Answers to Decibel Problems 7–9

7. The intensity level at a distance 1 ft from a siren radiating in all directions is 80 dB. What is the intensity level at each of the following distances from the siren? a) 2 ft, b) 10 ft, c) 20 ft, d) 100 ft.

Answers: (a) 74 dB, (b) 60 dB, (c) 54 dB, (d) 40 dB.

8. If two sirens of the type in problem 9 are radiating together, what is the intensity level of the two at a distance 1 ft away from both? Hint: The intensities simply add.

Answer: 83 dB. Two identical sirens have twice the intensity of one alone, which corresponds to an increase of 3 dB.

9. How many sirens sounding together have an intensity level 10 ft away that is 6 dB lower than that 1 ft away from one such siren?

Answer: twenty-five. One siren 10-ft away would have an intensity level of 60dB. We need to add sirens to make the intensity level 80 – 6 = 74 db, which is an additional 14 dB corresponding to an intensity ratio of about 25. therefore, we would need approximately twenty-five sirens.

©2013 Robert Chuckrow

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