18
7.1.3
Advanced Indicators
Extraneous Sources
The extraneous sources index (Called F
3
-F
2
in ISO 9614-1 and F
+/-
in ISO 9614-2) indicates the
difference between the average sound intensity level and the average of absolute sound intensity
levels.
𝐸𝐸𝐼
.
𝑆𝑆𝑆
. =
𝐿
|
𝐿
|
− 𝐿𝐿
When the extraneous sources indicator is close to zero (< 1dB), it means that the sign of the
intensity does not change significantly during the measurement.
When the extraneous sources indicator is high (> 6dB), it means that the sign of intensity changes
significantly from negative to positive during the measurement. This is a strong sign of reactive
field.
P|I| index
This index is called F2, or Surface Pressure-Intensity indicator in ISO 9614-1. It represents the
difference between the average sound pressure level and the average of absolute intensity levels.
It can be used to determine the extraneous sources index and is generally not used by itself.
Variability
This indicator shows the time variability of sound intensity. It is used in ISO 9614-1 and 3 to
evaluate the time variability of the sound field and determine the minimal duration of a
measurement to be representative of the sound field. The time variability should be lower than 0.6.
