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Measuring Maximum Continuous SPL with Multi-
Tone Stimuli
Gregor Höhne
Agenda
• Introduction/Motivation
o What is max SPL and why do we need to specify it
o Limiting factors for max SPL
• Existing Measurement Techniques
o Overview on ANSI/CEA-2010-A and B
• Proposed Improvements
o Multi-tone Stimuli
o Compression Prediction
o Advanced Interpretation
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What limits the Sound Pressure Output ?
for a sinusoidal input with different On/Off Cycle
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20 50 100 200 500 1k 2k 5k
SOUND PRESSURE LEVEL
dB
- [
V]
(rm
s)
Frequency [Hz]
Maximal SPL (short term)
1 s on, 1 min off
Maximal SPL (long term)
Stepped sine 1 min on, 1 min off
Limited by voice coil heating
Limited by peak
displacement
Limited by maximal acceleration handled by
the coil, cone, glue
03/01/2017 Measuring Maximum Continuous SPL with Multi-Tone Stimuli 3
Factors Limiting Maximal Output
Peak displacement
• Transducer nonlinearities distortion, compression
• Mechanical protection system attenuation of bass signal
Acceleration
• Cone, coil Damage of the mechanical parts
Peak voltage at amplifier output (active loudspeaker system)
• Amplifier clipping impulsive distortion
• Limiter, gain control, protection
Coil temperature
• Increase of Re Power compression thermal damage
• Thermal protection system attenuation of total signal
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Large Signal Performance
• Maximal SPLmax rated at reference point (1 m, on-axis) in specified frequency range (or corresponding maximum input umax)
• Compression of fundamental component (thermal and nonlinear effect)
• Harmonic distortion (THD + nth-order components, equivalent input distortion measured with steady-state tones or burst, sweep)
• Intermodulation distortion (two-tone IMD, multi-tone MTD)
• Impulsive distortion (PHD, CHD) indicating rub&buzz, loose particles
• Modulated noise (MOD) indicating air leakage
• Durability verified in long-term and accelerated life testing (100 h)
Specifications for Active and Passive Loudspeaker Systems
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Agenda
• Introduction/Motivation
o What is max SPL and why do we need to specify it
o Limiting factors for max SPL
• Existing Measurement Techniques
o Overview on ANSI/CEA-2010-A and B
• Proposed Improvements
o Multi-tone Stimuli
o Compression Prediction
o Advanced Interpretation
03/01/2017 Measuring Maximum Continuous SPL with Multi-Tone Stimuli 7
ANSI/CEA-2010-A/B
Targets: • How loud in volume and how low in frequency is the speaker capable of
operation?
• measure and report maximum usable sound pressure level • required output power of amplifier
• for Manufacturer - capability of the speaker
• for Consumer - Simple rating „…to select, purchase and enjoy a subwoofer,
that will complement their main full-range loudspeaker system.“
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Burst Test (CEA-2010-A)
-2
-1
0
1
2
0 200 400 600
Stimulus
Limit Check
[ms]
[V]
Maximum SPL (last passed Peak value)
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0 200 400 600
[ms]
[Pa]
Microphone Signal
Peak Value
Increase Voltage
FAIL
PASS
10 100 1k 10k [Hz]
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-10
0
Thresholds
[dB]
fundamental 2nd
3rd 4th
5th
higher order + noise
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Burst Test Results
Tone Burst Center Frequency (Hz)
Maximum SPL CEA-2010 Rating
20 79
84 25 85
31.5 89
40 96
100 50 101
63 104
SPLMAX rated to 1m and 2.83V
65
70
75
80
85
90
95
100
20 30 40 50 60
Input V
oltage
Maximum SPL
[dB
SP
L]
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SPL Continuous (ANSI/CEA 2010 B)
FAIL
PASS
1 min Heating
35
40
45
50
55
60
[ d B
]
3dB compression
Reference
30
10 2 10 3 10 4
Frequency [Hz]
Increase Voltage
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-5
0
10 2 10 3 10 4
DEBUG U(f)
U(f)
Shaped Pink Noise
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10 2 10 3 10 4
