SOUND.

Specific objectives.

• Define sound.
• Describe and perform simple experiment to show that sound is produced by vibrating bodies.
• Perform and describe experiment to show that sound requires a material medium for its transmission.
• Explain the properties of sound.
• Determine the speed of sound in air by echo metal.
• State the factors affecting the speed of sound.
• Solve numerical problems involving speed of sound.

                                    Sound.

• It is a rate of compressionate wave frequencies at which the human ear is sensitive.
• It is a form of energy that can be detected by using the human ear. This range frequencies is called audio frequency and extend from about 20Hz to 20KHz.
• Those frequencies below the audio range (20KHz) are called infra sonic frequencies and those above audio range (20KHz) are called ultra sonic frequencies.
• Sound is a progressive longitudinal and mechanical wave.

                        Sources or products of sound.

• Sound is produced by any vibrating material medium with frequencies within the audio range.
• Energy is required to make the material vibrate.
• The nature of vibration determines the type of sound produced (high or low sound).
• Sound is produced by;

1. Stretched skin or the drum head.
2. Air in an organ pipe.
3. A stretched wire when plucked.
4. Turning fork.
5. Wings of some insects.
6. Water flowing in a river.
7. Vocal card.

Transmission or propagation of sound.

• Sound is an elastic or mechanical wave hence require material medium for its transmission.
• Sound requires matter for its transmission (solid, liquid and gases).
• Solid transmit sound better than liquid and gas.
• The speed of sound in solid i.e. steel is 15x that in air and in water is 5x that in air.
• Speed in air under room temperature is 340m/s and for water is 1500m/s.
• The speed in sound varies with the temperature of the medium.

                        Speed of sound as mechanical wave.

EXP: SOUND REQUIRES A MATERIAL MEDIUM FOR ITS TRANSMISSION.

Requirements.

1. Electric bell.
2. Bell jar.
3. Vacuum pump.
4. Power source.

                        Procedure.

1. Set the apparatus as shown below.
2. Switch on the current to start the bell ring.
3. Switch on the vacuum pump to remove air from the bell jar.
4. Stop the pump and re- admit the air into the jar.

                        Observations.

• When the bell jar is switched on with air inside, the sound is heard.
• As the air is reduced in the jar, the sound becomes fainter or the pitch reduces.
• When all the air is removed, the hammer is still seen strucking the bell but no sound is heard.
• When the air is admitted into the jar, the sound becomes audible again.

                        Conclusion.

• Sound does not travel through a vacuum but requires a material medium.

                        Properties of sound.

• Sound as a wave have the same properties as waves which include

1. Reflection.
2. Refraction.
3. Diffraction.
4. Interference.

                                Reflection.

• Reflection id the bouncing of sound as it falls on a surface.
• Sound can be reflected and obey the laws of reflection. The reflected sound is called echo.
• It is more profound on a hard surface such as a wall and a metal surface.
• Hard surface reflect sound more that soft surfaces.
• For an echo to be heard separately, the reflecting surface should be 17m or more away from the source.

EXP: TO SHOW REFLECTION OF SOUND.

                        Requirements.

1. 2 long tube, 1m.
2. Source of sound clock.
3. Clamp and stand.
4. A reflecting surface.

                        Procedure.

1. Set the apparatus as shown below.
2. Place your ear close to the second tube and listen to the reflected sound at different angles.

                        Results.

• It is found that the reflected sound is loudest when;

1. The angle of reflection r is equal to the angle of incident i.
2. Both tubes and the normal to the reflecting surface lie in the same plane.

Use of reflection of sound.

1. To measure the velocity of sound.
2. Pulse- echo technique to measure the depth of sea.
3. Speaking tubes used in ships for communication.
4. Special type of spectacles used by blind people to tell how far objects are ahead of them. They have a transmitter that emits ultra sound and receivers that collect the echo and converts them to audible sounds.

EXP: TO MEASURE THE SPEED/VELOCITY OF SOUND.

                        Requirements.

1. Tape measure.
2. Stop watch.
3. Source of sound.
4. Reflecting surface (a tall building or wall).

                      Procedure.

1. Measure a distance of 50cm from the tall building.
2. Produce sound by clapping your hands until an echo is heard.
3. Make the time interval between the original sound and the echo to be at least 0.5 seconds.
4. Time for 20 intervals

                        Results.

• Total time for 20 intervals t___________(s)
• Time for one echo clap T= t/20 sec
• Distance between source and reflector d _____________
• Total distance covered by the sound D= 2d

            The velocity V= Total distance/time

                                 V= 2d/t

                        Factors affecting the speed of sound.

1. Temperature.
2. Density.
3. Humidity.
4. Elasticity.

       1. Temperature.

• It determines the average kinematic energy of molecules of material medium through which the sound is transmitted.
• If the temperature increases, the speed of sound increases as the particles will vibrate more and more faster.

2. Density.

• The velocity of sound varies inversely as proportional to the square of density of the transmitting medium.
• If the molecules are heavy, they move less readily and sound progresses through such a medium more slowly.

3. Humidity.

• Is the amount of water vapor in the atmosphere.
• Sound travels lightly faster in moist areas than dry areas because moist areas contain a greater number of lighter molecules.

                                    Refraction.

• Sound can be refracted and the most common occurrence is known as atmospheric refraction and it is caused by temperature changes in the atmosphere.
• Sound is more audible at night than during the day because the speed of sound in warm air exceeds that in cold air hence refraction occurs.

                                    At night.

• The air is usually colder near the ground than it is higher up. Refraction towards the earth occurs.
• Sound waves coming from a source close to the ground are therefore bent downwards.
• The upper portion of the wave front now travels faster and the wave front is refracted towards the ground.
• Under this condition, sound energy is not dispersed but concentrated to the ground hence sound is heard at a large distance from the source.

                                    During the day.

• The layer of the air close to the ground is much warmer than the air above.
• When sound is produced from a source close to the ground, the lower part of the wave front moves faster than the upper part due to high temperature in the bottom layers. The wave front is gradually charged in direction so that sound is refracted away from the ground and dispersed into the space.

                        Stationary wave in sound.

Vibrating string.

• When a stretched string is plucked at some point, a wave is produced which is reflected at one end.
• The incident and reflected sound interfere and produces nodes and antinodes (stationary waves).
• If the frequency of the node heard is 256Hz and speed of sound is 340m/s, determine the length of the vibrating wire.