Acoustics: Back to basics

Fundamentals of Acoustics Presented by Tim Reynolds

HERRING STORER ACOUSTICS Suite 34, 11 Preston Street, Como, W.A. 6152 P.O. Box 219, Como, W.A. 6952 Telephone: (08) 9367 6200 Facsimile: (08) 9474 2579 Email: hsa@hsacoustics.com.au

Fundamentals To Cover

Some Basic Definitions

Basic Terminology

Criteria

Noise Sources and Typical Examples

Basic Noise Controls

Noise Source Decibel Level dB(A) Jet Takeoff (100m) 130 Live rock band 120 Chain Saw 110 Lawn Mower 100 Diesel Truck, food blender 090 Dishwasher 080 Freeway Traffic at 15m 070 Conversation 060 Quiet Suburb 050 Library, living room 040 Quiet rural area 030 020 010 Threshold of hearing 000

Common Noise Sources

Definitions

dB(lin) Decibel measurement without any adjustment.

dB(A) Decibel measurement according to the "A" weighted scale. The human ear does not respond equally to the same sound pressure at different frequencies. The "A" weighting has predetermined adjustments made at certain frequencies to make the measured sound level approximate to the response of the ear.

Decibel (dB) The human ear has a vast sound-sensitivity range of a thousand billion to one. The decibel is a logarithmic unit that allows this range to be compressed into a somewhat more comprehensible range. The decibel is ten times the logarithm of the ratio of a sound level to a reference sound level. Decibel noise levels cannot be added arithmetically since they are logarithmic numbers.

LA1 Represents the A-weighted noise level which is exceeded for 1 per cent of the measurement period.

LA10 Represents the A-weighted noise level which is exceeded for 10 per cent of the measurement period.

Sound Pressure Level (SPL) The sound pressure level is the noise level of a sound measured at a point using a standard noise level meter relative to a standard noise level (2x10-5 N/m2). Sound Power Level (SWL) The sound power level of a source is the sound energy emitted by the source. The sound power level of a source cannot be measured directly, but is calculated based on measured sound pressure levels at a known distance (10-12W).

Leq Equivalent Continuous Sound Level. The Leq measurement integrates these fluctuations in noise to give the equivalent continuous level containing an equal amount of energy over the time of measurement.

LA90 Represents the A-weighted noise level which is exceeded for 90 per cent of the measurement period. Also often used to represent background noise level.

Noise level measurements can be broken down into Octave or 1/3 Octave Bands, with the following centre frequencies.

40

42

44

46

48

50

52

54

56

58

60

62

64

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59

SPL

(dBA

)

Time (Sec)

Measurement 1 - Time History Noise Level LA10 Overall LAEq Overall LA90 Overall

40

42

44

46

48

50

52

54

56

58

60

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55

SPL

(dBA

)

Time (Sec)

Measurement 2 - Time History Noise Level LA10 Overall LAEq Overall LA90 Overall

CRITERIA Environmental Noise Environmental Protection (Noise) Regulations 1997

Internal Noise Levels AS/NZ 2107:2000 Acoustics - Recommended design sound levels and reverberation times for building interiors

Others Building Code of Australia (BCA) – Part F5 GreenStar

ENVIRONNEMENTAL PROTECTION (NOISE) REGULATIONS 1997

The allowable or assigned noise level at the surrounding locales is prescribed by the Environmental Protection (Noise) Regulations 1997.

The influencing factor is calculated for the usage of land within two circles, having radii of 100m and 450m from the premises of concern.

Premises Receiving Noise Time of Day Assigned Level (dB)

LA10 LA1 LAma

x

Noise sensitive premises

0700 - 1900 hours Monday to Saturday (Day)

45 + IF

55 + IF

65 + IF

0900 - 1900 hours Sunday and Public Holidays (Sunday / Public Holiday Day)

40 + IF

50 + IF

65 + IF

1900 - 2200 hours all days (Evening) 40 + IF

50 + IF

55 + IF

2200 hours on any day to 0700 hours Monday to Saturday and 0900 hours Sunday and Public Holidays (Night)

35 + IF

45 + IF

55 + IF

Commercial premises All hours 60 75 80

Baseline Assigned Outdoor Noise Level

Penalties are also applied for annoying characteristics. • Tonality • Modulating • Impulsiveness

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

dB(A

)

25 H

z

31.5

Hz

40 H

z

50 H

z

63 H

z

80 H

z

100

Hz

125

Hz

160

Hz

200

Hz

250

Hz

315

Hz

400

Hz

500

Hz

630

Hz

800

Hz

1 kH

z

1.25

kHz

1.6

kHz

2 kH

z

2.5

kHz

3.15

kHz

4 kH

z

5 kH

z

6.3

kHz

8 kH

z

10 k

Hz

Noi

se L

evel

(dB(

A))

1/3rd Octave Band Centre Frequency (Hz)

Tonality

Where tonality is present Where modulation is present Where impulsiveness is present

+5 dB(A) +5 dB(A) +10 dB(A)

Penalties

AS/NZ 2107:2000

AS2107 has the following recommended noise levels within an office building:

Satisfactory Maximum

Boardrooms and conference rooms 30 dB(A) 40 dB(A) Corridors and Lobbies 45 dB(A) 55 dB(A) General Office Areas 40 dB(A) 45 dB(A) Private Offices 35 dB(A) 40 dB(A) Reception Areas 40 dB(A) 45 dB(A) Tea Rooms 40 dB(A) 45 dB(A) Toilets 50 dB(A) 55 dB(A)

