basics of acoustic measurement

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Acoustic Calibration Techniqueand basics of Sound Measurement

Erling Frederiksenef-consult

[email protected]

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Who am I?

Brüel & Kjaer 1958 – 2010

Microphone Development Engineer and Production Manager 1958 – 1969Development Manager for microphones and calibration products 1969 – 1995Development and Product Managing of Microphone Calibration Systems

1995 – 2010

Manager of DPLA Microphone Calibration Department – B&K branchDPLA: Danish Primary Laboratory of Acoustics 1990 – 2010

Member of IEC/TC29WGs for microphones, calibration, calibrators, sound intensity, ..

Technical Specialist of CCAUV (BIPM)

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Who are you?

I guess that some of you

- work with acoustic calibration or measurements- are responsible for acoustic activities- intend to start up acoustic activities- make products to be tested acoustically- deal with related topics – vibration?

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Contents

- Measurement Microphones- Principles of operation- Interaction with the sound fields

- Acoustic Calibration - Primary Calibration (NMIs and High-tech Units)- Secondary Calibration (Test and Service Centers)- Field Calibration (Users)- Additional Calibration Techniques

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Places where sound is measured

Electro-acousticTesting Lab

Car Testing LaneConstruction Site

Machine Building Hall

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Car Cabin Crash Test

Rocket Launch Ramp Very High Levels and Infrasound

Wind MillSite

Places of Extreme Conditions

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Units and Levels

Sound Pressure Unit: Pa or N/m2

Sound Pressure Level: dB

where: Pa (threshold of hearing at 1000 Hz)

refp p

pL log20 ⋅=

61020 −⋅=refp

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Audible RangePressure: 20 µPa – 60 PaLevel: 0 dB – 130 dB

Frequency: 20 Hz – 20 kHz

‘Full’ Measurement RangePressure: 3 µPa – 60 kPaLevel: -16 dB – 190 dB

Frequency: 0.1 Hz – 150 kHz

200 dB

160 dB

120 dB

80 dB

40 dB

0 dB

-40 dB

0.01 Hz 1 Hz 100 Hz 10 kHz 1000 kHz

Dynamic - and FrequencyRanges

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• Well-suited for calibration (reciprocity, actuator)• Well-suited for theoretical analysis• Highly stable• Essentially flat frequency response• Interaction with sound fields can be measured

and calculated• Low vibration sensitivity

Some reasons:

Condenser Microphones are dominating

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Traditional Condenser Measurement Microphone

Diaphragm

Backplate

InsulatorElectrical Terminals

Static PressureEqualization Tube

Airgap – 20 µm

Rear Cavity

Diameter: 23.77 mm

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uout

∆d

do

R

C

R C⋅ >> 1sec

U C QU C U C Q

U U CC

U U Sd

d dS

U U dd

u U dd

o

out

⋅ =⋅ = ⋅ =

= ⋅

= ⋅⋅

⋅+⋅

= ⋅ +

= ⋅

0 0 0

00

00

0

0

00

1

εε

∆

∆

∆

U0

S

Transduction Principle of Condenser Microphone

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Pre-polarised Measurement Microphones

½” and ¼”

Today this principle is used in a large fraction of measurement microphones

PolarisationPrinciple

• More complex• More expensive• Very reliable

Electret

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Damping of Microphone Diaphragm

Q=10

Q=1

Q=0.316

Diaphragm Dampingdepends ondistance to back-plateand number of holes

Pressure Field MicrophoneQ = 1

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Typical Line of Microphone Types

Ø 3.175 mm Ø 12.7 mmØ 6.35 mm Ø 23.77 mm

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Effects of Scaling Microphone Diameter

Responses of ’Pressure Microphones’

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Principal Types of Sound Field

• Free-fieldopen spacewith no reflecting surfaces

• Diffuse-fieldrooms with hard surfacesrooms with many sources

• Pressure-fieldsmall enclosureswith uniform pressure

Lawn Mover

Workshopwith twonoise sources

Hearing Aid andTest Coupler

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Microphone Body in Propagating Sound Wave

