Latest advances of in situ (PU)Latest advances of in-situ (PU) absorption measurementsp

Emiel TijsEmiel TijsMicroflown Technologies

ContentsStandardized laboratory based methods:

Contents• Standardized laboratory based methods:

– Reverberant room & alpha cabin– Kundt’s tube

• in situ methods– Single microphone methods– Impedance based methodsImpedance based methods

• Sound field models• Experienced problems• Extrapolation methodExtrapolation method

– PU in situ method• Setup• Low influence reflection and background noiseLow influence reflection and background noise• Spatial resolution• Asphalt measurements whilst driving

• ConclusionConclusion

Standardized laboratory based methods yReverberant room & alpha cabin

• Main principle• Main principle– An acoustic absorbing sample will reduce

the pressure amplitude in time. The decay rate is a measure for the absorption

• Advantages– Standardized under ISO 354 & ASTM C423

• Disadvantages– Large, expensive facilities required– Large sample size: several square meters– Only absorption is measured (not the

complex impedance)“Diff ” fi ld i h d b th t i l– “Diffuse” field is changed by the material

– Absorption values higher than 100% are experienced, because of finite sample size and edge effectsand edge effects.

Standardized laboratory based methods yKundt’s tube

• Main principle• Main principle– Measurement of the impedance with one or

multiple microphones or by PU probes

• Advantages– Standardized– Straight forward equationsg q– Small sample size

• Disadvantagesg– Sample cut-out: acoustic properties are

affected– Sound leakage due to mounting problems– Only normal incidence– Cannot be used on materials with a high flow

resistivity and a low stiffness(like many multi layer materials)(like many multi-layer materials)

– Only uniform samples

in situ methodssingle microphone methodssingle microphone methods

• Main principle microphone• Main principle– Direct and reflected sound wave are separated by

subtraction- and/or time window techniques

• Advantages– in situ– Adrienne / Extended surface method standardized

(ISO 13472-1)

• Disadvantages

sound source

– Frequency limitations– Large sample size– Background noise

sound source

– For road measurements: cannot be usedwhile moving

microphone

in situ methodsimpedance based: sound field modelsimpedance based: sound field models

Pl d l• Plane wave modelθ

Sound source

2cZ cosr1

θ1θ2

Impedance

r221 Ra

cZcZ

Rs

s

coscos

• Mirror source model h

Impedance probe

110

1cosreZ

Zrik

m

1

2

2201

20

10

1cos11 re

re

rikrik

rr

ZZ

ZR rik

m

ff

102 1rikrZ ff

in situ methodsimpedance based: sound field modelsimpedance based: sound field models

The Q term model: an example of an exact solution for aThe Q-term model: an example of an exact solution for apoint source above an infinitely thin impedance layer:

0 2

'0

21 '2 2002010

dqiqr

eeZk

re

re

Z

iqrikZk

qhikhik

0 20

20

2

'0

20

20

210

10

1

0 221

1'

211 2002010

dqiqrik

iqrikiqr

eeZk

rikrik

re

rikrik

re

qZ

iqrikZk

qrikrike

Ref: J. Alvarez, F. Jacobsen, “An Iterative Method for Determining the Surface

0 202202101 qq

gImpedance of Acoustic Materials In Situ”, Internoise, 2008

in situ methodsimpedance based: experienced problemsimpedance based: experienced problems

• Low frequency deviations with plane wave models• Results form the mirror source- and Q-term model are often similar• Negative absorption values are found because no correction for spherical

waves inside the sample is made (the sample is assumed to be infinitely thin).p ( p y )• Both the complex material impedance and wavenumber need to be known.

1

0.6

0.8

ient

[-]

Plane wave model / PU foamPlane wave model / Flamex

0.2

0.4

ptio

n co

effic Plane wave model / Linoleum

Mirror source model / PU foamMirror source model / FlamexMirror source model / LinoleumQ-term model / PU foam

-0.2

0abso

rp Q-term model / FlamexQ-term model / Linoleum

102 103 104

Frequency [Hz]

in situ methodsintensity: extrapolation modelintensity: extrapolation model

1)( IreIIre reflectedincident

01)(

IreIrer

incident

reflectedincident

Intensity based absorption for several source probe distances r:Intensity based absorption for several source-probe distances r:

Measured Simulated

in situ methodsintensity: extrapolation modelintensity: extrapolation model

Three (or more) measurements with nearfield effects are combined to solve

rBh m

( )B, m and α:

r m

1 1

0.6

0.8

ffici

ent [

-]

0.6

0.8

ffici

ent [

-]

0.2

0.4

Abs

orpt

ion

coe

mirror source model( r=0.3 m)100 & 325 mm (m=1)100, 150 & 325 mmr0

0.2

0.4

Abs

orpt

ion

coef

mirror source model (r=0 3m)

103 104-0.2

0A

Frequency [Hz]

r =

102 103 104

-0.2

0

Frequeny [Hz]

A

mirror source model (r=0.3m)Extrapolated resultKundt tube

Flamex PU foamq y [ ]y

PU in situ methodSetup

• Small probe-sample and probe-source distanceB d b d d• Broad banded

• Fast• Small sample size• Extremely small spatial resolution• Low susceptibility to background noise• Impedance and absorption coefficientp p

PU in situ methodLow influence reflection and background noise

1

0 4

0.6

0.8

oeffi

cien

t [-]

Front window - door closed.matF t t d l d t

0

0.2

0.4

abso

rptio

n co Front seat - door closed.mat

Foot area - door closed.matRoof - door closed.matLaboratory reference.mat

103

104

-0.2

Frequency [Hz]

PU in situ methodSmall spatial resolution

Velocity contour plot @2253Hzplot @2253Hz

Foam

Steel

Foam

Ab tiAbsorption @2376Hz

PU in situ methodAsphalt measurements whilst driving

spherical speaker

wind protection

Vibration free mountingelastics to reduce

vibrations

ConclusionsConclusions• There are several methods to measure the acoustic absorption of

materials, but these have their specific weaknesses. Some of them are only laboratory based. Single microphone in situ techniques have limitations in terms of band width, sample size and signal to (background) noise ratio.

• Throughout the years increasingly advanced models have been proposed to correct for a monopole above an impedance layer. However, errors are found for small sample source distances because of spherical waves insidefound for small sample-source distances because of spherical waves inside the sample

• An intensity based extrapolation method has recently been proposed that• An intensity based extrapolation method has recently been proposed that combines several measurements with near field effects in order to obtain the plane wave absorption coefficient.

• Advantages of using small PU probes are the small sample size requirement, the high spatial resolution and the low influence to background noise and reflections. bac g ou d o se a d e ec o s