acoustics in building services

1. BUILDING SERVICES IV ACOUSTICS & NOISE CONTROL - 18 ARC 7.3
MODULE 4 – NOISE REDUCTION AND CONTROL

2. 18 ARC 7.3 BS IV
The World Health Organization (WHO) defines environmental noise as noise from all sources with the exception
of workplace noise. Noise is all unwanted sound or set of sounds that causes annoyance or can have a health
impact.
Environmental noise is the noise produced by
● Transport
● Industrial activities
● Construction site activities
● Public works and services
● Cultural, sporting and leisure activities
● Neighbourhood noise - Indoor and Outdoor

3. 18 ARC 7.3 BS IV
NOISE CLASSIFICATION
● Outdoor and Indoor Noise
Noise from outdoor noise producing equipments like gardening
equipments etc or indoor noise like home appliances, fans, music
systems etc
● Airborne Noise
This type of noise is transmitted by air and atmosphere such as the
radio, the barking of dogs or people carrying on conversations.
When sound waves traveling through the air reach a building
element they hit it and cause it to vibrate. These vibrations travel
through the structure or building and are radiated out the other
side. Any loud music played in an adjacent building will feel like it
is reverberating within your building. This is due to airborne noise
traveling through windows and doors which are a major source of
sound leakage. Examples of Air borne noise are music and
television, conversation, traffic noise

4. 18 ARC 7.3 BS IV
● Structure Borne Noise
Structure-borne noises are transmitted when sound arises from the
actual impact of an object on a building element such as a wall,
floor or ceiling. Example - footsteps on upper floors or stairs.
Structure-borne sound occurs because the impact causes both
sides of the building element to vibrate, generating sound waves.
This can often be the hardest to isolate.
STEPS TO REDUCE AIR BORNE AND STRUCTURE BORNE SOUND
● The best way to stop sound is having mass, an air gap and mass
again. Sound waves have a difficult time penetrating through
that space and continuing indoors.
● DGU for windows are costly but not very effective for sound
compared to thermal insulation. Double pane windows have an
average STC rating of about 26.
● Sound travels through flanking through doors and windows - air
and sound follow the same travel paths.
● Therefore weather stripping is used as added seal.

5. 18 ARC 7.3 BS IV
● Impact Noise
Impact Noise is the physical impact on
buildings or solid materials. Examples being
footfall, doorsbanging, walking and furniture
moving.
Impact sound occurs because the impact
causes both sides of the building element to
vibrate, generating sound waves. This can
often be the hardest to isolate as impact
vibrations are stronger and travel further
through dense materials.
IMPACT NOISE Vs AIR BORNE NOISE

6. 18 ARC 7.3 BS IV
● Noise from Mechanical systems like Ventilation system
and HVAC system
The vibrations from mechanical equipments in buildings -
(< 20 Hz) can be felt and heard by building occupants.
Vibrations along with noise can travel great distances
through building elements (structure borne noise)and may
be reradiated as air borne sound at a great distance from
the source.
Mechanical vibrating equipments should be placed on
resilient mounts to isolate the vibration from the building
structure.
Preferably vibrating equipments should be placed near
structural elements rather than on the md span of a slab.
● Community noise
Community noise is the environmental noise from all
sources except from the Industrial areas.

7. 18 ARC 7.3 BS IV
● Construction noise
The noise generated at construction sites due to
construction activities like rock breaking,
machinery working, welding , metal cutting etc.
These need to be properly regulated so that it
does not affect the residents around a
construction site.
● Industrial noise
The noise generated by machinery and
equipments in an Industrial setting is classified
as industrial noise.
It can affect the auditory sensibilities of people
working in such a setting. These are
occupational hazards.

8. 18 ARC 7.3 BS IV
NOISE TRANSMISSION
Transmission loss (TL) is measured in dB. It is a
measure of how much sound energy is reduced in
transmission through materials. The more massive a
material, higher is the TL.
However in addition to the weight, other factors also
affect the TL. For E.g certain natural frequencies for
bending waves can exist depending on the stiffness
of the construction. When such waves are excited by
sound energy impinging on them , the resistance to
sound transmissions are greatly reduced (a
phenomenon called coincidence effect).
Transmission loss of sound is a measurement of the reduction in sound level of a sound source as it passes through an acoustic
barrier. It is the number of decibels that are stopped by the acoustical barrier or the wall and is measured at different
frequencies. TRANSMISSION LOSS is a measurement of the dB (volume) difference on either side of a wall.

9. 18 ARC 7.3 BS IV
Transmission Loss can be expressed as
TL = L1 - L2
Where
TL= Transmission Loss (dB)
L1 = Sound level in Lab Source room (dB)
L2= Sound level in Lab Receiving room (dB)
And TL = 10log 1/τ
Where
TL = Transmission Loss (dB)
τ= sound transmission coefficient (no units)
τ (tau) is the ratio of the sound energy transmitted by a material to the incident sound energy.

