acoustical measurement of sound system equipment according … · 2020. 5. 6. · klippel-live #3:...
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KLIPPEL-live #3: Extracting meaningful data from 3D output, 1
Acoustical Measurement of Sound System Equipment
according IEC 60268-21
KLIPPEL LIVEa series of webinars presented by
Wolfgang Klippel
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1. Modern audio equipment needs output based testing2. Standard acoustical tests performed in normal rooms3. Drawing meaningful conclusions from 3D output measurement4. Simulated standard condition at an evaluation point5. Maximum SPL – giving this value meaning 6. Selecting measurements with high diagnostic value7. Amplitude Compression – less output at higher amplitudes8. Harmonic Distortion Measurements – best practice9. Intermodulation Distortion – music is more than a single tone10. Impulsive distortion - rub&buzz, abnormal behavior, defects11.Benchmarking of audio products under standard conditions 12.Auralization of signal distortion – perceptual evaluation 13.Setting meaningful tolerances for signal distortion14.Rating the maximum SPL value for a product15.Smart speaker testing with wireless audio input
Acoustical testing of a modern active audio device
Previous Sessions
1st Session
2nd Session
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KLIPPEL-live #3: Extracting meaningful data from 3D output, 3
3rd KLIPPEL LIVE:Drawing meaningful conclusions from 3D output
measurement
Topics today:
1. Standard Measurement Techniques2. Far field directivity (e.g. professional application) 3. Mean value at selected angles (spin-o-rama) (e.g.
consumer-home application)4. Mean value of a listening zone in 3D space (e.g.
personal audio devices)5. Accurate complex data for beam steering (e.g.
loudspeaker panels)
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Poll:
Is loudspeaker directivity relevant for yourwork?
• Always• Depends on the particular application• Not really
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Conventional Directivity Measurement
PUSH
AMP
InputOutput
Turntable
Multiplexer Analyzer
Amplifier
Loud-speaker
anechoicroomThe sound pressure is measured at
multiple measurement points in the far field located on a sphere with radius r.
Angular Resolution depends on the number of measurement points placed in the far field.
5 degree 2592 points2 degree 16200 points1 degree 64800 points
r
Not practical
θ
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Question:How much angular resolution would you like to see in the measured directivity?
A. Only on-axis dataB. A few measurement point at selected angles (e.g. 30
degree) off-axisC. Vertical and horizontal polar plots D. Balloon data with at least 5 degree resolutionE. Balloon data with more than 5 degree resolution
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0°
+30°
+60°
+90°
+120°
+150°
+/-180°
-150°
-120°
-90°
-60°
-30°
-25 -20 -15 -10 -5-30
Polar Plot4 kHz
KLIPPELvertical horizontal
0°
+30°
+60°
+90°
+120°
+150°
+/-180°
-150°
-120°
-90°
-60°
-30°
-25 -20 -15 -10 -5-30
Polar Plot4 kHz
KLIPPELvertical horizontal
0°
+30°
+60°
+90°
+120°
+150°
+/-180°
-150°
-120°
-90°
-60°
-30°
-25 -20 -15 -10 -5-30
Polar Plot4 kHz
KLIPPELvertical horizontal
0°
+30°
+60°
+90°
+120°
+150°
+/-180°
-150°
-120°
-90°
-60°
-30°
-25 -20 -15 -10 -5-30
Polar Plot4kHz
KLIPPELvertical horizontal
5° Angular Sampling
1° Angular Sampling
Linear Interpolation between PointsConventional Directivity Measurement
Linear (complex) interpolation can generate a significant (aliasing) error if the angular sampling can not describe the complexity of the directivity pattern
10° Angular Sampling20° Angular Sampling
Vertical directionHorizontal direction
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Holographic MeasurementNear Field Scanning + Wave Expansion
General solutions of the wave equation are used as basic functions in the expansion
Total number of coefficients = (N+1)2
monopole
dipoles
quadrupoles
BASIC FUNCTIONS
),( rB f+
Results
