Question bank › IGCSE Physics › Waves

Sound: longitudinal waves, hearing range, oscilloscopes, pitch and loudness

Answer on paper first, then reveal the mark scheme one mark at a time.

1Grade 6–74 marks

A ship uses pulses of ultrasound to find the depth of the sea. A pulse returns to the ship 0.80 s after it was sent out.

The speed of sound in sea water is 1500 m/s.

Calculate the depth of the sea. [4]

distance = speed × time (in any form)(1)
$1500 \times 0.80 = 1200$ (m); allow $1500 \times 0.40$ if the time is halved first(1)
dividing the distance by 2 / halving the time to 0.40 s (as the pulse travels down and back)(1)
600 m(1)
Answer: 600 m
Examiner tip: The echo time is for the journey there and back — forgetting to halve it gives 1200 m.

2Grade 6–7Triple only5 marks

A microphone connected to an oscilloscope detects a sound. The diagram shows the trace on the screen.

The horizontal scale is set so that one square represents 2.0 ms.

(a) Determine the frequency of the sound. [3]

(b) The sound is changed so that it is louder and has a higher pitch. The oscilloscope settings are not changed.

Describe how the trace changes. [2]

(a) one wave = 4 squares, so time period = 4 × 2.0 ms = 8.0 ms(1)
(a) $f = \dfrac{1}{T} = \dfrac{1}{0.0080}$ (ms converted to s)(1)
(a) 125 Hz allow 130 Hz; ecf from incorrect period(1)
(b) taller waves / greater (vertical) amplitude / peaks higher(1)
(b) more waves on the screen / waves closer together / each wave narrower(1)
Answer: (a) 125 Hz (b) waves taller and more of them across the screen
Examiner tip: Convert ms to s before using f = 1/T: 1 ÷ 8.0 = 0.125 is a power-of-ten error.

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