18. Sound : Production of 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.

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.

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.

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.

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.

Time period of oscillation and frequency The time required by an oscillator to complete one oscillation is called the time period of the oscillator. In the previous activity, the oscillator traverses the distance from the extreme position B to central position A and from there to the position C, then back again to A and from A to B. The time required by the oscillator to traverse this distance B-A-C-A-B, is the time period of oscillation (T) of the oscillator. The number of oscillations completed by an oscillator in one second is called the frequency of oscillation.

Take a plastic ruler and hold one of its ends pressed down on the table, as shown in the picture, so that a large portion of the ruler is off the table. Now ask your friend to press the free end of the ruler down and release it. Observe what happens. Now press the ruler with your finger at such a point that it stops making any sound. Now pull in 10cm more of the ruler onto the table and repeat the action. Listen to the difference in the first and the second sound. Take note of the difference in the frequency and pitch of the sounds. Also, take note of what happens by further decreasing the length of the free part of the ruler little by little.

High and low pitch of sound

Take a wooden board, 80 to 90 cm long and 5 cm wide. Hammer two nails into it, each a few centimeters away from its two ends. Tie the ends of a thin metal wire to the nails so that the wire is stretched tightly between them. Insert a wedge-shaped wooden or plastic block under the wire near each of the nails and pluck the wire lightly.

 Did you hear any sound ? Observe whether the wire vibrates. Now insert two or three small rectangular blocks below the wedge-shaped block on one side in such a way that the length of the wire does not change. Observe whether there is any change in the tension in the wire. Now pluck the wire with your finger and listen to the sound. Also watch its vibration. Note the difference you perceive in its frequency. What can you infer from this ? When tension in the wire is increased, the frequency is seen to increase and when it is reduced, frequency is also seen to decrease. We also perceive a difference in the sounds generated. When the tension is increased the resulting sound is shriller. When the tension is reduced the sound is also less shrill. This is what is called the high and low pitch of sound.

  1. Which of the sounds, the roar of a lion or the hum of a mosquito has the higher pitch?
  2. What structures in the sitar help to produce higher or lower pitched sounds ?

Intensity of sound – sound level

Loudness or softness of sound is indicated by two terms, namely, intensity of sound and sound level. Sound level is the intensity of sound as perceived by our ears. The intensity of sound is proportional to the square of the amplitude of vibration. For example, if the amplitude is doubled the intensity of sound becomes four times as much.

Audible sound

 The frequency of the sound audible to human beings is between 20Hz and 20,000Hz. We can only hear sounds in this frequency range.

 Infrasonic sound

 Have you ever heard the sound of the movement of our hands or of the movement of leaves falling from a tree ?

That the oscillator makes 3 to 4 oscillations in one second means that the frequency of the sound is 3 to 4 Hz. Humans cannot hear sounds of frequency less than 20 Hz. In all the above examples, oscillations did take place, but no sound was heard. It means that this sound is of a frequency less than 20 Hz. A sound with a frequency less than 20 Hz is called infrasonic sound. Sounds with a frequency less than 20 Hz are produced by some animals, namely, whales, elephants and rhinoceros.

Ultrasonic sound

A sound with a frequency higher than 20,000 Hz is called ultrasonic sound. Human beings cannot hear such sounds. However, some animals, for example, a dog, can hear such sounds.