airborne sound insulation in buildings by tong yeung wilson acoustics limited

Airborne Sound Insulation in Buildings

By Tong Yeung

Wilson Acoustics Limited www.wal.hk

Content

• Sound Insulation Rating

• Sound insulation– Wall– Door– Window– Floor


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


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



• 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


Poor transmission loss at low frequencies limits overall STC improvement suggested by better performance at higher frequencies


STC Examples


Weighted Sound Reduction Index

• Similar to Sound Transmission Class, but follow ISO 717 standard

• Frequency range: 100-3150Hz


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)


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


Floor Tapping Machine


Wall

• Single-Leaf Partition– All kinds of solid homogenous panels

• Drywall• Plywood• Glass• Solid concrete• Concrete block

• Performance Mainly depends on:– Mass– Stiffness


Wall insulation performance

• Mass– Transmission loss increases with mass

• Heavier the panel, less it vibrates

– Apply to thin panel at frequencies below coincidence frequency


100mm concrete, 16mm plywood, 13mm gypsum board


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


Coincidence effect


Critical frequency calculation

E.g. Concrete of 100mm:A = 18700Coincidence frequency =18700/100

=187Hz


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 (80-4000Hz)

– Increase damping: separate 1 thick wall into 2 thinner walls, loosely glued together to allow sliding friction in between and increase transmission loss


• Gypsum board glued together (STC 31)• Gypsum board screwed together (STC 31)• Single layer (STC 28)


Wall insulation performance - Corrugating panels

• Adding ribs, joists, studs increases stiffness in one direction

• Also introduces additional coincidence dip


Two-leaf Partition

• Weight of the layers

• Depth of air space

• Sound absorbing material in layers

• Any connection between the layers


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



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)



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.


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


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


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


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


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)


Insulation of Door – Automatic door bottom

• Control leakage under door


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)


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


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


Insulation of floor

• Airbone 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 73Wilson Acoustics Limited www.wal.hk

Insulation of floor – floating floor

• Floating floor: when hard floor surface cannot be avoided


END


airborne sound insulation in buildings by tong yeung wilson acoustics limited

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