SoundIB MYP Physics: Revision notes
Section 1
What is sound?
Sound is made by vibrations. A guitar string, a drum skin or your vocal cords vibrate, and push on the air next to them. This makes air particles bunch together (compressions) and spread apart (rarefactions) in a pattern that travels outwards.
Sound is a longitudinal wave: the particles vibrate parallel to the direction the wave travels. The particles do not travel from the source to your ear. They only vibrate backwards and forwards and pass the energy on.
Sound is not a transverse wave. In a longitudinal wave the vibrations are along the direction of travel, not at right angles to it.
Section 2
Sound needs a medium
Sound passes on its vibrations from particle to particle, so it needs a medium (solid, liquid or gas). There are no particles in a vacuum, so sound cannot travel through it. This is why there is no sound in space.
Sound travels fastest in solids, slower in liquids and slowest in gases because the particles are closer together in solids and can pass the vibration on faster: solids > liquids > gases.
- Air: about 340 m/s
- Water: about 1500 m/s
- Steel: about 5000 m/s
Section 3
Pitch and loudness
- Pitch is how high or low a note sounds. It depends on the frequency (the number of vibrations each second, measured in hertz, Hz). High frequency means high pitch.
- Loudness depends on the amplitude (the size of the vibrations). A bigger amplitude means a louder sound.
On an oscilloscope, a higher note shows more waves squeezed onto the screen, and a louder note shows taller waves. Changing the amplitude does not change the pitch.
Section 4
Echoes and the speed of sound
An echo is sound that has been reflected from a hard surface. The sound travels to the surface and back, so the distance it covers is twice the distance to the surface.
speed = distance / time
Worked example. A ship sends a sound pulse to the sea bed and the echo returns after 0.80 s. The speed of sound in water is 1500 m/s.
Distance there and back = 1500 × 0.80 = 1200 m. Depth = 1200 / 2 = 600 m.
In echo questions, do not forget that the sound goes there and back. Halve the total distance to find how far away the surface is.
Section 5
Hearing range and ultrasound
Humans can hear frequencies from about 20 Hz to 20 kHz (20 000 Hz). Sounds above 20 kHz are called ultrasound. Young people hear higher frequencies than older people.
Uses of ultrasound:
- Scanning: ultrasound pulses reflect from boundaries between tissues and the echoes make an image, for example of an unborn baby. It is safe because it is not ionising.
- Sonar: ships send pulses down and use the echo time to find the depth of the sea bed or a shoal of fish.
- Cleaning: ultrasound makes dirt shake loose from jewellery and surgical instruments in a liquid bath.
Section 6
The ear and hearing damage
In outline, the ear works like this:
- Sound waves enter the ear canal and make the eardrum vibrate.
- Three tiny bones (ossicles) pass the vibrations on to the cochlea.
- Hair cells in the cochlea turn the vibrations into electrical signals.
- The auditory nerve carries the signals to the brain.
Very loud sounds (large amplitude) and long exposure can damage the eardrum or the hair cells. Hair cells do not grow back, so the damage is permanent. Ear defenders and turning music down help to protect your hearing.
Must know
- Sound is a longitudinal wave caused by vibrations and needs a medium.
- Speed: solids > liquids > gases; about 340 m/s in air.
- Pitch depends on frequency; loudness depends on amplitude.
- Echo = reflected sound; speed = distance / time (remember there and back).
- Human hearing: about 20 Hz to 20 kHz; above this is ultrasound.
- Ear: ear canal, eardrum, ossicles, cochlea, auditory nerve; loud noise causes permanent damage.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Sound
- In a science-fiction film, a spacecraft explodes in the vacuum of space and the audience hears a loud bang. A physics teacher says this could not happen in real life, although a crew member inside the spacecraft could hear a tapping sound travelling through the metal hull.Explain why the bang could not be heard in space.2 marks
- A student records two notes on a guitar. Note X has a frequency of 220 Hz and is plucked firmly. Note Y has a frequency of 440 Hz and is plucked gently. She then blows a dog whistle that makes sound of frequency 30 kHz. Her dog reacts to the whistle straight away but she hears nothing.Explain why the dog can hear the whistle but the student cannot.2 marks
- Two students measure the speed of sound in air. Hannah stands 85 m from a large, flat wall and claps her hands once. Raj, standing next to her, starts a stopwatch at the clap and stops it when he hears the echo. He repeats this five times and records the times 0.50 s, 0.54 s, 0.48 s, 0.52 s and 0.51 s.Calculate the mean time and use it to calculate the speed of sound in air.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).