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Introduction to Sound Waves

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Introduction to Sound Waves

Use the information and diagrams on this sheet to investigate how sound is made and travels. Show working for calculations and use suitable units.

A concert in the hall

At a school concert, a speaker vibrates and makes the air around it vibrate. These vibrations travel outwards as a longitudinal sound wave. Air particles move back and forth in the direction the wave travels, forming compressions and rarefactions. The particles do not travel all the way from the speaker to the listener; they transfer energy to neighbouring particles. Sound needs a medium, such as air, water or a solid, and cannot travel through a vacuum. When sound reaches the ear, it makes the eardrum vibrate.

1.A student says, “The air particles travel from the speaker into my ear.” Explain what is wrong with this statement. Use the idea of energy transfer in your answer.

2.Which statement best describes sound travelling through air?

Air particles move in circles perpendicular to the direction of travel.
Air particles vibrate parallel to the direction the sound wave travels.
Air particles move from the sound source to the listener.
Sound travels through empty space without particles.

3.Sound can travel through air, water and solids, but not a vacuum. Explain why a vacuum prevents sound from travelling.

Reading wave diagrams

For a sound wave, frequency is the number of complete vibrations each second, measured in hertz (Hz). Pitch depends on frequency: a higher frequency means a higher pitch. Amplitude describes the size of the pressure variation; a greater amplitude is heard as a louder sound. Wavelength is the distance between matching points, such as the centres of two neighbouring compressions. In the diagrams, each dot marks a compression; the distance between adjacent dots is one wavelength.

Sound A |● ● ● ● ●|

Sound B |● ● ● ● ● ● ● ● ●|

4.Which sound has the higher frequency? Explain how the spacing of its compressions supports your answer.

5.A speaker’s frequency increases while the wave speed stays constant. What happens to its wavelength, and how would the pitch change?

6.Match each wave term to its description: frequency, amplitude, wavelength.

Frequency
Amplitude
Wavelength

Size of the wave’s pressure variation; linked to loudness
Distance between neighbouring matching points
Number of complete vibrations each second

Sound data challenge

A sound wave in air has a frequency of 500 Hz and a wavelength of 0.68 m. Use the relationship wave speed = frequency × wavelength.

7.Calculate the speed of this sound wave. Include your working and the unit.

8.A second sound travels through the same air at 340 m/s and has a frequency of 250 Hz. Calculate its wavelength.

9.A sound becomes louder, but its pitch stays the same. Which wave property changes, and which stays the same?

10.During a demonstration, a vibrating tuning fork is held above a bowl of water. The water surface moves more strongly when the fork is struck harder. Explain this observation using sound-wave ideas.