Communication Systems - Engineering

Q1:

For a good public address system the sound intensity at the farthest point should be at least

A 20 dB over threshold of hearing

B 40 dB over threshold of hearing

C 80 dB over threshold of hearing

D 120 dB over threshold of hearing

ANS:C - 80 dB over threshold of hearing

Noise that is 100 db at one meter will have an intensity of only 1/100 as much at ten meters. That's 2 bels, or 20 decibels less, since a bel corresponds to a factor of ten. So at 10 meters the sound is 80 db. At 100m the intensity is down to 60 db.