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I'm reading about the WW2 North Atlantic submarine war. In particular about the anti-submarine weapons availbale to the allies.

This article includes a statement that I'm trying to understand.....

When a depth charge explodes it can take 15 minutes before the disturbance can settle down enough that sonar becomes effective.

What was the nature of the 'disturbance' that could effect sonar for 15 minutes? I assume it's a reference to some form of underwater echos and/or shockwaves, but if so why so long? I know that sound travels much faster and further in water than air, but does that not mean that any echos should die down more quickly?

Or maybe the ASDIC was very sensitive and needed re-calibration (or even repairs) after a nearby explosion?

Or is there some other phenomena that I've missed?

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    It might be better to ask this in physics – user32121 Oct 28 '18 at 20:07
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    Bubbles......... – Steve Oct 28 '18 at 21:39
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    Because the echo of the explosion reverberates between sea surface and sea floor in the vicinity of sub & hunter, due to low transmission coefficient of sound from water into both air and rock, until the sound energy is absorbed by the water as heat through friction. See my answer below. – Pieter Geerkens Oct 29 '18 at 3:23
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An underwater explosion creates turbulence in the water, creates bubbles, and perhaps mixes waters of varying temperatures or salinities, all of which affect the refraction of sound in water. For ASDIC to work best the water should be homogeneous, with laminar flow only.

  • all true, but would it really take 15 minutes to settle back to normal? – ConanTheGerbil Oct 28 '18 at 15:27
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    There are obviously many factors involved, the rule-of-thumb-summary of which seems to have been 15 minutes, according to WIkipedia's sources. Isn't your question really an applied physics question and not a history question? – kimchi lover Oct 28 '18 at 15:31
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    The bubbles are probably the thing that take longest to clear. A single depth charge will release several hundred kilograms of gasses (that's what its charge turns into) and while the big ones will rise quite quickly, the small ones reflect and distort sounds, and take much longer to rise. – John Dallman Oct 28 '18 at 22:07
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    @Hemel It doesn't seem surprising to me that several hundred kilograms of high explosive can stir water up enough that it takes 15 minutes to settle again. – David Richerby Oct 29 '18 at 13:36
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Your expectation "does that not mean that any echos should die down more quickly" is largely inaccurate for that portion of the explosive energy directed vertically, or near vertically.

Precisely because sound travels so much faster in water than air, and that water is much denser than air, the transmission coefficient from water to air for sound is very close to zero even at an incident angle of zero degrees (vertical).

enter image description here
...
enter image description here

Here:
- c is the speed of sound for the medium
- ρ is the density of the medium
- subscript l represents the second medium (air), no subscript the origin medium (water)

Typical values for seawater are:
    ρ = 1020 kg/m3
    c = 1500 m/s;

and for air:
    ρ = 1.225 kg/m3
    c = 340 m/s

so from Eqn. 1.28 above
    m ~ 1.225 / 1020 = 0.0012
    n ~ 1500 / 340 = 4.41

Then even at an incident angle of 0 degrees (so cos θ == 1 and sin θ == 0), from Eqn. 1.30 above we obtain as transmission coefficient:

T ~ ( 2 . 0.0012 ) / ( 0.0012 + 4.41 . 1 )
   ~ 0.0024 / 4.4112
   ~ 0.00054

A similar analysis will show that a very small transmission coefficient also occurs with respect to the sea floor.

Therefore in the immediate vicinity of sub and destroyer/corvette, where the incident angle of the sound blast is very close to zero in respect to both sea surface and sea floor, the echo of the sound blast reverberates in a nearly vertical pattern until that echo slowly migrates off of the vertical or is absorbed by the water through heating.


Most of the Atlantic is between 3000 and 4000 metres deep, averaging about 3600 metres. This means that the echos of a depth charge explosion are racing back and forth between sea surface and seafloor in 2 -3 seconds each way. With near zero transmission to both air and bedrock, a loud reverberation would echo around the vicinity of the explosion, typically the immediate vicinity of both sub and hunter. Yelling out in a cave system with strong echos, and then waiting for enough silence to hear a pin drop, would be an analogous situation.

Typical frequency for sonar in WW2 was 20kHz to 30kHz. The wavelength for a 25kHz signal would be ~ 6 cm given the sea water properties above, so all features of sea surface or sea floor < ~3 cm would be invisible.


The claim is made in another answer:

For ASDIC to work best the water should be homogeneous, with laminar flow only.

While true, this is never the case. Rather there are always three thermoclines affecting the transmission, except in waters so shallow that one or more are squeezed out.

enter image description here

Therefore the notion that waiting 15 minutes or so for a return to homogeneity of the sea is absurd. What actually happens is that any great disturbance of the water will disturb the "lamination" of the ocean that facilitates the sonar shadow effect illustrated here, that occurs when a positive thermocline gradient closest to the surface lays above a negative one:

enter image description here

Other more complex propagation patterns must also be allowed for by the experienced sonar operator, as described in the reference.

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    It seems that the speeds c = 340 for water and 1500 for air are reversed. The result could also be more approachable if you calculated the amount of sound energy that will remain after 15 minutes. Your comments to the other answers come off as quite rude. While your answer is stronger on the physics calculations, the other answers do better on explaining what is happening. – jpa Oct 29 '18 at 5:13
  • @jpa: Thank you - the calculation was the right way around, but the two figures were listed backwards as properties. The answer in regards to bubbles is backwards - bubbles would speed energy dissipation not slow it. – Pieter Geerkens Oct 29 '18 at 5:22
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I think that the key to this is considering the reflected waves and their interactions with the environment and each other.

