Theory:

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.
 
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Calculation of distance using SONAR
  
The bottom of boats and ships are equipped with sonar, which consists of a transmitter and a detector. Ultrasonic waves are created and transmitted by the transmitter. These waves travel through water and are reflected in the detector after striking an object on the seabed. The detector converts ultrasonic waves into electrical signals, which are then appropriately interpreted. Knowing the speed of sound in water and the time interval between transmission and reception of the ultrasound can be used to calculate the distance of the object that reflected the sound wave. Allow for a time interval of 't' between the transmission and reception of an ultrasound signal.
 
This method is called echo-ranging. Sonar technique is used to determine the depth of the sea and to locate underwater hills, valleys, submarines, icebergs etc.
 
Example:
 
A ship sends out ultrasound, which is detected after \(3.42\) \(seconds\) when it returns from the seabed. What is the distance between the ship and the seabed if the speed of ultrasound through seawater is \(1531\) m/s?
 
Given data:
 
Speed of ultrasound through water \(=\) \(1531\) \(m/s\)
 
Time \(=\) \(3.42\) \(s\)
 
To find:
 
distance=speed×time2d=1531×3.42d=52362d=2618m
 
Thus, the distance of the seabed from the ship is \(2618\) \(m\) or \(2.618\) \(km\).
 
ECG (Electrocardiogram):
 
The electrocardiogram (ECG) is one of the most basic and long-standing cardiac tests available. It can provide a wealth of useful information and is still an important part of cardiac patient evaluation. The heart's sound variation is converted into electric signals in an ECG. As a result, an ECG is simply a representation of the heart muscle's electrical activity as it changes over time. It is usually printed on paper for ease of analysis. An ECG is the sum of this electrical activity when amplified and recorded for a few seconds.
Reference:
https://commons.wikimedia.org/wiki/File:Sonar_Sistemi_Mesafe_%C3%96l%C3%A7%C3%BCm%C3%BC.gif