Page 52 - Q & A STPM 2022 Physics

Physics Semester 3 STPM Chapter 21 Sound Waves
Answer: C
I
b = 10 log  
I 0
When power output is doubled, intensity I is doubled.
2I
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b’ = 10 log  
I 0
I
= 10 log   + 10 log 2 = b + 3.0 dB
I 0
Question 3

The intensity level of sound at a distance of 3.0 m from a source is 60 dB. What
is the sound intensity level at a distance of 5.0 m from the source?
A 14 dB C 36 dB
B 22 dB D 56 dB

Answer: D
1 I
Intensity, I ∝ , intensity level, b = 40 dB = 10 log  
r 2 I 0
When r = 5.0 m, I’ = 3.0 2 I = 0.36I
5.0 2

b' = 10 log  0.36I 
I 0
I
= 10 log   + 10 log 0.36
I 0
= 56 dB

Section B Structured Questions
Question 4

The power output of a radio is 12.0 W. The radio emits sound equally in all
directions.The threshold of pain is when intensity = 1.00 W m . At what
–2
maximum distance from the radio would the sound be painful to the ear?

Answer
P
Intensity, I = 4πr 2 = 1.00 W m –2
12.0
Semester
3 r = 4π(1.00) m
= 0.98 m

412

021 STPM Q&A Physics T3.indd 412 17/02/2022 10:24 AM