Physics / Unit 7: Waves and Sound
7.3 Sound
Notes
Sound is a longitudinal pressure wave made by vibrating objects.
- Speed of sound in air at about $20^\circ\text{C}$ is $343 \text{ m/s}$. Sound is faster in liquids and fastest in solids, and faster in warmer air.
- Pitch depends on frequency: higher frequency, higher pitch. Humans hear about 20 Hz to 20,000 Hz. Above that is ultrasound.
- Loudness depends on amplitude (intensity), measured in decibels (dB). Each $+10 \text{ dB}$ means 10 times the intensity.
- An echo is reflected sound: distance $\displaystyle = \frac{v \times t}{2}$ because the sound goes there and back.
- Example: a 686 Hz note in air has $\displaystyle \lambda = \frac{343}{686} = 0.5 \text{ m}$.
Practice
A hiker shouts toward a cliff and hears the echo 2.0 s later. How far away is the cliff? (Use $343 \text{ m/s}$.)
The sound travels $343 \times 2 = 686 \text{ m}$ round trip, so the cliff is half that.
Which change makes a sound higher in pitch?
Pitch is set by frequency.
A sound of 80 dB is how many times more intense than a sound of 60 dB?
Each 10 dB is a factor of 10, so 20 dB is $10 \times 10 = 100$.
Ready for more? Take the Unit 7 test.