UNITS 4 AND 5. SOUND AND LIGHT

1. WHAT IS SOUND?“Sound is a form of mechanical energy which is produced by the vibration

of an object.”Sound travels in waves through solids, liquids and gases, and cannot

travel through a vacuum.Sound travels much faster through solids than through gases. Therefore,

the sound of the train can be perceived through rails before it can be heardthrough the air.

2. HOW CAN WE MEASURE SOUND?

The “frequency” is the magnitude that measures the amount ofvibrations of an object per second. The human ear perceives sounds withfrequencies from 20 Hz to 20,000 Hz (hertz)

The “speed of sound” depends on the medium. In the air, the speed ofsound is 340 m/s. In water or in iron, the speed of sound is much greater.

3. HOW IS SOUND PERCEIVED BY THE EAR?

The air is composed of particles separated from each other. When an object vibrates, it pushes the particles closer together and the air pressure increases. On the other hand, when the particles are separated more they forman area of low pressure.

When these pressure fluctuations reach our ears, the eardrum vibrates, and it transmits this vibration to the ossicles (maleus, incus and stapes), and finally to the cochlea.

Here, the auditory nerve picks up the vibration and sends information to the brain.

4. HOW IS SOUND REFLECTED?Sound is reflected when a sound wave meets an obstacle. The sound

wave bounces off and changes direction.The reflection of sound is used in many different fields. For example,

• ultrasonic waves are used to scan inside to the human body.• Sonar uses the reflection of sound to detect objects at sea. Reflected

sound is used to map the sea floor by measuring the depth of the ocean.

The reflection of sound produces two phenomena: Echoes: An echo is produced when a sound is reflected by an obstacle.

The obstacle must be situated at least 17 m away. Both the initial soundand the reflected sound are heard.

Reverberations: Reverberations occur when a sound is reflected by anobstacle which is less than 17 m away. The initial sound and the reflected

sound are heard, but they are mixed together.

5. SOUND POLLUTIONSound or noise pollution occurs when people or animals are exposed to

levels of sound that are stressful or damaging to hearing. Noise is caused byroad traffic, aircraft, trains, industries, and entertainment centres, such as barsand discotheques.

How to reduce sound pollution• Buildings can be insulated to keep out loud noises.• Aircraft flight paths can be diverted away from cities.• Traffic can be reduced in city centres.• Legislation can establish limits on the intensity of noise from

entertainment centres.

ACTIVITIES about SOUND

1. How does sound travel? Can it travel in a vacuum?

2. You know the speed of the sound in the air. How long does your voicetake to travel 200 metres through the air?

3. Who would hear the sound of a motor boat first: a diver under water or afisherman on a boat? They are at the same distance from the motor boat.

4. Describe what happens when sound waves hit an obstacle: Sound isreflectedwhen.................................................................................................................The soundwaves...............................................................................................

5. In a storm, a few seconds after lightning strikes, thunder is heard. If youhear thunder four seconds after a bolt of lightning, how far are you fromthe storm?

6. A person standing opposite a mountain makes a sound. She hears theecho two seconds later. How far is she from the mountain?

7. A fishing boat uses sonar to detect a whale. The receptor makes a soundwave and 0.2 seconds later it detects the echo. Sound travels throughwater at 1,500 m/s, so calculate...a) the distance travelled by the sound wave in 0.2 seconds.b) the distance form the boat to the whale.

8. If you touch a bell when it is ringing, the sound stops immediately. Why isthis?

9. In you opinion, why do architects take the reflection of sound intoaccount when they are designing an auditorium?What can happen if theauditorium does not have the right structure?

10. Does sound travel faster through a block of wood , or an inflatedballoon?

11. If from the summit of a mountain you scream "Hello!" and three and five

seconds, respectively, the two mountains (in front of you) return to you the greeting, how far away are you from each mountain?

12. Imagine you're between two mountains which are far away from you800 m and 1500 m,a) If you scream, how long afterdo you hear the two echoes?b) Do you think that there would be more echoes? If so, how long after doyou hear them?

13. Copy and complete the following sentences:a) It is called “frequency” to per second and the unitis called .b) Human beings can perceive sounds with frequencies between and Hz.c) the sound waves need to be able propagate and they never can be transmitted .

14. If a storm is located 3 km away from us, how long does the thunder takes to reach us?