Building Environment 1: Acoustics (conclusion)

David Coley (6E2.22, d.a.coley@bath.ac.uk)

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