Direct SoundExtrapolation
NEAR FIELD DATA
),( rfH
Scanning
( )TFE fTotal Fitting Error
)( fCCOEFFICIENTS
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N=0 N=2N=1 N=3 N=10
f in Hz
Sou
nd P
ower
in d
B
Total Sound Power
Directivity patterns at 200 Hz:
Target
sound field is completely described by order N=3 (16 Coefficients)
Holographic Measurement Example: Wave Expansion of a Woofer
Higher orders
n=0
n=1
n=2
n=3
monopole
dipoles
quadrupoles
can be estimated by a few scanning points (M > 16)
monopole
dipoles
quadrupoles
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Order of the Expansion Depends on the Loudspeaker Properties
The directivity can be modeled by single coefficients and a single scanning would be required if• the expansion point is in the acoustical center• the dipole axis is aligned with the coordinate system
Example: Woofer in a Vented Box far below port resoance
woofer
port -q
+q
Dipole
internal coordinate system
Angular resolution is much higher than the sampling on the scanning grid
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Holographic MeasurementExtrapolation of the Direct Sound
General solutions of the wave equation
)( fCCOEFFICIENTS BASIC FUNCTIONS
),( rB f+
Results
Scanning
Wave Expansion Sound pressure distribution (3kHz) generated by a laptop outside the scanning surface
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Comparison of the Standard Methods according IEC 60268-21
Conventional Measurement
Holographic Measurement
Sound pressure measurement
direct far field measurement (turntable)
near field scanning (robotics)
Simulated Free Field Condition
Windowing restricted to higher frequencies
• direct sound separation at lower frequencies
• Windowing applied at higher frequencies
Model not required (spherical) wave model
Results sound pressure depending on angles and distance r
• sound pressure depending on point r in 3D space
• model parameters C
Angular resolution Limited by number and placing of measurement point
Higher than scanning grid (interpolation based on spherical wave modeling)
Extrapolation within far-field (1/r law is valid) near and far field (outside scanning surface)
Self-test not possible based on total fitting error
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Far field DirectivityExample Laptop r=1m
Vertical (phi = 0 degree)
Horizontal (phi=90 degree)
The left and right speaker generate a complex directivity pattern !
left
right
2 kHz
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CEA 2034 Standard using Spino-a-rama
Application : Home audio devices, Hifi-Loudspeaker
CEA 2034 - Window
60
65
70
75
80
85
90
95
100
105
110
10 2 10 3 10 4
f / Hz
On Axis
Listening Window
Early Reflections
Sound Power
Sound Power DI
Early Reflections DI
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POLL:
Which integral values based on CEA 2034 do youuse for your work?(multiple answers)
• None• Sound power • Directivity index • Listening window• Early reflections
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KLIPPEL
0
50
100
150
200
250
0.01 0.1 1 10
Apparent Sound Power vs. Distance at f=501 Hz
Lw/d
B
DISTANCE r / m
N=0 N=1 N=2 N=3 N=4 N=5N=6 N=7 N=8 N=9 N=10 N=11N=12 N=13 N=14
Near-Field or Far-Field?
Determining the location of the near and far-fields is important for personal and handheld audio devices !!
Far-field
Near-field
user
monopoledipolesquadrupoles
Multipoles of nth-order
Power is independent of distance
Wave Expansion
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Evaluation Point in the Near Field
rerMeasurement distance
Near field Far field
Far-field extrapolation
a
Scanning surface
Problem: • Large Speaker • Reference distance re>rfar (far field distance)Solutions: a) True near-field SPL L(re) measured at the evaluation distance re (1/r
Law is not applicable)b) Assumed far-field SPL Lfar(re) referenced to evaluation distance re
(discrepancy to reality, extrapolation into far field is possible)
rfar
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POLL:Are the near field properties relevant for your work?
