latest advances of inlatest advances of in-situ (pu)situ
TRANSCRIPT
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:
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eeZk
re
re
Z
iqrikZk
qhikhik
0 20
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1
0 221
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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