The Principles of Acoustical The Principles of Acoustical AttenuationAttenuation
Edward R. Green, Ph.D.Edward R. Green, Ph.D.Roush Industries, Inc.Roush Industries, Inc.
OutlineOutline
Sound Transmission Loss (TL)Sound Transmission Loss (TL) Mass LawMass Law CoincidenceCoincidence Dual Panel ConstructionDual Panel Construction LeakageLeakage
Sound AbsorptionSound Absorption Layered MaterialsLayered Materials Biot ModelBiot Model Scrim FacingScrim Facing
Sound Transmission Loss (TL)Sound Transmission Loss (TL)
Airborne noise enters the vehicle Airborne noise enters the vehicle interior through the body panels, glass, interior through the body panels, glass, and leakageand leakage
Transmission Loss (TL) is the Transmission Loss (TL) is the resistance to airborne noise entryresistance to airborne noise entry
TL=10LogTL=10Log1010(Power Incident/Power (Power Incident/Power Transmitted)Transmitted)
Single Panel TLSingle Panel TL
0
10
20
30
40
2
0
3
1
.
5
5
0
8
0
1
2
5
2
0
0
3
1
5
5
0
0
8
0
0
1
2
5
0
2
0
0
0
3
1
5
0
5
0
0
0
8
0
0
0
1
2
5
0
0
2
0
0
0
0
Third-Octave Frequency (Hz)
T
L
(
d
B
)
StiffnessStiffnessControlledControlledRegionRegion
CoincidenceCoincidenceControlledControlledRegionRegion
MassMassLawLawRegionRegion
6 dB/Octave6 dB/Octave
Single Panel TL Single Panel TL The Mass LawThe Mass Law
0
10
20
30
40
1
0
0
1
6
0
2
5
0
4
0
0
6
3
0
1
0
0
0
1
6
0
0
2
5
0
0
4
0
0
0
6
3
0
0
1
0
0
0
0
Third-Octave Frequency (Hz)
T
L
(
d
B
)
TL(dB) = 20LogTL(dB) = 20Log1010{[Surface Weight (kg/m{[Surface Weight (kg/m22)][Freq (Hz)]} )][Freq (Hz)]} 47.247.2
Applies to singleApplies to single--layer, impermeable materialslayer, impermeable materials
Not valid at low frequency due to mounting effectsNot valid at low frequency due to mounting effects
Not valid at high frequency due to coincident effectsNot valid at high frequency due to coincident effects
Random IncidenceRandom Incidence
Single Panel TL Single Panel TL CoincidenceCoincidence
In air, the speed of sound, c, is In air, the speed of sound, c, is independent of frequency.independent of frequency.
In a solid panel, the speed of sound, In a solid panel, the speed of sound, ccbb, is proportional to the square root of , is proportional to the square root of frequency (dispersion).frequency (dispersion).
Single Panel TL Single Panel TL CoincidenceCoincidence
0
100
200
300
400
500
0
1
0
0
0
2
0
0
0
3
0
0
0
4
0
0
0
5
0
0
0
6
0
0
0
Frequency (Hz)
S
p
e
e
d
o
f
S
o
u
n
d
(
m
/
s
)
AirSolid
SubsonicSubsonicRegionRegion
SupersonicSupersonicRegionRegion
Single Panel TL Single Panel TL CoincidenceCoincidence
0.0
0.1
0.2
0.3
0.4
0.5
0
1
0
0
0
2
0
0
0
3
0
0
0
4
0
0
0
5
0
0
0
6
0
0
0
Frequency (Hz)
W
a
v
e
l
e
n
g
t
h
(
m
)
AirSolidSubsonicSubsonic
RegionRegion
SupersonicSupersonicRegionRegion
In subsonic region, wavelength in air isIn subsonic region, wavelength in air islonger than wavelength in solidlonger than wavelength in solid
In supersonic region, wavelength in air isIn supersonic region, wavelength in air isshorter than wavelength in solidshorter than wavelength in solid
Single Panel TL Single Panel TL CoincidenceCoincidence
SolidSolid
AirAir
WavelengthWavelength
WavelengthWavelength
In subsonic region, In subsonic region, wavelength in air iswavelength in air islonger than wavelength in longer than wavelength in solid solid poor coupling, good TLpoor coupling, good TL
In supersonic region, In supersonic region, wavelength in air iswavelength in air isshorter than wavelength in shorter than wavelength in solid solid good coupling, poor TLgood coupling, poor TL
DecouplerDecoupler LayerLayer
Mass LayerMass Layer
Mass LayerMass Layer0
20
40
60
80
1
0
0
1
6
0
2
5
0
4
0
0
6
3
0
1
0
0
0
1
6
0
0
2
5
0
0
4
0
0
0
6
3
0
0
1
0
0
0
0
Third-Octave Frequency (Hz)
T
L
(
d
B
)
Single Layer
Air Gap
Bonded Decoupler
UnbondedD l
6 dB/Octave6 dB/Octave
Dual Panel TLDual Panel TL
Dual panel construction Dual panel construction dramatically increases TL with dramatically increases TL with minimal mass penaltyminimal mass penalty
Effect of Air LeakageEffect of Air Leakage
0
20
40
60
80
1
0
0
1
6
0
2
5
0
4
0
0
6
3
0
1
0
0
0
1
6
0
0
2
5
0
0
4
0
0
0
6
3
0
0
1
0
0
0
0
Third-Octave Frequency (Hz)
T
L
(
d
B
)
Dual Layer
Dual Layer w/Leakage
Leakage causes roll off of high frequency Leakage causes roll off of high frequency TL sometimes mistaken for coincidenceTL sometimes mistaken for coincidence
TL SummaryTL Summary
Sound transmission loss is improved by:Sound transmission loss is improved by:
Optimized dual wall treatments (carpet, Optimized dual wall treatments (carpet, dashmat, etc.)dashmat, etc.)
