BRUEL & KJAER 4297 User Manual page 12

4 Sound Intensity Calibrator Type 4297 – User Manual
Fig.1.2 Type 4297 Sound Intensity Calibrator with a sound intensity probe in place. This configuration is used for both
sound pressure calibration and pressure-residual intensity index measurement
Fig.1.3 Simplified block diagram of an intensity measuring instrument. The signals from two pressure microphones,
p1 and p2, are used to determine the pressure midpoint of the probe axis, p, and the particle velocity along
the probe axis, v Multiplying p and v gives the intensity reading I
p1
Preamp.
p2
Preamp.
Residual Intensity Level
If a pressure-residual intensity index spectrum is to be used to assess the accuracy of sound
intensity measurements, then the mean sound pressure spectrum in the field must also be
measured. The residual intensity level is then quickly established by subtracting the pressure-
residual intensity index spectrum from the measured mean sound pressure spectrum.
The residual intensity level is then compared to the measured sound intensity level. It can be
shown that, for a certain frequency, the residual intensity level must be at least 7 dB lower to
ensure a measurement error of less than 1 dB when measuring intensity level. Refer to IEC 61043
for details.
Ch. 1
+
Phase
∑
ADC
correction
+
+
Phase
∑
ADC
-
correction
Ch. 2
Diff.
Sum
1/3-oct.
filters
p
v
1/3-oct.
X
filters
Int.
I
Averaging
980277/2e