Modelling and measuring the atmospheric excess attenuation
over flat terrain
Astrid Ziemann, Kati Balogh, Klaus Arnold
Institut für Meteorologie, Universität Leipzig, Stephanstr. 3, 04103 Leipzig, Germany
[email protected]
Summary
The model SMART (Sound propagation model of the atmosphere using ray-tracing) simulates the modified sound propagation due to atmospheric sound-ray refraction. For validation
of the simulated data a measuring campaign was carried out over flat terrain in autumn 2004. The comparison of the modelled with measured data during clear night conditions with
strong temperature inversion shows a satisfactory agreement, which leads to the conclusion that the main effects on outdoor sound propagation are reliably described by SMART.
Ray-model SMART
Measurements in Melpitz 2004
Measurements or simulations:
Vertical profiles of temperature, wind velocity and wind direction
Calculation of >2000 sound ray paths in 36 horizontal directions
SMART
Calculation
S of sound level attenuation
S
x-z-charts with
sound ray paths
Horizontal maps of sound attenuation
Output
Direct sound level measurement (Brüel&Kjaer Front ends):
Recording signals from the tomography sound sources
using additional microphones at nearly the same places as
receivers of the tomographic system
R
R
Fig. 1: Flowchart of geometrical acoustics model SMART.
SODAR/RASS
Example for SMART simulation of attenuation:
S measured vertical
Input:
profiles
R
S
30
20
10
0
0
1 2 3 4 5 6 7 8 9 10 11
-1
Temperature [K]/Wind velocity [ms ]
Fig. 2: Smoothed vertical profiles of air temperature and wind
velocity, measured on 8th October 2004, 1:30 a.m. (local time), at
the Melpitz test site using mast and SODAR/RASS.
Fig. 5: Scheme of the measurement system(s). Total measurement
area: ca. 500 x 500 m².
downwind
60
40
30
20
10
00:00
Fig. 6: Microphone with windbreak (left) and sound source (right).
Experiment at the Melpitz test site of the Institute for
Tropospheric Research Leipzig :
Distance from sound source [m]
Fig. 3: Selected sound ray paths up- and downwind from the
sound source at a height of ca. 2 m above ground surface over
horizontally homogeneous grassland (see Fig. 2: SMART input).
• Test site (51°32’ N, 12°54’ E, 86 m a.s.l.) for air-chemical
and micrometeorological investigations (flat, grassland)
¾Major influence of temperature profile: downward
refracted sound rays in up- and downwind direction
• Example: 8th October 2004 (nighttime), stable stratification,
small wind velocity, western wind directions
200
wind direction
0
-200
-400
-200
0
200
400
Distance from sound source [m]
Fig. 4: Horizontal map of sound level attenuation in dB (sound
level at the point (x,y) in relation to sound level in 1-m-distance
from the sound source at the point (0, 0)); (see Fig. 2).
¾Increased sound level (decreased attenuation) at a
height of ca. 2 m especially in downwind direction
8
8
temperature A-TOM
temperature mast
wind speed A-TOM
wind speed mast
7
6
7
6
5
5
4
4
3
3
2
2
1
1
0
00:00
0
01:00
02:00
03:00
04:00
Local time
Fig. 7: Temperature and wind speed measured at a height of ca. 2 m
using a profile mast and acoustic tomography A-TOM (applying
reciprocal sound paths)
¾Agreement between area and point measurement
¾Horizontally homogeneous meteorological fields
02:00
03:00
Fig. 8: Atmospheric excess attenuation derived from sound level
measurements and SMART simulations for a distance of ca. 75 m in upwind
and downwind direction and vertical gradient of effective sound speed
(temperature-dependent sound speed + wind component in sound path
direction) at the Melpitz test site on 8th October 2004.
6
4
2
0
-2
-4
-6
-8
-10
-12
-14
-16
-18
-20
00:00
upwind
A-TOM
SMART
01:00
downwind
A-TOM
SMART
02:00
03:00
04:00
Local time
Fig. 9: Atmospheric excess attenuation derived from tomography A-TOM and
SMART simulations for a distance of ca. 175 m in upwind and downwind
direction at the Melpitz test site on 8th October 2004.
-1
-400
dB
39
41
43
45
47
49
51
53
55
57
59
Temperature [°C]
400
Wind speed [ms ]
Distance from sound source [m]
Results (2): SMART calculation of sound level
attenuation (modified spheric wave divergence
caused by atmospheric refraction, reflection at the
ground surface and sound absorption at 1 kHz)
• Comparison of spatially averaged (tomography using
reciprocal sound travel time measurements along 4
directions, see Fig. 5) and point measurements (at a
meteorological mast) of temperature and wind
01:00
1,0
down- 0,9
wind 0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
-0,1
04:00
Local time
Excess attenuation [dB]
0
500 400 300 200 100 0 100 200 300 400 500
Measurement
SMART
∆ceff/∆z
-1
Height [m]
50
6
4
2
0
-2
-4
-6
-8
-10
-12
-14
-16
-18
-20
∆ceff/∆z [ms /m]
upwind
Local time
Excess attenuation [dB]
Results (1): SMART calculation of sound ray paths
using a refraction law for a two-dimensional stratified
moving medium
70
-1
Height [m]
40
∆ceff/∆z [ms /m]
Source
Receiver
50
• Sound level measurement: measured travel time from
tomography ⇒ identification of signal amplitudes at two receivers
between two sources ⇒ relative sound level between these
receivers in down- and upwind direction
Excess attenuation [dB]
Wind velocity
Temperature
• Tomography: measured signal amplitudes at two receivers
between two sources ⇒ relative sound level between these
receivers in down- and upwind direction
Comparison of atmospheric excess attenuation between
measurements and SMART simulations
1,0
6
4
upwind 0,9
2
0,8
0 Reduced immission
0,7
-2 Increased immission
0,6
-4
-6
0,5
-8
0,4
-10
0,3
-12
0,2
-14
Measurement
-16
0,1
SMART
-18
0,0
∆ceff/∆z
-20
-0,1
00:00
01:00
02:00
03:00
04:00
75 m
70
60
Calculation of atmospheric excess attenuation:
• SMART: difference between total attenuation including all effects
on sound propagation in a stratified atmosphere and total
attenuation in an atmosphere without vertical gradients of
meteorological quantities
Meteorological
measuring mast
75 m
Input
Results and Analysis
Acoustic tomography: 5 loud speakers (1000 Hz signal) and 8
microphones (see Fig. 5)
¾Agreement between measurements and simulations (differences
caused by not included effects like ray interference, turbulence…
and measurement errors)
¾Remarkable influence of atmospheric structure on sound
propagation, e.g. increased sound immission (negative
attenuation) during strong temperature inversion in up- and
downwind direction
For further information: Balogh et al., 2006: Influence of atmospheric refraction on pulse propagation over a flat ground surface, Acustica.
This work was partly funded by the DFG (German Research Foundation). Special thanks go to T. Conrath und G. Spindler (Institute for Tropospheric Research in Leipzig) for
the preparation of the measurement values of the mast and SODAR/RASS system.
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