10. 18 ARC 7.3 BS IV
What is STC (Sound Transmission Class) rating in Acoustics ?
Sound Transmission Class (STC ) is an integer rating of how well a building partition (partition assembly)
attenuates airborne sound.
It is widely used to rate interior
partitions, ceilings and floors, doors,
windows and exterior wall configurations
(see ASTM International Classification
E413 and E90).
STC is considered for an assembly and
not for a particular material.
STC is calculated by taking the
Transmission Loss (TL) values tested at
16 standard frequencies over the range
of 125 Hz to 4000 Hz and plotted on a
graph.
Note: STC only considers frequencies down to 125Hz and can be misleading since sound isolation complaints are mostly from
sources below 125Hz

11. 18 ARC 7.3 BS IV
● STC rating - how much sound is blocked from going through a product
● NRC Rating - how much sound is absorbed by a product.
● The STC rating is important when trying to reduce the amount of sound entering or leaving a room.
● The NRC rating is important when trying to reduce echo and improve the sound quality in a room.
● STC rating for walls may be
different from the acceptable
STC rating for floors. The higher
the rating, the more sound is
blocked from going through the
material.
With a Sound transmission class rating of 50, speech cannot be heard through the walls, and loud sounds are only
faintly audible.

12. 18 ARC 7.3 BS IV
STC What can be heard at this level
25 Soft speech can be heard and understood
30 Normal speech can be heard and understood
35 Loud speech can be heard and understood
40 Loud speech can be heard, but not understood
45 The threshold at which privacy begins
50 Loud sounds can be heard, but are very faint
60+ At this level, good soundproofing begins.
Neighbors generally are not disturbed by very
loud speech from inside.
STC RATING CHART

13. 18 ARC 7.3 BS IV
An STC rating of 45 - get into the world of soundproofing and privacy. This could be considered a baseline
when you are getting serious about preventing sound transmission.
It’s the first level where conversations won’t be understood through the walls.
The International Building Code requires an STC of 50 for multi family construction, which is the point at
which noise is reduced to a point that people generally feel like their homes are adequately insulated
from noise.
The STC rating of walls has to do with multiple variables. Things like the thickness and air space within
the wall can improve the rating greatly.
Soundproofing - usually defined by weakest point.
A high STC rating wall can be rendered ineffective by a hollow core door, or a single pane window.
The STC rating works well for things like speech and the daily incidental sounds associated with living or working in
a space, but isn’t incredibly accurate when evaluating music or heavy machinery, since those sounds live in lower
frequencies, and will vibrate structures differently.

14. 18 ARC 7.3 BS IV
STC does not measure how many decibels a material can block
STC ratings cannot be added together.
STC ratings are calculated by measuring the transmission loss values of 16 - 18 different frequencies between
125 Hz and 4000 Hz and plotting those values as a curve on a graph. That curve is then compared to standard STC
rating curves to determine a score.
NOTE:

15. 18 ARC 7.3 BS IV
MASS LAW AND TRANSMISSION LOSS
According to the Mass Law for homogeneous building materials like glass, wood , concrete, TL and sound
transmission class rating (STC rating) increase by about 5 for each doubling of surface weight (in pounds per
sqft).
Sound Transmission Class (STC) system rates the entire TL curve at speech frequencies from 125Hz to 4000
Hz(frequency for speech range) using a standard contour as a reference.
STC is a single number rating of TL performance for a construction element tested over standard frequencies
as mentioned above.
The higher the STC, the more efficient the construction is for reducing sound transmission.
Heavier Materials provide better sound isolation - this is the fundamental principle of sound isolation for
architectural acoustics.

16. 18 ARC 7.3 BS IV
Mass law follows the law of diminishing returns.
As seen in the adjacent figure , the STC of a
homogeneous construction increases by approx 5
for each doubling of weight.
Initial doubling provides the most practical
improvement. Each successive doubling produces
less STC improvement per unit weight and a
greater increase in cost per unit STC increase.
Therefore complex constructions are required
when it is necessary to achieve high STC and TL
improvements.
Sound isolation also depends on the stiffness and mass of the materials

17. 18 ARC 7.3 BS IV
The sound isolation depends on the stiffness as well
as mass
The adjacent graph shows 2 plywoods of equivalent
total weight.
One has grooves the other is stiff.
As per Mass Law the TL should be same.
However the graph shows that the less stiff plywood
(with grooves)has higher TL performance at the mid
and high frequencies compared to the stiff one.
Stiff panels are needed if high performance is required at low frequencies.
The best barriers are heavy, high mass, limp, highly damped materials with a high weight to stiffness ratio such as our
soundproofing mats.

