Page 54 - Q & A STPM 2022 Physics

Physics Semester 3 STPM Chapter 21 Sound Waves

(c) Two small speakers emit sound waves of different frequencies equally in all
directions. Speaker A has an output of 2.5 mW, and speaker B has an output
of 4.50 mW. Determine the sound level at point C in the figure.
C
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4.00 m
A B

3.00 m 2.00 m
(i) when only speaker A emits sound,
(ii) when only speaker B emits sound, and
(iii) when both speakers emit sound.

Answer
(a) Intensity, I is the energy per second incident normally on a unit surface
I
area. Sound intensity level in dB = 10 log   where I is the intensity, I 0
I 0
is the threshold of hearing 10 W m .
–12
–2
1
(b) (i) At P, I P ∝
r 2
1
At Q, I Q ∝
(2r) 2
Hence I P = 4I Q
I Q
(ii) Intensity level at Q, b Q = 10 log  
I 0
Intensity level at P, b P = 10 log  (4I Q ) 
I 0
I Q
= 10 log   + 10 log 4
I 0
b P – b Q = 0.60 dB
Intensity level at P is 0.60 dB greater than intensity level at Q.

Semester
3

414

021 STPM Q&A Physics T3.indd 414 17/02/2022 10:24 AM