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Date November 2020 Marks available 2 Reference code 20N.2.SL.TZ0.4
Level Standard level Paper Paper 2 Time zone 0 - no time zone
Command term Predict Question number 4 Adapted from N/A

Question

Two loudspeakers, A and B, are driven in phase and with the same amplitude at a frequency of 850Hz. Point P is located 22.5m from A and 24.3m from B. The speed of sound is 340ms-1.

Deduce that a minimum intensity of sound is heard at P.

[4]
a.

A microphone moves along the line from P to Q. PQ is normal to the line midway between the loudspeakers.

 

The intensity of sound is detected by the microphone. Predict the variation of detected intensity as the microphone moves from P to Q.

[2]
b.

When both loudspeakers are operating, the intensity of sound recorded at Q is I0. Loudspeaker B is now disconnected. Loudspeaker A continues to emit sound with unchanged amplitude and frequency. The intensity of sound recorded at Q changes to IA.

Estimate IAI0.

[2]
c.

Markscheme

wavelength=340850=0.40«m»

path difference =1.8«m»

1.8«m»=4.5λ OR 1.80.20=9«half-wavelengths» ✓

waves meet in antiphase «at P»
OR
destructive interference/superposition «at P»

 

Allow approach where path length is calculated in terms of number of wavelengths; along path A (56.25) and
path B (60.75) for MP2, hence path difference 4.5 wavelengths for MP3
 

a.

«equally spaced» maxima and minima

a maximum at Q

four «additional» maxima «between P and Q»

b.

the amplitude of sound at Q is halved
«intensity is proportional to amplitude squared hence» IAI0=14 

c.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.

Syllabus sections

Core » Topic 4: Waves » 4.4 – Wave behaviour
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