How do you come to know that a ‘period’ is over in your school? You come to know easily that someone is at your door when he knocks or you hear the sound of the doorbell. Most of the time you can make out that someone is approaching you by just hearing the foot steps.
Touch the school bell when not in use.
What do you feel? Again touch it when
producing sound. Can you feel it
Take a metal plate (or a shallow frying pan). Hang it at a convenient place in such a way that it does not touch any wall. Now strike it with a stick (Fig.13.2). Touch the plate or pan gently with your finger. Do you feel the vibrations?
Speak loudly for a while or sing a song,
or buzz like a bee. Put your hand on
your throat as shown in Fig. 13.8. Do
you feel any vibrations?
In humans, the sound is produced by the voice box or the larynx. Put your fingers on the throat and find a hard bump that seems to move when you swallow. This part of the body is known as the voice box. It is at the upper end of the windpipe. Two vocal cords, are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air (Fig.13.8).
When you call up your friend who is
standing at a distance, your friend is
able to hear your voice. How does the
sound travel to her?
Take a metal glass tumbler. Make sure that it is dry. Place a cell phone in it. (Remember that the cell phone must not be kept in water.) Ask your friend to give a ring on this cell phone from another cell phone. Listen to the ring carefully. Now, surround the rim of the tumbler with your hands (Fig. 13.10). Put your mouth on the opening between your hands. Indicate to your friend to give a ring again. Listen to the ring while sucking air from the tumbler.
The shape of the outer part of the ear is like a funnel. When sound enters in it, it travels down a canal at the end of which a thin membrane is stretched tightly. It is called the eardrum. It performs an important function. To understand what the eardrum does, let us build a tin can model of the eardrum.
We have learnt that the to and fro motion
of an object is known as vibration. This
motion is also called oscillatory motion.
You have already learnt in earlier classes
about the oscillatory motion and its
The number of oscillations per second is called the frequency of oscillation. Frequency is expressed in hertz. Its symbol is Hz. A frequency of 1 Hz is one oscillation per second. If an object oscillates 20 times in one second, what would be its frequency?
We know that we need a vibrating body
for the production of sound. Can we
hear the sound of all vibrating bodies?
The fact is that sounds of frequencies less than about 20 vibrations per second (20 Hz) cannot be detected by the human ear. Such sounds are called inaudible. On the higher side, sounds of frequencies higher than about 20,000 vibrations per second (20 kHz) are also not audible to the human ear. Thus, for human ear, the range of audible frequencies is roughly from 20 to 20,000 Hz.
We hear different types of sounds
around us. Is the sound always
pleasing? Does a sound sometimes
cause discomfort to you? Some sounds
are pleasant to the ear, whereas some
Suppose construction work is going on in your neighbourhood. Are the sounds coming from the construction site pleasing? Do you enjoy the sounds produced by horns of buses and trucks? Such unpleasant sounds are called noise. In a classroom, if all the students speak together, what would the sound produced be called?
You already know about air pollution. Presence of unwanted gases and particles in air is called air pollution. Similarly, presence of excessive or unwanted sounds in the environment is called noise pollution. Can you list some sources of noise pollution? Major causes of noise pollution are sounds of vehicles, explosions including bursting of crackers, machines, loudspeakers etc. What sources in the home may lead to noise? Television and transistor radio at high volumes, some kitchen appliances, desert coolers, air conditioners, all contribute to noise pollution.