Class 9, Science, Chapter-11, Lecture-4, Characteristics of Sound (Notes)
SOUND:
Sound is a form of energy produced due to the vibration of objects that causes a sensation of hearing in our ears.
Vibration:
A kind of rapid to and fro motion of an object about a central position is called vibration or oscillation.
Example: a stretched rubber band when plucked vibrates and produces sound.
Activity to show that vibrating bodies produces sound:
Strike the prongs of a tuning fork on a rubber pad and bring it near the ear. We can hear a sound.
If a suspended table tennis ball is touched with the vibrating prong, the ball is pushed away repeatedly.
This shows that the prong is vibrating and vibrating objects produces sound.
Examples of how certain sounds are produced:
In flute, the air column vibrates to produce sound.
In stringed musical instruments like sitar, veena etc, there is a tightly stretched string which vibrates to produce sound. In humans, sound is produced by the voice box or the Larynx. Vocal cords in the larynx vibrate and produce sound.
When a bird flaps its wings, a sound is produced.
Activity to show that sound needs a material medium for its propagation:
Suspend an electric bell in an airtight bell jar. Connect the bell jar to a vacuum pump. If the switch is pressed, we can hear the sound of the bell.
If air is pumped out through the vacuum pump, we cannot hear the sound of the bell.
This shows that sound needs a medium to travel and sound cannot travel in vacuum
Sound Waves are Longitudinal.
Justification
The particles of sound wave do not move from one place to another.
They just oscillate back & forth about their position of rest.
The disturbance is carried forward in the direction parallel to the direction of oscillation.
Hence, sound waves are longitudinal waves.
CHARACTERISTICS OF SOUND WAVES:
Sound waves have 4 characteristics
- Amplitude
- Wavelength
- Frequency
- Speed
DEPENDENCE OF SPEED OF SOUND:
- Nature of the medium:
Speed of sound depends on the nature of the medium(material) through which it travels.
Speed of sound is different in different mediums
The speed of sound is more in solids, less in liquids and least in gases. - Temperature of medium:
The speed of sound also depends on the temperature of the medium.
If the temperature of the medium is more, the speed of sound is more - Humidity of medium:
Speed of sound increases with increase in humidity.
CHARACTERISTICS OF SOUND:
- Loudness:
Loudness of sound is the measure of sound energy reaching the ear per second.
Loudness or softness of a sound wave is the sensation that depends upon its amplitude.
When we strike the top of a table with more force, it vibrates and produces a loud sound.
But, when we strike the top of a table with lesser force, the vibrating table top produces soft sound waves.
Louder sound has more amplitude. - Pitch:
Pitch is the sensation (brain's interpretation) of the frequency of an emitted sound.
The pitch of sound (shrillness or flatness) depends on the frequency of vibration.
A high pitch sound is called a shrill sound
A low pitch sound is called a hoarse sound or heavy sound - Quality or Timbre:
Quality or timbre of sound wave is that characteristic which helps us in distinguishing between two sounds of same loudness and same pitch.
Music vs Noise:
Music is the sound that is pleasant to the ears and Noise is the sound that is unpleasant to the ears.
Tone vs Note:
A pure sound of single frequency is called tone.
An impure sound produced by mixture of many frequencies is called a note. It is pleasant to hear.