chapter 18 sound transmission jean-louis...

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 1

Chapter 18Sound transmissionJean-Louis Migeot

1. Sound transmission and insulation: definitions and measurement techniques

2. Low frequency model: rigid panel on elastic support

3. High frequency model: mass law and coincidence

4. General model

5. Double walls

6. Some comments

7. Application to a windshield






4. General model

5. Double walls

6. Some comments



Some Terminology

Reflection - Absorption Transmission - Insulation Refraction

Radiation Scattering Propagation - Attenuation


Acoustic Transparency

➢ Acoustic transparency generally defines the ability of a component to isolate a volume from external noise sources

➢ The standard tests of acoustic transparency generally involve 2 rooms acoustically connected by the system to be tested

➢ The typical indicator used for acoustic transparency is the Transmission Loss index (TL)


Transmission Loss

Reverberant Room

with Diffuse Sound FieldAnechoic Receiving

Room

Measurement of

Radiated Power

Measurement of

Incident Power

Partition


What is the transmission loss index?

➢ Transmission loss:

Symbols used in the literature: TL, STL or R

expressed in dB

Intrinsic property of system (does not depend on the coupled rooms of the set-up)

with

➢ The transmission loss is the logarithmic representation of the ratio of powers: What part of the incident power is transmitted through the structure ?

dBTL )1(log10 10 =

incidentdtransmitte WW /)( =


Typical Measurement Set-up (1)

➢ Anechoic and reverberant rooms can be associated: side by side (classic transmission loss measurements) or on top of each other (building impact noise).

➢ Most used set-up: two side-by-side reverberant rooms


Typical variation of TL with frequency

Resonancescontrolled

St

iffn

ess

con

tro

lled

Mass law

Coincidence

Asymptoticmass law

Low damping

High damping

TL –

Tra

nsm

issi

on

Lo

ss

Frequency (log f)






4. General model

5. Double walls

6. Some comments



Elastically mounted rigid panel

X

Incident

Reflected

Transmitted


Transmission coefficient

Normal velocity continuity:

Dynamic equations


Transmission coefficient (Ctd)

Transmission coefficient:


0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80 90 100

k=10.000, m=1

k=20.000, m=1

k=10.000, m=2

k=10.000, m=1, c'.10

k=10.000, m=1, c'/10

Sensitivity to stiffness and mass






4. General model

5. Double walls

6. Some comments



Infinite flexible plate

qq

X

Y

q2

Normal velocity continuity:


Dynamic equations of the plate


Attenuation

Consider two identical fluids:


Insulation curves

0

20

40

60

80

100

120

140

100 1,000 10,000 100,000

15°

30°

45°

60°

q

q

Coincidence: damping control


Free bending waves in an infinite plate


Coincidence

q

Coincidence occurs when


Coincidence frequency / incidence

Free bending waves:

Coincidence angle:

Coincidence frequency:


Coincidence

q1

(b)

q2


Transmission under diffuse incidence

0.0000E+00

2.0000E+01

4.0000E+01

6.0000E+01

8.0000E+01

1.0000E+02

1.2000E+02

100 1000 10000

Champ diffus

Incidence normale

Incidence oblique 45°

6dB/oct.

contrôle par la masse

9dB/oct.

contrôle

par la r

igidité






4. General model

5. Double walls

6. Some comments



Transmission through finite baffled plates

x

z

y

baffleplaque

1

2


Finite vs. infinite plate models

Frequency (Hz)

Finite plate model

Infinite plate model


Typical variation of TL with frequency

Resonancescontrolled

St

iffn

ess

con

tro

lled

Mass law

Coincidence

Asymptoticmass law

Low damping

High damping

TL –

Tra

nsm

issi

on

Lo

ss

Frequency (log f)






4. General model

5. Double walls

6. Some comments



Double-walls – Mass-air-mass resonance

m1=rvh1

Resonance frequency:

m2=rvh2


Double-walls

I R

T

M1 M2


Coincidence 1

Coincidence 2






4. General model

5. Double walls

6. Some comments



Real vs. ideal structures

Figure 1. Transmission losses of typical single-leaf walls, A: 16 mm plywood, 10 kg/m², STC 21; B: 13 mm wallboard, 10 kg/m², STC 28; C: 1.3 mm steel, 10 kg/m², STC 30; D: 100 mm concrete, 235 kg/m², STC 52. Credit: A.C.C. Warnock.

Figure 2. Effect of air space on ideal double walls with 0.5 mm steel on each face, sound absorbing material in the cavity and no rigid mechanical connections between the faces. A has an airspace of 100 mm, a resonance dip at 135 Hz, and an STC of 29; B has an airspace of 5 mm, a resonance dip at 630 Hz, and an STC of 24. Curve C represents mass law predictions for a single 1 mm steel sheet and has an STC of 28. Credit: A.C.C. Warnock.


Weak link principle of acoustic insulation

1.5 mx

2.5 m

1mm gap


Airborne vs. structure-borne transmission in buildings

From Vèr & Sturz in Harris C.M., Handbook of Acoustical Measurements and Noise Control, McGraw Hill, New-York, 1991

Airborne transmission Structure borne transmission






4. General model

5. Double walls

6. Some comments



Windshield transmission

Glass Glass

CeramicPVB

Car

Exterior

Car

Interior

Absorption

Transmission

Acoustic Transmission Path


Correlation or simulations with measurements: NSG

➢ Measurement following ISO 140

Reverberant

room

Anechoic Room

Test Piece :

single layer glass

Nippon Sheet Glass


Correlation or simulations with measurements: Glaberbel (AGC)

T ransparence acoust ique - Comparaison simulat ions ACT R AN/mesures

BMW Sér ie 3

-10

-5

0

5

10

15

20

25

30

35

40

0 20

0

40

0

60

0

80

0

10

00

12

00

Fréquence [Hz]

Tra

nsm

issi

on

Lo

ss (d

B)

ESSAI 1 ESSAI 2 ACT RAN


20

25

30

35

40

45

50

100 1000 10000

Frequence (Hz)

TL

(d

B)

=

tr

in

W

WTL log10

Critical

frequency

Effect of seals on windshield TL performance


Structural model

glass run

channel glass

incident

power

transmitted

power

air (I-FEM)


e

15

20

25

30

35

40

45

50

100 1000 10000

Fréquence (Hz)

TL

(d

B)

Epaisseur de 3.15 mm



Effect of glass thickness


3 lips

2 lips

15

20

25

30

35

40

45

50

100 1000 10000

Fréquence (Hz)

TL

(d

B)

Design 3 lèvres

Design 2 lèvres

Glass thickness of 3,85 mm

2-lips seal vs. 3-lips seal


Key Takeaways

➢ TBD






4. General model

5. Double walls

6. Some comments


chapter 18 sound transmission jean-louis...

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