Prep Guide 2020 #151

9/29/2020

BM: Looking for clarification on problem 151. Why do you use the full length of the room (12.8m) when the strobe is centered on that wall (6.4m)? Wouldn’t this make the farther distance for the sound to travel be the 9.2m? Yielding answer b, 34cd?

1 Comment

Chris Campbell

9/30/2020 06:59:44 am

If the strobe appliance output light equally in all directions, then the proposed approach of looking at one half of the room length would work. However, strobe light output diminishes as the angle between the measurement point and the normal axis to the strobe increases. In other words, if you draw a line perpendicular from the wall where the strobe is mounted, the light output will be highest on this line. As you get further away from that line (meaning the angle between you and the line increases), the light output diminishes.

For this reason, you need to use the greater of the length or width of the room, even though the strobe is mounted in the middle of the wall. Please refer to the SFPE Handbook reference in the problem solution for a more detailed explanation on this.

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