Physical Science / Unit 9: Waves, Sound and Light
9.3 Sound
Notes
Sound is a longitudinal mechanical wave produced by vibrations.
- It needs a medium and cannot travel through a vacuum.
- Speed in air is about 343 m/s at room temperature; faster in liquids and solids (about 1,500 m/s in water), because particles are closer.
- Pitch depends on frequency; loudness depends on amplitude (measured in decibels, dB).
- Humans hear about 20 Hz to 20,000 Hz. Ultrasound is above 20,000 Hz.
- An echo is reflected sound. Distance to a reflecting surface $\displaystyle = \frac{vt}{2}$.
- Doppler effect: the pitch changes when a source moves toward or away from you.
Practice
Which property of a sound wave determines its pitch?
Higher frequency means higher pitch.
In which medium does sound travel fastest?
Particles are closest together in solids.
A student hears an echo 2 s after shouting. Sound travels at 340 m/s. How far away is the wall?
The sound travels there and back: $\displaystyle \frac{340 \times 2}{2} = 340$ m.
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