AMPLITUDE, TIME PERIOD AND FREQUENCY OF A VIBRATION

AMPLITUDE, TIME PERIOD AND FREQUENCY OF A VIBRATION

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 oscillatory motion and its time period.

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?

You can recognise many familiar sounds without seeing the objects producing them. How is it possible? These sounds must be different to enable you to recognise them. Have you ever thought what factors make them different? Amplitude and frequency are two important properties of any sound. Can we differentiate sounds on the basis of their amplitudes and frequencies?

Source: This topic is taken from NCERT TEXTBOOK

LOUDNESS AND PITCH

LOUDNESS AND PITCH

Activity 13.11:

*  Take a metallic tumbler and a tablespoon.

*  Strike the tablespoon gently at the brim of the tumbler.

*  Hear the sound produced.

*  Now bang the spoon on the tumbler and hear the sound produced again.

*  Is the sound louder when the tumbler is struck hard?

*  Now suspend a small thermocol ball touching the rim of the tumbler (Fig. 13.17).

*  Vibrate the tumbler by striking it. See how far the ball is displaced.

*  The displacement of the ball is a measure of the amplitude of vibration of the tumbler.

Figure 13.17: Thermocol ball touching vibrating tumbler

Now, strike the tumbler gently and then a little harder. Compare the amplitudes of vibrations of the tumbler in the two cases. In which case is the amplitude larger?

Loudness of sound is proportional to the square of the amplitude of the vibration producing the sound. For example, if the amplitude becomes twice, the loudness increases by a factor of 4. The loudness is expressed in a unit called decibel (dB). The following table gives some idea of the loudness of sound coming from various sources.

Above 80 dB the noise becomes physically painful.

The loudness of sound depends on its amplitude. When the amplitude of vibration is large, the sound produced is loud. When the amplitude is small, the sound produced is feeble. Compare the sound of a baby with that of an adult. Is there any difference? Even if two sounds are equally loud, they differ in some way. Let us see how.

The frequency determines the shrillness or pitch of a sound. If the frequency of vibration is higher we say that the sound is shrill and has a higher pitch. If the frequency of vibration is lower, we say that the sound has a lower pitch. For example, a drum vibrates with a low frequency. Therefore, it produces a low-pitched sound.

On the other hand, a whistle has a high frequency and therefore, produces a sound of higher pitch (Fig. 13.18). A bird makes a high-pitched sound whereas a lion makes a low-pitched roar. However, the roar of a lion is very loud while the sound of the bird is quite feeble.

Figure 13.18: frequency determines pitch of sound

Every day you hear the voices of children and adults. Do you find any difference in their voices? Can you say that the frequency of the voice of a child is higher than that of an adult? Usually, the voice of a  woman has  a higher frequency than that of a man

Source: This topic is taken from NCERT TEXTBOOK

AUDIBLE AND INAUDIBLE SOUNDS

AUDIBLE AND INAUDIBLE SOUNDS

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 the human ear, the range of audible frequencies is roughly from 20 to 20,000 Hz.

Some animals can hear sounds of frequencies higher than 20,000 Hz. Dogs have this ability. The police use high-frequency whistles which dogs can hear but humans cannot.

The ultrasound equipment, familiar to us for investigating and tracking many medical problems, works at frequencies higher than 20,000 Hz.

Source: This topic is taken from NCERT TEXTBOOK