With the evolution of Submarine Warfare and the enemy's advances in U Boat technology and numbers, it became necessary to investigate methods whereby we could regain the advantage over them.

To this end the Anti Submarine Detection Investigation Committee was created, as you will note the initials spell "ASDIC" to achieve this object, it became clear that that the use of sound (SONAR), was going to be the best course.

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The method used is often referred to as "Underwater Radar" and this is certainly a justifiable synonym, as the radio wave used in Radar is replaced by the sound wave used in Asdics.

The unit used to transmit this sound wave was lowered through the keel, much as a periscope is used above water.

It consisted of a Quartz Crystal housed in a unit which could be rotated, either continuously or in steps through 360 Degrees, with it's compass position being precisely recorded in the Control Room, from where the instruction to rotate would originate.

The Quartz Crystal would be energised by applying current to it, it was necessary for this Current to be AC (Alternating Current), the ship's normal current was DC (Direct Current). this was converted by  a unit called a High Frequency Motor Alternator (HFMA), this unit was rather clever as, at that time other nations used a full ship's compartment to generate AC, whereas this unit was not much larger than a vacuum cleaner and could achieve the same result using a fraction of the space.

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The Quartz Crystal acted as both a Transmitter and Receiver, on applying the required current to it the Crystal would expand underwater thus generating a sound wave, with no current applied the returning sound wave would strike the Crystal which would generate a small electrical current, this would be amplified and the resulting sound broadcast through a speaker in the Control Room.

The unit would normally transmit continuously on a 360 degree cycle, stopping at each transmission point only long enough to receive the returning signal, if there was no signal it could be assumed that there was no Submarine at that point, in the event of a signal being received, the exact compass direction would be noted, the sweep would continue until it ceased to return a signal, a reverse sweep would then take place, which would confirm the presence of an object.

The size of the object could be estimated by the number of compass degrees between each end of the sweep, the density of the object could be determined by the crispness of the returning echo, the direction of travel of the object could be determined by use of the Doppler Effect, the Doppler principle being that a stationary object will return an echo at the same pitch whereas an object travelling away from the ship would return an echo at a lower pitch, accordingly an object travelling toward the ship would return an echo at a higher pitch, this would occur on the basis that the sound waves which struck an object travelling toward would squash the sound wave because of the additional force given by a forward movement, the same principle would apply to the other directions of travel, that is same course returns a sound wave the same as was transmitted, while travelling away would have the effect of extending the sound wave, thus it returns at a lower pitch.

Radar's affinity with Torpedoes is obvious when you consider that you can obtain a precise position from the Radar, you may set your Torpedo in that particular course, however the same affinity can be found between Asdic and Depth Charges, of whatever kind in that, you would track the target with your Asdic equipment and simply steer towards a position directly above your target, then release the Depth Charges thus giving you a greater likelihood of striking your target.

There are, of course, also peaceful uses for Sonar, such as locating shoals of fish, or studying the nature of or warning of  the depth to, the seabed, just as with most other Military Equipment.

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