Measurement Point

Undesturbed Sound Field

Desturbed Sound Field

Direction of SoundPropagation

Direction of SoundPropagation

Microphone Body

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Microphone Size and Influence on Pressure Level

ø 24 mm ø 12 mm ø 6 mm

Free-field

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Influence of Microphone Protection Grid

Typical Influence0o Incidence

ø 12 mm

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0

5

10

15

0.1 1 10 100frequency [kHz]

dB 3

Direction of Sound Incidence (0o)

Combined Influence of Microphone Body and Grid

Free-field conditions and body with protection grid

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Response of Microphone optimized for Free-field

-20

-15

-10

-5

0

5


dB

1

2 3Frequency response of diaphragm systemIncrease of pressureon the diaphragm

Resulting Microphone Response

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Principal Types of Sound Field

• Free-fieldopen spacewith no reflecting surfaces

• Diffuse-fieldrooms with hard surfacesrooms with many sources

• Pressure-fieldsmall enclosureswith uniform pressure

Lawn Mover

Workshopwith twonoise sources

Hearing Aid andTest Coupler

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Microphone Size and Influence on Pressure Level

0.1 k 1 k 10 k Frequency 100 k

ø24 mm ø12 mm ø6 mm

Diffuse-field

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Microphones are Optimized for Types of Sound Field

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Other Microphones and Parts to be Calibrated (1)

Microphonefor permanentoutdoor use(probe mic.)

Turbulence Suppression Screen

RainCover

NoseCones

WindScreens

Probe Microphonewith tiny tube (ø1.25mm)

Microphonewith very lowInherent Noise(0 dB SPL)

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For Noise and Turbulence Measurementsin Wind Tunnels and during driving or flying

Microphones onaircraft wing


Surface Microphones

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Microphones with nearly identical Phase Responses are required - especially for Sound Intensity Measurements


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Examples of Microphone Preamplifiers

12.7 mm 6.35 mm

Input Impedance: up to15 GΩ in parallel with 0.3 pFOutput Resistance: down to 50 ΩGain: 0 dB

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Contents

- Measurement Microphones- Principles of operation- Influence on sound fields

- Acoustic Calibration - Primary Calibration (NMIs and High-tech Units)- Secondary Calibration (Test and Service Centers)- Field Calibration (Users)- Additional Calibration Techniques

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Calibration Hierarchy

National Metrology InstituteCountry B

National Metrology InstituteCountry A

National Metrology InstituteCountry C

Calibration ServiceCenter A

Calibration ServiceCenter B

Calibration ServiceCenter C

User A

User B

User C

User X

User A

User B

User C

User X

User A

User B

User C

User X

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Methods for Primary Calibration Laboratories

IEC 61094 – 1 Laboratory Standard MicrophonesIEC 61094 – 2 Primary Method for Pressure CalibrationIEC 61094 – 3 Primary Method for Free-field CalibrationIEC 61094 – 7 Free-field Corrections, LS1/LS2 Microphones

Low-frequency Microphone CalibrationHigh-level Microphone CalibrationIEC 61043 Sound Intensity InstrumentsMeasurement of Inherent Microphone NoiseIEC 61094-8 Free-field Comparison Calibration (draft)Diffuse-field Calibration

Note: The standard titles are not fully correct – they are shortened

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Laboratory Standard Microphones – IEC 61094-1

LS1 LS2a

ø 23.77 mm ø 12.7 mm

ø 18.6 mm

ø 9.3 mm

ø 13.2 mm

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Pressure Reciprocity Calibration

IEC 60194-2

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Reciprocal Transducers

Reciprocity calibration requires reciprocal transducers

Sound Receiver Sound Transmitter

The condenser microphone is a reciprocal transducer

uoc q ipoc

ip

qu ococ =

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Principle of Pressure Reciprocity Calibration (1)

iin

uout

mic(A)

mic(B)

air-filled coupler

uout

iin

where

Ze =

Mp(A) x Mp(B) =Ze

Za

Za = acoustical transfer impedance of the air-filled coupler terminated by the microphones (A and B)(acoustic transmission line)

