In-situ Production Test of Car Audio Systems , 1
In-situ Production Test
of
Car Audio Systems
Tutorial
AES 2017 International Conference
on Automotive Audio
Stefan Irrgang
KLIPPEL GmbH, Dresden, Germany
In-situ Production Test of Car Audio Systems , 2
Scope
• „In-Situ“:
in situ may describe the way a measurement is
taken, that is, in the same place the phenomenon
is occurring without isolating it from other systems
or altering the original conditions of the test. (Wikipedia)
• Production Test: 100% testing
• Car Audio System:
Complete chain: Source, Processing, Amp,
Speaker, Enclosures, Radiation
In-situ Production Test of Car Audio Systems , 3
Car Production EOL test
Headunit
Balance / Fader
LF RF
RRLR
Control
Commands
window buzz
grill
buzz
Needs from market:
• More bass
• More channels
• Higher level
Problem:
Parasitic vibrations:
Very annoying
Easy detectable
Related to product
quality / reputation
Complains are
expensive
In-situ Production Test of Car Audio Systems , 4
Why End-Of-Line Testing?
Basic Goal Separate defect units from delivered goods
simple PASS/FAIL decision using limits
Ultimate Goal Avoid manufacturing of defect units
use diagnostics to get meaningful characteristics used as feedback in controlling the production process and life cylce
In-situ Production Test of Car Audio Systems , 5
Causes of Critical Defects
Headunit
Balance / Fader
LF RF
RRLR
Control
Commands
window buzz
grill
buzz
Survey: 1. Mounting
• Speaker • Panels • Plastic Clips
2. Parts
• Speaker • Enclosure • Sub-Assemblies
Supplier test is not sufficient!
In-situ Production Test of Car Audio Systems , 7
Input
Signal
Output
Signal
Desired
SystemNoise
Defects
StimulusMeasured
Signal
Undesired Defects • Buzzing panels
• Cable beat
• Loose particles
• Air leak noise
Designed Performance • Frequency Response
• Directivity
• Non-linear Distortion
Reproduced Sound Quality Generation of Signal Distortion in an Audio System
Non-correlated signals • Production noise
• Sensor noise
• Car noise (engine, AC)
In-situ Production Test of Car Audio Systems , 8
Left Audio
Channel
Engine
RightAudio
Channel
Excitation of Parasitic Vibration by the Mechanical Transfer Path
loudspeaker
car engine
tire-road
Excitation by
mechanical
Vibration
Sources depend on
• distance between
source and
resonators
• audio channel (left,
right)
independent of
• size of the resonator grill buzz
In-situ Production Test of Car Audio Systems , 9
Excitation of Parasitic Vibration via Acoustical Transfer Path
Left
Audio
Channel
Engine
Right
Audio
Channel
car engine
Excitation by Sound Pressure
Sources
all
loudspeakers
tire-road
interial panel
door buzz
depend on
• size of the resonator
• total sound pressure
independent of
• distance of source
and resonator
• audio channel (left,
right)
In-situ Production Test of Car Audio Systems , 10
Loose Panel: Buzz problem
Most defects behave as a nonlinear oscillator
• active above a critical amplitude
• powered and synchronized by stimulus Loose joint
(Nonlinearity)
parasitic resonator
Externally excited
spring
mass
Loose joint
(Nonlinearity)
parasitic resonator
Externally excited
spring
mass
time one period
vibration
distortion signal
In-situ Production Test of Car Audio Systems , 11
Re-Design
Production
Testing
Problem?
Subjective or Objective Testing?
PASS
FAIL
OR
In-situ Production Test of Car Audio Systems , 12
Human Testing at the Assembling Line? Consequences
• Protect and avoid fatigue of the human ears
• regular breaks • low SPL
• Need for trained operators
• should be more sensitive than the end user
• Need for sufficient time for human inspection • flexible and long cycle times
• Good for double check at sample basis • Use off-line diagnostics station
Human testing is expensive, not critical, not reliable since highly subjective !
