MECH 482 - Week 10 - Noise Partitions and Sound Barriers

MECH 482 – Noise Control
Week 10
Noise Partitions, Enclosures and Sound
Barriers
March 16, 2015
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Partitions, Enclosures and Barriers
•
•
•
•
Acoustics constitutes an important factor in building design and
in layouts of residences, plants, offices, and institutional
facilities.
A building not only protects against inclement weather; it must
also provide adequate insulation against outside noises from
transportation and other sources.
Interior walls and partitions need to be designed to prevent the
intrusion of sound from one room into another.
Exposure of workers to excessive occupational noises can be
decreased by construction of appropriate barriers or enclosures
around noisy machinery.
March 16, 2015
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Partitions, Enclosures and Barriers
• Partitions, enclosures and barriers are used for
insulating sound, reducing sound transmission,
keeping sound in and out and controlling sound
outdoors and indoors.
March 16, 2015
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Partitions, Enclosures and Barriers
• Sound absorption materials tend to be light and
porous (dissipative – absorb broad band acoustic
energy at high frequencies).
• Sound absorption panels (and mufflers) need to be
hollow and rigid (reactive - absorb acoustic energy at
specific frequencies).
• Sound isolation materials, however, generally are
massive and airtight, thereby forming effective sound
insulation structures between the noise source and
the receiver.
March 16, 2015
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Partitions, Enclosures and Barriers
Transmission Loss
• When sound is incident upon a wall or partition, some
of it will be reflected and some will be transmitted
through the wall.
• The ratio of transmitted energy to the incident energy is
called the transmission coefficient τ (usually defined
empirically). The transmission loss (TL) is then
defined as
TL  10 log10 
(dB)
  Wtransmitted WIncident
March 16, 2015
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Partitions, Enclosures and Barriers
Transmission Loss
• The power transmitted through a wall (area = A)
is…
Pi A 
Wt 
4 c
2
• The sound pressure in the receiving room is
Pr 
2
March 16, 2015
4W t  c 1   

S
Pi A  1  
S
2

Page 6
Partitions, Enclosures and Barriers
Transmission Loss
• The noise reduction is then…
 Pi 2 
 A 1    
NR  10 log 10  2   TL  10 log 10 

 S

 Pr 
• In a highly reverberant room (  < 0.1) the
equation for TL may be approximated as…
 A 
TL  NR  10 log 10 

 S 
March 16, 2015
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Partitions, Enclosures and Barriers
Transmission Loss
• In general, the transmission coefficient and thus the
transmission loss will depend upon the angle of
incidence of the incident sound.
• However, at low frequencies, the transmission loss is
controlled by the stiffness of the panel.
• At the frequency of the first panel resonance (called the
critical frequency), the transmission of sound is high,
i.e. the transmission loss is small.
March 16, 2015
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Partitions, Enclosures and Barriers
Sound transmission through panels
• The critical frequency of a panel is:
c2
fc 
2
B
m
Above and above the critical frequency, the panel radiates sound.
March 16, 2015
B: bending stiffness of the panel
2
m: surface density   thickness (kg/m )


At the critical frequency, the disturbance is local.
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Partitions, Enclosures and Barriers
Sound transmission through panels
• The panel bending stiffness is:

B  EI  / 1 
2
  Eh
3

/ 12 1  
2
 kg m
2
s
-2
h = panel thickness
E = Young’s modulus
 = Poisson’s ratio
March 16, 2015
Page 10
Partitions, Enclosures and Barriers
Transmission Loss
• At frequencies above the first panel resonance, a
generally broad frequency range is encountered, in
which transmission loss is controlled by the surface
density of the panel.
• This is called the “mass law” region.
• At very high frequencies, the transmission loss rises
again and is damping controlled.
March 16, 2015
Page 11
Partitions, Enclosures and Barriers
Transmission Loss
Coincidence region = single critical frequency
March 16, 2015
Page 12
Partitions, Enclosures and Barriers
Transmission Loss
Coincidence region = critical frequency range
March 16, 2015
Page 13
Partitions, Enclosures and Barriers
Transmission Loss
• The resonance frequencies of a simply supported
rectangular isotropic panel of width a, length b, surface
density m, and bending stiffness per unit width, B, may
be calculated as
fi ,n 

