Transcript E-WM-2 - orbee

Environmental Science
By Tim South – licensed under the Creative Commons Attribution – Non-Commercial – Share
Alike License
http://creativecommons.org/licenses/by-nc-sa/2.5/
Why it matters
Tim South
Leeds Metropolitan University
What are the acoustic issues in a
completed building?
Sound insulation
– Between dwellings
– Between rooms in a building
Reverberation in rooms
Internal noise levels
– From building services
– From outside
Noise emitted from the building
Any extra issues during
construction?
Noise exposure of the workforce
Hand-arm vibration
Construction noise
Vibration eg from piling
Objectives
By the end of this lecture you should be able to:
• Understand the properties of sound.
• Identify the various noise problems that
arise in buildings. Understand how
reflection, absorption and reverberation
affect room acoustics.
• Understand how good sound insulation can
be achieved.
• Be aware of the requirements of Building
Regulations Part E.
What is sound?
A disturbance in the atmosphere
Travels in three dimensions
Pressure fluctuations are small
Sound is measured in pascals (Pa); 1
pascal is equivalent to 1 newton per
square metre
Normal atmospheric pressure is about
100,000 Pa
A noisy environment may involve a sound
pressure of about 1 Pa
What is sound?
Sound pressure is the deviation from
atmospheric pressure due to the passage
of a sound wave
A 1% fluctuation is caused by a very loud
sound indeed.
For comparison, normal weather
variations may cause a pressure change of
3,000Pa or 3% in the course of a day
Noise is the same as sound
It is normally called noise if it is unwanted
Sound waves
Are longitudinal waves
ie the chunks of air move backwards
and forwards in the direction the
wave travels
This is very difficult to draw
So in practice they are often
represented as graphs of pressure
against either time or distance
Frequency and wavelength
The frequency of a
wave is the
number of waves
arriving at a fixed
point in one second
Normal symbol f
The unit of
frequency is the
hertz (formerly the
cycle per second)
The wavelength is the
length of one
complete cycle of the
wave
Normal symbol 
The unit of
wavelength is the
metre
Frequency
The human ear can hear sound with
frequencies from 20 Hz to 20,000 Hz
We are most sensitive to the middle range,
particularly 1-3 kHz
These frequencies are also the most
damaging
A system called A weighting is used for
many measurements to approximate
human hearing
Velocity, frequency and
wavelength
If you multiply the frequency of any wave
by the wavelength, you get its velocity
The velocity of sound in air varies with
temperature
At “normal” temperatures it is about 340
ms-1
So frequency can easily be converted to
wavelength and vice versa
Audible wavelengths are from a few
metres to a few millimetres
Wave properties
Sound waves demonstrate the
following properties, which are
common to all waves;
Refraction
Diffraction
Reflection
Interference
Refraction
Waves bend when passing from one medium to
another
They bend towards the normal if their velocity is
greater in the
Normal
second medium
Observer
Water looks
Air - higher speed
shallower than
it is
Water - lower speed
Apparent depth
Actual depth
Refraction of sound waves
The velocity of sound is different in layers
of air at different temperatures
Normally air temperature decreases with
height, and sound waves bend away from
the ground
A temperature inversion causes them to
bend towards the ground. Sound is the
audible at much greater distances.
Diffraction
The tendency of waves to bend round corners
More pronounced at long wavelengths
Not always obvious with light (but try
squinting at a sodium street light)
Sound waves have longer wavelengths and
readily bend round corners
So you can hear a fire alarm in the corridor
Or someone talking about you in the next
room
Limits the effect of roadside noise barriers
Reflection
Familiar in the case of light waves
Surfaces may reflect in various
ways;
– Partial
– Total
– Specular
– Diffuse
Focusing by a concave mirror
Reflection of sound waves
Noise levels increased if there is a hard
surface behind the source
A large building can make an aircraft
appear to be in the opposite direction
Sound levels in rooms a result of multiple
reflections from surfaces
Concave surfaces can focus sound
Surfaces may reflect different frequencies
to different extents
Interference
Two sound waves can combine to set
up a pattern of standing waves
Particularly at low frequencies
This is the principle on which some
musical instruments work
It can cause problems in rooms
The decibel scale?
