Building Environment 1: Acoustics (conclusion)
David Coley (6E2.22, [email protected])
Topics
1. How loud is a dB?
2. Adding dBs
3. Accounting for the sensitivity of the ear
4. Sound outdoors
5. Sound indoors
6. Keeping sound out
1. How loud is a dB?
Sound pressure levels of common noises
dBA
THRESHOLD OF PAIN 120
Disco noise 105
Full orchestra, loud passage 95
Working environment without ear defenders (8-hour day)
The decibel can be used as an absolute measure of how loud sound is,
with values between 0 and 120 dB
It can also be used as a relative measure, say for a car silencer
Silencer
Acoustic efficiency
15 dB
Note: a doubling of intensity (energy) = +3 dB
How loud is a decibel?
Minimum audible change 1 3 dB
Minimum change worth spending money on 5 dB
Subjective doubling of loudness ~ 10 dB
2 twice
Threshold of hearing and frequency
Music and speech: frequency and level
Loudness measurement using the dBA
We do this by introducing a filter which means the sensitivity to low frequencies is less
(the filter is similar to a filter used to convert a tungsten-light image for a daylight film)
We need a number which represents how loud we judge a sound to be
Our ears are less sensitive to low frequencies
2. Adding dB
Procedure for combining two sound levels
0 2 4 6 8 10
+3
+2
+1
0
Difference between the two levels in dB
Ad
d t
o t
he
hig
he
r le
ve
l d
B
Example: (60 dB) + (62 dB) = 64.1 dB
+ =
Decibel addition of last row gives 63 dBA
Frequency (Hz): 63 125 250 500 1000 2000 4000Hz
True spectrum 78 69 59 58 59 55 48
dBA correction -26 -16 -9 -3 0 +1 +1
dBA spectrum 52 53 50 55 59 56 49
Octave
band
frequencies
dBA Linear (unfiltered)
3. Accounting for the sensitivity of the ear the dBA
Calculating dBA value
1. Apply dBA corrections to octave spectrum
2. Add octave levels together to get dBA value, use graph for decibel addition of
pairs of dB values. Start with smallest numbers first.
3. 50 and 49 are the smallest, difference = 1 dB, from graph add 2.5 dB to largest
giving 52.5 dB.
4. Repeat process until you arrive at a single value, the dBA.
Building Environment 1: Acoustics (conclusion)
Frequency (Hz): 63 125 250 500 1000 2000 4000
Level 78. 69 59 58 59 55 48
dBA correction -26 -16 -9 -3 0 +1 +1
A-weighted level 52 53 50 55 59 56 49
Combining pairs 52 52.5
53 55.3 57.3 55 59.3 56 61.0 59 dBA value: 63.1
Situation and sound source
sound power
Pac
watts
sound power
level Lw
dB re 1012 W
Rocket engine 1,000,000 W 180 dB
Turbojet engine 10,000 W 160 dB
Siren 1,000 W 150 dB
Heavy truck engine or
loudspeaker rock concert 100 W 140 dB
Machine gun 10 W 130 dB
Jackhammer 1 W 120 dB
Excavator, trumpet 0.3 W 115 dB
Chain saw 0.1 W 110 dB
Helicopter 0.01 W 100 dB
Loud speech,
vivid children 0.001 W 90 dB
Usual talking,
Typewriter 105 W 70 dB
Refrigerator 107 W 50 dB
4.1 Sound outdoors: sound power
Sound in free space
Many sound sources can be considered as point sources. A good example is a human speaker.
If you think of a short burst of sound, the energy will spread out from the source at the speed of sound. The energy will be
concentrated on a spherical shell whose surface area is 4r2.
This leads directly to the inverse square law, which is also relevant to lighting, heat radiation etc.
r1
r2
In decibels, we get L2 = L1 20.log ( r2/r1) dB, where r1 and r2 are
distances from the point source. L1 is the sound level in dB at
radius r1.
If r2 is twice r1, then we have a 6 dB reduction per doubling of distance
NOTE: log10 2 = 0.3, so 10.log 2 = 3 dB
10.log 4 = 10.log 22 = 20.log 2 = 6 dB
4.1 Sound outdoors: sound pressure
Learn the two equations in red
Free field: SPL2=SPL1 20log10(R2/R1)
For a sound with a given sound power (SWL) at
distance R: SPL=SWL-20log10(R)-11
On a reflecting surface, e.g. concrete: SPL=SWL-20log10(R)-8
Line source, such as a road: SPL=SWL-20log10(R)-5
What happens to sound in here?
Durham Cathedral
Borrow one of these
5. Sound indoors
In a space like a cathedral, sound persists but gradually gets quieter over perhaps 6 7 seconds.
Bath Abbey
Reverberation occurs in all rooms, but in smaller rooms which include porous, sound absorbing
materials, reverberation is much less obvious
Simple experiments show that stone reflects sound, so sound must have been bouncing
between surfaces in the cathedral, most of
which are of stone.
Sound travels 2.4 km in 7 seconds, where has it been?