Sensitivity Product (IEC61094-2)

(measured)

Coupled Pair of Microphone

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Principle of Pressure Reciprocity Calibration (2)

C

A

uCiT

uR

Cs

A

B

uCiT

uR

Cs

B

C

uCiT

uR

Cs

coupler coupler coupler

Receiver

Source

The 3 unknown microphone sensitivities are found by solvingthe 3 equations, which are related to the above 3 pairs of microphone

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Impedance of 'Plane Wave Coupler'

'Plane Wave Coupler'

lxy

lxy < 0.4 . λ d = ddiaphragm

mic.x mic.y

( ) ( )

⋅⋅

⋅++⋅⋅

+⋅= xy

ya

oa

xa

oaxy

ya

oa

xa

oa

oaxya

lZZ

ZZ

lZZ

ZZ

ZZγγ sinh1cosh11

,

,

,

,

,

,

,

,

,,

is determined by the transmission line theoryZa,xy

Dimension requirements:

dZa,x Za,y

Za,o

Model of Acoustic Transfer Impedance

(IEC 1094-2)

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B&K Type 9699 Pressure Reciprocity Calibration Systembased on PULSE Multi-Analyzer Type 3560 C

Measurement Chamberfor pressurization and

noise suppression

Automated Reciprocity Calibration System

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Magnitude and Phase Pressure Responses

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Repeatability History

0

1.5

1-1-

91

1-1-

92

1-1-

93

1-1-

94

1-1-

95

1-1-

96

1-1-

97

1-1-

98

1-1-

99

1-1-

00

1-1-

01

1-1-

02

1-1-

03

years

0.1

dB p

er d

ivis

ionn

n

250 Hz

25 kHz

1 kHz

16 kHz

8 kHz

4 kHz

2 kHz

12.5 kHz

63 Hz

31.5 Hz

500 Hz

125 Hz

20 kHz

20 Hz

Type 4180 No. 1124043

DPLA Standard MicrophoneType 4180 No.1124043

0.1

dB p

er d

ivis

ion

half-inch

Period: 1991 – 2002

Pressure Reciprocity CalibrationResults normalised with AverageSensitivity of last 5 years

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Uncertainty Calculation Type 4180 – LS2 - DPLA

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0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

10 100 1000 10000 100000Hz

dB

DPLA Client CalibrationsUncertainty (k=2) for Type 4180

calculated

specified

Resulting Uncertainty Type 4180 – LS2 - DPLA

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DPLA Calibration Uncertainty, Type 4160 and 4180

0.00

0.04

0.08

0.12

0.16

0.20

10 100 1000 10000 100000Hz

dB

4160

4180

Specified DPLA Uncertainties (k=2)October 2000

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Free-field Reciprocity Calibration

IEC 60194-3

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Free-field Reciprocity Calibration Set-up, Half-inch

Transmitter MicrophoneReceiver Microphone

Anechoic Room with cubical glass-wool absorbers

Typical Distance200 mm

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Expression for Free-field Sensitivity Calculation

12

1

2122,1,

2 dvff e

iU

fdjMM ⋅⋅⋅−=⋅ρ

Symbol Parameter Mf,1, Mf,2 Sensitivities of microphones ‘1’ and ‘2’

d12 Distance between acoustic centers of microphones ‘1’ and ‘2’ ρ Density of air f Frequency v Complex sound propagation coefficient

U2 Output voltage of receiver microphone i1 Input current of source microphone

)2(1

2

1

2 CfjUU

iU π−⋅=

Symbol Parameter Z12 Transfer impedance of microphones ‘1’ and ‘2’ valid for the parameters of equation (4) U1 Voltage across the series capacitor of source microphone ‘1’ C Capacitance of series capacitor of the source microphones

