Range of Hearing

(i) Range of hearing in human is 20 Hz to 20000 Hz.

→ Children younger than 5 years and dogs can hear upto 25 Khz.

(ii) The sounds of frequencies lower than 20 Hz are

known as ‘infrasonic sounds’

→ A vibrating simple pendulum produces infrasonic sounds.

→ Rhinoceroses communicate each other using frequencies as low as 5 Hz.

→ Elephants and whales produces infrasonic waves.

→ Earthquakes produces infrasonic waves (before shock waves)

which some animals can hear and get disturbed.

(iii) The sounds of frequencies higher than 20 KHz are known as ‘ultrasonic waves’.

→ Dogs, parpoises, dolphins, bats and rats can hear ultrasonic sounds.

→ Bats and rats can produce ultrasonic sounds.

Hearing Aid

→ It is battery operated electronic device used by persons who are hard of hearing.

Microphone convert sound into electrical signals, than those are amplified by amplifier. Ampified

signals are send to the speaker of hearing aid. The speaker converts the amplified signal to sound

and sends to ear for clear hearing.

Applications of Ultrasound

(i) It is used to detect cracks in metal blocks in industries without damaging them.

(ii) It is used in industries to clean ‘hard to reach’ parts of objects such as spiral tubes, odd shaped

machines etc.

(iii) It is used to investigate the internal organs of human body such as liver, gall bladder, kidney

uterus and heart.

(iv) Ecocardiography: These waves are used to reflect the action of heart and its images are

formed. This technique is called echocardiography.

(v) Ultrasonography: The technique of obtaining pictures of internal organs of the body by using

echoes of ultrasound waves is called ultrasonography.

(vi) Ultrasound is used to split tiny stones in kidneys into fine grains.



→ The word ‘SONAR’ stands for ‘Sound Navigation And Ranging’.

→ SONAR is a device which is used to find distance, direction and speed of underwater objects

→ SONAR consists of a transmitter and a receptor or detector and installed at the bottom of a sea

→ The transmitter produces and transmits ultrasonic waves.

→ These waves travel through water and after striking the objects on the bottom of sea, are

reflected back and received by detector.



→ These reflected waves are converted into electric signals by detector.

→ The sonar device measures the time taken by ultrasound waves to travel from ship to bottom of

sea and back to ship.

→ Half of this time gives the time taken by the ultrasound waves from ship to bottom.

• Let the time interval between transmission and reception of ultrasound signal is t.

Speed of sound through sea water is v

Total distance travelled by waves = 2d.

→ The sonar is used to find the depth of sea, to locate underwater hills, valleys, submarines, ice

and sunken ships etc.

→ Bats fly in the dark night by emitting high pitched ultrasound waves which are reflected from

obstacle or prey and returned to bats ear.

→ The nature of reflection tells the bat where the obstacle or prey is and what it is like.