1. Reflection
Do you know what is the use of the large plates suspended above the stage?
They are used to reflect sound.

2. The reflection of sound waves obeys the laws of reflection,
Sound waves are reflected by smooth surfaces in the same ways as light waves.
The reflection of sound waves obeys the laws of reflection,
i.e. angle of incidence = angle of reflection
The reflected sound is called an echo.

3. Refraction
Sound waves are refracted when they enter at an angle into a medium in which their speed changes.
e.g. When a sound wave travels through a balloon filled with carbon dioxide,
the wave is refracted and focused at a point.
balloon filled with carbon dioxide
signal generator
C.R.O.
loudspeaker
microphone

4. Sound waves are refracted when they travel through air of uneven temperatures.
On a hot day,
cooler air
warmer air
air near ground is warmer than that above
sound waves travel faster in warm air than in cool air
refracted away from ground

5. ∴ Sound can be heard over longer distances at night.
warmer air
cooler air
sound waves are refracted towards ground
air near ground is cooler than that above
∴ Sound can be heard over longer distances at night.

6. Diffraction
Sound waves have a much longer wavelength than light waves.
They undergo diffraction when travelling around corners or through doorways.
We can hear voice around corners or doorways.

7. Interference Demonstration of interference of sound:
loudspeaker
signal generator
loudspeaker
The two loudspeakers are connected to the same signal generator to act as two coherent sources.

8. When you walk across in front of the loudspeakers, the loudness of the sound heard varies.
When the sound waves interfere constructively – a loud sound is heard.
When the sound waves interfere destructively – a soft sound is heard.

9. That’s the end of Section 16.3
Key Ideas
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