Where d - distance
V - velocity
t - time taken
d = \(\frac {V \times t}{2}\)
d = \(\frac {3 \times {10}^{8} \times 2.5}{2}\)
Distance of the moon from the earth = 3.75 x 10⁸ m

For hearing distinct echo sound, the time interval between the original sound and the reflected sound must be at least 0.1 second.

Sound is defined as the vibrations that travel through the air or another medium as an audible mechanical wave.
It is produced from a vibrating body (water, air etc) around it to vibrate thus producing sound.

When an object travels at a speed higher than that of sound in air, it produces shock waves. These shock waves carry a large amount of energy. The air pressure variations associated with this type of shock waves produce a very sharp and loud sound called sonic bloom or supersonic speed.

The number of vibrations (complete waves or cycles) produced in one second is called frequency. Its SI unit is hertz.

An object should- vibrate to produce sound. The vibrating body causes the medium around it to vibrate.

Tuning fork

1. SONAR stands for Sound Navigation and Ranging.
2. It is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects.
3. It consists of a transmitter and a detector and is installed at the bottom of the boats and ships.
4. The transmitter produces and transmits ultrasonic waves.
5. These waves travel through water and after striking the object on seabed, get reflected back and are sensed by a detector.
6. The detector converts the ultrasonic waves into electrical signals which are appropriately interpreted.
7. The distance of the object that reflected the sound wave can be calculated by knowing the speed of sound in water and the time interval between transmission and reception of the ultrasound.
8. Let the time interval between transmission and reception of ultrasound signals be 't' and the speed of sound through sea water be 2d/t = v. This method is called echo ranging.
9. SONAR technique is used to determine the depth of the sea and to locate underwater hills, valleys, submarine icebergs etc.,

1. Musical instruments such as shehnai, trumpets and Nathaswaram, Mega phones, loud speakers uses the property of reflection of sound to guide most of the sound waves from the source in the forward direction towards the audience.
2. In stethoscopes, sound produced in the body reaches doctor's ears by multiple reflection of sound that happen in the connecting tube.
3. Echo is used to find the distance between two distance objects.
4. Ultrasound is used in examing inner parts of the body and also in marine surveying.
5. Ultrasound is used in cleaning technology, to detect cracks and flaws to form the image of the heart through 'echo cardiography'.
6. Ultrasound is employed to break stones formed in the kidney.
7. 'SONAR' is a device uses ultrasonic waves to measure the distance, direction and speed of the underwater objects.
8. In Electrocardiogram (ECG) the sound variation produced by the heart is converted into electrical signals, which represents the electrical activity of the heart muscle.

The frequency of 600Hz means that the source of sound vibrates 600 times in 1 second.
1 minute = 60 sec.
Number of vibrations in one second = 600
∴ Number of vibrations in 60 seconds = 60 x 600
= 36000
Hence the source of sound vibrates 36000 times in a minute.