1. Airborne Sound Insulation in Buildings
By Tong Yeung
Wilson Acoustics Limited

2. Content Sound Insulation Rating Sound insulation Wall Door Window
Floor
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3. Sound Insulation Rating
Sound Transmission Class (STC)
Noise Isolation Class
Impact Insulation Class
Normalized Sound Pressure Level Difference
Weighted Sound Reduction Index
Weighted Normalized Impact Sound Pressure Level
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4. Sound Transmission Class
Sound transmission losses in 16 1/3 octave bands from 125 to 4000Hz
Values are compared with a reference contour such that no individual transmission loss may lie more than 8dB below the contour
Sum of negative discrepancies may not exceed 32
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5. Sound Transmission Class
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6. Sound Transmission Class
Usually designed with higher STC, e.g. 5dB
Construction often perform less well in building than in lab
Higher STC, less chance of complain but not necessarily increase the costs
Limitation at low frequency below 125Hz
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7. Poor transmission loss at low frequencies limits overall STC improvement suggested by better performance at higher frequencies
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8. Wilson Acoustics Limited www.wal.hk

9. Wilson Acoustics Limited www.wal.hk

10. STC Examples
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11. Weighted Sound Reduction Index
Similar to Sound Transmission Class, but follow ISO 717 standard
Frequency range: Hz
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12. Noise Isolation Class
Sound Isolation between 2 enclosed spaces that are acoustically connected
Normalized Noise Isolation Class
Adjusted space to furnished rooms such that RT60=0.5s
Adjustment factor = 10log(T1/T0)
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13. Impact Insulation Class
By a standardized tapping Machine with 5 brass hammers at 10Hz
Measure in the room below the floor
1/3 octave band from 100 to 3150Hz
Compare value with reference contour similar to STC calculation method
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14. Floor Tapping Machine
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15. Wall Single-Leaf Partition Performance Mainly depends on:
All kinds of solid homogenous panels
Drywall
Plywood
Glass
Solid concrete
Concrete block
Performance Mainly depends on:
Mass
Stiffness
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16. Wall insulation performance
Mass
Transmission loss increases with mass
Heavier the panel, less it vibrates
Apply to thin panel at frequencies below coincidence frequency
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17. 100mm concrete, 16mm plywood, 13mm gypsum board
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18. Wall insulation performance - Coincidence dip
Stiffness and the Coincidence dip
At a critical frequency, wavelength of free bending waves in panel coincides with wavelength of sound in air.
Coincidence dip: below the critical frequency to an octave or more
Stiffer and thicker, lower the frequency
Less effect with higher damping
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19. Coincidence effect
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20. Critical frequency calculation
E.g. Concrete of 100mm:
A = 18700
Coincidence frequency =18700/100
=187Hz
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21. Wall insulation performance
Avoid the effect of coincidence dip
Use different thickness so that critical frequency does not fall in the range of important for building acoustics ( Hz)
Increase damping: separate 1 thick wall into 2 thinner walls, loosely glued together to allow sliding friction in between and increase transmission loss
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22. Gypsum board glued together (STC 31)
Gypsum board screwed together (STC 31)
Single layer (STC 28)
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23. Wall insulation performance - Corrugating panels
Adding ribs, joists, studs increases stiffness in one direction
Also introduces additional coincidence dip
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24. Two-leaf Partition Weight of the layers Depth of air space
Sound absorbing material in layers
Any connection between the layers
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25. Two-leaf Partition – Cavity Depth
Larger the space, higher the transmission loss
Mass-air-mass resonance occur reducing
Trapped air acts as spring
Larger the air space or heavier the material, lower the resonance frequency
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26. Two-leaf Partition – Cavity Depth
Solid curve: double 3mm glass with airspace of 6mm (STC 28)
Dotted curve: double 3mm glass with airspace of 19mm (STC 32)
Dash curve: single layer of 3mm glass (STC 29)
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27. Two-leaf Partition – Cavity Depth
fmam=K[(m1+m2)/dm1m2]0.5
where fmam = mass-air-mass resonance frequency, Hz
m1 = surface mass of the first layer, kg/m2
m2 = surface mass of the second layer, kg/m2
d = their separation, mm
K = 60 for an empty cavity, 43 for a cavity filled with sound absorptive material
Density of sound-absorptive material is not very important to transmission loss
- increase transmission loss above resonance frequency
- limit negative effect from cracks or leaks at the partitions.
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28. Two-leaf Partition – Mechanical Coupling
Mechanical connection transfers vibrations and is more effective to transfer sound than in air
Use resilient connection if mechanical connection cannot be avoided
Sound absorptive material becomes useless with mechanical connections
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29. Two-leaf Partition – Design of Mechanical Coupling
Lightweight steel studs
Wood studs with resilient metal channels on one side and both side
Staggered wood stud
Steel studs with resilient metal channels
Double wood studs
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30. Different wall material
Concrete-block wall
porous and cannot give STC as great as the mass law
Can be improved with suitable sealing such as a layer of drywall, plaster
Masonry Wall
Very high sound insulation in principle
Two blocks are not solidly connected
Mount with drywall surfaces
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31. Insulation of Door Similar principle as insulation of walls
Leakage of sound through the edge is important
Rubber or neoprene gaskets are effective sealing material
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32. Insulation of Door – Double door
Less expensive but more effective way to improve STC
Commonly used as communication room in hotel
Can be used with sound absorption material between doors (can provide STC of 45 or higher)
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33. Insulation of Door – Automatic door bottom
Control leakage under door
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34. Insulation of windows Follow mass law Have coincidence dip
Laminated glass provide greater transmission loss at frequency near coincidence dip
4mm glass (STC 28), 12mm glass (STC 36), laminated 12mm glass (STC 38)
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35. Insulation of windows – Double glazing
Not necessarily better performance than single glazing due to coincidence dip
Improve by increasing mass and separating distance
Double 3mm glazing with 6mm airspace: STC 31, no air space: STC 34, air space of 100mm: STC 42
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36. Insulation of windows – Other methods
Slanted glazing
Avoid multiple optical reflections
No enhancement of sound insulation than parallel glazing
Triple glazing
Nearly the same performance as double glazing
Higher transmission loss below mass-air-mass resonance
Heavy gas filling
STC rating is usually small
Significant improvement at mid-range frequency
Use with combination of thermal insulation
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37. Insulation of floor Airbone sound insulation Impact sound insulation
Heavy enough to provide good airborne sound insulation
Impact sound insulation
Controlled by resilience of the floor surface layers
Soft resilient layer. E.g. carpet increases IIC from 40 to 65 or more
Carpet increases IIC from 43 to 73
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38. Insulation of floor – floating floor
Floating floor: when hard floor surface cannot be avoided
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39. END
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