Sound Waves (Oxford AQA IGCSE Physics)

Sound Waves

• Sound waves are longitudinal waves

• A vibrating source causes vibrations in a medium (such as air) which are detected as sound

• Sound waves require a medium to travel through

  • This means that if there are no molecules, such as in a vacuum, then the sound can’t travel through it

• Humans can detect vibrations with frequencies between 20 Hz and 20 000 Hz

  • This is the range of human hearing

• Longitudinal waves consist of compressions and rarefactions:

  • A compression is a region of higher density i.e. a place where the molecules are bunched together

  • A rarefaction is a region of lower density i.e. a place where the molecules are spread out

Compressions and rarefactions in a column of air

• These compressions and rarefactions cause changes in pressure, which vary in time with the wave

  • Therefore, sound is a type of pressure wave

Pitch & Loudness

• The frequency of a sound wave is related to its pitch

  • Sounds with a high pitch have a high frequency (or short wavelength)

  • Sounds with a low pitch have a low frequency (or long wavelength)

• The amplitude of the disturbance is related to its volume

  • Sounds with a large amplitude have a high volume

  • Sounds with a small amplitude have a low volume

Graphs of amplitudes and frequencies

Reflection & Diffraction of Sound

Reflection of sound waves

• Sound waves reflect off hard surfaces

  • The reflection of a sound wave is called an echo

• Echo sounding can be used to measure depth or to detect objects underwater

  • A sound wave can be transmitted from the surface of the water

  • The sound wave is reflected off the bottom of the ocean

• The time it takes for the sound wave to return is used to calculate the depth of the water

  • This is the distance to the ocean floor plus the distance for the wave to return

  • The distance the wave travels is twice the depth of the ocean

Ship using radar

Diffraction of sound waves

• Sound can be diffracted through a gap or round an obstacle

  • This is why sound from another room is louder when the door is open

• The sound waves spread out as they pass through a gap or move around an object

Diffraction of sound