Current is determined by measuringvoltage across a series capacitor

Symbol Parameter

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Typical Measured Free-field Transfer Function

01020304050607080

1 10 100

dB

re. 1

µV

kHz

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System Instrumentation

Insert VoltageSwitch Box

WB3551

Generator

Ch.1 Ch.2Analyzer

PULSEType

3560C(100 kHz)

ConditioningAmplifier

Type 2690-W-008

ReciprocityCalibrationApparatusType 5998

LAN

PreamplifierType 2673-W-001

(20 dB gain)

Transmitter Unit ZE0796 withbuilt-in series

capacitor

PC

Microphones

IEEE AUXFree-field ReciprocityCalibration System

DFM-B&K

Receiver

Transmitter

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Removal of Sound Reflections1)

4)3)

2)

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Responses valid for the three pairs of microphone (AB, AC and BC)are displayed for evaluation of the operator prior to the calculationof the microphone sensitivities

The distance between the fronts of microphones (LS1) is 320 mm

Calculated Impulse Responses for one Distance

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Resulting Sensitivity (magnitude)

19-aug-09 Lin1453799 4160.1453799

-45

-35

-25

-15

100 1000 10000 100000

Average

Hz

dB

Type 4160 No. 1453799

Average of results obtainedwith 4 measurement distances

ø23.77 mm

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Resulting Sensitivity (phase)

19-aug-09 Lin1453799 4160.1453799

-90

-60

-30

0

30

60

90

120

150

180

210

100 1000 10000 100000

Average

Hz

dB

Type 4160 No. 1453799

Average of results obtainedwith 4 measurement distances

deg

ø23.77 mm

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19-aug-09 Lin1503933 4180.1503933

-0.12

-0.08

-0.04

0.00

0.04

0.08

0.12

100 1000 10000 100000

d 170 d 200 d 240 d 300

Hz

dB

Type 4180 No. 1503933

The results for the 4 measurement distancesare shown relative to their common mean value

Comparison of Sensitivity Results (magnitude)

ø12.7 mm

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DFM chamber and general criteriaDFM chamber

- volume: 6.0 m3

- free volume: 1.7 m3

- mineral wool: 90 kg/m3

(not especially critical)

General requirements: - Good sound isolation- Large enough fordelaying reflexions

Small-room features:- short cables (lower cross-talk)- precise and stablemicrophone mounting

- convenient operation Knud Rasmussen in front of the DFM room

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System Properties

The system operates with a repeatability better than +- 0.03 dBand an accuracy better than +- 0.1 dB up to 20 kHz (k=2)

Three one-inch and half-inch microphones may be thoroughly calibrated (4 distances) in about 4 and 8 hours respectively

The method is time-selective. Therefore, only a small an-echoicroom is required

PhD Thesis by Salvador Barrera Figueroa

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Free-field Calibration Uncertainty (k=2)

WS microphones may also be calibrated, but in general the uncertainty becomesLarger for less known types and for microphones of complex shapes

0.04

0.06

0.08

0.10

0.12

0.14

1 10 1000.04

0.06

0.08

0.10

0.12

0.14

1 10 100

LS1 LS2

kHz kHz

dB dB

k = 2 k = 2

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BIPM – CCAUV Key Comparisons

Regional Organizations- AFRIMETS- APMP- COOMET- EURAMET- SIM

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Free-field Response Determinationof

LS Microphonesby

Pressure Reciprocity (IEC 60194-2)and

Free-field Corrections (IEC 61094-7)

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An Alternative to Free-field Reciprocity Calibration

0

2

4

6

8

10

0.1 1 10 100

dB

kHz

Free-field CorrectionsDifference between

Free-field and Pressure Responses

Type 4160 Type 4180

0o Incidence

IEC 61094-7states

Free-fieldCorrections

for LS1 and LS2Microphones

Perform a Pressure Reciprocity Calibration and add correctionscalculated by the polynomial of IEC 61094-7

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Uncertainty of Free-field Calibrations – LS2a

