Sound Waves
Physics: Cambridge International A Level, topic 7.2.8
Sound Waves
Sound waves are longitudinal waves. Pingu is able to hear the music in his headphones because his ear drum vibrates.
Pingu's headphones
Sound Waves and our Ears
Our ears can detect vibrations (compressions and rarefactions) and transfer the information to our brain via our auditory nerve. Our ears are sensitive to (can hear) a range of frequencies between 20Hz and 20,000 Hz.
Age
- The range of frequencies that we can hear changes with age.
- Elderly people tend to become less sensitive to sounds with a higher frequency.
Ultrasound
- Ultrasound has a frequency above 20,000Hz. Humans cannot hear sounds with frequencies this high, but other animals can.
- Dog whistles have frequencies above 20,000Hz, which is why humans cannot hear them.
- Ultrasound is also used by doctors to perform scans of a developing foetus.
Speed of Sound
Sound needs to travel through a medium. The more rigid the medium is, the higher the speed of sound through it. The more compressible the medium is, the slower the speed of sound through it.
Gases
- Gases are readily compressible (easy to squash).
- So the speed of sound in a gas is very slow.
Liquids
- Liquids are more rigid and less compressible than gases.
- So the speed of sound in liquids is much higher than in gases.
Solids
- Solids are significantly more rigid than liquids and gases and are very hard to compress.
- Therefore, the speed of sound in solids is much higher than in liquids or gases.
Test yourself on Sound Waves
In the context of sound waves, a medium is...
Which of these sound frequencies can humans not hear?
Which of these is ultrasound not used for?
In science fiction films we often hear sound effects of battles taking place in outer space. Considering the conditions needed for sound waves to propagate, why are these sound effects unrealistic?