NCERT Solutions for Class 9 Science Chapter 12
Q1: What are the topics and exercises covered in this chapter?
A1: The topics and concepts included in this chapter are as follows
12.1 Production of Sound
12.2 Propagation of Sound
12.3 Reflection of Sound
12.4 Range of Hearing
12.5 Applications of Ultrasound
12.6 Structure of Human Ear
List of Exercises
Exercise Solutions 22 Questions (7 long answer questions, 5 numerical answer questions, 10 short answer questions)
Production of Sound- 2 Questions
Propagation of Sound- 3 Questions
Reflection of Sound- 4 Questions
Range of Hearing- 1 Questions
Applications of ultrasound- 4 Questions
Structure of Human Ear- 1 Question
Q2: What are wavelength, frequency, time period and amplitude of a sound wave?
A2: (a) Wavelength – Wavelength can be defined as the distance between two consecutive rarefactions or two consecutive compressions. The SI unit of wavelength is the meter (m).
(b) Frequency – Frequency is defined as the number of oscillations per second. The SI unit of frequency is hertz (Hz).
(c) Amplitude – Amplitude can be defined as the maximum height reached by the trough or crest of a sound wave.
(d) Time Period – The time period is defined as the time required to produce one complete cycle of a sound wave.
Q3: What is sound and how it is produced ? Give an example
A3: A sound is a form of energy that is produced by the vibration of objects. Vibrations refer to the rapid to and fro motion of an object. When a body vibrates, a disturbance is created in the medium by the particles. Sound is produced due to the vibration of the objects. For example, A stretched rubber band, when plucked, vibrates and produces sound. Here the disturbance produced by the vibrating body travels through the medium but the particles do not move forward themselves.
Ex: When a school bell is hit with a hammer, it moves forward and backwards producing compression and rarefaction due to vibrations. This is how sound is produced by the school bell.
Q4: Explain the working and application of a sonar.
A4: SONAR is an abbreviation for - Sound Navigation and Ranging. It is an acoustic device used in measuring the direction, speed, and depth of underwater objects. For ex: shipwrecks and submarines that use ultrasound. Along with it, it is also used to determine the depth of oceans and seas.
A beam of ultrasonic sound is produced and it is made to travel through the seawater which is transmitted by the transducer. When it reflects an echo is produced which is detected and recorded by the detector. This detected signal is then converted into electrical signals. The distance of the underwater is represented by - ‘d’ and it is calculated from the time - represented as ‘t’ - taken by the echo to return with speed - represented as ‘v’ - which is expressed as,
2d = v × t.
This method of measuring distance is also referred to as echo-ranging.