Acoustics basics (PA calculation use-case)

Acoustic basics

Public Address calculation example :

For example, a outside space. This outdoor area is 10m wide and 32m long. The background of this space is 65 dB.

The sound projector used in the first calculation is an IMPI IN 180T with a sensitivity of 94 dB 1w / 1m. Two power are available 8W / 15W and a directivity of 90 °.

We chose to power the speaker at 8W. This gives us then :

97dB 2W /1m
100dB 4W / 1m
103dB 8W / 1m

Now if we calculate the pressure level at 32m knowing that we are outside, we have 103dB 8W/ 1m :

97dB 8W / 2m
91dB 8W / 4m
85dB 8W / 8m
79dB 8W / 16m
73dB 8W / 32m

The 73dB pressure level obtained at 8W at 32m and the level of background noise is 65dB, we have a difference of 6 dB who is insufficient to provide an adequate understanding of the message.

If we now cable one sound projector IMPI 180T EN at 15W, you double the power and therefore is added 3dB which gives us 76dB 15W / 32m. Which is still insufficient for a proper understanding of the message.

If we change this projector for one CH 532 with a sensitivity of 95dB 1W / 1m and a power of 30W, we obtain 110dB 30W / 1m and therefore 80 dB 30W / 32m.

The difference with the background noise is 15 dB, which is sufficient for good intelligibility.