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8/20/2019 Attenuation and DB
1/9
Loses intensitydue to
Sound travelling through a material
Attenuation
• Sound beam comparable
to a torch beam
•Reduction differs for small
and large reflectors
• Energy losses due to
material
•Made up of absorption
and scatter
Beam Spread
8/20/2019 Attenuation and DB
2/9
Scatter
• The bigger the grainsize the worse the
problem
• The higher the
frequency of theprobe the worse the
problem
M!z " M!z
8/20/2019 Attenuation and DB
3/9
#eam Spread
The sound beam
spread out and the
intensity decreases
8/20/2019 Attenuation and DB
4/9
#eam spread and $ttenuation combined
%&'(S!
)&'
(S!
*o attenuation+only beam
spread, -d# reduction
%&'
(S!
.-'
(S!
$ttenuation and beamspread, -d#/ reduction
8/20/2019 Attenuation and DB
5/9
Sound 0ntensity
1omparing the intensity of 2 signals
1
0
1
0
P
P
I
I =
Electrical power proportional to the
square of the voltage produced
2
1
2
0
1
0
)(
)(
V
V
P
P =
2
1
2
0
1
0
)(
)(
V
V
I
I =!ence
8/20/2019 Attenuation and DB
6/9
Sound 0ntensity
2
1
2
0
1
0
)()(
V V
I I = 3ill lead to large ratios
21
2
0
10..1
0
10.. )(
)(
V
V Log
I
I Log =Therefore
dB
V
V Log
I
I Log
1
0
10..
1
0
10.. 20=
BELS
V
V Log
I
I Log
1
0
10..
1
0
10.. 2=
8/20/2019 Attenuation and DB
7/9
1
0
10..20
H
H Log dB =
2 signals at 2&' and )&' (S!,
3hat is the difference between them in d#4s5
2..2020
4020
1010.. Log Log dB ==
3010.020×=dB
dBdB 6=
8/20/2019 Attenuation and DB
8/9
1
0
10..20
H
H Log dB =
2 signals at &' and &&' (S!,
3hat is the difference between them in d#4s5
10..2010
10020
1010.. Log Log dB ==
120×=dB
dBdB 20=
8/20/2019 Attenuation and DB
9/9
$mplitude ratios in decibels
• 2 6 7 -d#
• ) 6 7 2d#
• " 6 7 )d#• & 6 7 2&d#
• && 6 7 )&d#