Minimized leakageMinimized leakage
Minimized coincidence Minimized coincidence
Sound AbsorptionSound Absorption Vehicle interior noise levels are reduced by the Vehicle interior noise levels are reduced by the absorption of noise in the vehicle interiorabsorption of noise in the vehicle interior
Underhood noise levels are reduced by absorption Underhood noise levels are reduced by absorption with corresponding reductions in the vehicle interiorwith corresponding reductions in the vehicle interior
Sometimes sound absorption and TL are traded Sometimes sound absorption and TL are traded for best interior noise levelsfor best interior noise levels
Sound AbsorptionSound Absorption
AirAir PorousPorousMaterialMaterial
IncidentIncidentSoundSound
ReflectedReflectedSoundSound
Material must be like air to Material must be like air to prevent reflection from the prevent reflection from the front surfacefront surface
Material must provide Material must provide attenuation to dissipate sound attenuation to dissipate sound in the materialin the material
These are competing These are competing demandsdemands
Sound Absorption Sound Absorption Layered Layered MaterialsMaterials
AirAir
PorousPorousMaterialMaterialOne One More More Air LikeAir Like
IncidentIncidentSoundSound
ReflectedReflectedSoundSound
PorousPorousMaterialMaterialTwo Two More More AttenuationAttenuation
Layered Layered materials can help materials can help satisfy competing satisfy competing demands to demands to optimize optimize absorptionabsorption
Sound Absorption Sound Absorption Biot Model for Biot Model for PoroPoro--Elastic MaterialsElastic Materials
AirAir Porous MaterialPorous Material
IncidentIncidentSoundSound
ReflectedReflectedSoundSound
AirborneAirborneWaveWave
StructureStructure--BorneBorneWaveWave
AirborneAirborneWaveWave
StructureStructure--BorneBorneWaveWave
Sound in the material Sound in the material is carried by airborne and is carried by airborne and structurestructure--borne wavesborne waves
Applies to porous Applies to porous materials including foams materials including foams and fibrous materialsand fibrous materials
Biot model provides Biot model provides insight into performance insight into performance of various materials as of various materials as shown in following slidesshown in following slides
Sound Absorption Sound Absorption Biot Model for Biot Model for PoroPoro--Elastic MaterialsElastic Materials
AirAir Porous MaterialPorous Material
IncidentIncidentSoundSound
ReflectedReflectedSoundSound
AirborneAirborneWaveWave
StructureStructure--BorneBorneWaveWave
AirborneAirborneWaveWave
StructureStructure--BorneBorneWaveWave
Airborne wave is Airborne wave is slower than the speed of slower than the speed of sound in airsound in air TortuosityTortuosity Pore shapePore shape Flow ResistanceFlow Resistance
StructureStructure--borne wave borne wave may be faster than may be faster than airborne waveairborne wave DensityDensity Youngs modulusYoungs modulus Loss factorLoss factor Poissons ratioPoissons ratio
PoroPoro--Elastic Material Elastic Material Bonded Bonded Film FaceFilm Face
AirAir Porous MaterialPorous Material
IncidentIncidentSoundSound
ReflectedReflectedSoundSound
AirborneAirborneWaveWave
StructureStructure--BorneBorneWaveWave
AirborneAirborneWaveWave
StructureStructure--BorneBorneWaveWave
Typical of closed cell foam and Typical of closed cell foam and foam & barrier constructionfoam & barrier construction
Almost all sound is carried by Almost all sound is carried by the structurethe structure--borne waveborne wave
Absorption due to mechanical Absorption due to mechanical resonance of materialresonance of material
Can produce lowCan produce low--frequency frequency absorption at a particular absorption at a particular frequency frequency
PoroPoro--Elastic Material Elastic Material Bonded Bonded Film FaceFilm Face
0.0
0.2
0.4
0.6
0.8
1.0
0
1
0
0
0
2
0
0
0
3
0
0
0
4
0
0
0
5
0
0
0
6
0
0
0
Frequency (Hz)
N
o
r
m
a
l
I
n
c
i
d
e
n
c
e
A
b
s
o
r
p
t
i
o
n
Low frequency Low frequency absorption peak due to absorption peak due to mechanical resonancemechanical resonance
PoroPoro--Elastic Material Elastic Material OpenOpen--Face, Face, Relatively StiffRelatively Stiff
0.0
0.2
0.4
0.6
0.8
1.0
0
1
0
0
0
2
0
0
0
3
0
0
0
4
0
0
0
5
0
0
0
6
0
0
0
Frequency (Hz)
N
o
r
m
a
l
I
n
c
i
d
e
n
c
e
A
b
s
o
r
p
t
i
o
n
Typical of open Typical of open cell foam, rigid cell foam, rigid fiberglass, rigid fiberglass, rigid fibrous materials, fibrous materials, etc.etc.
