A child hears an echo from a cliff 4 seconds after the sound from a powerful cracker is produced. How far away is the cliff from the child?

---

Time taken for the echo to reach the child's ear = 4 seconds

Therefore time taken by the sound to reach the cliff = 

Speed of sound in air, v = 340 m/s

Distance of cliff from the child = vt

---

First we calculate time t taken by the stone to reach the water level in the well by using the relation,

---

The ceilings of the concert halls are made curved so that sound after reflecting from the ceiling reaches all corners of the hall, as shown in Fig.


             .

---

Speed of sound in air, v = 342 ms^-1

Time taken for hearing the echo, t = 3 s 

Distance  travelled by the sound = vt

                                                 = 342 x 3

                                                 =  1026 m

In 3 s, sound has to travel twice the distance between the the cliff and the person.

Hence the distance between the cliff and the person is given by, d=  1730 m/2 = 865 m

---

Infrasonic waves: If the frequency of the waves is less than 20 Hz then the waves are called infrasonic waves. Infrasonic waves are produced by large vibrating bodies. 

The waves produced before the actual shockwaves during an earthquake is infrasonic waves. Animals ike whales and elephants produce infrasonic waves. Human beings cannot hear these waves. 

Ultrasonic waves: When the frequency of the waves is greater than 20,000 Hz they are called ultrasonuic waves. 

Bats, dogs, rats, cats, birds and insects can produce and also hear these waves.