Hydrophone VS Sonar

[SAJ]

These two confuse me. First of all I should ask are they the same thing?
If their NOT the same did U boats have both?
If the are different, How are they different. Did one have a longer range?

I know these may seem like easy questions for some but I am confused and would appreciate anyone helping me to understand the differences between the two if there are any.

[Polar bear]

hi,

AFAIK, they are not the same.

A hydrophone is a passive listening device which may detect another ship's or boat's propeller (or other man-made) noise from any direction.

A Sonar is an active searching device sending directed soundwaves of a special frequency. Of course, to receive the echoing sound waves (echoing from any underwater object), you need a passive device which is - technically - another hydrophone.

greetings, the pb

[vladalex]

A small observation: a submarine can not be detected with the most advanced hydrophone if
do not move and not emit any noise and from this simple example the sonar need to complete detection.
Regards,
VladAlex.

[SAJ]

Thank You for your replies. so Essentially sonar could or would be use on attack boats who are searching for a kill operating in a offensive manor.
So something like a "Milk Cow" who's purpose was to stay hidden would not necessarily have or use Sonar but rather use the hydrophones?
If so what is the range of Hydrophones vs Sonar.

I appreciate all the help!!!

[Polar bear]

hi,

If so what is the range of Hydrophones vs Sonar.

Simple physics : a hydrophone will have the better range as the sound is only one-way. The sonar's echo sound (i.e. energy) has to go both ways, i.e. back to the originator.
ALL passive detectors, not only those underwater, have this advantage.

greetings, the pb

[T. A. Gardner]

A hydrophone works more like a directional microphone and is passive only. It lacks the oscillator of an active sonar. That is there is no way to induce a "ping" using a hydrophone.

Here's an introduction from a US training manual of the period:

https://maritime.org/doc/fleetsub/sonar/chap1.htm

Sonars of this period all worked on one of two principles to induce the "ping:" Magnetostriction and piezoelectric induction. The first relies on the principle of an electromagnet. When a ferromagnetic material is exposed to an electrical field it changes shape slightly inducing a "hum" into the metal. By using the right frequency of electrical field of the right strength, you get the "ping." The piezoelectric method uses an electrical signal to make a quartz or Rochelle salt crystal vibrate as in a radio transmitter. That vibration becomes the "ping."

The hydrophone was usually much more sensitive than a sonar, but the down side was you had to have still water around the pick up head of the set or the noise of the water moving past it would interfere with the picking up of other sounds. With submarines this wasn't a big problem. They were slow submerged and water pressure and lack of wave action made hydrophones work well for them.
For surface ships, their movement and the wave action made hydrophones impractical unless they were nearly stopped. Sonar was less susceptible to movement, particularly once the sonar dome was introduced. The active ping was loud enough to overcome ambient noise. As the war progressed, sonar systems improved to a point where surface ships could move at upwards of 20 knots and use their gear without great penalty in range or performance.

The big German breakthrough in this field was the coupling of a large array of hydrophones together to get much better directional accuracy and sensitivity out of the set. The Allies developed sonars at the same time that could automatically scan up to 360 degrees around the ship whereas most WW 2 sonars were of the "searchlight" type with a roughly 11 degree beam that was manually rotated with each new search.