“Everyone” knows we measure sound
levels in decibels
Few people know much more about the
scale
It is based on logarithms, so
calculations can be complicated
The reasons for using this scale are not
very clear
But everyone else uses it, and…
…you get nice numbers (0-100 ±½)
Sound pressure level
 p
Lp  20  log  
 p0 
Lp is the sound pressure level
Measured in decibels
Some other quantities are measured in decibels
too
Lp is still sometimes called SPL
Addition of decibels
The decibel scale makes addition a bit
complicated
Suppose we measure the sound pressure level
at a point with one source operating – L1
Then we switch off the first source, switch on a
different one and measure the sound pressure
level at the same reception point – L2
What sound pressure level Lp would be
measured at that point if both sources operated?
Graphical addition
Addition of Decibels
3
2.5
2
dB to be added to
higher level
1.5
1
0.5
0
1
0
3
2
5
4
7
6
9
8
11
10
13
12
Difference in levels/dB
15
14
17
16
19
18
20
Graphical addition - example
Two noise sources
individually cause
Lps of 86 and 88
dB at a point.
What Lp will result
from both sources
simultaneously?
Addition of Decibels
3
2.5
2
dB to be added to
higher level
1.5
1
0.5
0
1
0
3
2
5
4
7
6
9
8
11
10
13
12
Difference in levels/dB
Add 2 dB to the higher
level; 88 = 2 = 90 dB
88-86 = 2dB
15
14
17
16
19
18
20
Simple addition of decibels
We have a noise source operating and the
meter reads 80 dB
If we add an identical noise source at the
same position, the meter reading will go
up to 83 dB
In general,
– Doubling the noise sources adds 3 dB to Lp
– Halving the sources reduces Lp by 3 dB
Sound power level
The sound power level
(LW) is a measure of
the noise emitted by a
source
Not the same as
sound pressure level
Often labelled on
outdoor equipment
Can be used to make
Lp predictions
Point sources
Small compared with their distance
In the free field, sound levels fall by 6 dB every
time you double the distance
Line
sources
Extended in one
dimension
In the free field,
sound levels fall by
3 dB every time
you double the
distance
Prediction of sound levels
Radiated from a point source outdoors
Lp = LW - 20logr -11
Radiated from a point source outdoors
Lp = LW -10logr - 8
Indoors, there are multiple reflections from
surfaces and predictions are more
complicated
Acoustics in buildings
Sound insulation - between rooms
Sound absorption - within a room
The two ideas are often confused by nonspecialists
Particularly in complex structures (eg
partition walls), the absorption and
insulation work together to control noise
Sound absorption
Panel materials (absorb at low
frequencies)
– eg plasterboard walls
Porous materials (absorb at high
frequencies)
– Stability of porous surfaces is a problem
Traditionally absorption comes mainly
from suspended ceilings
Combination absorbers can be “tuned” to
requirements across the frequency range
Reverberation
Reverberation time (RT) is a measure of
how long it takes sound to die away
It is the simplest measure of acoustic
conditions inside a room
Sound absorbing surfaces tend to reduce
the RT
Reflective surfaces tend to increase it
Reverberation times
Rooms for speech should have an RT less
than 1s (<0.8s required for school
classrooms)
For music the RT should be longer
(depends on the type of music)
Long RTs reduce speech intelligibility
Very short RTs stop sound propagating
around the room
Reverberation times
Rooms for speech should have an RT less than 1s (<0.8s required
for school classrooms)
For music the RT should be longer (depends on the type)
Long RTs reduce speech intelligibility
Very short RTs stop sound propagation around the room
Reverberation times
Rooms for speech should have an RT less than 1s (<0.8s required
for school classrooms)
For music the RT should be longer (depends on the type)
Long RTs reduce speech intelligibility
