Sound wavesIB MYP Sciences: Revision notes
Section 1
How sound is made and travels
Sound is produced by vibrations, for example a guitar string or a loudspeaker cone. The vibrating object pushes on nearby particles, which pass the energy on. Sound is a longitudinal wave, so it needs a medium (solid, liquid or gas) to travel through. It cannot travel through a vacuum, such as space, because there are no particles to vibrate.
Sound cannot travel through space. Light can, because it does not need a medium.
Section 2
Compressions and rarefactions
In a sound wave the particles are squashed together in some places and spread apart in others.
- Compression: a region where particles are close together (high pressure)
- Rarefaction: a region where particles are further apart (low pressure)
The particles vibrate backwards and forwards along the direction of travel. Each compression and the next rarefaction make one complete wave.
Section 3
Pitch and loudness
- Pitch depends on frequency. A higher frequency gives a higher pitch.
- Loudness depends on amplitude. A larger amplitude gives a louder sound.
On an oscilloscope, a high-pitched sound has waves close together and a loud sound has tall waves. Changing the loudness does not change the pitch.
A louder sound does not mean a higher pitch. Loudness is amplitude; pitch is frequency.
Section 4
Speed of sound
Sound travels fastest in solids, slower in liquids and slowest in gases, because particles are closer together and pass vibrations on more quickly.
- Air: about 340 m/s
- Water: about 1500 m/s
- Steel: about 5000 m/s
Sound in air takes time to travel, which is why you see lightning before you hear thunder.
Section 5
Echoes and distance
An echo is sound reflected from a hard surface. The sound travels to the surface and back, so the total distance is twice the distance to the surface.
distance = speed × time
Worked example: an echo returns after 0.50 s. Total distance = 340 × 0.50 = 170 m. Distance to the surface = 170 ÷ 2 = 85 m.
Sonar uses echoes to measure the depth of the sea.
Section 6
Hearing range and ultrasound
Humans hear frequencies from about 20 Hz to 20 kHz (20 000 Hz). Sound above 20 kHz is ultrasound.
Uses of ultrasound: scanning unborn babies (safe because it is non-ionising), cleaning jewellery and instruments, sonar and finding flaws in metal. Pulses are sent in and the echoes from boundaries are timed to build an image.
Must know
- Sound is made by vibrations and is a longitudinal wave needing a medium
- Compressions (particles close) and rarefactions (particles apart)
- Higher frequency means higher pitch; bigger amplitude means louder
- Speed: solids > liquids > gases; about 340 m/s in air
- Echo distance = speed × time ÷ 2
- Human hearing 20 Hz to 20 kHz; ultrasound is above 20 kHz
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Sound waves
- Astronauts on a spacewalk outside a space station cannot hear each other speak, even when they are only one metre apart, so they talk by radio. Inside the station, where there is air, the same astronauts can talk normally.Describe what is meant by a compression and by a rarefaction in a sound wave.2 marks
- A hiker in a valley in Switzerland shouts towards a large cliff and hears her echo 0.80 s later. The speed of sound in air is 340 m/s.The hiker walks to a new position, shouts again and hears the echo 1.2 s later. Calculate her new distance from the cliff.2 marks
- Students in Cairo measure the speed of sound in air. They stand 100 m from a large flat wall and clap their hands. A partner uses a stopwatch to time the delay between the clap and the echo. Five repeat readings are 0.66 s, 0.54 s, 0.62 s, 0.55 s and 0.58 s. The students know that the accepted speed of sound in air is about 340 m/s.Calculate the mean time and use it to find the speed of sound in air from the students' results.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).