DPLA Calibration by Free-fieldReciprocity IEC 61094-3

DPLA Calibration by PressureReciprocity IEC 61094-3andStandardized Free-field correctionsIEC 61094-7

LS2aø12.7 mm

LS2aø12.7 mm

k=2

k=2

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Infra-sound and Low-frequency Calibration

Various non-standardized principles

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Low Frequency Calibration of Reference Microphone

Laser-pistonphone0.1 Hz – 100 HzUncertainty 0.25 dB at 1 Hz

[ ]V

SdPrmsp o ⋅⋅

⋅⋅=2

γ

γ: ratio of specific heatsd: displacement of pistonS: area of pistonV: volume of cavity

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Pressure Reciprocity2 Hz – 30 kHz Uncertainty 0.5 dB at 2 Hz

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Pressure Reciprocity2 Hz – 30 kHz Uncertainty 0.5 dB at 2 Hz

Related Microphones Method0.1 Hz – 250 HzUncertainty 0.08 dB at 0.1 Hz

New B&KMethod

B&K TechnicalReview

No. 1 - 2008

Equal Microphoneswith different

Diaphragm Tension

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Low-frequency Comparison Calibration

WB3570

Coupler designed for :1) exposure of diaphragm and vent2) diaphragm only

Filename, 67

High-level Microphone Calibration

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Pistonphone Principle for High Pressure Calibration

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High-pressure Calibration System Type 9719

B&K PULSEMulti-Analyzer System

Type 3560

with

High-pressure UnitType 4298

and

Linearity-measurementSoftwareWT9643

Operation frequency: 500 Hz

Filename, 70

System of Coupled Tubes

MicrophonesM1 and M2

T3 T2 T1 Loudspeaker

Loudspeaker

~

T1

T3

T2

M1 M2

C R L

Equivalent Circuit Model

Physical Model

Loudspeaker

Filename, 71

Generation of High Pressure with Low Distortion

• Operation at lowest system resonance (500 Hz)• Relatively high input power (25 W)• Transfer function minimizes distortion from source

-120

-110

-100

-90

-80

-70

-60

-50

-40

0 1000 2000 3000 4000

Lp for constant Input Voltage

10

0

-10

-20

-30

-40

-50

-60

-70

Operation Frequency

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Dynamic Linearity Reference Microphones

B&K Condenser Microphone Type 4941 (ø 6.3 mm)

ENDEVCO Ceramic Microphone Type 2510 (ø 19 mm)Heat-protector

Filename, 73

Operation Range of High-pressure System

180

dB

160

140

120

100

80

SP

L (R

MS

)

SP

L P

eak

(+)

SP

L P

eak

(-)

2nd

Har

mon

ic

3rd

Har

mon

ic

Tota

l Har

mon

ic

1 Pascal (94 dB)

10000 Pascal (174 dB)So

und

Pres

sure

Lev

el re

. 20

µPa

Filename, 74

Capability of High-pressure Calibration System

Range: 94 dB (RMS) and 174 dB (Peak):

• RMS-linearity +- 0.02 dB

• Peak-linearity (+ and -) +- 0.04 dB

• Harmonic Distortion- Sound Pressure < 0.3 %- Reference Microphones < 0.5%

• Vibration Excitation < 2.5.10-4 m/s2 per Pa

Filename, 75

Phase Response Comparison Calibrationfor

Microphone Arrays and Sound Intensity Probes

Filename, 76

Arrays for Sound Mapping, Beamforming and STSF

60 – 200 mics

36 – 108 mics

36 – 64 mics

36 – 50 mics64 – 114 mics

(12 cameras)

