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Measured Pressure Level
L
20log
=
p
Measured Intensity Level
p
p
1
L
10log
-------------------------------- -
=
1
I
ω ρ
ref
where φ = φ
+ φ
f
e
φ
f
φ
e
p
p
,
1
2
ω
p
ref
I
ref
The pressure-residual intensity index of a measurement system is defined as the difference
between the indicated pressure and intensity levels for equal pressure applied to the
microphones (φ
f
For a small phase difference between the channels of a measurement system, the pressure-
residual intensity becomes:
I
ref
L
10 log
---------------------------------- -
=
K
0 ,
p
✐
The pressure-residual intensity index is a function of the selected system variables for gas
Please note:
density and microphone separation. Thus, the index should be determined for those
values of the variables that are to be used for the measurements.
The curves in Fig.5.3 indicate that a measured pressure-intensity index should be at least 7 dB
higher than the residual pressure intensity index of the measurement system to ensure an error
due to phase mismatch of less than 1 dB.
Thus, we could define the dynamic capability of an intensity measurement system by subtracting
7 dB from the residual pressure-intensity index.
1 2 ⁄
2
2
p
p
p
p
2
cos
(
φ
)
+
+
1
1
2
2
--------------------------------------------------------------------- -
p
2
ref
sin
φ
2
dB
r
Δ
0
0
: phase difference between measured pressures
: phase error of measurement system
: measured pressures (RMS values)
: angular frequency
: reference pressure (2 × 10
: reference intensity (10
= 0).
r
ω ρ
Δ
0
0
dB
2
sin
φ
ref
e
Pressure-Residual Intensity Index
dB
–5
Pa)
–12
2
W/m
)
CHAPTER 5
25
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