The behaviour is known as reverberation and the reverberation time in Durham Cathedral is
about 7 seconds.
Behaviour of one ray of sound
Source
Many, many rays of sound leave a source, so the picture of sound in a room quickly becomes complicated.
Sound reflection from a smooth surface obeys the law of
reflection which also applies to light.
The energy in a ray diminishes as it travels due to
spherical radiation (inverse square law) and absorption
at room boundaries.
Very quickly there are so many reflections that hit us that we
can not consider them individual echoes but a reverberant
field.
The reverberation time of a space is defined as the time
taken for a loud sound to become almost inaudible (more
precisely to drop by 60dB). This can be measured, or
calculated easily.
Sound absorbing materials
Most sound absorbing materials are porous, that is they have pores.
Typical examples are fabrics: curtains
carpets
clothing and people
acoustic ceiling tiles
BEWARE: these last three materials also have thermal properties, they are
thermal insulators as well as being acoustic absorbers. Insulation and absorption
are different things !
The most efficient sound absorbers are:
mineral wool
fibreglass
open cell foam
A sound absorbing material
A cheap and cheerful sound absorbing panel!
Perforated finishes are also acceptable
Others use holes with a void behind
The effects of adding absorbing material
Adding sound absorbing material reduces the persistence of sound, in technical terms it reduces the reverberation time
Reverberation time (Tr) is proportional to the volume/amount of absorbing material. A cathedral has a large volume and little sound absorbing material.
A domestic living room has a small volume, a relatively large amount of
absorbing material and so has a short reverberation time (about 0.5 seconds).
We need a short reverberation time for speech, about 0.8 seconds for a lecture theatre or classroom. We may need to add absorbing material in this case.
Acoustic ceiling tiles are a common solution.
Tr (seconds) = (0.16 x room volume) / (sum of (area
of surfaces x mean absorption of surfaces))
=0.16
The effects of adding absorbing material
A space which includes absorbing material is quieter.
The larger the amount of absorbing material, the lower is the sound level in a room,
except that there is always sound coming directly from the source of sound.
Thus, absorbing material is often useful in
large public spaces, like atria.
Terminal at
Bristol Airport
Absorbing
material
The effects of adding absorbing material (3)
Adding absorbing material suppresses sound reflections.
Adding sound absorbing material in rooms has two effects:
Reduces sound level
Reduces reverberation time
In some circumstances, these effects can conflict. For instance, in a large volume
to be used for speech, we need to add absorbing material to bring down the
reverberation time but can end up having too quiet a speech level
The absorption coefficent () is a number from 0 to 1
and supplied by the manufacturer.
Change in background noise = 10log10 (Safter/Sbefore)
Where S is the absorption of the space = A.
This means that twice the absorption will reduce the background noise of 3dB. If the space had little
absorption initially, the results of adding absorbing
material will be dramatic.
Reverberation is only part of the story: Echoes to watch out for
Early reflections
70 ms
at 340 m/s
= 23.8 metres
Solution: a acoustically soft back wall.
Flutter echoes
If no scattering sound can bounce back and
forth and miss being
absorbed
Solution: either acoustically soft wall,
or add some scattering
Reverberation is only part of the story: Noise
Is an issue if:
Noise is excessive; or
Communication is unclear.
Solution:
Remove source
Enclose source
Tackle reverberation
Improve speech intelligibility (possibly via
loudspeakers).
A major misunderstanding To use absorption and insulation interchangeably
Sound absorption and sound insulation are not the same
6. Keeping sound out
Incident
Reflected
Porous
material
Transmitted Incident
Absorption Transmission
Absorption = NOT reflection Poor transmission = Good insulation
Effect of absorption is seen on same Effect of insulation is seen on the other side as the incident sound side to the incident sound
Intelligibility and concentration
Place/activity Qualification Sound pressure level (dB(A)) (measured over a
suitable period)
Optimal Maximum
Factory Very low 75 80
Cleaning Low 65 75
Reception Moderate 55 65
Laboratory Reasonable 45 55
Teaching/study High 35 45
Sound insulation
Mineral wool or fibreglass are good sound absorbers but
not good sound insulators.
