Sound: Complete O Level Physics Cheatsheet
O Level Physics, Chapter 12 · Read time: ~8 minutes
These notes come from TGC Academy's O Level Physics tuition in Singapore.
Key Formulas
How sound is produced and travels
Sound is produced by vibrating sources, for example a guitar string, a loudspeaker cone or your vocal cords.
Sound needs a medium (solid, liquid or gas) to travel. It cannot travel through a vacuum. In the classic bell jar experiment, the ringing fades as air is pumped out, even though the bell can still be seen vibrating.
Sound is a longitudinal wave. The vibrating source pushes air particles together to form compressions (regions of higher pressure) and pulls them apart to form rarefactions (regions of lower pressure).
Sound generally travels fastest in solids, slower in liquids and slowest in gases. In air it travels at roughly 330 to 340 m/s.
Trap: In the bell jar experiment, the light from the bell still reaches you because light does not need a medium. Only the sound disappears. Don't write that the bell stops vibrating.
Pitch and loudness
Loudness depends on amplitude. A larger amplitude gives a louder sound.
Pitch depends on frequency. A higher frequency gives a higher pitch.
Humans can hear frequencies from about 20 Hz to 20 kHz.
Trick: On an oscilloscope trace, taller waves mean louder, more tightly packed waves mean higher pitch. Compare the two features separately, one at a time.
Echoes and ultrasound
An echo is a reflected sound. Timing an echo lets you measure distance: the sound travels to the surface and back, so 2d = vt.
Ultrasound is sound with a frequency above 20 kHz, too high for humans to hear.
Sonar: a ship sends ultrasound pulses down and times the echo from the seabed or a shoal of fish to find its depth.
Medical scanning: ultrasound reflects off boundaries between soft tissues, building up an image, for example of a baby in the womb. It is used because it is non-ionising and safe for soft tissue.
Trap: The most common echo error: forgetting to halve. The time measured is for the round trip, so the distance to the surface is half of v × t.
Worked Example
A ship's sonar sends an ultrasound pulse straight down. The echo from the seabed is received 0.80 s later. The speed of sound in seawater is 1,500 m/s.
(a) Calculate the depth of the sea below the ship. [2]
Total distance travelled = vt = 1,500 × 0.80 = 1,200 m
This is a round trip, so depth = 1,200 ÷ 2 = 600 m
(b) The ultrasound used has a frequency of 50 kHz. Calculate its wavelength in seawater. [2]
λ = v / f
= 1,500 ÷ 50,000
= 0.030 m
(c) Explain why the pulse would take longer to return if the same depth were measured through air instead of water. [1]
Sound travels more slowly in air than in water, so it takes longer to cover the same distance.
Why this question is a good test of the topic: it checks the round trip halving, unit conversion from kHz to Hz, and the link between the medium and the speed of sound in one short scenario.
Frequently Asked Questions
Why can't sound travel through a vacuum?
Sound is a vibration passed between particles. A vacuum has no particles to carry the vibration.
What determines the pitch of a sound?
Its frequency. Higher frequency means higher pitch.
What determines the loudness of a sound?
Its amplitude. Larger amplitude means a louder sound.
What is ultrasound?
Sound with a frequency above 20 kHz, beyond the range of human hearing. It is used in sonar and medical scanning.
How do I calculate distance from an echo?
Use 2d = vt. The sound goes to the surface and back, so halve the total distance travelled.
Echo questions still tripping you up?
Small group O Level Physics classes at TGC Academy's Bishan, Bukit Timah and Potong Pasir centres, helmed by Andrew Seah, MOE Award-Winning Teacher and Marshall Cavendish textbook author.