acoustics: back to basics - airah - home · acoustics: back to basics fundamentals of acoustics ......
TRANSCRIPT
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: [email protected]
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
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