2
2
2

B  i  n 
     
m  a   b  
i, n  1, 2,3,...
(Hz);
The fundamental frequency corresponds to i=n=1.
Recall that:
March 16, 2015

B  Eh 3 / 12 1  
2
 kg m
2
s -2
Page 14
Partitions, Enclosures and Barriers
Transmission Loss
• The resonance frequencies of a simply supported
rectangular orthotropic panel of width a, length b, may be
calculated as
f i ,n 

2m
 Bai
Bb i
B ab i n
 4  4 
4 4
a
b
a
b

4
1/ 2
4
4
4



1/ 2
; i, n,  1, 2, 3, ...
The fundamental frequency corresponds to i=n=1.
March 16, 2015
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Partitions, Enclosures and Barriers
Transmission Loss
• Where…

B ab  0 . 5 B a  B b  Gh 3 / 3

G  E / 2 1   
B a  bending stiffness per unit length in direction a
Bb  bending stiffness per unit length in direction b
March 16, 2015
Page 16
Partitions, Enclosures and Barriers
Transmission Loss
• Mass law transmission loss at normal
incidence:
   fm 2 
TL0  10 log10 1  
 
   c  
• At field incidence (averaged over most likely
angles of incidence):
TL  TL0  5.5
March 16, 2015
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Partitions, Enclosures and Barriers
Transmission Loss Measurement
• The transmission loss of a partition is usually
measured in a laboratory by placing the partition in
an opening between two adjacent reverberant rooms
(source room and receiver room) designed for such
tests.
• The resulting mean space-average sound pressure
levels in the source and receiver rooms are
measured and the difference in levels is called the
noise reduction (NR).
March 16, 2015
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Partitions, Enclosures and Barriers
Transmission Loss
NR  LP1  LP 2
 A 
TL  NR  10 log10 

S



March 16, 2015
A: area of the test partition
S : Sabine absorption of the receiver room
Page 19
Partitions, Enclosures and Barriers
Transmission Loss Measurement
Receiver room
Sound source
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Partitions, Enclosures and Barriers
Transmission Loss Data
Frequency (Hz) / Sound Transmission Loss (dB)
Material
Lead
1/32 in. thick
1/64 in. thick
Plywood
¾ in. thick
¼ in. thick
Lead vinyl
Lead vinyl
Steel
18 gauge
16 gauge
Sheet metal
Plexiglas
¼ in. thick
½ in. thick
1in. Thick
Glass
1/8in. Thick
¼ in. Thick
March 16, 2015
125
250
500
1000
2000
4000
8000
22
19
24
20
29
24
33
27
40
33
43
39
49
43
24
17
11
15
22
15
12
17
27
20
15
21
28
24
20
28
25
28
26
33
27
27
32
37
35
25
37
43
15
21
15
19
30
25
31
34
28
32
27
32
35
40
39
48
47
42
53
52
47
16
21
25
17
23
28
22
26
32
28
32
32
33
32
34
35
.7
46
35
37
46
11
17
17
23
23
25
25
27
26
28
27
29
28
30
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Partitions, Enclosures and Barriers
Enclosures
• Enclosures may be categorized as being either full
enclosures or partial enclosures.
• These structures of varying sizes may enclose
people or noise generating machinery.
• It is advisable to never enclose more volume than
necessary.
• For example, an entire machine should not be
enclosed if only one of its components (such as a
gear box, motor, etc.) constitutes the principal noise
source.
March 16, 2015
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Partitions, Enclosures and Barriers
Enclosures
• The walls of an enclosure should be constructed of
materials that will provide isolation, absorption, and
damping—all necessary for effective noise reduction.
• Moreover, any presence of cracks or leaks can radically
reduce the noise reduction of an enclosure, so all
mechanical, electrical, utility, and piping outlets must be
thoroughly sealed.
March 16, 2015
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Partitions, Enclosures and Barriers
Enclosures
• The interior of the enclosures should be lined with highly
absorbing material so that the sound level does not
build up from reflections, thereby decreasing the wall
vibration and the resultant radiation of the noise.
• In some cases, a partial enclosure may suffice to shield
a worker from excessive exposure to noise.
March 16, 2015
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Partitions, Enclosures and Barriers
Enclosures
The noise reduction of an enclosure around a noisy item
of machinery is…
NR  TL  C
C  10 log10  0.3  S E (1   i ) / ( Si i ) 
 i : the mean Sabine absorption coefficient of the interior
of the enclosure;
S i : the interior surface area of the enclosure;
S E : the area of the external surface of the enclosure.
March 16, 2015
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Partitions, Enclosures and Barriers
Acoustic Barriers
• The term acoustic barrier (or noise barrier) refers to an
obstacle that interrupts the line of sight between a noise
source and receiver but does not enclose either source
or receiver.
• An acoustic barrier may be in the form of a fence, a
wall, dense foliage, or a building between the noise
source and the receiver.
March 16, 2015
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Partitions, Enclosures and Barriers
Acoustic Barriers
• Noise attenuation occurs from the fact that noise
transmission through the barrier is considered negligible
in comparison with refracted noise, particularly if the
barrier is solid, without holes or openings, and it is of
sufficient mass.
• The refraction phenomenon is highly dependent upon
the frequency content of the sound.
March 16, 2015
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Partitions, Enclosures and Barriers
Acoustic Barriers
• Because the sound reaches the receiver by an
indirect path over the top of the barrier, the sound
level will be less than the case would be if the sound
had traveled the (shorter) direct path.
• The calculations for barrier attenuation are based in
part on Fresnel’s work in optics.
March 16, 2015
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Partitions, Enclosures and Barriers
Acoustic Barriers
• Fresnel number N:
March 16, 2015
N 
2( A  B  d )

