WAVE

          Wave motion is a form of disturbance which travels through a material medium due to the repeated periodic motion of the particles of the medium about their mean positions.

SOUND

          Sound is a form of energy which produces the sensation of hearing. These are longitudinal mechanical waves.

Definitions

• Amplitude(a): The maximum displacement of a particles from the mean position is called amplitude. Its unit is metre.
• Time period (T): Time taken by a particles of the medium to complete one vibration is called time period. Its unit is second.
• Frequency (n): The number of vibrations completed by a particles in one second is called frequency. Its unit is hertz. N=1/T

Wave length (λ)

• Distance moved by a wave during the time a particle completes one vibration. Its unit is metre.

Velocity of a wave or Relation between Velocity, wavelength and Frequency

          Distance traveled by a wave in T second= λ

                   / v=n λ

• Sound travels almost five times faster through water and twenty times faster through iron than it travels in air.
• Speed of light (3×10⁸ m/s) is much faster than the speed of sound (340 m/s) . Light travels almost million times faster than sound.
• Due to this reason lightning flash is seen first and thunder sound is heard next.

Echo

• Therefore, the hear echo, the barrier reflecting the sound be least at a distance of 17 meters.
• Echoes may be heard more than once due to successive or multiple reflections.
• The rolling of thunder is due to the successive reflections of the sound from a number of reflecting surfaces such as cloud and land.

Reverberation

• The repeated reflection that results in the persistence of sound is called reverberation.
• To reduce reverberation, the roof and walls of auditorium are generally covered with sound absorbing materials like compressed fiber board.

Range of Hearing

• We can hear sound of frequencies ranging from 20Hz to 20,000 Hz.
• This range of frequencies, sensed by our ear is known as the audible range of sound for human.
• Sounds of frequencies above 20,000Hz are known as ultrasonic.
• Sometimes sound produced by bats, dolphins are ultrasonic.
• Sound of frequencies below 20 Hz are called infrasonic.

Heinrich Rudolf Hertz (1857-94)

• Hertz gave the first experimental proof of the existence of radio waves.
• The frequency which is measured in cycles/second was changed as hertz.

Audible range of sound (in Hertz) for Human and certain animals.

                   Human                  20-    20,000

                   Elephant               16-    12,000

                   Bat                     1000-1,50,000

                   Dolphins               70-1,15,000

                   Dog                      40-    46,000

Sonar (Sound Navigation and Ranging)

• Sound is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater object and depth of the sea.
• The detector converts the ultrasonic waves into electrical signal which are appropriately interpreted.

Echo Ranging

• It is used determine the depth of the sea and to locate under water hills submarine, icebergs, sunken ship, etc.

Doppler Effect in sound

• The phenomenon of the apparent change in the frequency of the source due to relative motion between the source and the observer is called as Doppler’s effect.
• He first proposed in concerning the colored light of double stars in 1842. This principle is known as the Doppler effect.

Uses of Doppler effect in sound

• RADAR (Radi Detection And Ranging) Doppler effect principle is used in RADAR to determine the velocities and movement of submarines and aircrafts.
• Traffic control vehicles direct microwaves on speeding vehicles. The waves reflected by the moving vehicles act as a moving source. From the Doppler shift in frequency, the speed of vehicles are detected.
• The Doppler shift of radar waves are used in airport to find the height speed and distance of approaching aircraft.
• Bats send out and receive ultrasonic wave reflected by the prey and obstacles. Bats detect the location, distance and movement of the prey by the Doppler shift.

Sound Produced by humans

• In human beings, the sound is produced by the “Voice Box” or the Larynx.
• The vocal cords in men are about 20 mm long. In women, these are about 15mm Children have very short vocal cords.
• The Voice Box has two “vocal cords” They are stretched across the voice box in such a way that it leaves a narrow slit between them for the passage of air.
• When we speak, the lungs force air through the slit and the vocal cards , vibrate, producing sound.

Human ear and Hearing

• Human ear has three important parts. Only one of its parts can be seen and felt by you, which is the outer ear.
• When sound enters the ear, it travels down a canal at the end of which a thin membrane is stretched tightly called “ear drum” which performs a very important function.
• The middle ear three tiny interlocked bones. The inner ear has a coiled organ of hearing semi circular canals and the auditory nerve.
• A vibrating body causes air molecules to vibrate. These vibrations reach our ear and are collected by the pinna and then funneled into the ear tube.
• These vibrations strike the eardrum and start vibrating. The ear drum sends vibrations to the inner ear From there the signal goes to the brain.
• A tuning fork is made of steel. The two upper ends of the tuning fork are called the ‘prongs’ while the lower end is called the ‘stem’.
• Frequency (n): The number of oscillations per second is called the frequency. Frequency is expressed by hertz-Hz.
• Time period (T): The time taken by the vibrating body to complete one vibration or oscillation is called the time period. The unit of period is second(s).
• Amplitude (a): The maximum displacement of a vibrating body from its mean position is called amplitude. The unit of amplitude is metre(m).
• The relation between frequency (n) and time period (T).
• The period of oscillation is the reciprocal of the frequency

(T=1/n)

• Sound waves of frequencies above 20,000Hz are called ultrasonic waves. Bats use ultrasonic waves for their flight.
• Some animals can hear the sound of frequencies higher than 20,000Hz. Dogs have this ability.

Noise

• The unpleasant sound is called as noise.

Noise pollution

• The unwanted sound from any source that causes discomfort of any kind is called noise pollution.

Science today

• An optical fiber is a device based on the principle of total internal reflection.
• Optical fibers are thin, flexible and transparent strands of glass which can carry light along them very easily.
• A bundle of such thin fibers forms a light pipe.
• Even if the fiber is bent or twisted, the light can easily travel through tithe fiber.
• The method of using optical fibers to carry images and messages is called fiber optics.