NCERT Solutions for Class 9 Science Chapter 12

Music:
1. It has a pleasant effect on the ear.
2. It consists of a series of sound impulses which follow one another regularly.
3. The frequency of musical sound is high
4. There are no sudden changes in amplitude (loudness) of the wave constituting a musical sound.
Noise:
1. It has an unpleasant effect on the ear.
2. The sound impulses do not follow one another regularly.
3. The frequency of noise is low.
4. There are usually sudden changes in amplitude (loudness) of the wave forming a noise.

It is an acronym which means Sound Navigation And Ranging.
Sonar is a device which measures the distance, direction and speed of objects lying under water ultrasonic waves.
The above method of finding the distance of an object is called echo-ranging as it is based on echo principle. The sonar technique is used to:
1. To locate the position of other ships or submarines. Ship to ship communication also uses ultrasonic waves. The advantage of using ultrasonic waves is that these waves cannot be heard without the aid of special instruments.
2. Determine depth of the sea , called echo depth ranging. When used for purpose , it is called fathometer as it directly fathoms the depth of the sea.
3. Locate underwater hills, valleys, icebergs, submarines and sunken ships.

250 m

0.0004 m

Sound is a form of energy and it is produced due to vibrations of different types of object. For example: A vibrating tuning fork, a bell, wires in a sitar and a guitar etc.

A sound wave is called a longitudinal wave because it travels in the form of compressions and rarefactions in the medium, where the particles of the medium vibrate in a direction which is parallel to the direction of propagation of the sound wave.

The timbre of sound is that characteristic which enables us to distinguish one sound from the other even when these are of the same pitch and loudness. Each person has its own timbre of sound and this characteristic helps us to identify a person from others even without looking at him (i.e., in a dark room).

Since the frequency of the source of sound is 100 Hz, Number of vibrations of the source in 1 second = 100 Number of vibrations of the source in 1 minute (i.e., 60 second) =100×60 = 6000

A sound created in a big hall will persist by repeated reflection from the walls until it is reduced to a value where it is no longer audible. The repeated reflection that results in this persistence of sound is called reverberation. To reduce reverberation, the roof and walls of the auditorium are generally covered with sound-absorbent materials like compressed fibreboard, rough plaster or draperies. The seat materials are also selected on the basis of their sound absorbing properties.

To clean any objects, it is placed in a cleaning solution and ultrasonic waves are sent into the solution. Due to the high frequency, the particles of dust, grease and dirt get detached and drop out. The objects thus get thoroughly cleaned.