Class 9 Science

Range of Hearing

  • Infrasound and Ultrasound
  • Structure and working of human ear

Human ear can hear the sound between frequencies of 20 Hz to 20,000 Hz. So, audible range or range of hearing is between 20 Hz to 20,000 Hz, for human beings. However, children under 5 years of age can hear sounds upto 25000 Hz. Some animals can hear sounds beyond or below the hearing range of humans.

Infrasonic Sound or Infrasound

Sound, below the frequency of 20Hz is called infrasonic or infrasound. Infrasound is produced because of very slow vibration. For example, simple pendulum produces sound below 20Hz. Human beings cannot hear infrasound as their ears are not adapted to hear the sound of such a low frequency.

Many animals such as whale, elephant, rhinoceros, etc. can produce and hear sound having frequencies below 20 Hz.

Ultrasound or Ultrasonic Sound

Sound, above the frequency of 20,000 Hz is called ultrasound. Humans cannot hear the ultrasonic sound. However, many animals such as dogs, cat, bat, monkey, deer, etc. can hear ultrasound.

Bats catch their prey by producing ultrasound. Bat produces ultrasound and detects the reflected sound waves coming from an obstacle, such as a prey. By detecting the reflection of ultrasound, bat understands the position and type of prey or of any obstacle in the way. Some aquatic animals, such as dolphin, also use ultrasound to catch their prey.

Uses of Ultrasound

Ultrasound is the sound wave of high frequency. Because of high frequency, ultrasound is associated with more energy and can penetrate upto a large extent. This characteristic of ultrasound makes it very useful for many purposes. Some of its uses are given here:

  • In detection of ailments in the human body.
  • In cleaning of machinery parts which are beyond reach without disassembling of parts.
  • Detection of any deformities in metal blocks.
  • Detection of any blockade in pipe lines.


The full form of SONAR is SOund Navigation And Ranging. This is a device which is used to measure depth of sea bed, locate scraps, wrecks, submarines of enemies, etc. in the water by producing ultrasound. It is fitted over ships and submarines.

Ultrasonic sound waves are produced by SONAR and when these waves return after reflection from anything in water, they are analyzed with the help of computer. The shape and position of objects under sea and ocean is detected on the basis of speed and nature of reflected ultrasound waves.

Human Ear

structure of human ear

The human ear can be divided into three main parts, viz. external ear, middle ear and internal ear.

External Ear

The external ear is outside the body and is also called pinna. It extends into the ear canal.

Middle Ear

The middle ear is composed of the ear drum or tympanum and the bone ossicles. There are three bone ossicles, viz. the hammer, the anvil and the strirrup.

Internal Ear

The internal ear is composed of a cochlea and three semi-circular canals. The cochlea makes the hearing apparatus and the auditory nerve from it goes to the brain.

Working of Human Ear

The external ear catches sound waves and channelizes them to the ear drum, via the ear canal. During compression, the pressure increases outside the ear drum which forces the eardrum to move inwards. During rarefaction, the pressure decreases outside the ear drum which forces the eardrum to move outwards. Thus, a vibration is produced in the eardrum. Further, the three bones amplify the sound wave, by vibrating in turns. In the inner ear, vibrations are converted into electrical signals. These signals are transmitted by the auditory nerve to the brain. Finally, the brain interprets those signals as sound.

Matter in Our Surroundings

Anything that has both mass and volume is called matter. You can also say that anything which has mass and which occupies space is called matter.

Is Matter Around Us Pure?

Elements and compounds are pure substances. All other substances are mixtures which means they are not pure substances.

Atoms and Molecules

Read about law of conservation of mass, law of constant proportions and Dalton's atomic theory.

Structure of Atoms

Atom is made of three particles; electron, proton and neutron. These particles are called fundamental particles of an atom or sub atomic particles.

Cell: The Fundamental Unit of Life

A cell is capable of independent existence and can carry out all the functions which are necessary for a living being.


A groups of cells which is meant for a specific task is called tissue. Tissues are the first step towards division of labour in complex organisms.

Diversity in Living Organisms

Without proper classification, it would be impossible to study millions of organisms which exist on this earth.


If an object changes its position with respect to a reference point with elapse of time, the object is said to be in motion.

Force & Laws of Motion

Force has numerous effects. Force can set a stationary body in motion. Force can stop a moving body.


Earth attracts everything towards it by an unseen force of attraction. This force of attraction is known as gravitation or gravitation pull.

Work & Energy

When force is exerted on an object and object is displaced, work is said to be done. It means work is the product of force and displacement.


Sound is a type of energy. Sound travels in the form of wave from one place to another.

Why Do We Fall Ill

Health is a state of physical, mental and social well being. A condition in which the affected person is unable to carry out normal activities is termed as disease.

Natural Resources

Resources which are obtained from nature are called natural resources. Examples: Air, water, soil, wood, etc.

Improvement in Food Production

Food security is said to exist when all people, at all times, have physical and economic access to sufficient, safe and nutritious food to meet their dietary needs and food preferences for an active and healthy life.