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On the other hand, the sound pressure level indicates the sound pressure energy regardless of the
direction of the flow of energy. Thus, with the energy flow vector in the reference axis, both sound
intensity level will give the same result. However, with the energy flow vector perpendicular to the
reference axis, the sound pressure remains unchanged but the sound intensity will be null.
The following image shows the directivity pattern of the sound intensity probe.
For example, a sound field yielding a sound pressure of 75 dB yields a sound intensity level of 75 dB
when the direction of propagation is in-line with the reference axis. It will be 72 dB when the
direction of propagation is 60° with respect to the reference axis, and –inf dB when it’s 90°.
7.1.2
PI index
In practice, the sound field is not perfectly perpendicular to the measuring surface. Hence, there
will always be a difference between the sound intensity level and the sound pressure level. The
absolute difference between Pressure and Intensity levels is known as the PI index.
𝑃𝐼
=
𝐿𝐿 − 𝐿𝐿
A high PI index (> 6dB) indicates a low intensity level compared to the pressure level. This means
that the sound field is either reactive or that its propagation is mostly perpendicular to the
reference axis.
A low PI index (< 2dB) indicates a sound intensity level close to the sound pressure level. This
means that the propagation of the sound field is mostly in-line with the reference axis.
0° = 0dB
60° = -3dB
90° = -inf dB
30° = -0.6dB
