MP Board · Class 9 · Science · Sound(a) What is SONAR? Write its full form. Explain its working principle and state two important applications of SONAR. (b) State the conditions necessary for a human being to hear a distinct echo. (c) Numerical: A SONAR device mounted on a research ship sends an ultrasonic signal directly downwards into the sea. The reflected signal (echo) is received back by the detector after $3.6\text{ seconds}$. If the speed of sound in seawater is $1530\text{ m/s}$, calculate the depth of the sea at that location.

Part (a): SONAR (Sound Navigation and Ranging)

• Full Form: SOund Navigation And Ranging.
• Working Principle: SONAR works on the principle of reflection of ultrasonic sound waves.
• Working Mechanism:
  • It consists of two main components installed at the bottom of a ship: a Transmitter and a Detector.
  • The transmitter produces and transmits powerful high-frequency ultrasonic waves into the water.
  • These waves travel through the seawater, strike the ocean bed or underwater objects, and get reflected back.
  • The reflected waves are picked up by the detector, which converts the ultrasonic sound signals into electrical signals that are analyzed to determine distance.
• Applications:
  1. Determining the depth of oceans and seas.
  2. Locating underwater hills, valleys, sunken ships, submarines, and icebergs.

Part (b): Conditions for Hearing a Distinct Echo

\nAn echo is the repetition of sound caused by the reflection of sound waves from a distant surface.

• Persistence of Hearing: The human brain retains the impression of any sound for about $0.1\text{ second}$ ($1/10\text{th}$ of a second). To hear a clear and distinct echo, the reflected sound must reach the ear after at least $0.1\text{ second}$ from the original sound.
• Minimum Distance of Reflecting Surface:
  • Speed of sound in air at $22^\circ\text{C} = 344\text{ m/s}$.
  • Total distance traveled by sound in $0.1\text{ s} = \text{Speed} \times \text{Time} = 344\text{ m/s} \times 0.1\text{ s} = 34.4\text{ m}$.
  • Since the sound travels to the obstacle and returns back, the minimum one-way distance between the source of sound and the reflecting obstacle must be half of $34.4\text{ m}$, which is $17.2\text{ meters}$.
• Size and Nature of Obstacle: The reflecting surface must be large enough relative to the wavelength of sound and must be hard/rigid.

Part (c): Step-by-Step Numerical Solution

Given:

• Total time taken for signal to return ($t$) = $3.6\text{ s}$
• Speed of sound in seawater ($v$) = $1530\text{ m/s}$

Formula: $$\text{Total distance traveled by wave} = 2 \times d = v \times t$$\nwhere $d$ is the depth of the sea.

$$d = \frac{v \times t}{2}$$

Calculation: $$d = \frac{1530 \times 3.6}{2}$$ $$d = 1530 \times 1.8$$ $$d = 2754\text{ meters}$$

Answer:\nThe depth of the sea at that location is $2754\text{ m}$ (or $2.754\text{ km}$).