Maharashtra Board Class 7 Science Chapter 18 Sound Production of Sound PDF Download

Sound: Production Of Sound

Some events are given below. Put a tick mark '✓' in the box if you have experienced the event. If not, put a cross '×' in the box.

1. Clapped your hands together.

2. Played a musical instrument.

3. Burst a fire cracker.

4. Knocked on a closed door.

5. Whistled using the cap of a pen.

6. Placed your palm on a mobile that is ringing.

7. Swung the clapper of the bell and the bell rang.

8. A metal utensil fell down with a clatter.

9. There was a thunderclap in the sky.

10. Put your hand on a speaker which is producing sound.

It is seen from the above examples that sound is generated due to a variety of events. In some examples, sound was generated due to the vibration of an object, for example, the bell, or the strings or diaphragm of a musical instrument; while in some examples like bursting a cracker, clapping, a lightning strike, vibrations are not actually felt. However, vibrations are produced in those cases as well. All these vibrations are imparted to the molecules in the air and sound is produced. You might have seen that, when a stone is thrown into the calm water of a lake, waves are generated and they reach up to the banks of the lake. Vibrations reach our ears through the air in a similar way and the sound is heard.

When a singer tunes the musical instruments before he starts singing, what exactly does he do? He ensures that the tanpura will produce the required notes, by adjusting the tension in its strings i.e. he 'tunes' the tanpura. A tabla player tunes the tabla by hammering the pegs to adjust the tension in the diaphragm of the tabla. The harmonium accompanist finds out beforehand the key in which the singer will sing. To tune an instrument is to adjust how high or low the pitch of the notes produced will be. The pitch of a sound depends upon its frequency. In Indian music, the musical notes, Sa, Re, Ga, Ma, Pa, Dha, Ni, are of increasingly higher pitch. In scientific terms frequency is a measure of pitch.

You have learnt how sound is generated, how it reaches us on travelling through some medium and is heard by us. You have also seen that vibration of an object is necessary for generation of sound.

In the present lesson we will learn more about vibration, pitch, intensity and level of sound.

Teacher's Note

Sound is made when things move back and forth very fast. Like when you clap your hands, they move and make sound. Just like when a drum is hit, it moves and makes sound.

Exam Trick

Remember: All sound needs vibration. No vibration = no sound. Think about a mobile phone on silent - it does not vibrate, so you cannot hear it.

Points to Remember

Sound is made by things that vibrate or shake.
Vibrations move through air and water to reach our ears.
Different sounds are made by different vibrations.
Pitch means how high or low a sound is.
Frequency tells us how fast something vibrates.

When the string of a musical instrument such as a tanpura is plucked, the string can be seen to vibrate but the two ends of the vibrating string are still. As it vibrates, the string moves to one side of the central position and comes back to the central position. This motion of the string is repeated again and again at fixed intervals of time. Such motion is called periodic motion.

Teacher's Note

Periodic motion means something moves back and forth in the same way again and again. Like a swing in a park - it goes forward and backward in the same way every time.

Exam Trick

Remember: Periodic = repeating. Like your school bell rings periodically - at the same time every day. Back and forth motion that repeats is periodic motion.

Points to Remember

A vibrating string moves side to side in the same pattern.
This back and forth motion happens again and again.
The motion follows a fixed time pattern.
This is called periodic motion.
Sound is made by this repeating motion.

Sound is generated by the rhythmic vibration of an object. We can hear the sound as long as the object vibrates. But, when we touch the vibrating object with our hand, the vibrations stop and we no longer hear the sound. Sometimes we can see the vibrations, but sometimes, the vibrations are so minute that we cannot see them with our eyes.

Make a list of musical instruments you are familiar with. Find out which part of the instrument vibrates and produces the sound.

Such vibrations, that produce sound, can be studied with the help of a simple 'oscillator'.

Oscillator, Oscillation And Oscillatory Motion

You must have seen children playing on a swing in a garden. Observe carefully the motion of the swing. Go to a swing at rest in a garden and mark its position on the ground below it. You can call this mark the central position of the swing. Now pull the swing to one side and let it go. Observe how it swings.

The swing will be seen to cross the central position again and again as it moves from one end to the other of its swing.

A swing that moves back and forth like this, is an oscillator. When the swing moves from one end to the other and returns to its starting point, it is said to have completed one oscillation. The back and forth motion of an oscillator on either side of a central position is called oscillatory motion.

Teacher's Note

An oscillator is anything that swings back and forth. A child on a swing is an oscillator. The swing keeps going back and forth in a regular pattern.

Exam Trick

Remember: One oscillation = one complete back and forth movement. Like a pendulum swinging from left to right and back to left is one oscillation. Count: left to right, then right to left = one complete oscillation.

Points to Remember

An oscillator moves back and forth regularly.
One oscillation is one complete back and forth movement.
The center point is where the oscillator starts.
Oscillatory motion is the back and forth movement.
Swings, pendulums, and vibrating strings are all oscillators.

Take an empty porcelain bowl or an empty steel glass. Stretch a rubber band and fix it on the bowl or glass as shown in the picture. Now give a jerk to the rubber band. Repeat this action applying a smaller or greater force. While doing this, observe the farthest distance to which the rubber band is stretched. Take note of the sound generated.

When the rubber band is stretched and released it vibrates. Compare the vibrations with the figure alongside. When the rubber is stretched from the original position A and comes to position B, it is seen to be curved. The maximum distance between the original position A and the position B on stretching the rubber, is called the amplitude of vibration.

When a greater force is applied to the rubber, it is stretched further, meaning that the amplitude increases. On releasing it, a louder sound is generated. When a smaller force is applied, the rubber is stretched less. Then the amplitude is smaller and the sound is softer, too.

Take a strong thread, about half a meter long. Tie a small iron or wooden ball to it and suspend it from a support as shown in the figure. Such an oscillator is called a pendulum.

Give an oscillatory motion to the pendulum. The maximum distance between the original positions A of the pendulum and the extreme position B or C is called the amplitude of oscillation. In the figure, AB or AC is the amplitude of oscillation.

Teacher's Note

Amplitude means how far something moves from its center. If you push a swing a little, it does not go far - that is small amplitude. If you push it a lot, it goes far - that is large amplitude.

Exam Trick

Remember: Amplitude = how far it goes from center. More force = bigger amplitude = louder sound. Less force = smaller amplitude = softer sound.

Points to Remember

Amplitude is the farthest distance from the center point.
When you push harder, the amplitude gets bigger.
When amplitude is bigger, the sound is louder.
When amplitude is smaller, the sound is softer.
You can see amplitude by watching how far something moves.

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