In-situ Production Test of Car Audio Systems , 13
Objective Measurement
Techniques
In-situ Production Test of Car Audio Systems , 14
Evaluation in Target Application Condition (in-situ)
Head-
unit
~
audio
signal
amplifier
u, i
microfone
wooferparasitic vibration
u, i
amplifier
internal
sound
sourcesambient
noise
Instrument
(R&D)
artificial test
signal
Instrument
(QC)
Field Measurements • applicable to all units
• ordinary audio signals (music)
• external sound sources
• using hardware in car
• operated by end-user
Lab R&D Measurements • prototypes + Samples
• not time critical
• optimal lab conditions
• using separate equipment
(artificial head, analyzer, ...)
• operated by engineer
EOL Measurements • applicable to all units
• as fast as possible
• external sound sources
• using hardware in car
(+ test microphone)
• automated
In-situ Production Test of Car Audio Systems , 15
State of the Art 100% EOL
Extremely heterogenous:
• Nothing
• rely on manufacturing process stability
• Subjective
• each car driven by test driver, also testing sound
• Objective
• Electrical:
• Field Diagnostics: Connection only
• Acoustic:
• Single Tone Tests: Connection only
• Multitone Tests: Response, Level, Distortion, Phase
• „burned fingers“ with rattling detection
In-situ Production Test of Car Audio Systems , 16
How to check Audio Systems?
Electrical Measurement • Simple current and voltage sensor
• Can be implemented as a diagnostic system in the audio amplifier
• Fast diagnostics of each transducer possible
• Full ambient noise immunity
• Poor detection of defects (no feedback from mechanics)
Acoustical Measurement • Test microphone in car interior required
• Ambient noise may corrupt measurement
• Detection of defects (Rub&Buzz, door buzzing)
advantage
disadvantage
In-situ Production Test of Car Audio Systems , 17
What can be measured with
electrical tests? Linear Distortion • Impedance • Thiele / Small Parameter (Re, fs)
Regular Distortion (caused by Nonlinearities) • Coil offset • Stiffness asymmetry • Polarity from inductance (ANC!)
Defects (Irregular Distortion) • Loose connections
EOL relevant
In-situ Production Test of Car Audio Systems , 18
What can be Measured with
Acoustical Tests?
Linear Distortion • Frequency Response • Sensitivity / Level • Phase / Delay / Polarity
Non-linear Distortion • Harmonic Distortion • Intermodulation Distortion (R&D)
Irregular Distortion (Defects) • Parasitic vibration (Buzz) • Loose Particles / Parts • Air Leakage
EOL relevant
In-situ Production Test of Car Audio Systems , 19
Frequency Response Test Tweeter disconnected
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Soun
d P
ress
ure
[dB]
Frequency [Hz]
Frequency Response QC-Frequency Response
Acoustically significant, distinct symptom
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Frequency Response Test Mat in front of speaker
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Sou
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ress
ure
[dB
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Frequency [Hz]
Frequency Response with Mat
Acoustically not significant, no Rub&Buzz
EQU Tuning test?
In-situ Production Test of Car Audio Systems , 21
• Ensure consistent excitation
• Adjust level in car to target value (94dB SPL)
• Define allowed deviation (+/- 0.5dB)
• Cope with different interiors
• Use existing EQU tuning (Level only)
• Repeated tests required
Volume
Control
Test
microphone
Level Alignment Automatic Adjustment of Average Level
In-situ Production Test of Car Audio Systems , 22
Optimal Stimulus For Testing
Signal Stimulus Output
1V
Output
2V
Output
3V
Music
Multi-Tone
20 Hz – 20 kHz
Multi-Tone
20 Hz – 1 kHz
Sinussoidal Sweep
1 s
Most critical Stimulus
Open question: Which stimulus excites most defects?