Very short RTs stop sound propagation around the room
Sound insulation
Traditionally insulation was
achieved by high surface mass
and high integrity
There is a move from masonry
walls towards lightweight
structures
Combining sound insulation and
sound absorption principles
means lightweight structures
can perform better than masonry
structures
Sound insulation
Laboratory
measurements
Field measurements
Two common quantities
Rw
Weighted sound
reduction index
Measured in a
laboratory
Relates to a particular
material or product
DnT,w
Weighted,
standardised level
difference
Measured in a real
building
Used to specify
building performance
The difference between Rw and
DnT,w depends on
Room shape and size
Weak spots where two building elements
join
Workmanship issues
Flanking transmission
Sound insulation
Often the sound insulation is decided by
the weak component in a partition. This
may be;
– There by design (eg a door)
– Due to faulty design or workmanship (eg a
continuous cavity above a suspended ceiling)
– Difficult to avoid (eg ventilation ducts)
Air leakage paths – an illustration
Air leakage paths – an illustration
Air leakage paths
Sound
insulation
in decibels
Good
walls
50
45
Improved
Building Regs
Original
40
Effect of small air gaps on the overall sound
insulation performance of a 3.0 metre high wall
45.0
40.0
Rw=25dB
Rw=30dB
35.0
Rw=40dB
30.0
25.0
Rw=50dB
Air gap in millimetres
20
18
16
14
12
10
8
6
4
2
20.0
0
Overall sound
reduction index
50.0
Flanking transmission
Direct sound
Flanking
transmission
Direct sound
Flanking paths become important once the
direct sound has been reduced
May need to introduce structure breaks etc
A continuous floor slab (as shown) would
not be allowed for party walls
Improving sound insulation
Increasing sound insulation performance
Increase
mass
Eliminate
gaps
Reduce
flanking
Sound insulation - door
considerations
A door plus frame will typically be rated at
about 28 dB of sound insulation
If it takes up 10% of the surface area, then
however good the wall system the
maximum sound insulation is 38 dB.
This assumes perfect installation
Doors should never be installed between
occupied rooms
Sound insulation is dead simple
L´nT,w
Dn,e
DnT
Dw
R´w
Rtr,s
D2m,n
Rw
Ctr
DnT,w
DnT,w+ Ctr
Part E
The Building Act
The Building Regulations 2000, as
amended in 2002
Approved Document E - 2003 edition
The law applying in Scotland is different
Purpose
“…securing reasonable standards of health
and safety …(and welfare and
convenience) …for persons in and about
the building.”
(Other regulations include energy
conservation and prevention of water
contamination in their purposes)
Regulation E1 (2002 version)
Dwelling-houses, flats and rooms for
residential purposes shall be designed and
constructed in such a way that they
provide reasonable resistance to sound
from other parts of the same building and
from adjoining buildings.
Regulation E2 (2002 version)
Dwelling-houses, flats and rooms for
residential purposes shall be designed
and constructed in such a way that
(a) internal walls between a bedroom or a
room containing a water closet , and
other rooms, and
(b) Internal floors,
Provide reasonable resistance to sound.
Regulation E3 (2002 version)
The common internal parts of buildings
which contain flats or rooms for residential
purposes shall be designed and
constructed in such a way as to prevent
more reverberation around the common
parts than is reasonable
Regulation E4 (2002 version)
(1) Each room or space in a school building shall
be designed and constructed in such a way
that it has the acoustic conditions , and the
insulation against disturbance by noise
appropriate to its intended use.
(2) For the purpose of this part – “school” – has
then same meaning as in section 4 of the
Education act 1996, and “school building”
means any building forming a school or part of
a school.
