Course: Separate Physics only | Tier: Higher | Linked equation: v = fλ | See also: Sound, Ultrasound and Seismic Waves
Notes
How sound travels
Sound is a longitudinal wave. It is made by vibrating objects and needs a medium (solid, liquid or gas) to travel through, so it cannot travel through a vacuum. Sound travels fastest in solids and slowest in gases.
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- Sound waves can travel through solids, causing vibrations in the solid.
- In the ear, sound waves make the ear drum and other parts vibrate, which causes the sensation of sound.
- This conversion of sound waves into vibrations of solids only works over a limited frequency range, so humans can only hear from about 20 Hz to 20 kHz.
- When sound passes from one medium to another, its speed and wavelength change, but its frequency stays the same.
Pitch and loudness
- A higher frequency gives a higher-pitched sound.
- A bigger amplitude gives a louder sound.
Echoes
An echo is a reflected sound wave. To find the distance to a reflecting surface, use distance = speed × time, then halve it, because the sound travels there and back.
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) Why can’t sound travel through space?
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Space is a vacuum. Sound needs particles (a medium) to vibrate, and there are none.
Q3 (F) A dog whistle produces a sound of 25 kHz. Explain why a person can’t hear it.
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25 kHz is above the upper limit of human hearing (20 kHz).
Q4 (F/H) Describe how we hear sound.
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Sound waves enter the ear and make the ear drum vibrate. These vibrations are passed on to other parts of the ear, causing the sensation of sound.
Q5 (F/H) Two notes have the same amplitude, but note B has a higher frequency than note A. How do they sound different?
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They are equally loud, but note B has a higher pitch.
Q6 (H) A student claps near a cliff and hears the echo 1.2 s later. The speed of sound in air is 340 m/s. How far away is the cliff?
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Total distance = 340 × 1.2 = 408 m (there and back)
Distance to cliff = 408 ÷ 2 = 204 m
Q7 (H) A 500 Hz sound wave travels from air (340 m/s) into water (1500 m/s). Calculate its wavelength in each medium and state its frequency in water.
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F: λ = v ÷ f
In air: λ = 340 ÷ 500 = 0.68 m
In water: λ = 1500 ÷ 500 = 3.0 m
Frequency in water: still 500 Hz (frequency doesn’t change).
Q8 (H) Explain why humans can only hear a limited range of frequencies.
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Hearing depends on sound waves being converted into vibrations of solid parts of the ear, such as the ear drum. This conversion only works over a limited range of frequencies.
