Measuring Maximum Continuous SPL with Multi- Tone Stimuli Maximum... · • Amplifier clipping...

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

03/01/2017 Measuring Maximum Continuous SPL with Multi-Tone Stimuli 2

What limits the Sound Pressure Output ?

for a sinusoidal input with different On/Off Cycle

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135

140

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


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


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


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


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.“


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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-4

-2

0

2

4

6

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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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Thresholds

[dB]

fundamental 2nd

3rd 4th

5th

higher order + noise


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]


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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-10

-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)


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


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


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)


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)


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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10 2 10 3 10 4

[ d B

]

Frequency [Hz]

Noise Floor

Distortion

Fundamental

microphone spectrum Relative multitone distortion from voltage

Inp

ut V

olta

ge

1V

6V


The Causes of Multi-Tone Distortion

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50 100 200 500 1k 2k 5k 10k

[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)


Proposed Measurement Setup

• Measurement of voltage at terminals gives easy clipping detection

• Measurement of driver impedance allows additional interpretations


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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1 2 5 10 20 50 100 200 500 1k 2k

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


Measuring Voice Coil Temperature


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


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

]

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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-0,5

0,0

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]


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


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


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


Measuring Maximum Continuous SPL with Multi- Tone Stimuli Maximum... · • Amplifier clipping...

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