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.

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.

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.

Infrasonic waves. Sound waves with frequencies less than 20 Hz are called infrasonic waves. Such waves are produced by large vibrating bodies. The earthquakes, volcanic eruptions, big animals like whales and elephants produce infrasonic waves. We cannot hear these waves.
Ultrasonic waves. Sound waves with frequencies greater than 20,000 Hz are called ultrasonic waves or ultrasound. Bats, dogs, rats, cats, birds and insects can produce and also hear these waves. We cannot hear these waves.

Frequency of source = 100Hz = 100s^-1
Number of vibrations in 1s = 100
Number of vibrations in 1 minute or 60s = 100 x 60 = 60000

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.

First, we find the time taken by the stone to reach the base of the tower. We use the relation,
\(s=ut+{1\over2}gt^2\)
But,  s = 500m,  u = 0,   g = 10m s^-2
500=0+\({1\over2}\)x10xt²
or \(t^2={500\over5}=100\)  or  t = 10s
Time is taken by sound to travel from the base of the tower to its top,
\(f'={Distance\over Speed\ of\ sound}={500m\over 340ms^{-1}}=1.47s\)
Total time after which the splash is heard
= t + t' = 10 + 1.47 = 11.47 s

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.

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.

Differences between transverse and longitudinal waves:

Speed, wavelength, and frequency of a sound wave are related by the following equation:
Speed (v) = Wavelength (λ) × Frequency (v)
v = lambda x xupsilon.