Other Criteria Used are Noise Ratings (NR) Curves

NR CURVES

0

20

40

60

80

100

120

140

160

31.5 62.5 125 250 500 1000 2000 4000 8000

Noi

se L

evel

(dB)

Octave Band Centre Frequency (Hz)

NR 20

NR 30

NR 40

NR 50

NR 60

NR 70

NR 80

NR 90

NR 100

NR 110

NR 120

NR 130

0

20

40

60

80

100

120

140

160

31.5 62.5 125 250 500 1000 2000 4000 8000

Noi

se L

evel

(dB)

Octave Band Centre Frequency (Hz)

NR 20

NR 30

NR 40

NR 50

NR 60

NR 70

NR 80

NR 90

NR 100

NR 110

NR 120

NR 130

Return Air

GREENSTAR For Example, under Office Design there are point available for Acoustics. Under IEQ-12 there are up to two points available where it is demonstrated that for 95% of the project’s NLA does not exceed the ‘Satisfactory’ ambient internal noise levels in accordance with AS/NZS 2107:2000. One point is awarded for building services design, where within the entire base building general office space, noise from the building services does not exceed 40 dB(A) Leq. One point is awarded for the overall building, where within the base building office space, the sound level does not exceed 40 dB(A) Leq (assuming open plan offices).

Building Code Of Australia

Part F5 of the BCA 2013 'Sound Transmission and Insulation'. Part F5 of the BCA 2013 details the requirements for sound transmission and insulation of residential type buildings. It is noted that Part F5 of the BCA 2013 is only applicable to the residential sections of the development.

Basic Criteria Walls between Apartments Rw + Ctr not less than 50 Walls to corridor Rw not less than 50 Floors Rw +Ctr not less than 50

ENVIRONMENTAL NOISE

Typical Example of Exhaust Fan Being 67 dB(A) at 3 metres (This is Sound Power of 87 dB(A)) Neighbour at 50 metres away Reduction for distance is 24 dB(A) Noise level at residence is 43 dB(A) However, with penalty of +5 dB(A) for tonality, assessable noise level would be 48 dB(A).

Say Influencing Factor was 1 dB(A), then assigned noise level would be :

Premises Receiving Noise

Time of Day Assigned Level (dB)

LA 10 LA 1 LA max

Noise sensitive premises

0700 - 1900 hours Monday to Saturday 46 56 66

0900 - 1900 hours Sunday and Public Holidays 41 51 66

1900 - 2200 hours all days 41 51 56 2200 hours on any day to 0700 hours Monday to Saturday and 0900 hours Sunday and Public Holidays

36 46 56

Noise emissions from mechanical services are : - Steady state - Would operate for more than 10% of time

Assessable Noise Level, dB(A) Applicable Times of Day

Applicable Assigned LA10 Noise Level

(dB)

Exceedance to Assigned Noise

Level (dB)

48

0700 - 1900 hours Monday to Saturday 46 +2 0900 - 1900 hours Sunday and Public Holidays 41 +7 1900 - 2200 hours all days 41 +7 2200 hours on any day to 0700 hours Monday to Saturday and 0900 hours Sunday and Public Holidays 36 +12

Possible Noise Control : • Barrier • Enclose in plantroom • Reduce speed • Located equipment further away from neighbours • Attenuators • Acoustic Louvres

Noise Reductions with Unpodded Silencers

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

dB(A

)

63 125

250

500

1 K

2 K

4 K

8 K

SWL (Linear) SWL (dB(A) Level at residence

With 1D unpodded Silencer With 2D unpodded Silencer

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0dB

(A)

63 125

250

500

1 K

2 K

4 K

8 K

SWL (Linear) SWL (dB(A) Level at residence

With 300mm Acoustic Louvre With 600mm Acoustic Louvre

Noise Reductions with Acoustic Louvres

NOISE PATHS

Also need to consider regenerated noise.

INTERNAL DESIGN TO CONTROL NOISE

Supply Air Bends Internal Insulation Acoustic Type Flexible Ductwork Add attenuators Also consider : Air velocity Design to registers

0

5

10

15

20

25

30

35

63 125

250

500

1 K

2 K

4 K

8 K

Inse

rtio

n Lo

ss d

B

Octave Band Centre Frequency (Hz)

INSERTION LOSS OF FLEXIBLE DUCT 300 Diameter, 3m length

Plain Flex Acoustic Flex

0.05.0

10.015.020.025.030.035.040.045.050.055.060.065.070.075.080.085.090.095.0

dB(A

)

63 125

250

500

1 K

2 K

4 K

8 K

SWL (Linear) SWL (dB(A) With Plain Flexible Duct With Acoustic Flexible Duct

Noise Reductions with Flexible Duct

RETURN AIR Bends without Turning Vanes Return Air plenums Add attenuators

0.05.0

10.015.020.025.030.035.040.045.050.055.060.065.070.075.080.085.090.095.0

dB(A

)

63 125

250

500

1 K

2 K

4 K

8 K

SWL (Linear) SWL (dB(A) With Turning Vanes Without Turning Vanes

Noise Reductions - Bends

Breakout Need to consider : From flexible connections Flexible ductwork

Also need to consider Regenerated Noise

From Fantech Web Site