Knowledge Organiser: Sound Waves
Key Terms
- Longitudinal: particles vibrate parallel to wave travel
- Compression: high-pressure region
- Rarefaction: low-pressure region
- Ultrasound: frequency > 20,000 Hz
- Infrasound: frequency < 20 Hz
Key Facts
- Sound speed: gas < liquid < solid
- Sound in air: ~330 m/s
- Cannot travel through vacuum
- Louder = bigger amplitude on oscilloscope
- Higher pitch = higher frequency on oscilloscope
Key Equations
- v = f × λ
- distance = (speed × time) ÷ 2 (echo)
- f = 1/T
Exam Tips
- Echo formula: always ÷2 for return journey
- Ultrasound = non-ionising = safe for foetuses
- Sound = longitudinal; EM waves = transverse
- Amplitude affects loudness; frequency affects pitch
Common Mistakes
- Forgetting to halve in echo calculations: The measured time is for the sound to travel to the object AND back — divide by 2 before multiplying by speed to get the one-way distance
- Saying sound travels faster in air than solids: Sound travels fastest in solids, then liquids, then slowest in gases — denser packing means vibrations transfer more efficiently
- Confusing amplitude and frequency effects: Amplitude determines loudness (volume); frequency determines pitch — a louder sound has larger amplitude, not higher frequency
- Saying sound is a transverse wave: Sound is a longitudinal wave — particles vibrate parallel to the direction of wave travel, creating compressions and rarefactions
- Saying ultrasound is ionising: Ultrasound is non-ionising — it is safe for medical imaging including foetal scans; X-rays are ionising and have associated risks
Memory Aid
- Amplitude = how tAll the wave is = how Loud (A and L both go together)
- Frequency = how Frequently packed the waves are = Pitch (F and P both go together)