Mr. Martin Lund from Denmark emailed us with the following question: While reading the Harpoon 4 sonar rules I came across the following things: Looking at the sonar propagation path diagram (page 4 - 6 in the H 4 rules); it looks like there should be a maximum range for direct path detection, and that this should be short of any CZ-detection, and If I understand sound propagation correctly (or at least enough of it), this should be independent of the sonar (BQQ-5D, TB-23, etc), because of the way sound tends to ‘curve’ in water (temperature/ pressure differential over depth). Better sonars are simply better at discriminating fainter sound sources. Is this correct? Chris Carlson answers: The reason why I group these two questions, is because the answer affects both. First, the direct path detection range is a function of many different things, but target source level is a big driver. The louder a target the further away you can detect it. Also, and this goes into the second question, sound propagation is NOT independent of the type of sonar. Lower frequencies travel further than higher frequency noise because there is less attenuation. This is a significant point, at higher frequencies several dB are lost for every mile of travel, for very low frequency (VLF) noise this loss is less than 1/2 dB for every mile. Thus towed arrays will detect targets, especially noisy ones, at considerable ranges. Second, although we don’t preclude towed arrays from using CZs, the fact of the matter is that VLF sound doesn’t bend well. What causes a CZ to be formed is the higher pressure at the lower part of a sound wave causes it to travel faster than the shallower part and this turns the sound wave back towards the surface. At VLF frequencies, the sound wave is so long that it doesn’t respond as well to the pressure effect and so it doesn’t turn as sharply. So while there is a CZ of a sorts, it merges in with the direct path to a much greater degree than a MF or even LF-MF sonar. Mr. Lund asked about calculating the detection range for a SKAT-3 towed array (passive range = 6.0 nm) against a loud target at 19 knots in sea state 1: This gives us a range modifier (for the 50% chance) of: 4 x 6 x 1 x 1 x 2 = 48, or a whopping 288nm(!) for a 50% direct path detection and a 25% chance out to 432nm(!) ; this is somewhere out beyond a theoretical 8th or 9th CZ (and out to the 14th CZ for the 25% chance). Chris Carlson answers: Yes, it looks like you did the math right and I’m sorry to say that this is not ludicrous. On my submarine, with a long towed array, we detected targets that turned out to be 200 - 300 nm away! Now, mind you there were a lot of targets that we had to filter through to find one of interest. SOSUS arrays, which are shorter than many long towed arrays, have had detections on the order of 500 to 800 nm against loud targets! True, they are taking advantage of the deep sound channel, but all this does is reduce the spreading loss by keeping the sound in the equivalence of a wave guide. The bottom line is, loud targets can be detected at huge distances with a modern VLF long towed array. The only correction I would make to your scenario is that most of today’s merchants would not be considered Loud, just Noisy. I would even say some of the most modern cruise ships are probably Quiet. This reduction has been driven by environmental laws on the amount of vibration that can be in the crew quarters and by the desire to improve fuel efficiency. Anyway, I hope this adequately addresses your questions. And remember, the sub must always win - oops, sorry, Larry... BT Back to The Naval Sitrep #22 Table of Contents Back to Naval Sitrep List of Issues Back to MagWeb Master Magazine List © Copyright 2002 by Larry Bond and Clash of Arms. This article appears in MagWeb (Magazine Web) on the Internet World Wide Web. Other military history and related articles are available at http://www.magweb.com