Green Final

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


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).


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


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


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


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


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



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



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








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








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

Documents

Green Final