ba 7666-11, 1 basic concepts of sound contents: definitions db conversion sound fields db ± db
TRANSCRIPT
BA 7666-11, 1
Basic Concepts of Sound
Contents:
Definitions
dB Conversion
Sound Fields
dB ± dB
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Sound
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Sound and Noise
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Terminology of Sound
dB
Leq
RMS
Sound Pressure
Constant percentage bandwidth
Noise Dose
Logarithmic scales
Pascal
FastSlow
Impulse
RMSPeak
Weighting
1/1 and 1/3 Octave Analysis
Percentile level
Statistical analysis
L10
L90
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Pressure vs. Power
Temperature t [°C]
Power P [W]
ElectricalHeater
Analogy
Pressure p [N/m2 = Pa]
SoundSource
Lp [dB]
Power P [W]
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Basic Parameters of Sound
Under free-field conditions:
Power: P [W]
Intensity: [J/s/m2] =
W/m2
Pressure: p [Pa = N/m2]
The Sound Intensity vector, describes the amount and directionof flow of acoustic energy at agiven position
P
r
p
c4 2
2
P
P2
P2
r1
r2
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Propagation of Sound
t = 0
t = 1/4 T
t = 1/2 T
t = 3/4 T
t = T
Compression
Extension
Minimum compression Maximum compression
No compression
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Sound Pressure
Pressure
[Pa]
100 000Pascal
Time
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Sound Pressure
100 000 Pascal
Atmospheric Pressure
Mexico City
New York
Pressure[Pa]
AcousticPressureVariations
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100
10
1
0.1
0.01
0.001
0.000 1
0.000 01
Range of Sound PressurePressure, p
[Pa]
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Basic Concepts of Sound
Contents:
Definitions
dB Conversion
Sound Fields
dB ± dB
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Sound Pressure, p[Pa]
Range of Sound Pressure Levels
140
120
100
80
60
40
20
0
Sound Pressure Level, Lp
[dB]100
10
1
0.1
0.01
0.001
0.000 1
0.000 01
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dB – decibel
Ex. 1: p = 1 Pa
Lp= 20 log
= 20 log 50 000
= 94 dB
1
20 10 6
Ex. 2: p = 31.7 Pa
Lp= 20 log
= 20 log 1.58 × 10-6
= 124 dB
317
20 10 6
.
(p0 = 20 Pa = 20 × 10-6 Pa)
Lp = 20 log dB re 20 Pa
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Perception of dBs
Change in Sound Level (dB)
Change in Perceived Loudness
3
5
10
15
20
Just perceptible
Noticeable difference
Twice (or 1/2) as loud
Large change
Four times (or 1/4) as loud
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Conversion to dB using Charts
dB re 20 Pa
200
150
100
50
0
0.0001 0.01 1 100 10 000
Pressure[Pa]
p0 = 20 Pa
94 dB
Lp
0.1 10 1000
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Conversion to dB using TablesdB to Pressure Ratio
PressureRatio
db PressureRatio
PressureRatio
db PressureRatio
1.000.9890.9770.9660.9550.9440.9330.9230.9120.902
0.8910.8410.7940.7080.6310.562
0.00.10.20.30.40.50.60.70.80.9
1.01.52.03.04.05.0
1.0001.0121.0231.0351.0471.0591.0721.0841.0961.109
1.1221.1891.2591.4131.5851.778
0.5010.4470.3980.3550.3160.2510.2001.1580.1260.100
0.03160.01000.0032
10-3
10-4
10-5
6779
101214161820
3040506080
100
1.9952.2392.5122.8183.1623.9815.0126.3107.943
10.000
31.62100
316.2103
104
105
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Simple Rules for Conversion
Sound Pressure Level, Lp
dB re 20 Pa
Sound Pressure, p[Pa]
114
74
80
8486
94
10
0.1
0.2
0.3
1
20
2× 100
× 10
× 2× 3
× 46 dB 10 dB
20 dB
40 dB
12 dB
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Basic Concepts of Sound
Contents:
Definitions
dB Conversion
Sound Fields
dB ± dB
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Types of Sound Sources
r: Lp
2r: Lp 6 dB
r: Lp
2r: Lp 3 dB
r: Lp 2r: Lp
Plane source
Point source
Line source
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Anechoic and Reverberant Enclosures
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Pressure Field
Loudspeaker
Microphone
Enclosure
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Sound Fields
Near
field
Far field
Free field Reverberant field
Lp
Distance, rA1 2 × A1
6 dB
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Directivity Index
Source Location
Free field L = Lp
L = Lp + 3 dB
L = Lp + 6 dB
L = Lp + 9 dB
On a flat plane
At a junction of two planes
At a junction of three planes
DirectivityFactor
1
2
4
8
DirectivityIndex, dB
0
3
6
9
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Pressure Increase at Walls
L = Lp + 3 dB
L = Lp + 6 dB
Lp
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Basic Concepts of Sound
Contents:
Definitions
dB Conversion
Sound Fields
dB ± dB
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Two Sound Sources
Lp1 = X dB
Lp2 = X dB
Lp1 + Lp2 = X + 3 dB
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Addition of dB Levels
Example:L+
dB
3
2
1
00 5 10 15
LdB
L1
L2
L
L+
Lt
55 dB
51 dB
4 dB
1.4 dB
55 + 1.4 = 56.4 dB
=
=
=
=
=
4 dB
1.4 dB
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Subtraction of Noise Levels
LS+N
[dB]LN
[dB]
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Subtraction of dB Levels
Ex:
LS+N
LN
L
L
LS
60 dB
53 dB
7 dB
1 dB
60 1 = 59 dB
=
=
=
=
=
LdB
6
4
2
LdB
1
3
5
1 102 3 4 5 6 7 8 9
LS ~ LS+N
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Addition of many dB values
Addition of sound levels :
For L1 = L2 = L3..... = LN
Examples:
N = 2: LTotal = L1 + 3 dB
N = 10: LTotal = L1 + 10 dB
0
2
4
6
8
10
12
0 5 10 15
10 log N
N
L1 + L2 ..... + LN = ?
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Conclusion
Sound pressure level in dB is related to 2 * 10 -5 Pascal.
The range of human hearing is 130 dB.
To add or subtract dB-values use charts or formulas
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Literature for Further Reading
Acoustic Noise Measurements
Brüel & Kjær (BT 0010-12)
Noise Control - Principles and Practice
Brüel & Kjær (188-81)