Filename, 77

Intensity Probes and Dedicated Microphones

Method is described in IEC 61043

Filename, 78

Intensity Microphones – B&K Type 4197

0,001

0,01

0,1

1

10

10 100 1000 10000

deg

Hz

Intensity Microphones - B&K Type 4197

Tolerances

Probe (Mics + PAs)Mics

Required Measurement Capability

Tolerances are from IEC 61043

Filename, 79

New Wide-band Phase Comparison Couplers

WA1544Half-inch

Phase ResponseComparison CouplerWA1545

Quarter-inchPhase ResponseComparison Coupler

WA0817Broard-bandSound Sourcefor the couplers

Filename, 80

Phase Comparison Result - Type 4197 (Intensity Pair)

Measured with the Calibration SystemType 9721

Blue and Redindicatenegative andpositivedeviationsrespectively

Filename, 81

Array Microphone Type 4958 (batch A)

-12

-6

0

6

12

10 100 1000 10000 100000

12345671234568258155425818302592624259262525926262592628259262925926312592632259263325926342592635259263625926372611371261137226113732611374261137526113762611377261137826113792611380261138126113822611383261138426113852611386

Phase Response Spread within batch (A)(microphones for selection to arrays)

The responses are displayed relative to their average

deg

Hz

Filename, 82

Test Tube for Intensity ProbesLength: 6.2 mInner Diameter: 288 mm

Sledge for Intensity Probe

S.W.R.:24 dB from40 to 500 Hz

Filename, 83

Measurement of Inherent Microphone Noise

Filename, 84

Brownian Noise of Condenser Microphone

e

sn

n

VpTkp

CTkp

⋅⋅⋅=

⋅=

γ

2

k : Boltzmanns ConstantT : absolute temperatureγ : ratio of specific heatsps : static pressureVe : equivalent diaphragm volume

Ve = 148 mm3

6.6210148

10013.14.12961038.19

523

=⋅

⋅⋅⋅⋅⋅= −

−

np µPa

or 9.9 dB SPL

Filename, 85

Noise Spectra of Type 4160/44 and Preamp

Filename, 86

Noise Spectra of ½” Microphone and Preamplifier

dB SPL

Filename, 87

-10

0

10

20

30

40

50

dB (A)

1/2"

1/1"

1/4"

12.5 mV/Pa 50 mV/Pa

50 m

V/P

a

50 m

V/P

a

4 m

V/P

a

1.6

mV

/Pa

Noise Levels of Microphonesand of Microphones combined

with Preamplifiers

Equivalent A-weightedSound Pressure Level

100

mV

/Pa

Filename, 88

Chamber for Measurement of Inherent Microphone Noise

Volume: about 1.5 liter

Weight: about 5 kg

Covered with vibration

damping material

Damping material inside

Filename, 89

Free-field Comparison Calibrationof

LS and WS Microphones

IEC 60194 - 8(draft)

Filename, 90

Free-field Comparison Calibration – IEC 61094-8 (Draft)

Non-critical Reflection

Critical ReflectionDirectSound

SoundSource

Microphone

Calibration requires:

1) a very good andlarge an-echoic roomor

2) a time-selective orpost processingmeasurement techniqueto eliminate influenceof reflections

Many an-echoic rooms are good enough for testing purposes,but few are good enough for calibration !

Reflections and measurement distance are main topics of the IEC draft

Filename, 91

Diffuse-field Comparison Calibrationof

LS and WS Microphones

IEC 60194 - 8(draft)

Filename, 92

ReferenceMicrophone

UnknownMicrophone

Diffuse-field Comparison Calibration

Room with hard Sound Reflecting Walls

Filename, 93

Obtaining Diffuse-field response by FF-measurements

v

Microphone Body

v = 0o is perpendicularincidence on the diaphragm

12

8

4

0

- 4

- 8

- 12

-16 1 kHz 10 kHz 100 kHz

0o

0o – 180o in 5o stepsDiffuseSound Field

Curve for Diffuse Field iscalculated in accordancewith IEC 61183

Microphone (12.7 mm)without Protection Grid

Filename, 94

Methods for Service Centers and Test Laboratories

IEC 61094 – 4 Working Standard MicrophonesIEC 61094 – 5 Pressure-field Comparison CalibrationIEC 61094 – 6 Electrostatic Actuator Calibration

