The object to be cleaned 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 the object gets thoroughly washed.

Time taken by sound to travel from child to cliff  \(t=\frac { 4 }{ 2 } =2\quad s\)
Speed of sound in air, v = 340 m/s
Distance of cliff from the child = \(vt=340\times 2=680\quad m\) 

Relationship between frequency, wavelength and wave velocity. Since wavelength is the distance traveled by the wave during the time a particle of the medium completes one vibration, therefore if  \(\lambda \) be the wavelength and T the time-period, then the wave travels a distance \(\lambda \) in time T. Hence \(Wave\quad velocity=\frac { Distance }{ Time } \) or \(v=\frac { \lambda }{ T } \)
or  \(\upsilon =v\lambda \) \(\left[ \because \quad \frac { 1 }{ T } =frequency\quad \left( v \right) \right] \)
∴ Wave velocity=Frequency x Wavelength.
The wave velocity in a number remains constant under the same physical conditions.

When the school bell vibrates, it forces the adjacent particles in the air to vibrate. This disturbance gives rise to a wave and when the bell moves forward, it pushes the air in front of it. This creates a region of high pressures known as compression. When the bell moves backward, it creates a region of low pressure known as rarefaction. As the bell continues to move forward and backward, it produces a series of compressions and rarefactions. This makes the sound of a bell propagate through air.

Periodic motion. The motion of a body repeats itself regularly after a fixed interval of time is called a periodic motion.
Examples of periodic motion:
(i) Motion of a planet around the sun.
(ii) Motion of the hands of a clock.
(iii) Motion of a ball being rotated in a circle at the end of a string.

Echo. Echo is the phenomenon of repetition of a sound due to its reflection from the surface of a large obstacle. When a loud sound is produced in an empty hall, the same sound is heard again after some time.
Conditions for the production of an echo:
(i) Sufficient time gap between original and reflected sounds. Due to persistence of hearing, we keep on hearing a sound for 0.1 s, even after reflected sound must reach the ear 0.1 s after the direct original sound.
(ii) Sufficient distance between the source of sound and the obstacle. The speed of sound in air is 344 m/s. The distance traveled by sound in\(0.1\quad s=344\times 0.1\times 0.1=34.4\quad m\) . So echo will be heard if the minimum distance between the source of sound and the obstacle =34.4/2=17.2 m.
(iii) Nature of the obstacle. For the formation of echo, the reflecting surface must be rigid such as a building, hill or cliff.
(iv) Size of the obstacle. Echoes can be heard if the size of the reflecting surface is quite large.

Time period. The time taken by an oscillating body to complete one oscillation is called its time period. It is denoted by T. Its SI unit is second (s).
Frequency. The number of oscillations or vibrations completed by an oscillating body in one second is called its frequency. It is denoted by v (Greek letter nu).
SI unit of frequency=per second (s^-1) = cycles per second (cps) = hertz (Hz).
Relation between time period and frequency:
Let T=time period of an oscillating body. Then number of oscillations completed in T second = 1
Number of oscillations completed in 1 second =\(\frac { 1 }{ T } \)
But number of oscillations completed in 1 second = frequency (v)
∴ \(v=\frac { 1 }{ T } \)
Hence frequency is equal to the reciprocal of time period.

(a) For an average human ear,the audible range of frequency extends from 20 Hz to 20 kHz.
(b) Sound of frequency less than 20 Hz is called infrasound.
(c) Sound of having a frequency of 40 kHz is called ultrasound It is used in the traetment of muscular pains.  
(d) The word SONAR stands for Sound Navigation and Ranging
(e) SONAR is used to determine the depth of the sea and to locate underwater hills,submarine,icebergs,sunken,etc

The time between transmission and detection of SONAR pulse,  t = 1.02
Speed of sound in salt water, v = 1531 m s^-1
Distance of the cliff = d (say)
Then the distance travelled by sound = 2d
But, 2d = Speed of sound \(\times \) time = vt
= 1531\(\times \)1.02 m
∴ d = \( \frac { 1531\times 1.02 }{ 2 } =780.81\quad m\). 

Reverberation. When a sound is produced in a big hall or auditorium, the sound waves suffer multiple reflections from the walls, ceilings and other materials present in the hall. Due to this, the sound persists for some time even after the source has stopped producing sound. This persistence of sound due to repeated reflection is called reverberation.
Methods of reducing reverberation. In a big hall or auditorium, excessive reverberation is highly undesirable. It can be reduced by the following methods:
(i) By covering the walls and roof of the auditorium with sound absorbent materials like compressed fireboard, rough plaster, etc.
(ii) Providing open windows in the space.
(iii) Providing heavy curtains with folds.
(iv) By using good sound absorbing materials for the seats.

Sonic boom. When a body moves in air with a speed greater than the speed of sound, it is said to have a supersonic speed. Bullets and jet aircrafts etc. often travel at supersonic speeds.
When a source of sound travels through air at a supersonic speed, it leaves behind itself a conical region of disturbance which spreads continuously. A region consisting of a very high pressure layer followed by a lower pressure layer travels through the space together with the cone. This is called a shock wave. It carries a large amount of energy.
When a shock wave reaches a person, the variations in its air pressure produce a sharp and sound in his ears. This is called sonic boom.
The sonic booms from a high speed jet aircraft can break glass dishes, windowpanes and can even cause serious damage to a building.

Differences between mechanical and electromagnetic waves.

Characteristics of wave motion:
(i) It is the disturbance which travels forward through the medium and not the particles of the medium, the particles of the medium merely vibrate about their mean positions.
(ii) Each particle receives vibrations a little later than its preceding particle.
(iii) The velocity with which wave travels is different from the velocity of the particles with which they vibrate about their mean positions.
(iv) The wave velocity remains constant in a given medium while the particle velocity changes continuously during its vibration about mean position.

The audible range of an average human ear lies between 20 Hz to 20,000 Hz. Humans cannot hear sounds having frequency less than 20 Hz and greater than 20,000 Hz.

The time interval between two successive compressions is equal to the time period of the wave. This time period is reciprocal of the frequency of the wave and is given by the relation:
\(T=\frac{1}{\text { Frequency }}=\frac{1}{500}=0.002 \mathrm{~s}\)

Sound needs a medium to propagate. Since the moon is devoid of any atmosphere, you cannot hear any sound on the moon.