Civil Engineering Reference
In-Depth Information
6.3.1.1
Examples using idealized sources
We shall start using the idealized type of sources, monopoles and dipoles, to illustrate the
concept of radiation factor. For a monopole we found in section 3.4.1 that the radiated
power could be expressed as
22
ka
2
Wc u
=
ρ
S
,
(6.26)
00
a
22
1
+
ka
where
k
is the wave number and
a
the radius of the sphere with area
S
= 4π
a
2
. Inserting
this expression into Equation
(6.25)
giving the radiation factor, we get
22
ka
ka
σ
=
22
.
(6.27)
monopole
1
+
Examples on the radiation factor for a monopole source having radii of 5 and 25 cm,
respectively, are shown in
Figure 6.8
. The radiation factor is given on a logarithmic scale
as 10⋅lg σ, a quantity commonly denoted
radiation index
.
0
-10
25
-20
5
-30
-40
-50
10
20
50
100
200
500
1000
2000
5000
Frequency (Hz)
Figure 6.8
Radiation factor of a monopole (pulsating sphere) and a dipole (oscillating sphere) with radius 5 and
25 cm, respectively. Solid curves - monopole. Dashed curves - dipole.
when calculating the radiated noise from machinery based on measured vibration levels.
In this report it is expressed as
2
Currently (2007) under revision.