The initial explosion will send out shockwaves in all directions. These shockwaves will bounce off any surface, especially the sea floor and the surface with the air. Importantly, these reflected waves will also reflect as they hit any surface (for example, the initial reflection from the seabed will bounce up and reflect from the surface) and those reflections will do the same. As these surfaces are generally going to be irregular so the reflected shockwaves will very quickly become a jumble of noise.

It's also worth considering that depth charges usually dropped in patterns, so there wouldn't be just a single explosion but a series of them. The combination of shockwaves and reflections (and interference between the shockwaves and reflections) will make things very noisy indeed.

The initial explosions are orders of magnitude stronger (i.e. louder) than the ASDIC sound waves, and although each reflection loses energy, it takes time for the reflections to lose sufficient energy that they become insignificant in comparison to the ASDIC signals.

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To release depth charge ship would have to go below sonar's minimum range

Sonars (or ASDIC in British terminology) were relatively simple in WW2. Sound would be sent in one direction, it would propagate trough the water, and would bounce back from any underwater object in its path (submarine for example). Knowing the speed of sound in the water it was possible to roughly calculate range and bearing of that object. But if the object gets to close to the sonar, it could not detect it. The article about Hedgehog you posted explains that :

The system was developed to solve the problem of the target submarine disappearing from the attacking ship's ASDIC when the ship came within the sonar's minimum range. Due to the speed of sound in water, the time taken for the 'ping' echo to return to the attacking ship from the target submarine became too short to allow the human operator to distinguish the returning audible echo from that of the initial sound pulse emitted by the sonar – the so-called "instantaneous echo", where the output sound pulse and returning echo merge. This "blind spot" allowed the submarine to make evasive manoeuvres undetected while the ship was out of range for depth charge attack. Hence, the submarine was effectively invisible to the sonar as the ship came within the sonar's minimum range. The solution was a weapon mounted on the foredeck that discharged the projectiles up and over that carrying ship's bow, to land in the water some distance in front of the ship while the submarine was still outside the sonar's minimum range.

Depth charges on the other hand were more cumbersome weapons. As you can see from the link, they were usually dropped in the wake of the ship. This means that attacking ship had to cross over submarine, i.e. loose contact for some time. Unlike hedgehogs, depth charges would explode every time (at predetermined depth). Attacking ship would lay pattern of them, wait till they explode, get out of the minimum range and then try to reacquire target providing the cavitation has settled down.

Providing all necessary steps, the fact that sonars rarely worked at speeds above 15 kts and range under 300 yards, plus limitations of sending pings at intervals that lasted around 5 seconds in 5 degree arc (to allow sound to return back) , it could really last up to 15 minutes before you could start search again if first depth charge attack was unsuccessful.

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    The quote I provided seems to be saying that the sonar physically wouldn't work for 15 minutes, whereas you seem to be saying it might to 15 minutes to re-aquire the target - two subtly different things! – ConanTheGerbil Oct 28 '18 at 15:30
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    @Hemel Read your quote again : "it can take 15 minutes before the disturbance can settle down enough that sonar becomes effective" . Sonar is working but is not effective. – rs.29 Oct 28 '18 at 15:37
  • Some of the ahead-throwing weapons you mentioned (like Hedgehog) fired charges that would only explode on contact, thereby not messing with the sonar conditions. – David Thornley Oct 29 '18 at 17:48
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The quote may be referring to the physical systems implementing ASDIC. Shock and vibration shielding are major issues in naval architecture. Insufficiently shock shielded systems may have required maintenance after being shocked, for example by being near a recently executed depth charge attack. At least some British commonwealth WWII vessels lacked full shock protection on their ASDIC systems from the shocks induced by their own depth charge attacks.


Anti-submarine warfare division, Navy Office, Melbourne (1943-07) “SOUTH-WEST PACIFIC ANTI-SUBMARINE REPORT JULY, 1943” ACB0233/43(2) [ http://www.navy.gov.au/sites/default/files/documents/1943_July.pdf ] p. 9:

"KALGOORLIE" made an attack on a possible contact at 1808, and two minutes later "WARRNAMBOOL" had an echo on the bear­ing reported by "KALGOORLIE", range 700 yards. Four charges were dropped, the starboard thrower again misfiring. The concussion of the last charge put the Asdic set out of action. One valve had been jarred from its socket and the heterodyne condenser had moved from setting "E" to setting "B".

  • I’ve read a number of post incident reports—primary sources—on British WWII vessels noting essential systems insufficiently shock shielded. I’ve deleted the reference to other answers in terms of acquisition. – Samuel Russell Oct 29 '18 at 5:29
  • The only "internally accessible" portion of the system is the sonar operator's ears - which is why the remove their headphones when the depth charge is launched. The rest of the system is electronic, except for parts only accessible from outside the ship. Unless vacuum tubes are breaking, this explanation seems unlikely. If you have a reference for a specific effect I would be interested to see it. – Pieter Geerkens Oct 29 '18 at 5:39
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    @PieterGeerkens cheers, done – Samuel Russell Oct 29 '18 at 5:50
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    Perfect. I had forgotten how easy it was for vacuum tubes (or valves as here, in British English) to dislodge. I was thinking just of breakage. I don't know enough about heterodynes to comment beyond just noting that a setting change likely doesn't take more than a minute or two to identify and correct. I doubt the effect you are noting would take 15 minutes to be cleared, but it is certainly real. – Pieter Geerkens Oct 29 '18 at 5:57

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