Page 60 - Q & A STPM 2022 Physics

Physics Semester 3 STPM Chapter 21 Sound Waves

Section B Structured Questions

Question 6
An ambulance with its siren on is approaching a tall building with a speed of
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25 m s . The ambulance is moving away from an observer who is in front of
–1
the building as shown in the figure. The frequency of the siren is 1200 Hz and
the speed of sound is 330 m s .
–1
Building

Ambulance Observer
25 m s –1
Calculate (a) the frequency of sound received by the observer,
(b) the frequency of sound reflected from the building, and
(c) the frequency of the beat detected by the observer.

Answer
(a) The ambulance (source) is moving away from the observer.
The frequency received by the observer is less than 1200 Hz.

f 1 =  v  f =  330  1200 Hz
v + u s 330 + 25
= 1115 Hz
(b) The source is approaching the wall. Frequency of sound incident on the
wall is higher than 1200 Hz. The reflected sound has the same frequency.

f 2 =  v  f =  330  1200 Hz
v – u s 330 – 25
= 1298 Hz

(c) Interference between the reflected sound and the sound from the
ambulance received by the observer produces beats of frequency
f = f 2 – f 1 = (1298 – 1115) Hz
= 183 Hz

Semester
3

420

021 STPM Q&A Physics T3.indd 420 17/02/2022 10:24 AM