NCERT Class 9 Science Solutions: Chapter 12 – Sound Part 7 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 15:

Q. What is reverberation? How can it be reduced?


Persistence of sound (after the source stops producing sound) due to repeated reflection is known as reverberation. As the source produces sound, it starts travelling in all directions. Once it reaches the wall of a room, it is partly reflected back from the wall. This reflected sound reaches the other wall and again gets reflected partly. Due to this, sound can be heard even after the source has ceased to produce sound.

To reduce reverberations, sound must be absorbed as it reaches the walls and the ceiling of a room. Sound absorbing materials like fibreboard, rough plastic, heavy curtains, and cushioned seats can be used to reduce reverberation.

Question 16:

Q. What is loudness of sound? What factors does it depend on?



A loud sound has high energy. Loudness depends on the amplitude of vibrations. In fact, loudness is proportional to the square of the amplitude of vibrations.

Question 17:

Q. Explain how bats use ultrasound to catch a prey.


Bats produce high-pitched ultrasonic squeaks. These high-pitched squeaks are reflected by objects such as preys and returned to the bat՚s ear. This allows a bat to know the distance of his prey.

Question 18:

Q. How is ultrasound used for cleaning?


Objects to be cleansed are put in a cleaning solution and ultrasonic sound waves are passed through that solution. The high frequency of these ultrasound waves detaches the dirt from the objects.

Question 19:

Q. Explain the working and application of a sonar.


SONAR is an acronym for Sound Navigation and Ranging. It is an acoustic device used to measure the depth, direction, and speed of under-water objects such as submarines and ship wrecks with the help of ultrasounds. It is also used to measure the depth of seas and oceans.

Q_19_Image_Showing_Sound_Navigation_Ans_Ranging (SONAR)

A beam of ultrasonic sound is produced and transmitted by the transducer (it is a device that produces ultrasonic sound) of the SONAR, which travels through sea water. The echo produced by the reflection of this ultrasonic sound is detected and recorded by the detector, which is converted into electrical signals. The distance (d) of the under-water object is calculated from the time (t) taken by the echo to return with speed (v) is given by . This method of measuring distance is also known as ‘echo-ranging’ .

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