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Demystifying the Science of Sound: Practical Acoustics for Building ServicesDr Emmet English
G2 The Steelworks, Dublin 1
www.resonate-consultants.com
Dr Emmet English vs Dr Emmet Brown
Overview
An introduction to sound
Sound transmission through partitions
Building services noise
Sound power vs sound pressure
Controlling noise and vibration
Good and bad practice
What is sound
Sound is simply a pressure wave in air
travelling at approximately 340 m/s.
Frequency refers to ‘how often’ the air
particles vibrate
Range of audible sound is approximately
20 Hz to 20,000 Hz
Hertz (Hz) = 1 cycle per second
What is Acoustics?
Sound (Good) Noise (Bad) Vibration (Bad)
Effects of Noise
Noise Induced Hearing
Loss
Environmental Health
Hazard
Sleep Disturbance
Hypertension
Cognitive Impairment in
Children
Focus, Stress, Privacy,
Annoyance
Standards and Guidelines
EN ISO BS Standards (Recommendations)
ASHRAE Handbook (Chapter 48)
EPA Policy
BREEAM/LEED/HPI Criteria
Project Specific Criteria – i.e. Universities, Hotels, Hospitals etc
Acoustic Criteria• Environmental noise level
• Internal ambient noise level
• Sound isolation between spaces
• Reverberation time within the spaces
The Basics
Decibels
The human ear has incredible sensitivity and
dynamic range!
2 to 3 dB change is barely perceptible
5 dB change is easily noticeable
10 dB change is a doubling or halving of
loudness
Ref.
+1dB
+2dB
+3dB
+5dB
+10dB
Adding Noise Sources: 1+1=3
+= 1010 1010log10
BA
pL
Adding Noise Sources: 1+1=3
40dBA
Adding Noise Sources: 1+1=3
43dBA
Adding Noise Sources: 1+1=3
46dBA
Adding Noise Sources: 1+1=3
49dBA
Adding Noise Source:
40 + 40 = 43 (equal numbers add 3)
40 + 37 = 42 (difference of 2 – 4, add 2)
40 + 32 = 41 (difference 5 – 10, add 1)
40 + 25 = 40 (difference >10, add 0)
40 + 40 + 40 = 45 (3 equal numbers add 5)
40 + 40 + 40 + 40 = 46 (4 equal numbers add 6)
What is frequency?
0
-20
-10
-30
-45
-5
-25
-40
-15
-35
-50
31.5
One-third octave band centre frequency (Hz)
Level w
eig
hin
g (
dB
)
63 1k250 4k125 2k500 8k 16k
A
B
C
100Hz 1,000Hz 10,000Hz
Noise Descriptors
Standard Noise Descriptors
• A-weighting (dBA), C-weighting (dBC),
Building Services CIBSE/ ASHRAE Guides
1. Perceived Loudness
2. Sound Quality
• Noise Criteria Curve (NC)
• Noise Rating (NR)
• Room Criteria (RC)
• Room Criteria Mark II (RC 2)
Building Acoustics
© RICHARD BARRETT
Acoustics in Building Design
Architecture
ServicesStructure
Acoustics
&
Vibration
Sound wave interaction with structures
Noise impinging on a wall
Transmitted
Absorbed
Reflected r
IncidentStructure-borne
to other connected
elements
Incident
Diffraction
Shadow zone
Sound transmission
The extent sound is transmitted between adjacent spaces is defined both airborne and impact (structure-borne) sound
Airborne Sound Insulation is defined in:
▪ Rw (Laboratory Value used for Design),
▪ DnTw (Site Value, includes Flanking and Build Quality).
Impact Sound Insulation defined as
▪ Ln,w - Lab Value
▪ L’nt,w - Site Value
All values have units in dB!
Sound transmission factors
• Mass, density
• Stiffness
• Thickness
• Vibration isolation of skins
• Size of air gap between skins
• Structure borne noise
• Flanking
Flanking Transmission
© TMSoundproofing
Auralisation –
Walls To illustrate the impact acoustic decisions
play on project costs and outcomes.
