Course: Separate Physics only | Tier: Foundation + Higher | Linked equations: v = fλ, s = vt
Notes
Sound
- Sound is a longitudinal wave. It needs a medium – it can’t travel through a vacuum.
- Sound travels at about 330 m/s in air, and faster in liquids and solids. When it changes speed its wavelength changes but its frequency stays the same.
- Human hearing ranges from about 20 Hz to 20 kHz. Sound makes the eardrum vibrate; the conversion of sound waves into vibrations of solids only works over a limited frequency range.
- An echo is a reflected sound wave. For echo timings, the sound travels there and back, so halve the total distance.
Ultrasound
Ultrasound has a frequency above 20 kHz. It is partially reflected at a boundary between two different media. The time taken for reflections to return can be used to work out how far away the boundary is. Uses: medical imaging (e.g. pre-natal scans) and industrial imaging (finding cracks in metal). Echo sounding (sonar) uses high-frequency sound to find the depth of water.
Seismic waves
Earthquakes produce seismic waves. P-waves are longitudinal and travel through solids and liquids. S-waves are transverse and can only travel through solids. S-waves are not detected on the opposite side of the Earth to an earthquake (the S-wave shadow zone), which is evidence that the Earth’s outer core is liquid. Studying seismic waves gives evidence for the structure of the Earth, which we can’t observe directly.
Questions
Q1 (F) What is the range of human hearing?
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About 20 Hz to 20 kHz (20 000 Hz).
Q2 (F) What is ultrasound?
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Sound with a frequency higher than 20 kHz – too high for humans to hear.
Q3 (F) Why can’t sound travel through space?
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Sound needs particles (a medium) to vibrate. Space is a vacuum, with no particles to carry the vibrations.
Q4 (F) A student claps and hears an echo from a cliff 2.0 s later. Sound travels at 330 m/s. How far away is the cliff?
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Total distance: s = v t = 330 × 2.0 = 660 m (there and back)
Distance to cliff = 660 ÷ 2 = 330 m
Q5 (F/H) A ship’s sonar pulse returns 0.80 s after it is sent. Sound travels at 1500 m/s in seawater. How deep is the sea?
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Total distance = 1500 × 0.80 = 1200 m
Depth = 1200 ÷ 2 = 600 m
Q6 (F/H) Give the differences between P-waves and S-waves.
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P-waves are longitudinal and travel through solids and liquids. S-waves are transverse and travel only through solids.
Q7 (H) An ultrasound echo returns from a boundary inside the body 1.2 × 10−4 s after the pulse is sent. The speed of ultrasound in tissue is 1500 m/s. How deep is the boundary, in cm?
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Total distance = 1500 × 1.2 × 10−4 = 0.18 m
Depth = 0.18 ÷ 2 = 0.09 m = 9.0 cm
Q8 (H) Explain how seismic waves provide evidence that the Earth’s outer core is liquid.
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S-waves can only travel through solids. After an earthquake, S-waves are not detected on the opposite side of the Earth (the shadow zone), so they must have been stopped by a liquid layer – the outer core. P-waves still get through, but are refracted.