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Partitions, Enclosures and Barriers
Acoustic Barriers
• Barrier attenuation:
March 16, 2015
Page 30
Partitions, Enclosures and Barriers
Outdoor Barriers
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Partitions, Enclosures and Barriers
Outdoor Barriers
March 16, 2015
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Partitions, Enclosures and Barriers
Acoustic Barriers
• The overall attenuation caused by a barrier is calculated
by combining the noise reductions due to all paths
between the source and the receiver, including those
reflected by the ground.
• This sound level may be compared to a direct path
sound level.
• The reflection loss, if any, due to the ground is added to
each path that involves ground reflection.
March 16, 2015
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Partitions, Enclosures and Barriers
Indoor Barriers
• For outdoor barriers we are concerned mostly with
directly transmitted or once (at most twice) reflected
sound waves.
• For indoor barriers we must consider the reverberant
field (reflections from many surfaces).
• Before the sound loss due to the insertion of a sound
barrier into a large room can be determined, the
following assumptions are implicit.
March 16, 2015
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Partitions, Enclosures and Barriers
Indoor Barriers
• The transmission loss of the barrier must be at least 20dB
(to cause significant loss through the barrier).
• The sound power radiated by the source is not directly
affected by the barrier (the barrier is not too close to the
source).
• The receiver is in the shadow zone of the barrier.
• Interference effects between waves diffracted around the
edges of the barrier and reflected waves are negligible
(octave band analysis).
March 16, 2015
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Partitions, Enclosures and Barriers
Indoor Barriers
• The barrier insertion loss, IL, is then…
 Df
4
IL  10 log 10 

2
S 0 0
 4 r

 D fF
4 K1K 2 
  10 log 10 


2
S 1  K 1 K 2  
 4 r

Df  Directivit y Factor of source
r  distance from source to receiver w ithout barrier
S 0  surface area of room without barrier
 0  absorption coefficien t of room without barrier
S  open area between barrier and walls and ceiling
F  diffractio n coefficien t
March 16, 2015
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Partitions, Enclosures and Barriers
Indoor Barriers

1
F   
i  3  10 N i



• Ni is the Fresnel number for diffraction around the ith
edge of the barrier.
March 16, 2015
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Partitions, Enclosures and Barriers
Indoor Barriers
S
K1 
S  S 1 1
S
K2 
S  S 2 2
S  open area between barrier and walls and ceiling
S1 and S 2  total room surface areas on the
t wo sides of the barrier (not including barrier)
S1  S 2  S 0  (area of two sides of barrier)
1 and 1  the absorption coefficients for areas
S1 and S 2 respectively
March 16, 2015
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Next Time
Silencers and Muffling Devices
March 16, 2015
Page 39