18. 18 ARC 7.3 BS IV
The ideal sound isolating construction would be heavy, limp
and airtight.
Graph showing TL performance based on equal
surface weight for several materials
The graph has 3 basic parts
● The low frequency mass-controlled region at about 6dB
per octave slope
● The plateau region of relatively constant TL which
depends on bending stiffness and internal damping of
the material
● And the Critical frequency and mass controlled region
above the plateau usually at 10 dB octave slope.
● The stiffer the wall, lower the plateau height (poor sound isolating performance)
● Limper the material higher the plateau height ( better sound isolating performance)
● The more damping a wall has(energy loss from internal friction) the narrower the plateau width (better sound isolating
performance)

19. 18 ARC 7.3 BS IV
A noise problem starts with a noise source such as a stream of
traffic on a highway. The noise is transmitted through a path and
then arrives at the receiver. The noise will be perceived as a
problem when the noise is so high as to be a nuisance to the
receiver.
The severity of the problem depends on the strength of the noise
source (such as heavy or light traffic) or the length of the path,
that is, how large is the separation between the noise source and
the receiver.
DESIGN PRINCIPLES FOR NOISE MITIGATION
To reduce environmental noise the following methods can be considered
1. Control at noise sources
2. Noise reduction at transmission path
3. Protection at receiver end.

20. 18 ARC 7.3 BS IV
Control at source
● Primary consideration is to reduce noise at source.
● The sound producing machinery or equipment may be enclosed in an acoustical enclosure to reduce the
noise at source.A noise enclosure for reducing machine noise is commonly made of an exterior metal
skin, an interior perforated sheet, with some absorptive materials such as fiberglass filled in between.
● Certain new and quieter technologies have enabled some construction works to be done much quieter
as compared with conventional noisy equipment. For instance, some building demolition projects have
adopted the more environmentally-friendly hydraulic crusher instead of the conventional mounted
breaker.
Noise reduction at transmission path
● Noise sources can be distanced from the receiver to mitigate noise.
● Obstruction through landscaping features like earth bund or screening with natural landscape or
structures of noise tolerant uses (like carpark , commercial spaces, )
Protection at the receiver end
● Orientation of building and noise sensitive spaces
● Specification of materials to help mitigate noise effects like glazing and insulation for acoustic purpose.

21. 18 ARC 7.3 BS IV
POOR
BETTER
OUTDOOR BARRIERS FOR NOISE CONTROL

22. 18 ARC 7.3 BS IV
BEST

23. 18 ARC 7.3 BS IV
● A self protecting building configuration -
act as screens by interrupting the acoustic
line of sight.
● Windows and doors are the weak point
as it allows sound to travel.
● Self protecting building features are
atriums, recessed floors and podium
bases.
● Balconies and overhangs can be used to
isolate surface traffic noise.
● Underside of the overhang can be treated
with sound absorbing materials
● Solid balconies and overheads- reduce
noise transmission by 5-10 dB.

24. 18 ARC 7.3 BS IV

25. 18 ARC 7.3 BS IV
Sound leaks like water and travels through any small openings.
So when sound isolation is required
● Seal cracks and open joints in partitions
● Avoid back to back electrical sockets
● Eliminate all potential sound leaks
Sound Control - Enclosures and Barriers
PARTITION LEAKS
2 layers of gypsum boards on steel studs

26. 18 ARC 7.3 BS IV
ELECTRICAL OUTLET LEAKS DOOR LEAKS
An opening of 0.01% of wall surface
area can reduce the TL from 50dB
to 39dB.

27. 18 ARC 7.3 BS IV
When a weaker element like a door or window is used in a construction the composite TL for the combination
is usually closer to the TL of the weaker element than to the stronger .
Composite TL = 10 Log ΣS/ΣτS
Where
TL = Transmission Loss (dB)
S = Surface Area (sqft)
τ = Sound transmission coefficient (No units)

28. 18 ARC 7.3 BS IV
Combination of glass and brick walls and its effect
on TL

29. 18 ARC 7.3 BS IV
NOISE REDUCTION NR BETWEEN ROOMS
NR = L1 - L2
NR = Noise reduction (dB)
L1= Sound level in source room (dB)
L2= Sound level in receiving room (dB)
NR at a given frequency is independent of the
noise level in the source room.
NR is dependent on 3 different factors
● Area of wall transmitting sound (S in sqft)
● Absorption in receiving room (a2 in Sabins)
● Transmission loss of common wall (TL in dB)
NR = TL + 10Log a2/S
Where TL= Sound transmission loss of common
barrier
a2= absorption in receiving room (Sabins)
S = Surface area of common barrier (sqft)
Note : 10Log a2/S may range between 6dB to - 6dB