Almost all sound Almost all sound is carried by the is carried by the airborne waveairborne wave
Absorption due Absorption due to flow resistanceto flow resistance
Can produce lowCan produce low--frequency frequency absorption only if absorption only if material is thickmaterial is thick
PoroPoro--Elastic Material Elastic Material OpenOpen--Face, Face, Limp StructureLimp Structure
Typical of limp Typical of limp fibrous materialsfibrous materials
Performs as a Performs as a loss fluid because loss fluid because structure moves structure moves with the airwith the air
0.0
0.2
0.4
0.6
0.8
1.0
0
1
0
0
0
2
0
0
0
3
0
0
0
4
0
0
0
5
0
0
0
6
0
0
0
Frequency (Hz)
N
o
r
m
a
l
I
n
c
i
d
e
n
c
e
A
b
s
o
r
p
t
i
o
n
PoroPoro--Elastic Material Elastic Material OpenOpen--Face, Face, Intermediate StiffnessIntermediate Stiffness
Typical of Typical of partiallypartially--reticulated reticulated foamsfoams
Strong Strong contribution of both contribution of both airborne and airborne and structurestructure--borne borne waveswaves
Strong low Strong low frequency frequency absorption due to absorption due to combined wavescombined waves
0.0
0.2
0.4
0.6
0.8
1.0
0
1
0
0
0
2
0
0
0
3
0
0
0
4
0
0
0
5
0
0
0
6
0
0
0
Frequency (Hz)
N
o
r
m
a
l
I
n
c
i
d
e
n
c
e
A
b
s
o
r
p
t
i
o
n
PoroPoro--Elastic Material Elastic Material Scrim FacedScrim Faced
Porous Scrim can Porous Scrim can be used to increase be used to increase flow resistanceflow resistance
System System absorption is not absorption is not sum of scrim and sum of scrim and material absorptionmaterial absorption
0.0
0.2
0.4
0.6
0.8
1.0
0
1
0
0
0
2
0
0
0
3
0
0
0
4
0
0
0
5
0
0
0
6
0
0
0
Frequency (Hz)
N
o
r
m
a
l
I
n
c
i
d
e
n
c
e
A
b
s
o
r
p
t
i
o
n
Cotton Fiber Only
Cotton Fiberw/ScrimScrim Only
PoroPoro--Elastic Materials Elastic Materials SummarySummary Both airborne and structureBoth airborne and structure--borne waves present borne waves present depending on the BCs (boundary conditions) and material depending on the BCs (boundary conditions) and material typetype BCs can have a strong influence on absorption BCs can have a strong influence on absorption characteristicscharacteristics FilmFilm--face materials can have best low frequency face materials can have best low frequency absorption; high frequency absorption penaltyabsorption; high frequency absorption penaltyPartiallyPartially--reticulated foams and scrim faced materials can reticulated foams and scrim faced materials can have improved low frequency sound absorption; high freq have improved low frequency sound absorption; high freq penaltypenalty Limp and rigid porous materials can have excellent sound Limp and rigid porous materials can have excellent sound absorption properties; thick to absorb low frequencyabsorption properties; thick to absorb low frequency Layers improve absorption; balance being air like and Layers improve absorption; balance being air like and attenuating soundattenuating sound
The Principles of Acoustical AttenuationOutlineSound Transmission Loss (TL)Single Panel TLSingle Panel TL The Mass LawSingle Panel TL CoincidenceSingle Panel TL CoincidenceSingle Panel TL CoincidenceSingle Panel TL CoincidenceDual Panel TLEffect of Air LeakageTL SummarySound AbsorptionSound AbsorptionSound Absorption Layered MaterialsSound Absorption Biot Model for Poro-Elastic MaterialsSound Absorption Biot Model for Poro-Elastic MaterialsPoro-Elastic Material Bonded Film FacePoro-Elastic Material Bonded Film FacePoro-Elastic Material Open-Face, Relatively StiffPoro-Elastic Material Open-Face, Limp StructurePoro-Elastic Material Open-Face, Intermediate StiffnessPoro-Elastic Material Scrim FacedPoro-Elastic Materials Summary
Recommended