(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

(a) On the basis of the medium,waves are classified as mechanical arid non-mechanical waves.
(b) Non-mechanical waves do not require a material medium for their propagation.
(c) This is because sound needs a material medium like air, water,steel,etc for its propagation.
(d) No,the moon has no atmosphere or material medium through which sound can propagate.

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 = \( \frac { 1531\times 1.02 }{ 2 } =780.81\quad m\). 

First we calculate time t taken by the stone to reach the water level in the well by using the relation, \(s=ut+\frac { 1 }{ 2 } gt^{ 2 }\)
But   \(s=44.1\quad m,\quad u=0,g=9.8\quad m/s^{ 2 }\)
∴  \(44.1=0+\frac { 1 }{ 2 } \times 9.8\times t^{ 2 }\)
or  \(t^{ 2 }=\frac { 44.1\times 2 }{ 9.8 } =9\)
or t = 3 s
Time taken by sound to reach the mouth of the well 't' = 3.13 - 3 = 0.13 
\(Speed\quad of\quad sound=\frac { Distance }{ Time } =\frac { 44.1\quad m }{ 0.13\quad s } =339.23\quad m/s.\)

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

Let the length of the aluminium rod=d metre
Sound in air = 346m s^-1
Time is taken by sound in air, t(air) = \({d\over 346}s\)
Speed of sound in aluminium = 6420m s^-1
Time is taken by sound in aluminium rod t (aluminium) = \({d\over 6420}s\)
The ratio of times taken by sound wave in air and aluminium,
\({t(air)\over t(aluminium)}={d\over346}\times{6420\over d}={6420\over 346}=18.55\)

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:

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.