IEC 61672 – 1 SLM Specifications (Class 1 and 2)IEC 61672 – 2 SLM Pattern Evaluation Tests IEC 61672 – 3 SLM Periodic TestsIEC 61183 Diffuse-field Calibration of SLM

IEC 60942 Sound Level CalibratorsIEC 62585 Calibration Corrections for SLM (draft)

Note: The standard titles are not fully correct – they are shortened

Filename, 95

Secondary Microphone Calibrationby

Comparison Couplers (IEC61094-5)and

Electrostatic Actuators (IEC61094-6)

Filename, 96

System for calibration of various brands of microphone

ReferenceStandard

Type 4180

Type 9721Diameter

1/8” to 1/1”

Filename, 97

Type 9721 Microphone Calibration

-20

-15

-10

-5

0

5


dB

1

2 3

1. Sensitivity at 250 Hz2. Electrostatic Actuator Response3. Sound-field Correction

Filename, 98

Comparison Calibration Method – IEC61094-5

9.3

16

8.0 1.80.5

18

MicrophoneInterfaces

SourceCavity

SourceDiaphragm

Comparison Couplerwith built-in Sound Source

20 Hz – 16 kHz

SourceCavity

Interfaces toMicrophones

CylindricalDiaphragm

ofSource

WA0817

Filename, 99

Active ComparisonCoupler WA0817

(90 dB SPL for 1V)Calibration Fixture

WA0852

MonitorMicrophoneType 4192

Coupler and fixture mountedfor calibration of half-inch and smaller microphones

Microphone Fixture and Active Coupler WA0817

For ½” microphones

Filename, 100

One-inch CouplerUA1609

Calibration Couplers and Fixture for microphonesActive Coupler

WA0817

The active coupler WA0817 supplies thesound for the passive one-inch coupler

For 1/1” and smaller microphones by adapters

Filename, 101

Chart forcalculationof sensitivityuncertainty

Type 9721 Sensitivity Uncertainty Calculation Sheet

Filename, 102

0

0.04

0.08

0.12

0.16

4180

4192

4193

4133

4134

4149

4191

4197

4190

4145

4161

4144

4160

4176

4188

4938

4939

4189

4136

4165

4166

4135

4155

Calculated UncertaintySpecified Uncertainty

Valid at Reference ConditionsType 9721 Uncertainty of Sensitivity Calibration (250 Hz) dB

Type of Microphone

Type 9721 Sensitivity Uncertainty (k=2)

Filename, 103

Electrostatic Actuator

Glass legs Metal Plate

InsulatorElectrostatic Actuatorfor half-inch andsmaller microphones

Filename, 104

PULSE Analyser

Preamplifier

Actuator VoltageSupply

Actuator

Microphone

800 V DC + 100 V AC

Setup for Electrostatic Actuator Calibration

Filename, 105

CUdF ⋅=⋅2

2

dSUdF actgas ⋅

⋅=⋅ε

2

2

dia

actgas

dia SS

dU

SF

⋅⋅⋅

= 2

2

2ε

222

Ud

ap gas ⋅

⋅⋅

=ε

Electrostaticactuator

d

UF Sdia

Sact

Microphonehousing

Holes

( ) ( )( )202 sin

2tuU

da

tp pgas ω

ε⋅+⋅

⋅⋅

=

( ) ( ) ( )

−

⋅+⋅⋅⋅+⋅⋅

⋅=

22cos1sin2

22

0202

tutuUUd

atp pp

gas ωωε

Equivalent Sound Pressure of Electrostatic Actuator

Filename, 106

Microphone Calibration Module Type 5001

Driver unit forActive Coupler

and ElectrostaticActuator

Filename, 107

Type 9721 Microphone Calibration

-20

-15

-10

-5

0

5


dB

1

2 3

1. Sensitivity at 250 Hz2. Electrostatic Actuator Response3. Sound-field Correction

Filename, 108

-0.4

-0.2

0

0.2

0.4

10 100 1000 10000 100000

Uncertainty of Type 9721

Valid for One- and Half-inch Microphones (S > 25 mV/Pa)

