You are in a park when you spot your friend 100 m away.At what lowest decibel should you shout your greetings so that your friend can just hear you? Assume that your friend has normal hearing.
A)30 dB
B)40 dB
C)50 dB
D)60dB
E)70 dB

Question

Quizplus · Accepted Answer

The sound level decreases with distance according to the inverse square law, which means that the intensity of the sound at a distance of 100 m is 1/10000 of the intensity at a distance of 1 m (assuming no other factors such as absorption or reflections).

To calculate the minimum decibel level required for the friend to hear the greeting, we need to know the minimum audible threshold for a person with normal hearing at a distance of 100 m. The threshold of hearing for an average person is around 0 dB at 1 kHz, but it increases with decreasing frequency and increasing distance due to factors such as atmospheric absorption and spreading.

The typical rule of thumb for outdoor sound propagation is that a sound level of 60 dB at a distance of 1 m would result in a sound level of around 40 dB at a distance of 100 m. However, since we want to ensure that the friend can "just hear" the greeting, we should add a safety margin of around 10 dB, which brings the required decibel level to 50 dB.

Therefore, the correct answer is C) 50 dB.

You Are in a Park When You Spot Your Friend