REFLECTION OF SOUND
After reading this section you will be able to do the following:
- Observe the experiment below and explain why the wave reacts differently depending on what surface it hits.
- Discuss how echoes are made.
The Multi-Material Room
- What happens when a sound wave hits a concave shaped surface?
- Is the sound reflected back to the source from a concave shaped surface more or less than that reflected from a flat surface?
- What happens when a sound wave hits the porous surface?
- What happens when a sound wave hits an irregular surface?
When sound reflects off a special curved surface called a parabola, it will bounce out in a straight line no matter where it originally hits. Many stages are designed as parabolas so the sound will go directly into the audience, instead of bouncing around on stage. If the parabola is closed off by another curved surface, it is called an ellipse. Sound will travel from one focus to the other, no matter where it strikes the wall. A whispering gallery is designed as an ellipse. If your friend stands at one focus and you stand at the other, his whisper will be heard clearly by you. No one in the rest of the room will hear anything.
Reflection is responsible for many interesting phenomena. Echoes are the sound of your own voice reflecting back to your ears. The sound you hear ringing in an auditorium after the band has stopped playing is caused by reflection off the walls and other objects. A sound wave will continue to bounce around a room, or reverberate, until it has lost all its energy. A wave has some of its energy absorbed by the objects it hits. The rest is lost as heat energy.
Everything, even air, absorbs sound. One example of air absorbing sound waves happens during a thunderstorm. When you are very close to a storm, you hear thunder as a sharp crack. When the storm is farther away, you hear a low rumble instead. This is because air absorbs high frequencies more easily than low. By the time the thunder has reached you, all the high pitches are lost and only the low ones can be heard. The best absorptive material is full of holes that sound waves can bounce around in and lose energy. The energy lost as heat is too small to be felt, though, it can be detected by scientific instruments.
How does sound reach every point in the room?
Since sound travels in a straight path from its source, how does it get around corners? You already know that if you and your friend are standing on either side of a wall and there is an open door nearby, you will be able to hear what your friend says. Because you would not hear your friend if the door was closed, sound is not traveling through the wall. Instead, it must be going around the corner and out the door.
You hear your friend because of sound diffraction. Diffraction uses the edges of a barrier as a secondary sound source that sends waves in a new direction. These secondary waves overlap and interfere with each other and the original waves, making the sound less clear. Working together, diffraction and reflection can send sounds to every part of a room.