Calculated

SpecifieddB

Hz

Type 9721 Uncertainty of Actuator Response (2)

Filename, 109

Sound Level Meter Calibration

Pattern Evaluation TestsAcoustic Test Laboratories

Periodical TestsCalibration Service Centres

Field CalibrationUsers

Filename, 110

Pattern Evaluation Tests Acoustic Test Laboratories

IEC 61672-2 prescribes:

Checking of mandatory facilities and general requirements ( 23 points)

Environmental, electrostatic and radio-frequency tests (17)

Electrical performance tests (16)

Acoustical performance tests (5):- Measurement of Directional Responses- Measurement of Frequency Response of Reference Direction- Verification/determination of ’Free-field Correction Values’- Verification/determination of ’Adjustment Value(s)’- Measurement of Self-generated Noise

Filename, 111

Periodical Tests Calibration Service Centers

IEC 61672-3 prescribes:

- General Inspection

- Electrical Performance Tests (7)

- Acoustical Performance Tests (3):- Check of Indication at the reference frequency (1 kHz)- Check of Frequency Response (calibrator, actuator or sound field) - Check of Self-generated Noise

B&K SLMCalibration System

Type 3630

Filename, 112

Remarks about SLM Testing

Pattern Evaluation Test- is a very thorough, time consuming and costly test- requires advanced equipment for environmental testing- requires a good and large an-echoic room- ensures proper design quality- provides proved correction data for simple Periodic Testing

Periodic Test- intended to be cheap and widely used- requires no specific acoustic facilities - checks critical parameters- ensures proper performance - considering approval of Pattern

Filename, 113

SLM

Soft Spring

NoiseisolatedCavity

Vibration Isolation Table

Rear port sound isolator

Test Table for Self-generated Noise of SLM

SLM Microphone

Low-noise MonitorMicrophone

Filename, 114

Sound Calibratorsfor

Field – and Laboratory Use

IEC 60942

Filename, 115

Pistonphone Type 4228

Precision Calibratorand

Reference StandardSound Source

Frequency 250 Hz124 dB at 1013.25 hPa

SPL is proportional to ambientpressure, but essentially independent of temperature and humidity.

Filename, 116

Design Details of Pistonphone Type 4228

( ) ααα 8cos4sin ⋅−⋅+= CBAr

Retaining Spring Cam Disc

Coupler CavityPistons

Ruby Bushing

Filename, 117

Principle of Feed-back Calibrator- Significantly Improved Stability- Essentially independent of Static Pressure- Essentially independent of Loading Microphone

Type 4226 Multi-frequencyCalibrator

First feed-back Calibrator1988

Type 4230 – 1000 Hz, 94 dB

Filename, 118

Multi-frequency Calibrator

Calibrators produce Pressure-fields, but maycalibrate SLMs and Instruments for free-field use.

They generate a level higher than the nominal tocompensate for the lack of pressure-increase thatoccurs on the microphone in a free-field.

Filename, 119

Hint about use of Calibrators

Calibrating an Instrument with a Calibrator is very easy

However, just connecting the calibrator and adjusting the instrument toproper reading imply a risk of

overlooking microphone, analyzer or calibrator defects

Therefore,- read the microphone sensitivity on its calibration chart- set the system parameter for sensitivity accordingly- connect and start the calibrator- notice the deviation between display reading and calibrator level- if > 0.4 dB look for the reason- if < 0.4 dB adjust the reading (or leave the settings)

Filename, 120

An Introduction has been given to:

- Principal Types of Sound Field, Measurement Microphonesand the need for many models

- Calibration and Test Methods applied at Hierarchical Levels- SLM Calibration and Testing- Acoustic Calibrators- Acoustic Standards- Uncertainties of Methods- Uncertainty Budgets and Accreditation

Thanks for your attention

basics of acoustic measurement

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