Sound is produced when sound waves are emitted by a vibrating source and they are propagated in a material medium with certain velocity.
Sound waves are longitudinal waves with alternate compressions and rarefactions. Sound waves always require a medium to travel and so they can never travel in vaccum.
Propagation of sound waves :-
- Transverse Waves : – They have alternate crests and troughs. Thus, their propagation involves change in shape of a body (medium). As solids provide largest resistance to change of shape, so transverse waves propagate best in solids.
- Longitudinal Waves : – They have alternate compressions and rarefactions. So, their propagation involves change in volume or density of a body(medium). As volume or density can be changed in solids, liquids or gases, so longitudinal waves can propagate in any of these mediums.
Sound Waves in different mediums
- Solids : Sounds coming through walls, windows, floors etc of a building, sound of an approaching rail when heard in rail tracks.Liquids : sound waves coming from submarines underwater.Gases : When we speak in air.
Speed of sound :- Speed of sound depends on the medium. It travels with different velocities in different mediums.
Speed of sound depends on :
- Nature of the medium
- Atmospheric humidity
Speed of sound is generally more in solids than in liquids or gases. This is because Restoring Force is very high in solids.
Note : – study the examples yourself
- Horn of a car
- Lightning and thunder
- Hitting of ball in cricket
Range of Hearing in Humans : –
Loudness : The sensation produced in the ear which enables us to distinguish between a faint sound and a loud sound.
Humans can hear sounds from frequency 20 Hz to 20 Khz. This is known as Audible Range.
Ultrasonic Waves : Waves having frequency more than 20 khz are known as Ultrasonic waves. Sound produced by these waves is known as UltraSound. Bats can produce and hear ultrasonic waves, other examples are dogs, cats, some insects etc. These organisms can hear sound of frequency below 20 Hz also.
Supersonic Waves : When an object travels with speed greater than speed of sound, then it is said to have supersonic speed and waves produced are known as supersonic waves. When supersonic aircrafts fly, then they produce these waves and then they are known as Shock Waves.
Sonic Boom : The sound produced due to shock waves. Shock waves are very high energy waves. The sound produced is very unpleasant and can break glass panes and produce pain in ears.
Reflection of Sound
Whenever a sound wave hits an obstacle and reflects back, this phenomenon is called ECHO. For reflection of sound we need attended objects a large obstacles.
Practical Application of reflection of sound
1 MegaPhone : It is a horn-shaped instrument (tube). This shape prevents spreading prevents spreading of sound waves and allows multiple reflections of sounds. This concept is used in loud-speakers.
2 Hearing Aid : Persons who can’t hear clearly use hearing aids. These hearing aids reflect sound inside the ear. This enhances the amplitude of vibrating layer of air inside the air and helps in improving hearing.
3 Sound Boards : These are concave shaped boards that are placed behind the speaking area in a big auditorium. Speakers are placed on the focus of this concave shape. Sound waves that originate from this speakers reflects from the surface and move parallel to each other thus not interfering with each other. This ensures better hearing at large distances.
Reflection back of sound from obstacles is called an echo.
The sensation of sound remains in our brain for about 0.1 sec.
For an echo, the sound waves should reach us after 0.1 sec of hearing that sound.
Velocity of sound in air = 344 m/s
Distance travelled by waves = 344 X 0.1 = 34.4 m
Þ Distance of an obstacle should be = 34.4/2 = 17.2 m
Multiple Echoes : When waves reflect from many surfaces repeatedly.
Rolling of Thunder :When thunder occurs, the waves get reflected repeatedly from clouds, water vapours, dust particles etc. This causes a rolling of Thunder.
Sound Navigation And Ranging. This principle is based on reflection of sound. Here, we use ultrasonic waves. The main use of SONAR is to detect submerged submarines, rocks, sea bed (depth of the ocean), hidden icebergs etc.
Total distance travelled = 2d
Velocity of wave = v
Time, t = 2d/v
d = vt/2
Notional Use of SONAR :-
It is done by Bats. They use ultrasonic waves to find an obstacle in the way. They hunt like that also for its prey. Using this, they know the distance of their prey.
USES OF SONAR
- To find cavities in metals.
- To find level of a liquid in a metallic tank.
Applications of ultrasonic waves
- Ultrasound is used to clean parts located in
hard-to-reach places. example- Spiral tube, odd shaped parts, electronic
- In detecting cracks and flaws in metal blocks.
- Ultrasonic waves are made to reflect from various parts of the heart. This technique is called ‘echocardiography’.
- Ultrasound scanner uses ultrasonic waves for getting images of internal organs of the human body.
- Ultrasound is used in breaking small ‘stones’ formed in the kidneys into fine grains.