Buildings and
structures
covered
Residential buildings
Schools
Party walls
(airborne sound)
Party floors and stairs
(airborne and impact
sound)
Some internal walls
Common areas
Buildings and
structures not
covered
Hospitals, offices,
prisons
Most internal walls
within a dwelling
External walls
Ways of complying with the
Regulation – converted
buildings
Test 10% of party walls and floor as
they are completed
Internal walls and reverberation in
common areas are approved on the
basis of the plans submitted (no testing)
Ways of complying with the
Regulation – New buildings
Test 10% of party walls and floor as
they are completed
Or
Use Robust Details
Internal walls and reverberation in
common areas are approved on the
basis of the plans submitted (no testing)
Ways of complying with the
Regulation – School buildings
Submit plans to the building inspector
showing how the requirements are to
be met. They are approved on the
basis of the plans submitted (no
testing)
Standards for sound insulation –
new buildings
Airborne DnT,w+ Ctr ≥ 45 dB for walls and
floors in flats and houses
Airborne DnT,w+ Ctr ≥ 45 dB for floors in
rooms for residential purposes
Airborne DnT,w+ Ctr ≥ 43 dB for walls in
rooms for residential purposes
Impact LnT,w≤ 62 dB in flats, houses and
rooms for residential purposes
Standards for sound insulation –
converted buildings
Airborne DnT,w+ Ctr ≥ 43 dB for walls and
floors in flats and houses
Airborne DnT,w+ Ctr ≥ 43 dB for floors in
rooms for residential purposes
Airborne DnT,w+ Ctr ≥ 43 dB for walls in
rooms for residential purposes
Impact LnT,w≤ 64 dB in flats, houses and
rooms for residential purposes
Standards for sound insulation –
internal walls and floors
Rw ≥ 40 dB in each case
Reverberation standards
Cover an area equal to the floor area with an
absorbing material meeting absorption class D
or better
Or 50% of the floor area with an absorber
meeting absorption class C or better
Or
Make sure that from 250 Hz to 4 kHz the
absorption area is at least 0.2 m2 per cubic
metre (for entrances) or at least 0.25 m2 per
cubic metre (for corridors or hallways)
Conversions
In the case of a historic building it may not
be practical to meet the required standards
within the requirements of the listed Building
Regulations
In this case it may be agreed that the sound
insulation performance is measured and is
then declared via a notice fixed to the building
in a conspicuous place.
BB93 requirements for school
buildings - sound insulation
between rooms
Rooms are divided (table 1.1) into
categories according to
– how much noise they generate
– How sensitive they are to noise
Sound insulation is specified as DnT,w
A value of DnT,w is specified for each pair
of rooms - table 1.2
BB93 DnT,w requirements
Classroom - classroom
45 dB
Music room – music room
55 dB
Hall – drama studio
55 dB
Between open-plan teaching 40 dB
areas
Sound insulation between
rooms and circulation areas
Very difficult to measure, so this is
specified in terms of Rw (ie manufacturer’s
data)
Two specifications - music rooms and
everywhere else
Normally determined by the size and
specification of the door
Robust Details
The Robust Details are contained in a
manual published by Robust Details Ltd
Plot must be registered with Robust
Details in advance
RD inspectors can check on the work
Sample testing to monitor each RD
No completion testing on each
development
Robust details exist for
Walls
– Masonry
– Steel
– Timber
Floors
– Concrete
– Timber
– Steel-concrete composite
Robust Details
Detailed requirements
Particularly for junctions
Use generic materials
– In theory
Specifications include density etc
Checklists
Example – masonry walls
Separating walls - masonry
E-WM-1
E-WM-2
Masonry - dense aggregate blockwork (wet plaster)
Masonry - lightweight aggregate blockwork (wet plaster)
E-WM-3
Masonry - dense aggregate blockwork (render and gypsum-based
board)
E-WM-4
Masonry - lightweight aggregate blockwork (render and gypsum-based
board)
E-WM-5
Masonry - Besblock "Star Performer" cellular blockwork (render and
gypsum-based board)
E-WM-6
Masonry - aircrete blockwork (render and gypsum-based board)
E-WM-7
Not Currently Available
E-WM-8
Masonry - lightweight aggregate blockwork British-Gypsum-Isover
ISOWOOL (gypsum-based board)
Robust Details problems
Checklists
Some bad failures
Detailing – recent advice on mortar in
cavity walls
Problems with the Building
Regulations
Was the move from DnT,w to frequency
noise) DnT,w+ Ctr justified?
Is Regulation E too limited in scope?
Do they discourage the development of
new materials and techniques which
provide higher levels of sound
insulation?
Do Building Inspectors have the
necessary specialist expertise?
Further information
Approved Document E. HMSO, 2003
Resistance to the Passage of Sound
http://www.planningportal.gov.uk/uploads/br/
BR_PDF_ADE_2003.pdf
DfES (2003). BB93; Acoustic Design of
Schools TSO
Smith, Peters and Owen (1996) Acoustics
and Noise Control 2nd Edition. Longman