calculation of directivity library for various ABC
Library can be easily extended
Computationally efficient simulation
HYBRID PSM-BEM MODEL
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Procedure:
Measure normal component of particle velocity in front of cone and ports of subwoofer
Make finite boundary element model of subwoofer array
Calculate pressure distribution on boundaries using either full-space or half-space version of Helmholtz Integral Equation (HIE)
From the measured velocity and the calculated pressure distribution, calculate directivity balloons for active subwoofer
BEM CALCULATIONS
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BEM CALCULATION EXAMPLE
Set-up
Free-Field 3U1 full-space 3U1 half-space
) ) ) ) ) ) ) ) )
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BEM CALCULATION EXAMPLE
Normal particle velocity @80 Hz
Free-Field 3U1 full-space 3U1 half-space
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BEM CALCULATION EXAMPLE
SPL @80 Hz
Free-Field 3U1 full-space 3U1 half-space
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BEM CALCULATION EXAMPLE
Balloon @80 Hz
Free-Field 3U1 full-space 3U1 half-space
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BEM CALCULATION EXAMPLE
Sensitivity
Front-to-back ratio
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VALIDATION PSM-BEM MODEL
R=7 m
D=10º
2x B-121
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VALIDATION PSM-BEM MODEL
Cardioid Hyper-cardioid Dipole
Theoretical
Predicted
(63 Hz octave)
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MEASUREMENT RESULTSCardiod setting
Mean array parameters:
DI = 4.9 dB
Garray = 1.4 dB
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MEASUREMENT RESULTSDipole setting
Mean array parameters:
DI = 5.3 dB
Garray = -0.5 dB
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HOW DOES IT WORK IN PRACTICE?DDS Geo method
Realised direct SPL
Weights
Desired direct SPL
Directivity balloon
DDA Software
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HOW DOES IT WORK IN PRACTICE?DDS Balloon method
Realised direct SPL Desired radiation pattern
Directivity balloon
DDA Software
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Hybrid PSM-BEM model handles
– Full-space
– Half-space
– Various array lengths
Very accurate modelling of beam-shaped differential subwoofer arrays
Large front-to-back ratio of cardioid subwoofer arrays
Good Robustness, i.e. array response not sensitive to small deviations in sensitivity of individual drivers
Summary & Conclusions