Page 53 - Q & A STPM 2022 Physics

Physics Semester 3 STPM Chapter 21 Sound Waves
Question 5

The intensity level of sound at a distance of 3.00 m from a source is 120 dB. At
what distance is the sound intensity level (a) 100 dB and (b) 10.0 dB?

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Answer
I
(a) r = 3.00 m, b = 120 dB = 10 log  
I 0
I = (10 )(10 –12 W m ) = 1.0 W m –2
–2
12
When b = 100 dB
–2
10
I = (10 )(10 –12 W m )
1
= 10 W m ∝
–2
–2
r 2
1
1.0 W m –2 ∝ 3.00 2
r = 1.0 (3.00 m) = 30.0 m
10 –2
(b) When b = 10 dB, I = (10 )(10 W m ) ∝ r I 2
1
–12
–2

I
–11
–2
10 W m ∝
r 2
1
1.0 W m –2 ∝ 3.00 2
1.0
r = (3.00 m) = 94.9 km
10 –11

Section C Essay Question

Question 6

(a) Distinguish between intensity of sound and intensity level of sound.
(b) Sound is emitted from a small speaker uniformly in all directions. Points P
and Q are at distances of r and 2r from the speaker.
Compare Semester
(i) the intensity of sound at P and Q,
(ii) the sound intensity level at P and Q.
3

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021 STPM Q&A Physics T3.indd 413 17/02/2022 10:24 AM