Question
A loudspeaker emits sound waves of frequency f towards a metal plate that reflects the waves.
A small microphone is moved along the line from the metal plate to the loudspeaker. The intensity
of sound detected at the microphone as it moves varies regularly between maximum and
minimum values.
The speed of sound in air is 340ms−1.
(a) (i) Explain the variation in intensity. [3]
(ii) Adjacent minima are separated by a distance of 0.12m. Calculate f . [2]
(b) The metal plate is replaced by a wooden plate that reflects a lower intensity sound
wave than the metal plate.
State and explain the differences between the sound intensities detected by the same
microphone with the metal plate and the wooden plate. [3]
Answer/Explanation
Ans.
(a)(i)«incident and reflected» waves superpose/interfere/combine $\checkmark$ «that leads tow standing waves formed $O R$ nodes and antinodes present $\checkmark$ at antinodes / maxima there is maximum intensity / constructive interference / “displacement” addition / louder sound $\checkmark$
at nodes / minima there is minimum intensity / destructive interference / “displacement» cancellation / quieter sound $\checkmark$
(a)(ii) wavelength $=0.24 \ll \mathrm{m} »$
$f=« \frac{340}{0.24}=» 1.4 « \mathrm{kHz} » O R 1400 « \mathrm{~Hz} » \checkmark$
(b)relates intensity to amplitude $\checkmark$
antinodes / maximum intensity will be decreased / quieter $\checkmark$
nodes / minimum will be increased / louder $\checkmark$
difference in intensities will be less $\checkmark$
maxima and minima are at the same positions $\checkmark$