Modelling and measuring the atmospheric excess attenuation

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Modelling and measuring the atmospheric excess attenuation
over flat terrain
Astrid Ziemann,Kati Balogh,Klaus Arnold
Institut für Meteorologie,Universit?t Leipzig,Stephanstr.3,04103 Leipzig,Germany
ziemann@uni-leipzig.de
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.
Ray-model SMART Measurements in Melpitz 2004 Results and Analysis
S
S S
R
R
R
:Flowchart of geometrical acoustics model SMART.
:Selected sound ray paths up-and downwind from the
source at a height of ca.2 m above ground surface over
ntally homogeneous grassland(see Fig.2:SMART input).
jor influence of temperature profile:downward
ted sound rays in up-and downwind direction
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.
mmary
model SMART(Sound propagation model of the atmosphere using ray-tracing)simulates the modified sound propagation due to atmospheric sound-ray refraction.For validation
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
ong temperature inversion shows a satisfactory agreement,which leads to the conclusion that the main effects on outdoor sound propagation are reliably described by SMART.
Acoustic tomography:5 loud speakers(1000 Hz signal)and 8
microphones(see Fig.5)
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
ple for SMART simulation of attenuation:
t:measured vertical profiles
:Horizontal map of sound level attenuation in dB(sound
t the point(x,y)in relation to sound level in 1-m-distance
he sound source at the point(0,0));(see Fig.2).
reased sound level(decreased attenuation)at a
t of ca.2 m especially in downwind direction
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)
Agreement between area and point measurement
Horizontally homogeneous meteorological fields
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.
Agreement between measurements and simulations(differences
caused by not included effects like ray interference,turbulence…
and measurement errors)
Remarkable influence of atmospheric structure on sound
propagation,e.g.increased sound immission(negative
attenuation)during strong temperature inversion in up-and
downwind direction
Fig.5:Scheme of the measurement system(s).Total measurement
area:ca.500 x 500 m2.
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atenuat
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educed immission
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xcessE
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SODAR/RASS
Meteorological
measuring mast
Source
Receiver
Calculation of>2000 sound rayp aths in 36 horizontal directions
Calculation of sound level attenuation
Horizontal maps of sound attenuationx-z-charts with
sound rayp aths
t
RT
ut
Measurements or simulations:
Vertical profiles of temperature,wind velocitya nd wind direction
500 400 300 200 100 0 100 200 300 400 5000
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eH
giht
[m]
Distance from sound source[m]
upwinddownwind
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iDt
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dB
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Wind velocity
Temperature
Temperature[K]/Wind velocity[ms-1]
eigH
th
m][
:Smoothed vertical profiles of air temperature and wind
ty,measured on 8th October 2004,1:30 a.m.(local time),at
lpitz test site using mast and SODAR/RASS.
lts(1):SMART calculation of sound ray paths
ing a refraction law for a two-dimensional stratified
oving medium
lts(2):SMART calculation of sound level
tenuation(modified spheric wave divergence
used by atmospheric refraction,reflection at the
ound surface and sound absorption at 1 kHz)
Fig.6:Microphone with windbreak(left)and sound source(right).
Experiment at the Melpitz test site of the Institute for
Tropospheric Research Leipzig:
?Test site(51°32’N,12°54’E,86 m a.s.l.)for air-chemical
and micrometeorological investigations(flat,grassland)
?Example:8th October 2004(nighttime),stable stratification,
small wind velocity,western wind directions
?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
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emT
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indW
peeds
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Local time
temperatureA -TOM
temperaturem ast
winds peedA -TOM
winds peedm ast
Calculation of atmospheric excess attenuation:
?Tomography:measured signal amplitudes at two receivers
between two sources?relative sound level between these
receivers in down-and upwind direction
?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
?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
Comparison of atmospheric excess attenuation between
measurements and SMART simulations
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A-TOM A-TOM
SMART SMART
Local time
xcessE
ttenua
tia
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