DEBUG p(f)
p(f)
microphone spectrum
SPLMUCO (Maximum Usable Continuous SPL)
𝑆𝑃𝐿𝑀𝑈𝐶𝑂
= 10 ∙ lg 𝑝(𝑓𝑖)2
𝑛
𝑖=1
Transfer Function H(f)
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Pink Noise Stimulus
• Musik-like Signal
• Allows only interpretation of transfer function o THD and IMD only visible when having severe impact on fundamental
o Usually only thermal compression is visible
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Input Signal Generation According to ANSI/CEA 2010-B
HP 20Hz LP 4.4kHz LP 8kHz Pink Noise
Overview - ANSI/CEA 2010 A & B CEA 2010 A – Burst Test CEA 2010 B - SPL Continuous
Stimulus • burstshort excitation • 6.5 periodes • single frequency
• shaped pink noise • full band excitation • additional preheating (1 min)
Limits • harmonic burst distortion (e.g. 2nd -10dB, 3rd -20 dB etc. )
• Compression in passband of magnitude within 3 -3.5 dB
Results • Maximum SPL of each frequency bands
Peak of microphone signal
• SPLMUCO – Maximum Usable Continuous SPL
Summed full band sound pressure
Conclusion Harmonic distortion checked x Intermodulation distortion not
considered x thermal effects not considered x impulse distortion not
detected (loose particle)
variation of amplitude response considering thermal effects x distortion not considered
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SPL Continuous with Multi-Tone Stimulus
Properties - Full band excitation (music like)
- Sparse spectrum
- Deterministic stimulus
- Finite crest factor
Benefits - Persistent excitation of the transducer (reproducible)
- Separation of distortion without a model
- Defined pdf(x)
03/01/2017 Measuring Maximum Continuous SPL with Multi-Tone Stimuli 15
How to generate the Stimulus ?
Signal source
loudspeaker
~ FTMicrophone
Spectrum
AnalyzerAmplifier
Filter
f f
Spectrum of
distortion
components
f
N
i
iii tffUtu1
2cos)(
NiwithfTT
f Ristarti ,...,12int1 /
Frequencies of the sparse line spectrum logarithmically spaced
m
i
i
ma
mmod
2
*21
Random phase
Benefits:
- ensure comparability of the results measured by different instruments
- easy to generate (by software implementation)
- Modification of the stimulus should be possible (bandwidth , resolution R)
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Multi-Tone Measurement
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Spectrum p(f) of microphone signal
[dB
]Frequency [Hz]
Signal lines Noise + Distortions Noise floor
Signal level MTND
Distortion f Sparse multi-tone complex
Stimulus Output signal
• distortion at fundamental frequencies • harmonic components • difference-tone components • summed tone components
• Music-like signal • Good for quick testing
of overall performance
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Spectrum p(f) of microphone signal
[dB
]
Frequency [Hz]
Signal lines Noise + Distortions Noise floor Signal level
MTND Driver (1) - SPL Continuous - p(f)
Multitone Distortion (MTND)
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[ d B
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Frequency [Hz]
Noise Floor
Distortion
Fundamental
microphone spectrum Relative multitone distortion from voltage
Inp
ut V
olta
ge
1V
6V
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The Causes of Multi-Tone Distortion
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[dB]
Frequency [Hz]
Fundamental Multi-tone Distortion
Distortion
Fundamental
resonance frequency Cone Vibration
L(i) (rising with frequency)
Bl(x) (independent of frequency)
Kms(x)
L(x) (rising with frequency)
Doppler (rising with frequency)
Kms(x)
Bl(x)
L(x)
L(i)
Cone Vibration
Doppler Effect
Rms(v) Rms(v)
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Proposed Measurement Setup
• Measurement of voltage at terminals gives easy clipping detection
• Measurement of driver impedance allows additional interpretations
03/01/2017 Measuring Maximum Continuous SPL with Multi-Tone Stimuli 21
Signal Source Loudspeaker MicrophoneAmplifier
A
V
~
Current Measurement
Voltage Measurment
Checking Amplifier by Measured Voltage
