Find out the power required to talk over a movie.

2. CONSIDER THIS:
You work at Science World, and an IMAX movie is about to start. You have to let the
audience know about the rules of watching the movie in the globular theatre, which is
5 storeys (16.5m) high and has a diameter of 27m. The movie, producing a sound
that is 80 dB loud, how much power would you require in order to talk over the movie?

3. CONSIDER THIS:
Understand that the speakers are arranged in a way that the entire audience hears
80 dB, but the maximum distance to the audience is 13.5m (due to the dome shape
of the theatre).

4. HINT 1: Use the relationship
between dB and I to figure out the
sound intensity
0 dB = 10-12 W/m2
*remember I = I0 • 10β/10

5. HINT 2: Assuming that the power is being
radiated uniformly in all directions
(isotropically), use the relationship between I
and P
I = P/4πr2

6. SOLN:
HINT 1: Use the relationship
between dB and I to figure out
the sound intensity

7. SOLN:
HINT 1: Use the relationship
between dB and I to figure out
the sound intensity
80 db
_____
10
= 8

8. SOLN:
HINT 1: Use the relationship
between dB and I to figure out
the sound intensity
80 db
_____
10
= 8 108

9. SOLN:
HINT 1: Use the relationship
between dB and I to figure out
the sound intensity
80 db
_____
10
= 8 108
108 • 10-12

10. SOLN:
HINT 1: Use the relationship
between dB and I to figure out
the sound intensity
80 db
_____
10
= 8 108
108 • 10-12 = 10-4

11. SOLN:
HINT 2: Assuming that the
power is being radiated
uniformly in all directions
(isotropically), use the
relationship between I and P

12. SOLN:
HINT 2: Assuming that the
power is being radiated
uniformly in all directions
(isotropically), use the
relationship between I and P
I = P/4πr2

13. SOLN:
HINT 2: Assuming that the
power is being radiated
uniformly in all directions
(isotropically), use the
relationship between I and P
I = P/4πr2
Rearrange
the formula!

14. SOLN:
HINT 2: Assuming that the
power is being radiated
uniformly in all directions
(isotropically), use the
relationship between I and P
I = P/4πr2
I = P/4πr2
P = I • 4πr2

15. SOLN:
HINT 2: Assuming that the
power is being radiated
uniformly in all directions
(isotropically), use the
relationship between I and P
I = P/4πr2
I = P/4πr2
P = I • 4πr2
P = 10-4 • 4π(13.5)2
r = d/2
= 27/2
= 13.5
P = 10-4 • 2290.22
P = 0.229 W

16. THE
END