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Lecture 17Noise Reduction by Absorption

Sound PathsSound IsolationSound Privacy

Structure Borne Sound

Lecture 17Noise Reduction by Absorption

Sound PathsSound IsolationSound Privacy

Structure Borne Sound

Noise Reduction by Noise Reduction by AbsorptionAbsorption

Increased absorption reduces ambient noise

S: p. 771, F.18.2

Noise Reduction Noise Reduction by Absorptionby Absorption

Reverberation increases ambient sound levels

S: p. 798 F.19.1

69 db

Sound PathsSound PathsAir-Borne SoundStructure-Borne Sound

S: p. 806, F.19.10

Sound TransmissionSound Transmission

Barriers reduce sound transmission to reduce noise intrusion

S: p. 807, F.19.11

Air-Borne SoundAir-Borne Sound

Transmission Loss (TL): sound energy lost through a construction assembly

S: p. 813, F.19.17

Noise ReductionNoise Reduction

Combined effect of TL and absorption

NR=TL-10 Log (S/AR)

NR: noise reduction (db)TL: transmission loss (db)S: area of barrier wall (ft2)AR: total absorption of receiving room

(sabins, ft2)

Noise Reduction ExampleNoise Reduction Example

What is the noise reduction at 500 hz between two rooms separated by a 2x4 stud wall with ½” gyp. bd. on each side.

Receiving room absorption: 450 sabins, ft2

Separation wall: 10’ x 20’


Find TL500

TL500=35 db

S: p. 813, F.19.17


Find NR500

NR500=TL500-10 Log (S/AR)

=35-10 Log (200/450) =35-10 Log (0.444) =35-10(-0.35) =35+3.5=38.5 say 39db

Sound Transmission ClassSound Transmission Class

Sound Transmission Class (STC): defines overall transmission loss across center band frequencies

Barrier STC Descriptor/ ApplicationRanking

25 Poor/1 Space divider30 Fair/2 Room divider35 Very Good/4 Offices near quiet space42-45 Excellent/5 Party walls46-50 Total Privacy/6 Quiet next to noisy

from S: p. 828. T.19.6


STC can be compromised by:

Combination with lesser sound barriers

Gaps in construction

“An air path is a good sound path”


STC of a wall assembly

2x4 stud with gyp. Bd.

S: p.1712, Index K.1

Sound Sound Transmission Transmission ClassClass

Wall assembly description

2x4 stud with gyp. bd.

S: p. 1716, Table K.3


Wall assembly selection TL data

2x4 stud with gyp. bd.

S: p. 1713, Index K.2


Floor assembly follows same process

S: p. 1724, Index L.1

Noise CriteriaNoise Criteria

Noise Criteria (NC): accepted noise level for a given space.

Defined for center band frequencies

62.5 125 250 500 1000 2000 4000

Noise Criteria CurvesNoise Criteria Curves

Accepted noise level for a given space.

S: p. 759, F.17.17

Noise CriteriaNoise Criteria

Noise Criteria for various applications

S: p.831, T.19.8

Sound PrivacySound PrivacyPrivacy is affected by initial sound source, TL, NC and absorbtion

Privacy Maintained Privacy Violated

NC2

SPL

NC1

TL TL

Room A Room B

Sectional View

Sound Privacy—Example 1Sound Privacy—Example 1

Room A: NC500=35

Room B: NC500=25


NC2=25

SPL=70

NC1=35

TL=30 TL=30

Room A Room B

Sectional View

SPL=40SPL=40

Noise Criteria CurvesNoise Criteria Curves

Privacy is affected by background noise level

NC 35 @ 500 hz

Max SPL500=40 db

S: p. 759, F.17.17


Room A: NC500=35, SPL500 ≤ 40 db

Room B: NC500=25, SPL500 ≤ 31 db

Initial check based on TL alone


NC2=25

SPL=70

NC1=35

TL=30 TL=30

Room A Room B

Sectional View

SPL=40SPL=40


Combine effect of TL and absorption for final analysis

NR=TL-10 Log (S/AR)

NR: noise reduction (db)TL: transmission loss (db)S: area of barrier wall (ft2)AR: total absorption of receiving room (sabins, ft2)


For each frequency:

If SPL-NR ≤ NCmax privacy achieved

If SPL-NR > NCmax privacy violated

Sound Privacy EnhancementSound Privacy EnhancementTL data can be enhanced by standard enhancements

Wood stud wall (STC 35) Add gyp bd to one side +2 db Add gyp bd to both sides +4 db Double thickness insulation +6 db Staggered studs +9 db Double studs +13 db

from S: p. 816, T.19.1

Sound Privacy ReductionSound Privacy Reduction

Privacy is diminished by flanking noise, weaker STC constructions and/or gaps

S: p. 825, F.19.34

Sound PrivacySound Privacy

Privacy diminished by weaker STC constructions

S: p. 817, F.19.22


A 100 ft2 wall assembly has TL=50 [excellent]

What is the overall TL if a 20 ft2 door with TL=20 is part of the wall?

S: p. 817, F.19.22


S1= 100 ft2 S2= 20 ft2

S2/S1=20/100=20%

TL1-TL2=50-20=30 db

TL1-TLC=23

TLC=TL1-23=27 db

[fair-poor] S: p. 817, F.19.22

Sound PrivacySound Privacy

Privacy diminished by construction gaps

S: p. 817, F.19.23


A 10x10 (100 ft2) wall assembly has TL=35[very good]

What is the overall TL if a 1/16” gap exists along each vertical edge?

S: p. 817, F.19.23


S1= 14400 in2 S2= 15 in2

S2/S1=15/14400=0.1%

TL1=35 db

TLC=29 db

[fair-poor] S: p. 817, F.19.23

Sound Transmission Sound Transmission ReductionReduction

Impact Isolation

Structural isolation

Flexible connections

Spring mounts

Inertial dampening

S: p. 845, F.19.47

Sound TransmissionSound TransmissionReduction– Floors Reduction– Floors Sound Transmission Class (STC)

Impact Isolation Class (IIC)

S: p.1726, Index L.3

Impact Isolation Class—Impact Isolation Class—Enhancements Enhancements

Construction Tectonics

S: p. 842, F.19.43

Impact Isolation Class—Impact Isolation Class—Enhancements Enhancements

Resilient Floor Finishes: 1/16” vinyl tile 0 db 1/8” linoleum or rubber tile 4 +/- 1 db ¼” cork tile 10 +/- 2 db Low pile carpet on fiber pad 12 +/- 2 db Low pile carpet on fiber pad 18 +/- 3 db High pile carpet on foam pad 24 +/- 3 db

from S: p. 843

Mechanical IsolationMechanical Isolation

Moving parts (vibration sources) eliminated by mechanical isolation

S: p. 851, F.19.54

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