What is the submarine sonar sound 'made from' and why is it such special?

Question

I mean the well known deep 'boing' like in the 'Das Boot' movie.
And why is it like that? Is the frequency somehow special or it does contain all frequencies or why is it such special?

Answer 1

SONAR, sound navigation and ranging, is just a sound producing device coupled with a listening device. Most, if not all, sending devices consist of piezoelectric diaphragms that contract and expand when an electrical current passes through them. That sets the diaphragm to oscillating which is transmitted into the ocean as underwater sound.

Low frequency sound travels furthest under water. However, low frequency sound also requires larger sending devices than high freq. sound. So the size and thus frequency of the SONAR transmitter is often limited as the size of the platform (e.g., ship). For example, active sonar devices dropped into the water from ASW aircraft are relatively small with relatively high frequency and short range SONAR. SONAR off destroyers is larger, lower freq. and longer range.

As an approximation, we used 800 yards per elapsed second from when a SONAR pulse was transmitted to when its echo was received. Thus, if three seconds elapsed, we'd guesstimate 2,400 yards from the SONAR transmitter to the underwater target. This was only an approximation; however, as they always say, it was close enough for government work.

Answer 2

There are actually numerous types of pings. Like radar, sonar uses a variety of pulse types depending on the phase of the engagement. Some folks use the term DCL - Detect, Classify, Localize - to characterize the phase of the sonar engagement.

To detect a target at range, you need a long ping. This puts a lot of power into the water for a long time. While this can detect a target at a long distance, long pulses do not provide you accurate target position. This is the "bong" ping you describe.

As the target range gets shorter, the ping is made shorter. The power isn't needed and the shorter ping provide better range resolution.

As short range, the pings get very short and quick. With short pings, you can determine the target type (classify) and you can get a very accurate range and velocity (localize).

If you are hunting mines and slow targets, you use an FM sweep. This sounds like a whistle that increases in frequency.

To understand these pings, you need to understand the theory of optimal pulse shapes. Any good radar or sonar book covers this stuff.

Answer 3

Well, it's made by a specialized speaker called a sonar emitter.

And the optimal frequency depends on a lot of factors, including water temperature and salinity. But the "ping" that you're thinking of, the classic ping, is one of the frequencies that's best used in seawater. You probably hear the same one in the movies all the time because it's cheaper just to use a recording that's already been done than figure out which version of the ping to use in various situations depending on the plot.