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Optimal Signal Analysis
Symptoms of Impulsive distortion • generate new spectral components at harmonics and between
• cover wide frequency range (up to 40 kHz)
• are masked by fundamental and low-order harmonics
• have low spectral energy (close to the noise level)
cannot be detected by conventional Analysis (e.g. THD)
fundamental SPL
frequency
Sound Pressure Spectrum
Regular distortion
2nd
3rd
4th Symptoms of defects
-80 dB
noise level noise
In-situ Production Test of Car Audio Systems , 25
Time Domain Analysis
SPL
frequency
fundamental
Regular distortion
SPL fundamental
SPL fundamental fundamental
Regular distortion Regular distortion
FFT-1
rms-value
time
peak-value
phase spectrum
higher order
harmonics
360 deg
180 deg
0 deg
amplitude spectrum
Solution back to the time domain
• regular distortion generates low
crest factor
• most defects generate symptoms
with high crest factor
exploiting amplitude and phase of
higher-harmonics
peak value reveals small transients
(clicks)
High sensitivity for loudspeaker defects
and parasitic vibration
In-situ Production Test of Car Audio Systems , 26
Speed
Problems: – Available measurement time limited – Positioning of external microphone in the car – Parasitic vibration of high-Q resonators need sufficient excitation – Measurements corrupted by ambient noise require a repetition of the test – Diagnostics of Multichannel systems
Solution: – Use internal (ANC, free speech) microphone – Mobile (wire-less) microphone – As short as possible - excitation with optimal sweep peed profile – Parallel signal processing and failure detection – Automatic repetition of invalid measurements – Minimize number of test clusters (masking effects)
Typical Single Sweep Test including processing: 1s
In-situ Production Test of Car Audio Systems , 27
Measurement Without Defect
Left
Right
L+R
In-situ Production Test of Car Audio Systems , 28
Measurement With Defect
In-situ Production Test of Car Audio Systems , 29
Left
Audio
Channel
Engine
Right
Audio
Channel
OUT1 OUT2 MIC1 LINE1 LINE2 MIC2
Production Analyzer
Ambient Noise
Source
Test
Microphone
Defect
Source
Problem:
• Defect symptom can be 60-80dB below fundamental
• Noise is always present in plant
• More sensitive test more false rejects
Robustness in Production Environment?
In-situ Production Test of Car Audio Systems , 30
Noise Attenuation by Car Body
0
5
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15
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25
30
35
40
45
10 2 10 3 10 4
Upper Class
Medium-Sized
Compact Class
Frequency [Hz]
Att
enuatio
n [dB
]
• Not sufficient for
sensitive Buzz
detection
• No compensation
technique
applicable
Noise Detection
required!
In-situ Production Test of Car Audio Systems , 31
Reliable Detection of Invalid
Measurements
Solution:
1. Using a 2nd microphone (outside)
2. Predicting the effect at the test microphone (inside)
3. Applying the same analysis to test and to noise signal
4. Comparing the symptoms and detecting a corruption of the test signal
Left
Audio
Channel
Engine
Right
Audio
Channel
OUT1 OUT2 MIC1 LINE1 LINE2 MIC2
Production Analyzer
Ambient
Noise
Source
Noise
Microphone
Test
Microphone
Results: PASS, FAIL, NOISE (corrupted)
In-situ Production Test of Car Audio Systems , 32
Merging Technique repeating measurement automatically and accumulating valid parts
Ambient noise disturbances
28 % valid
85% accumulated
62% accumulated
100% valid
Full immunity against random noise
A simple repeat in case of FAIL is NOT optimal!
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Door Buzzing and Simulated Noise
ambient
noise
source
test microphone inside the car
Progress in Data Acquisition
fundamental
rub and buzz
In-situ Production Test of Car Audio Systems , 34
EOL Test Implementation
Stimulus connections:
1. None: Stored in Headunit / Amp
2. Cables (diagnostics station only)
3. Digital Audio Stream (automotive bus)
4. Asynchronous stimulus (CD, USB, Bluetooth with Synchronization)
5. FM-Transmission
6. Wireless Audio Transmission
Processing:
1. External Test-PC
2. Internal Portable Test-PC
3. Headunit (low MIPS demand)
4. Cloud (wave file based)
Microphone connections:
1. None: Internal mic
2. Cables (diagnostics station only)
3. Wireless Audio Transmission
Headunit
USB
Ambient Noise
Microphone
CD Player
Audio
Interface
OBD
AUX in
Test
PC
Production
Master
Control
Test Mic
In Car
Wireless
In / Out
FMFile
In-situ Production Test of Car Audio Systems , 35
Statistics
• Monitor long term characeristics
• Ensure Process Stability
• Combine all measures
• Find and remove outliers
• Predict Yield for changed limits
• Determine Golden DUTs
• Calculate reliable limits
Frequency response
Average level
In-situ Production Test of Car Audio Systems , 36
Online Process Control Rules
• Nelson & WECO rules – simple and common tool to check if process is under control
• Control charts – time course of scalar results (e.g. Re, fs…) based on statistical spread and moving average of past results
Upper Control
Limit
Lower Control
Limit
fs
1σ
2σ
3σ
Example: resonance frequency shows trend of six increasing values in a row
In-situ Production Test of Car Audio Systems , 37
Quantitative Measures
Process capability ratio
Estimates if process is capable of producing within specification limits (LSL/USL)
LTpkpkpp withCPCP ,If process is out of control:
use process performance
2pkC
e.g. standard normal
distribution,
6σ specification:
%9999998.99)6(
DPMO002.0
6
LSLUSLCp
3,
3min
LSLxxUSLCpk
If process is off center:
?
