The sound waves having frequencies greater than 20,000 Hz are called ultrasonics.
Applications of Ultrasonic Waves:
1. In the treatment of muscular pains.
2. In the detection of defects or flaws in metal blocks.

Ultrasound. A compression and rarefaction wave in a material medium with frequency above 20 kHz is called an ultrasonic wave or ultrasound. It travels freely in solids and liquids but its intensity decreases rapidly in gases.
Because of their high frequencies, ultrasonic waves have the following two important properties:
1. They have high power.
2. They are able to travel along well-defined straight paths, even in the presence of obstacles.
The above two properties make the ultrasonic waves highly useful for industrial and medical purposes.

Hearing aid. It is a battery operated electronic device used by persons who are hard of hearing. A hearing aid receives sound through a microphone. The microphone converts the sound waves to electrical signals. These electrical signals are amplified by an amplifier. The amplified electrical signals are given to a speaker of the hearing aid. The speaker converts the amplified electrical signal to sound and sends to the ear for clear hearing.

Earthquake produce low frequency infrasound before the main shock waves get set. These infrasonic waves are out of our audible range. But animals are able to detect these waves and hence some animals get disturbed before earthquakes and start behaving in a different manner.

The persistence of sound in a big hall or auditorium due to the repeated reflections of sound is called reverberation.
(i) By covering the walls and roof of the halls with sound-absorbing materials.
(ii) By providing heavy curtains with folds.

Speed of sound wave, v = 339m s^-1
Wavelength, λ = 1.5cm = 0.015m
Frequency, v = \({v\over \lambda}={339\over 0.015}=22,600Hz\)
As this frequency is higher than 20, 000 Hz, so it is not audible.

Yes, sound follows the same laws of reflection as light does. The directions in which sound is incident and reflected make equal angles with the normal to the reflecting surface and the three lie in the same plane. For verification of these laws, refer to activity 12.5 on page 479.

Speed of sound v = 344m s^-1
Frequencies, v₁ =v20Hz and v₂ = 20kHz = 20,000 Hz
Wavelength of sound waves corresponding to 20H
\(\lambda_1={v\over v_1}={344\over 20}=17.2m\)
The wavelength of sound waves corresponding to kHz,
\(\lambda_2={v\over v_2}={344\over 20,000}=0.0172m\) 

Sound waves travel through a medium in the form of alternate compressions and rarefactions.The particles of the medium thus move to and fro along the the direction of propagation of the sound wave.That is why th sound waves are called longitudinal waves. 

The frequency v of a wave is a characteristic of the source of oscillation that produces the disturbance. Different sources produce oscillations of different frequencies. The wavelength \(\lambda \) will be different for these sources. It will change in such a way that the product \(\left( v=v\lambda \right) \) remains constant in a given medium.

The wave velocity is the distance travelled by a wave per unit time. It remains constant in a given medium \(\left( v=v\lambda \right) \) . The particle velocity continuously changes with time. It is maximum at the mean position and zero at the extreme position.

The number of waves contained in unit length of the medium is called 'wave number'.
Wave number, \(\overset { - }{ v } =\frac { 1 }{ \lambda } \).                  

When the school bell is hit by a hammer, it begins to vibrate. The vibrating bell produces compression and rarefaction pulses, one after the other in the air. These pulses travel one behind the other as a sound wave. When the sound wave reaches our ear, it forces the tympanic membrane to vibrate and thus causing the sensation of hearing.

When a wave propagates along the surface of a medium, it generally has both transverse and longitudinal components. When such a wave travels through the medium, its particles move either along a circle or an ellipse. Examples of such waves are:
(i) Seismic waves travelling along the surface of the earth during an earthquake or explosion.
(ii) Ocean waves in deep water.
(iii) Wave pulses produced by beetle's motion along the sand's surface.

Transverse waves travel in the form of crests and troughs. They involve changes in the shape of the medium. So they can be transmitted through a medium which has elasticity of shape. As solids have elasticity of shape, so transverse waves can be transmitted through solids.
Longitudinal waves travel in the form of compressions and rarefactions. They involve changes in the volume and density of the medium. All media-solid, liquids and gases have elasticity of volume. Hence longitudinal waves can be transmitted through all the three types of media.