30. 18 ARC 7.3 BS IV
Sound travels through indirect paths as
shown in the adjacent pictures - that is
called FLANKING. Poor workmanship can
diminish the sound isolation and cause
sound leaks

31. 18 ARC 7.3 BS IV

32. 18 ARC 7.3 BS IV
● The TL of a wall can be
increased by separating
the wall layers with
airspace- to form double
wall construction.
● TL increases with cavity
air space.
● TL further increases with
sound absorbing
material added in the
cavity air space.
● Double stud
construction breaks
direct sound
transmission through
studs.
● Further improvement
with doubling the weight
of gypsum on either
side.
DRY WALL CONSTRUCTION WITH WOODEN STUDS

33. 18 ARC 7.3 BS IV
● Heavy Gypsum board construction on each side of studs, raises the STC
level.Doubling the layer on either side of the studs can increase the STC
by more than 5.
● Direct sound transmission path to be broken by staggering the wooden
studs.
● Lightweight metal channel studs (25 gauge or lighter) provide higher
STC values since they are less stiff than the wooden studs or heavy
metal studs.
● Sound absorbing blankets to be placed in the cavity between the 2 studs
which friction fit between studs with adequate fasteners so that they do
not sag.
● Seal all cracks, joints and penetrations. Seal perimeters of bottom
runners and sole plates with continuous beads of non hardening
caulking at edges on each side.
● Isolate perimeters of common walls from ceilings, intersecting walls and
structural members. Seal with caulking.
● Stagger electrical outlets by at least 2’-3’. Seal all the edges.
● Isolate plumbing from framing elements which act as sounding boards
for noise of flowing water.
DRY WALL CONSTRUCTION WITH METAL STUDS

34. 18 ARC 7.3 BS IV
COMPLEX MASONRY WALLS FOR SOUND INSULATION

35. 18 ARC 7.3 BS IV
Floated Floor
concrete slab
Wood Joist Floor
FLOOR CEILING CONSTRUCTION
● To prevent squeaking floors in wood joist constructions - nails
must be of proper size and spacing
● Install building paper or felt layer between sub floor and
finished floor.
● Use seasoned wood.
● To prevent flanking - install joists parallel to the party walls.
● Cut sub flooring at walls(>¼ “ gap)to interrupt flanking path
for airborne sound and vibrations.
● Ceiling should be an effective barrier to sound transmission
from room to room as the common wall.
● The ceiling materials should be of increasing mass and
decreasing porosity.

36. 18 ARC 7.3 BS IV

37. 18 ARC 7.3 BS IV
PLENUM BARRIERS
Gypsum board, lead sheet and mineral
fiber can be used as vertical barriers to
prevent flanking through ceiling
plenums.

38. 18 ARC 7.3 BS IV
SOUND LOCK
● Stagger doors on opposite sides
of double loaded corridors to
prevent noise from travelling
directly.
● Stub corridors or vestibules as
sound locks.

39. 18 ARC 7.3 BS IV
SOUND INSULATION AND VIBRATION ISOLATION FROM MECHANICAL EQUIPMENT
Poor isolation Better isolation

40. 18 ARC 7.3 BS IV
Vibration isolation - install vibrating equipment on resilient materials like ribbed or waffle shaped neoprene
pads, pre compressed glass fibre pads and steel springs.

41. 18 ARC 7.3 BS IV
Both supply and return air duct should contain
flexible canvas, rubber or loaded vinyl connections
to break the vibration path.

42. 18 ARC 7.3 BS IV

43. 18 ARC 7.3 BS IV
CHECKLIST FOR DUCT BORNE NOISE

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https://www.quebec.ca/en/health/advice-and-prevention/health-and-environment/the-effects-of-environmental-noise-on-
health/definition-environmental-noise
https://residential-acoustics.com/airborne-noise-vs-structure-borne-noise/
https://www.soundproofingstore.co.uk/whats-the-difference-between-impact-and-airborne-noise
https://www.constructionspecifier.com/controlling-mechanical-system-noise/
https://ocw.upj.ac.id/files/Textbook-ARS-405-TEXTBOOK-13-FISIKA-BANGUNAN.pdf
https://www.epd.gov.hk/epd/noise_education/web/text/ENG_EPD_HTML/m4/mitigation_1.html
https://www.soundassured.com/blogs/blog/what-is-transmission-loss-of-sound
https://amorimcorkcomposites.com/en/materials-applications/construction/knowledge-center/glossary/
https://www.acousticalsurfaces.com/blog/acoustics-education/sound-transmission-class-stc-rating/
https://www.soundproofingcompany.com/soundproofing_101/understanding-stc-and-stc-ratings
https://www.secondskinaudio.com/sound-blocking/stc-rating/
http://www.domesticsoundproofing.co.uk/sound_transmission_loss.htm