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Lecture 18Noise Reduction by Absorption

Sound PathsSound Isolation

Lecture 18Noise Reduction by Absorption

Sound PathsSound Isolation

Noise Reduction by Noise Reduction by AbsorptionAbsorptionNoise Reduction by Noise Reduction by AbsorptionAbsorption

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 PathsSound PathsSound Paths

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’

Noise Reduction ExampleNoise Reduction Example

Find TL500

TL500=35 db

S: p. 813, F.19.17

Noise Reduction ExampleNoise Reduction Example

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 IsolationSound IsolationSound IsolationSound Isolation

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

Sound Transmission ClassSound Transmission Class

STC can be compromised by:

Combination with lesser sound barriers

Gaps in construction

“An air path is a good sound path”

Sound Transmission ClassSound Transmission Class

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

Sound Transmission ClassSound Transmission Class

Wall assembly selection TL data

2x4 stud with gyp. bd.

S: p. 1713, Index K.2

Sound Transmission ClassSound Transmission Class

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