In-situ Production Test of Car Audio Systems , 38
Exploit Statistics
• Analyze large data
pools
• Filter data pool
(e.g. outliers)
• Calculate limits based
on mean or imported
reference
• Export to EOL Test
system
Cu
rve
Sta
tist
ics
...
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So
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ressure
[d
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Frequency [Hz]
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ressure
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Frequency [Hz]
new limits
log data
QC
tes
tin
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PASSFrequence
Response
Polari ty
THD
2nd Harmoni c
3rd Harmonic
Im pedance
Re
fs
In-situ Production Test of Car Audio Systems , 39
Root Cause Analysis
• Based on statistical clustering
• Assisted root cause analysis based on objective testing data
• Immediate and automatic classification – Optimize production
process to increase yield
– Feedback for R&D department improve design
In-situ Production Test of Car Audio Systems , 40
Fe
atu
re 1
(
Re
)
Feature 2 (fs)
Clustering of the Tests
Fe
atu
re 1
(
Re
)
Feature 2 (fs)
Prototype
P2
P1
P3
P4
P5
Volume (deviation)
Similarity (distance)
Clustering:
- Statistical process of
separating DUTs
- Building subgroups /
classes
- DUTs in one cluster have
similar properties
- DUTs of other cluster
have distinct differences
- Prototypes represent
cluster best
In-situ Production Test of Car Audio Systems , 41
Measurement
FAIL
PASS/FAIL
DetectionSelection
PASS
On-Line Off-Line Root
Cause
On-Line
Diagnostics
Clustering
Knowledge
Base
Prototype
Selection
Diagnostics
Station
Tagging
Prototypes
1st step: Automatic clustering of the raw data
2nd step: Prototype selection (best representative of the cluster)
3rd step: Human inspection and tagging of the defect
4th step: Updating the knowledge base for on-line diagnostics
Root Cause Analysis Automatic Defect Classification
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Clustering of cars
Raw Measurement Data (79 cars)
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Sound P
ressure
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Frequency [Hz]
SPL Fundamental
Rub & Buzz
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Sound P
ressure
[d
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Frequency [Hz]
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Std. Audio
3 Door
Std. Audio
3+5 Door
High
Level Audio
3+5 Door
High Level Audio
5 Door, Std. Audio
(low Buzz)
5 Door, Std. Audio (low Buzz)
Test of running production:
• 79 cars total
• All cars passed current EOL
• Subtypes:
3 & 5 door Compact
cars
Standard / High Level
Audio System
5 Door, Std. Audio (high Buzz)
5 Door, Std. Audio
(high Buzz)
In-situ Production Test of Car Audio Systems , 43
Operator View
Defect Class with highest rank
Overall Verdict
Classification Details • List of all possible classes incl.