Dntw Voice through
acoustic wall
35
40
45
50
55
Acoustics rated in DnTw ratings:
- Mass, density
- Stiffness
- Thickness
Example Partitions
DnTw 35 DnTw 45 DnTw 55
Reverberation
In an enclosed space, reflections will occur from the surfaces and a reverberant
field will be established in addition to the direct field
The absorption coefficients of the enclosing walls will determine the strength of
the reverberant field.
Reverberation
Glass/Concrete
Fully Absorptive
Plasterboard and acoustic tiles
Absorptive
Treatments
Sound Absorption
Sound Absorption is important in reducing the reverberation in a Room.
It can be introduced by Acoustic Ceiling Tiles and Wall Panels
Low reverberation times are important when clear communication is desired
e.g. Schools, Open Plan Offices, Conference rooms
Soft furnishings and carpets also help reduce the reverberation.
Note: Sound Absorption is different to Sound Insulation!
Building Services Noise
HOW BUILDING SERVICES AFFECT THE ACOUSTIC
ENVIRONMENT
• Services noise inside
• Services plant outside
• Hydraulic noise
• Sound insulation of walls (penetrations and
connecting aircon)
Power vs PressureSound Power - The rate at which a sound source emits energy.
Sound Pressure - The noise level measured at a distance
Both use the unit dB!
Understanding Data SheetsSound Data available for most mechanical plant:
Tips and Tricks
• Always specify a level that was used for design
• Specify a criteria Noise Rating & dB(A)…..
• Check if it is SWL or SPL?
• Look at the frequency content?
• Check if it is inlet, outlet, radiated?
• Never accept SPL without a distance!
Duct Acoustics
This Photo by Unknown Author is licensed under CC BY-SA-NC
Ducts and AcousticsWhen designed correctly ductwork can provide good attenuation.
Longer duct runs and internally lined ducts work best.
Ductwork needs to have flexible connections.
In-Duct Attenuation – Rough Guide
Element Size Low Freq High Freq
Duct 600mm sq x 1200, 50mm insulation 1 - 2 dB 10 dB
Bend 600mm sq, 50mm insulation 0 - 1 dB 10 dB
End Reflection 600mm sq 4 - 8 dB 0 dB
Branch Depends on takeoff size 1 - 10 dB 1 - 10 dB
Plenum 1.2 m cube, 600 x 600mm openings 7 - 8 dB 11 dB
Proprietary attenuator 600mm sq x 1200mm 5 - 10 dB 15 - 30 dB
Lined ducts and diffuser plenums
Acoustic Attenuators
Plenum Chamber
What to watch out for:• Size
• Regenerated noise
• Backpressure?
• Where is it located (i.e. above a meeting room)?
Piping
• Excessive fluid flow in pipes
• Primarily a structure-borne issue
• Inadequate vibration isolation
• Unsealed penetrations
• Direct contact with wall or floor
Piping
Pipework Noise Control
Service Penetrations
• Penetration gaps are a common
cause of sound transmission.
• Gaps less 10mm can be filled with
non setting flexible mastic.
• Larger holes require packing or a
pipe sleeve
Penetration Detail
Services IsolationMechanical plant needs to be isolated from structures to reduce
structure-borne noise.
Tips
• Vibration isolate all reciprocating and rotating equipment
• Specify inertia bases as required
• Vibration-isolate all ducts and pipes for at least 15 metres
from vibration isolated equipment
Sensitive Spaces
Lab Equipment
EMI, Noise and Vibration can affect
sensitive equipment, manufacturing
processes or specialist spaces
MRI Machines
This Photo by Unknown Author is licensed under CC BY-NC-ND
Site Examples
Isolator Types
Isolator Types
Cross talk
Isolator Types
Summary
Building acoustics design standards and regulations normally specify:
Reverberation times for different spaces
Sound Insulation Values for partitions, facades etc
Internal Ambient Noise Levels for different spaces
Sound insulation controls sound transmission through partitions and floors.
Sound absorption controls the reverberation (echo in a room).
Single number ratings are commonly presented in dB which can cause confusion.
Sound Power and Sound Pressure are different! Never accept a sound pressure level without a distance.
The noise level of a system can be predicted at design stage and noise control specified for the individual system
EMI, noise and vibration can affect sensitive equipment, manufacturing processes or specialist spaces
Questions
G2 The Steelworks, Foley Street, Dublin 1,
p+353 1 442 8991
Isolator Types
