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Class 9 Sound Waves: Characteristics and Applications: cheat sheet
CBSE Science. Sound is a longitudinal wave that needs a medium. Frequency, wavelength, speed, pitch and loudness, echoes, and uses of ultrasound.
Key ideas
- Sound travels as compressions and rarefactions; it cannot travel through a vacuum.
- Frequency is the number of waves per second (hertz); higher frequency means higher pitch.
- Amplitude decides loudness; the quality (timbre) lets us tell instruments apart.
- Sound travels fastest in solids, slower in liquids and slowest in gases, and faster in warmer air.
- Humans hear about 20 Hz to 20 000 Hz. Below is infrasound; above is ultrasound.
- An echo is heard only if the reflected sound comes back at least 0.1 s later (about 17 m away in air).
Remember
| Wave speed | v = fλ |
| Time period | T = 1 ÷ f |
| Echo or SONAR | distance = v × t ÷ 2 |
| Speed of sound in air | about 344 m/s at 22 °C |
Watch out
- Forgetting to halve the time (or distance) for an echo: the sound goes there and back.
- Linking loudness to frequency (it depends on amplitude).
- Mixing up wavelength and frequency in v = fλ.
Worked example
To hear a distinct echo, the reflected sound must arrive at least 0.1 s after the original. What is the minimum distance of the reflecting surface if sound travels at 346 m/s?
Working: sound travels 346 × 0.1 = 34.6 m there and back; ÷ 2
Answer: 17.3 m
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