Class nine NCERT Sol and Sound Important Points

Chapter 12: Sound

Important Points

Propagation of Sound

The source of sound creates a disturbance in the medium which travels through the medium. The particles of the medium do not move forward but the disturbance is carried forward. This is the propagation of sound in a medium, hence, Sound can be visualised as a wave.

Sound Needs a Medium to Travel: The substance through which sound travels is called a medium. It can be solid, liquid or a gas. A sound wave is a mechanical wave and requires a material medium like air, water, steel etc. for its propagation. A sound wave cannot travel in a vacuum.

Sound waves are longitudinal waves: A wave in which the particles of the medium oscillate (vibrate) to and fro about their mean position in the direction of propagation of the wave is called a longitudinal wave.

Longitudinal waves can be produced in all the above media such as solids, liquids and gases. This is exactly how a sound wave propagates, hence sound waves are longitudinal waves. There is also another type of wave, called a transverse wave. In transverse waves, the individual

particles of the medium move about its mean position in a direction perpendicular to the direction of wave propagation. For example, light is a transverse wave (but it is not a mechanical wave). A transverse wave can be produced only in solids and liquids but not in gases.

Characteristics of a Sound wave: A sound has four characteristics such as loudness, pitch (or shrillness) quality (or timbre) and intensity.

Class nine NCERT Sol and Sound Important Points

(i) Loudness: It is the measure of the sound energy reaching the ear per second. Greater the sound energy reaching our ears per second, louder the sound will appear to be. The loudness of a sound depends on the amplitude of the vibration producing that sound. Greater is the amplitude of vibration, louder is the sound produced by it. The amplitude of the sound depends upon the force with which an object is made to vibrate.

The loudness of sound is measured as the decibel (dB) unit. It depends on the sensitivity or the response of our ears.

(ii) Pitch or Shrillness: The characteristic of sound by which a shrill note can be distinguished from a flat note is called pitch. The pitch of a sound depends upon its frequency. The higher the frequency of a sound, the higher is the pitch.

The voice of a child or a woman has a higher frequency than the voice of a man. that is why the voice of a child or woman is more shrill as compared to the voice of a man. This is because men have

long vocal cords.

(iii) Quality (or Timbre): The quality of sound is the characteristic which enables us to distinguish between the sounds produced by different sources. The more pleasant sound is said to be of rich quality. A sound of a single frequency (called pure sound) is called a tone. The sound produced due to a mixture of several frequencies is called a note

and is pleasant in listening too. Noise is unpleasant to the ear, music is pleasant to the ear and is of rich quality.

(iv) Intensity: The amount of sound energy passing each second through unit area is known as the intensity of sound. The S.I. unit of intensity is watt per square metre (W/m2).