Physics Learning Objective

1. Measuring Sounds Levels Chapter 15.6

2. Decibel
• Intensity of sound is measured in decibels, dB
• On the decibel scale, the quietest sound is 0 dB, (close but not
complete silence)
• Unit for logarithmic comparisons of power or intensity. Ex. a sound 10
times more powerful than near total silence is 10dB, a sound 100
times more powerful than near total silence is 20dB
• Intensity level of 0 dB is I0= 10-12
W/m2
• To find any other sound intensity level (β) use equation
• β(I) = 0 dB + 10 log10 (I/I0)

3. Power and Intensity of Sound
• If you were to put a speaker in a sphere, the power is radiated
isotropically, meaning the same amount of power is radiated
in all directions and the intensity will also be uniform in all
directions over the surface of the sphere
• You can relate the power radiated and the intensity by the
equation
• P=I4πr2
• This formula can then be rearranged to find the intensity
• I= P/ (4πr2
)

4. Sample Question
• If you were front row (about 5m from the stage) at Katy
Perry’s half time show during the Super Bowl and the
sound intensity measures 5.00 W/m2
• a) Should you be concerned about your hearing?
• b) How much power are the speakers radiating?

5. Answer
• a) Use the equation β(I) = 0 dB + 10 log10 (I/I0)
• *** Hint I0 = 10-12 W/m2
• = 0 dB + 10 log10 ((5.00 W/m2)/(10-12 W/m2)
• = 126.99
• No because the threshold of pain is 130 dB

6. Answer Cont
• b) Because at the Super Bowl, speakers radiate power
isotropically therefore,
• P=I4πr2
• = (5.00W/m2)4π(5m2)
• = 1.57 x 103 W

7. Picture Sources
2014. Tonga. Alohajer's Blog. Web. 20 Feb. 2015

8. Thank you!