A massive partition is a good sound insulator
Mineral wool will be a good insulator if the sound passes
through it many, many times: how can we do this? create a cavity
Sound insulation: just subtract the dBs
30 dBA is reckoned to be acceptable for a good nights sleep
If you open a window, the transmission loss falls to about 15 dB
30 dB 70 dB
Sound insulation is measured in decibels: Transmission Loss (TL)
Sound Reduction Index (SRI)
Transmission Loss = 40 dB
Bedroom Busy road
Sound insulation: the mass law
For single partitions (without holes) the mass per unit area
determines transmission loss S
ou
nd
red
ucti
on
in
dex (
dB
)
The Mass Law for insulation by single partitions
Approximately 5 dB increase per doubling of mass
Sound insulation: examples
Examples of sound reduction index/transmission loss
Width (mm) Mass per unit area
kg/m2 Transmission
loss (dB)
Window 10% open - - 10
Single glazing 4 10 20
Double glazing 108 20 30
Independent stud partition 125 (25) 17 40
Cavity brick 280 350 50
Transmission loss figures have been rounded
Sound insulation
10%Glazing
10%
opened
of incident
energy passes through
= -10 dB
1
10
Singleof incident
energy passes through
= -20 dB
1
100
glazing
of incident
energy passes through
= -30 dB
1
1000
Acoustic
double
glazing
100mm
Acoustic impedance
Sound insulation: example
Plasterboard with
100mm spacing on
independent timber
of incident
energy passes through
= -40 dB
1
10,000
2 x 115mm brick
with 50mm cavity
studding
of incident
energy passes through
= -50 dB
1
100,000
Sound transmission class: 1 set the requirement
STC What can be heard
25 Normal speech can be understood quite easily and distinctly through wall
30 Loud speech can be understood fairly well, normal speech heard but not understood
35 Loud speech audible but not intelligible
40 Onset of "privacy"
42 Loud speech audible as a murmur
45 Loud speech not audible; 90% of statistical population not annoyed
50 Very loud sounds such as musical instruments or a stereo can be faintly heard; 99% of
population not annoyed.
60+ Superior soundproofing; most sounds inaudible
http://en.wikipedia.org/wiki/Sound_tra
nsmission_class)
2. Find a construction that meets the requirement
STC Partition type
33 Single layer of 1/2" drywall on each side, wood studs, no insulation (typical interior wall)
39 Single layer of 1/2" drywall on each side, wood studs, fiberglass insulation
44 4" Hollow CMU (Concrete Masonry Unit) [2]
45 Roxul Safe'n'Sound Insulation installed between wood 2 x 4 studs on 16" centers and 5/8" drywall (type x ) on
each side with resilient channels at 16" on one side
45 Double layer of 1/2" drywall on each side, wood studs, batt insulation in wall
46 Single layer of 1/2" drywall, glued to 6" lightweight concrete block wall, painted both sides
46 6" Hollow CMU (Concrete Masonry Unit)
48 8" Hollow CMU (Concrete Masonry Unit)
50 10" Hollow CMU (Concrete Masonry Unit)
52 8" Hollow CMU (Concrete Masonry Unit) with 2" Z-Bars and 1/2" Drywall on each side
52 Roxul Safe'n'Sound Insulation installed between steel 2 x 4 studs on 24" centres and 5/8" drywall (type x) on
each side
54 Single layer of 1/2" drywall, glued to 8" dense concrete block wall, painted both sides
54 8" Hollow CMU (Concrete Masonry Unit) with 1 1/2" Wood Furring, 1 1/2" Fiberglass Insulation and 1/2"
Drywall on each side
55 Double layer of 1/2" drywall on each side, on staggered wood stud wall, batt insulation in wall
59 Double layer of 1/2" drywall on each side, on wood stud wall, resilient channels on one side, batt insulation
63 Double layer of 1/2" drywall on each side, on double wood/metal stud walls (spaced 1" apart), double batt
insulation
64 8" Hollow CMU (Concrete Masonry Unit) with 3" Steel Studs, Fiberglass Insulation and 1/2" Drywall on each
side
72 8" concrete block wall, painted, with 1/2" drywall on independent steel stud walls, each side, insulation in
cavities
Attenuation of common wall constructions (STC). (Source: http://en.wikipedia.org/wiki/Sound_transmission_class)
There may be more than one transmission
path: Flanking
The role of holes
Rtotal=-10log[(1/Stotal)(S1 10-R1/10 + S2 10
-R2/10)]
where Stotal = S1 + S2, i.e. the sum of the two areas.
So if the wall area is 20m2 with R1 = 40 and the hole 0.01m2 with R
= 0, then
Rtotal=32 dB.
But now spend a lot of money on making the wall have an R of 60,
but dont repair the hole
Now Rtotal=33dB wall i.e. all that extra money only added 1 dB.
School acoustics (all photos from CEE, University of Exeter)
Plywood used in place of mineral wool in a plasterboard
partition cavity. This will reduce
the Rw of the partition by not
providing the sound absorption
necessary for this multi-pass
system to be effective.
FirstName LastName
(Affiliation)
Name of topic
Here we see a gap which is ~ 2 cm in width
between the two sheets
of plasterboard, and
which needs to be sealed
to reduce possible noise
ingress.
FirstName LastName
(Affiliation)
Name of topic
Plasterboard partitions in high
transit areas on
construction sites
can easily get
damaged in this
way, creating weak
spots for noise
transmission in the
final partition.
FirstName LastName
(Affiliation)
Name of topic
Multi-storey buildings often have penetrations
through party floor slabs
around laboratories and
WCs for gas, water and
waste pipes. These need
to be sealed to prevent
their becoming a route for
excessive noise ingress.
FirstName LastName
(Affiliation)
Name of topic
Conclusions
Acoustic absorption and insulation must not be confused
Sound insulation is considerably undermined by weak links (holes)
For good insulation, we need massive partitions or cavities
Most acoustic absorbers rely on porous materials or multiple holes
J.E. Moore Design for good acoustics and noise control