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24
Sound Intensity Calibrator Type 4297 – User Manual
3) In practice, if a sound wave is incident at 90°, then small differences between the phase
responses of the microphones cause a small phase difference between the microphone
signals. There now appears to be a flow of acoustic energy along the probe axis.
4) It is this apparent flow of acoustic energy that is detected and called 'residual intensity'.
Fig.5.2
Intensity measurement
principle
l
t
p t
u
( )
( )
×
( )
=
r
r
where
l
t
(
)
r
p
t
(
)
u
t
(
)
r
The pressure at the centre is represented by the average of two measured pressures. The particle
velocity is obtained by integration of the air disc's acceleration, which is a function of the
difference between the measured pressures:
p
t
p
t
( )
( )
+
1
2
l t
( )
---------------------------------
=
2
where
p
t
p
t
(
),
(
)
1
2
r
Δ
0
ρ
0
u
(t)
r
p
p(t)
p
(t)
1
t
: Instantaneous intensity in the direction of the probe axis
: Instantaneous pressure at the centre of the probe spacer
: Instantaneous particle velocity in the direction of the probe axis
p
t
p
t
( )
( )
–
1
2
dt
-------------------------------- -
r
ρ
Δ
0
0
: Measured instantaneous pressures
: System parameter representing the microphone
r
separation, Δ
(See Fig. 4.2)
: System parameter representing the air density
(t)
2
r
881781
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