NCERT 9th Class (CBSE) Science: Sound

Question: A sound wave causes the density of air at a place to oscillate 1200 times in 2 minutes. Find the time period and frequency of the wave.

Answer:

Question: Give 3 uses of ultrasound.

Answer: Use of ultrasound:

1. Ultrasound is used to detect cracks and flaws in metal blocks.
2. It is used in ‘echo-cardiography’, the ultrasonic waves are made to reflect from various parts of the heart and form the image of the heart.
3. It is used in ‘ultrasonography’, to detect the image of organs or to detect the abnormalities in the organs. It is also used to examine the foetus during pregnancy to detect congenital defects.

Question: What is the function of middle ear?

Answer: Middle ear consist of three small bones called hammer, anvil and stirrup. These three bones receive the sound vibrations and increase the strength of these vibrations to amplify the vibrations received by ear-drum. These amplified vibrations are further passed to the inner ear.

A ship sends out ultrasound that return from the seabed and is detected after 3.42 s.

Question: If the speed of ultrasound through seawater is 1531 m/s. What is the distance of the seabed from the ship?

Answer: Time between transmission and detection t = 342 s.

Speed of ultrasound in seawater = 1531 m/s.

Question: Distinguish between tone, note and noise.

Answer: Tone: A sound of single frequency is called a tone.

Note: The sound which is produced due to a mixture of several frequencies is called a note.

Noise: The sound which is produced due to a mixture of several frequencies but is unpleasant to the ear is called noise.

Question: Establish the relationship between speed, wavelength and frequency of sound.

Answer: Speed of sound —» The distance travelled by a wave or a point on a wave (compression or rarefaction) per unit time.

Question: Which wave property determines
(1) loudness? (2) pitch? Name the characteristic of the sound which help you to distinguish your friend’s voice while talking in a dark room.

Answer:

1. Loudness is determined by amplitude.
2. Pitch is determined by frequency.

The quality or timber of sound helps us to distinguish our friend’s voice while talking in a dark room.