membership value (probability, max. 1.0)
Car Production Test Classification (On-Line)
In-situ Production Test of Car Audio Systems , 44
Results of Root Cause Analysis
Re-Design
Production
Testing
Production Lot
Golden units
Golden Defects
Defect
Classification
In-situ Production Test of Car Audio Systems , 45
Auralization
Listening Tests
Perceptual Analysis
Objective Measurements
In-situ Production Test of Car Audio Systems , 46
Basic Idea of Auralization
• Sdis>0 dB Distortion emphasized
• Sdis<0 dB Distortion attenuated
• Management: acceptable scaling of distortions
• Engineers: selection of critical stimulus
MIXER
SEPARATOR
xRef
ddis GD
Test Signal
1st input file
2nd input file -
d dis
Reference Car
Test Car
yAur
Output wave file
In-situ Production Test of Car Audio Systems , 47
Audio Sdis/dB Signal
-∞ Reference (optimal quality)
-18
-15
-12
-9
-6
-3
0 Original test signal (current quality)
3
6
9
12
15
18
Systematic Listening Tests Audio Examples (Car Audio System)
In-situ Production Test of Car Audio Systems , 48
Perceptual Evaluation Simulation of Human Auditory system
• Excitation patterns for auralization output
• Simultaneous and temporary masking
[ITU-R BS.1387]
Calculation of Meaningful Perceptual Metrics
• Distortion to mask ratio
• Loudness
• Treble stressing
• Bass enhancement
• Discoloration
• Roughness
• Modulation
• …
In-situ Production Test of Car Audio Systems , 49
Limits
as tight as possible to get high quality
as wide as possible to reach maximal yield
In-situ Production Test of Car Audio Systems , 50
Rub&Buzz limit (1/2) Spec definition
• „Hard“ limit that must not be violated
• Relative to average level „clearance“
• Consider headroom (defects are instable!)
• Collect Data first
• Start with relaxed limit
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Sound P
ressure
[d
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Frequency [Hz]
Frequency Response Response Max
Response Min Rub+Buzz
Rub+Buzz Grades Max Rub+Buzz Max
Level Level
Avg. Level
Rub&Buzz
Limit
R&B Clearance
Rub&Buzz
Example:
R&B Headroom
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S o u n d P
r e s s u r e
[ d
B ]
Frequency [Hz]
Rub&Buzz limit (1/2) Spec definition
• „Hard“ limit that must not be violated
• Relative to average level „clearance“
• Consider headroom (defects are instable!)
• Collect Data first
• Start with relaxed limit
Avg. Level
Rub&Buzz
Limit
R&B Clearance
Rub&Buzz
Example:
R&B Headroom
In-situ Production Test of Car Audio Systems , 52
Rub&Buzz limit (2/2) Internal definition
• „Soft“ Limit for internal analysis
• Detect and analyse stray units
• Relative to reference car
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Sound P
ressure
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Frequency [Hz]
Frequency Response Response Max
Response Min Rub+Buzz
Rub+Buzz Grades Max Rub+Buzz Max
Level Level
• Consistency of production
• Use warning grades
• Record critical responses
Avg. Level
Rub&Buzz
Limit
R&B Specified
Clearance
Rub&Buzz
Rub&Buzz
Grades
Example:
In-situ Production Test of Car Audio Systems , 53
How to tune limits
Left
Audio
Channel
Engine
Right
Audio
Channel
Database
End-of-line testing
Definition of
Production Limits
Off-line Processing
Root Cause
Analysis
Door Buzzing
SDIS = 24 dB SDIS = 12 dB SDIS = -6 dB SDIS = 0 dB SDIS = -12 dB
Test with sweep for loudspeaker
defects and parasitic vibration
Recording of reproduced music
stimulus if defect is detected Auralization +
Perceptive
Evaluation
Reference
Measurement
results
Music recordings
in defective cars
CLUSTER
ANALYSIS
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Learning form End-of-Line Testing
Valuable information for Design and Manufacturing
R&D
Statistics
Production
Design Customer Pass
Automatic
Defect
Classification
Fail Re-Design
Production
Control
Golden
Defect Unit
Root Cause Analysis
Golden
Reference Unit
In-situ Production Test of Car Audio Systems , 55
Summary
• Customers expect high quality
• 100% testing of the components is not sufficient
• Parasitic vibrations are most critical for sound quality
• Fast, reliable detection of defects can be provided in a noisy production environment
• Start with relaxed limits!
• Exploit EOL for root cause analysis (localization and detection of the cause) and process control
• Close the loop from production back to design
In-situ Production Test of Car Audio Systems , 56
Thank you!
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