• Multi-tone spectrum allow easy observation of amplifier performance
• Clipping can be detected by rising distortion lines
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Spectrum U(f) of voltage at speaker terminals[d
B] 0
dB
= 1
V
Frequency [Hz]
Signal lines Noise + Distortion Noise floor
Fundamental components
Distortion generated by amplifier
Noise floor
Voice Coil Temperature
Measured via Electrical Resistance Re (t)
• Based on voltage and current
measurement
• Re (t) corresponds with mean
value of the temperature
• Local temperature varies in
overhang coils
Better cooling of inner coil
windings (Heat radiation to
the pole tips, convection
cooling, high air velocity)
Indications of a thermal damage:
• loose windings voice coil rubbing
• shortcut of windings with pole gap reduced resistance
• open coil maximal resistance limited by instrument
03/01/2017 Measuring Maximum Continuous SPL with Multi-Tone Stimuli 23
Measuring Voice Coil Temperature
03/01/2017 Measuring Maximum Continuous SPL with Multi-Tone Stimuli 24
Pilot Tone
• Pilot Tone at low frequencies can be used to measure the DC-resistance Re • Re is directly linked to voice coil temperature:𝜌 𝑇 = 𝜌(𝑇 0) 1 + 𝛼∆𝑇 • Can be used for temperature protection • Allows prediction of thermal compression
Fourier
Transform
loudspeaker
system
current
sensor
U(t) I(t)
-
Stimulus
voltage
sensor
power
amplifier
Pilot
Tone
Temperature
Calculation
Resistance of cold
speaker
Conductivity of Coil
Material
Increase of VC
Temperature
Predicting Thermal Compression
03/01/2017 Measuring Maximum Continuous SPL with Multi-Tone Stimuli 26
MMD CMD RMD-1Bl
Re (T0)
v=dx/dt
i
Blv
F=Bli
USd
p
j2XAR(f)
qa=Sdv
pSd
ZL(f)
2RAR(f)
ΔRe (ΔT)
• Increasing temperature leads to increasing DC-resistance • Additional resistance reduces force driving the transducer • High influence where total speaker impedance is low • Predicted thermal compression at temperature T:
𝐶(∆𝑇) = 20log10𝑍 𝑇0 + ∆𝑅𝑒(∆𝑇)
𝑍((𝑇0)
Separating Thermal Compression
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[ d B
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nonlinear effects
1V - Reference
6V – thermal comp. (predicted)
6V - measured
100 1k 10k
Frequency [Hz]
30
dominated by thermal compression
Transfer fucntion H(f)
5
10
15
20
25
30
35
40
45
1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0
Increase of voice coil temperature
[V]
[K]
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10 100 1k 10k
Thermal Compression
0°C 10°C 20°C 30°C 40°C
∆𝑇 = 10K
∆𝑇 = 20K
∆𝑇 = 30K
∆𝑇 = 40K
Incr
easi
ng
voic
e co
il te
mp
erat
ure
Frequency [Hz]
[dB]
03/01/2017 Measuring Maximum Continuous SPL with Multi-Tone Stimuli 27
Separating Distortion Sources with Nonlinear
System Identification
A
V
Digitalprocessing
unit
Power amplifier Speaker
Nonlinear System
Identification
Voltage & Current
Multi-tone complex
Stimulus
• Feeding measured state Signals allows identification of nonlinear loudspeaker parameters
• Non-linear model allows revealing contribution of different distortion signals to total distortion
03/01/2017 Measuring Maximum Continuous SPL with Multi-Tone Stimuli 28
Root Cause Analysis of Nonlinear Distortion
Exercise: Microspeaker
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Distortion Components
[ d B
]
Le(i)
fundamental components
20
10 2 10 3 10 4
Frequency [Hz]
Bl(x)
Kms(x)
Le(x)
out of band distortion
total distortion
Rms(x,v)
fs=600 Hz
showing the contribution of each nonlinearity
03/01/2017 Measuring Maximum Continuous SPL with Multi-Tone Stimuli 29
Summary
• Using multi-tone stimuli for determining the maximum continuous SPL allows:
Easy interpretation of distortion generated by amplifier and transducer
Possibility to utilize measured state signal for nonlinear system identification
• Measuring the voice coil resistance allows:
Implementation of temperature protection
Separation of nonlinear effects from thermal compression
03/01/2017 Measuring Maximum Continuous SPL with Multi-Tone Stimuli 30
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