• No• Yes, the listening zone is in the near field (personal,
mobile audio devices, studio monitors) • Yes, the loudspeaker size (professional equipment,
sound bars, sound panels) is too large for my anechoic room
• Yes, other reasons
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Comprehensive 3D Informationsupports the evaluation of special sound effects
KLIPPEL
60
65
70
75
80
85
90
95
100
1 10
Near Field SPL Response
dBS
PL
/ V
frequency / kHz
Left Ear
Wave front propagation
3kHz
SPL distribution
3kHz
Observation plane
ComprehensiveInformation
(Amplitude) (Phase)
Right EarListening Points
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Near-field Information is important for 3D sound effects
Wave front propagation (3kHz) Sound pressure distribution (3kHz)
KLIPPEL
30
40
50
60
70
80
90
100
1 10
Sound Pressure on axis
dBS
P L/
V
Frequency /kHz
r = 0.5m(near field)
Observation plane
2 m
0.5 m
ComprehensiveInformation
Variation versus distance
(Amplitude) (Phase)
r = 2m(far field)
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Application: Personal audio devices, Laptops, Tablets, etc.
50
55
60
65
70
75
80
85
90
95
100
105
10 2 10 3 10 4
dBSP
L
f / Hz
IEC62777 – Personal Acoustic Zones
PF (Front)
PL (Front-Left) PR (Front-Right)
PB (Rear)
PU (Front-Upper) PD (Front-Lower)
IEC 62777 Standard using Listening Zone
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POLL:
Do you use DSP for generating a desired directivitypattern of your speaker? (multiple answers possible)
• No• For two transducers (woofer, tweeter) at the
crossover region • For stereo system• For active beam forming and beam steering
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Measurement of line sources(sound bars, line array)
Particularities:• Large dimensions• Multiple tweeter• Wide spread
near field (r>>d)
Problems:• Sound field has high complexity• Fitting for high Frequencies (>5kHz)
requires high order N>50• Many measurement points,
long measurement time
Mul
tiple
xer
1) Measure each loudspeaker separately by using a multiplexer
Solution:
3) Super positioning of the multipoles
2) Wave expansion of each loudspeaker
Speaker 2
Speaker n
Speaker 1
+
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Measurement of line sources (2)Super positioning of the multiple measurements
Line Array:• 8 coaxial speakers• 24 tweeter• Super position of 8
multipoles
2kHz
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Controlled directivity
25
FilterDelay
𝜏𝜏0
𝜏𝜏0
𝜏𝜏0
𝜏𝜏0
𝜏𝜏0
𝜏𝜏0
𝜏𝜏0
𝜏𝜏0
DSP / Smart Amplifier
left
center
right
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Summary
• IEC 60268-21 defines conventional directivity measurements and new techniques for assessing the complete 3D sound output
• A holographic measurement models the sound pressure distribution outside of a scanning surface placed in the near-field of the speakers
• A spherical wave model is well suited for loudspeakers giving maximum resolution using a few parameters
• A holographic measurement performs a self-check and gives maximum accuracy for a minimum number of scanning points
• Traditional characteristics (e.g. spin-o-rama) simplify the interpretation
• Mean values, variances can be calculated for sound zones for the particular application
• Reliable complex 3D output data for each transducer can be provided for beam steering and other DSP processing
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Open Questions
However, most measurements (e.g. distortion, equalization) can be performed at a single evaluation point!
Can we perform our daily standard measurements at high accuracy outside the anechoic room?
The 4th KLIPPEL live webinar will address:• Simulation of free-field and far-field condition according IEC
60268-21• How to compensate for room influence, different positioning
and distance• Can we use a fixed room compensation functions for different
types of speakers?
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1. Modern audio equipment needs output based testing2. Standard acoustical tests performed in normal rooms3. Drawing meaningful conclusions from 3D output measurement4. Simulated standard condition at an evaluation point5. Maximum SPL – giving this value meaning 6. Selecting measurements with high diagnostic value7. Amplitude Compression – less output at higher amplitudes8. Harmonic Distortion Measurements – best practice9. Intermodulation Distortion – music is more than a single tone10. Impulsive distortion - rub&buzz, abnormal behavior, defects11.Benchmarking of audio products under standard conditions 12.Auralization of signal distortion – perceptual evaluation 13.Setting meaningful tolerances for signal distortion14.Rating the maximum SPL value for a product15.Smart speaker testing with wireless audio input
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