Sound: Class 9 Study Guide
Sound is made by vibrations and travels as a wave through a material.
Sound is produced by vibrating objects, and it needs a medium such as air, water or a solid to travel. It cannot travel through a vacuum. Sound waves in air are longitudinal waves: the particles vibrate back and forth in the same direction as the wave moves, forming compressions and rarefactions.
The frequency is the number of vibrations per second, in hertz (Hz), and decides the pitch. The amplitude is the largest displacement, and decides the loudness.
Wave quantities
Speed = frequency ร wavelength (v = fฮป). A sound of 500 Hz with wavelength 0.68 m has speed 340 m/s, close to the speed of sound in air at room temperature. Sound is fastest in solids, slower in liquids and slowest in gases.
| Quantity | Symbol | Unit |
|---|---|---|
| Frequency | f | Hz |
| Wavelength | ฮป | m |
| Speed | v | m/s |
| Time period | T = 1/f | s |
Echo and hearing range
An echo is a reflected sound. To hear it separately, the reflector must be at least about 17 m away, because the ear keeps a sound for about 0.1 s. A sound at 340 m/s travels 34 m in 0.1 s, so the one-way distance is 17 m. Humans hear from about 20 Hz to 20 000 Hz. Below 20 Hz is infrasound and above 20 kHz is ultrasound.
Clap or shout.
Back to you.
To the wall and back.
Find d.
Uses of ultrasound and sonar
Ultrasound is used in medical scans, in cleaning delicate parts and in detecting cracks in metal. Sonar uses ultrasound to measure ocean depth and find objects under water. Bats and dolphins use echoes to find their way and food.
Worked examples and common mistakes
Example: A sonar signal returns after 4 s in water where the speed is 1500 m/s. Depth = 1500 ร 4 รท 2 = 3000 m. Example: The frequency of a sound of period 0.01 s is 100 Hz. Common mistakes: forgetting to halve the time for an echo, thinking that loud means high pitch, and saying that sound can travel in space.
Check your understanding: model answers
1. 340 รท 680 = 0.5 m. 2. The larger amplitude. 3. For example medical scans or sonar.
โจ Key Takeaways
- Sound needs a medium and is a longitudinal wave.
- Frequency decides the pitch; amplitude decides the loudness.
- v = fฮป.
- Echoes need a reflector at least about 17 m away.
๐ฌ Questions for Discussion & Reflection
- Find the wavelength of a 680 Hz sound at 340 m/s.
- Which is louder, a wave with a larger amplitude or a smaller one?
- Name one use of ultrasound.