The acronym SONAR stands for Sound Navigation and Ranging.

Sonar is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects.

Principle: It uses the phenomenon of echoes in determining the sea-depth and locating the presence of under-water objects.

Working: 

i) Sonar consists of a transmitter and a detector and is installed in a boat or a ship.

ii) The transmitter produces and transmits strong ultrasonic waves.

iii) These waves travel through the water and after striking the object the beam is reflected from the seabed and is received by an under-water detector which is also mounted on the ship.

iv) The detector converts the ultrasonic waves into electrical signals which are appropriately interpreted.

v) The time interval between transmission and reception of the ultrasonic signal is noted. 

The SONAR method is also called echo ranging. This technique is used to determine the depth of the sea and to locate underwater hills, valleys, submarine, icebergs, sunken ship etc. 

Wavelength(λ): Wavelength is defined as the distance between two consecutive compressions or rarefactions of a sound wave. 

Frequency(v): Frequency is the number of complete oscillations per unit time.

Time period(T): The time taken by two consecutive compressions or rarefactions to cross a fixed point is called the time period of the sound wave.

Amplitude(A): The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave.

Time period: The time taken by an oscillating body to complete one oscillation is called its time period.

It is denoted by T.

SI unit of time period is second (s).

Frequency: The number of oscillations or vibrations completed by an oscillating body in one second is called its frequency.

Frequency is symbolically denoted by v (Greek letter nu).

SI unit of frequency = per  socond (s^-1)

                               = cycles per second (cps)

                               = hertz |(Hz.)


Relation between time period and frequency:

Let T be the time period of an oscillating body.

Then number of oscillations completed in T second = 1

So, number of oscillations completed in 1 second =$\frac{1}{\mathrm{T}}$

But frequecny($\mathrm{\nu }$) is the number of oscillations completed in 1 second.

Therefore, $\mathrm{\nu }$ = $\frac{1}{\mathrm{T}}$ 

Hence, number of oscillations is equal to the recoprocal of time period. 

Bats can produce and receive ultrasonic waves. During its flight, a bat emits ultrasonic waves. The bat receives back these waves after being reflected by the obstacle in its path. From the nature of the reflected waves, the bat gets information where the obstacle or prey is and what is its size. Porpoises also use ultrasound for navigation and location of food even in total darkness.

Human ear. It is a highly sensitive part of the human body which enables us to hear a sound. It converts the pressure variations in air with audible frequencies into electric signals which travel to the brain via the auditory nerve.

The huamn ear has three main parts. Their auditory functions are as follows : 1. Outer ear. The outer ear is called ‘pinna’. It collects the sound from the surrounding. The collected sound passes through the auditory canal. At the end of the auditory canal there is a thin membrane called the ear drum or tympanic membrane. When compression of the medium produced due to vibration of the object reaches the ear drum, the pressure

Fig. 12.27. Auditory parts of the human ear.

on the outside of the membrane increases and forces the eardrum inward. Similarly, the eardrum moves outward when a rarefaction reaches. In this way the ear drum vibrates.

2. Middle ear. The vibrations are amplified several times by three bones (the hammer, anvil and stirrup) in the middle ear which act as levers. The middle ear transmits the amplified pressure variations received from the sound wave to the inner ear.

3. Inner ear. In the inner ear, the pressure variations are turned into electrical signals by the cochlea. These electrical signals are sent to the brain via the auditory nerve, and the brain interprets them as sound.