Ears Under Water

At last week's end German submarines had sunk some 175,000 tons of Allied and neutral shipping, plus a British airplane carrier. The carrier was a fine trophy, but the total haul of merchantmen, for the first full month of World War II, was skimpy compared to the big bags of 1917, when the Kaiser's U-boats were sinking five, six, seven, eight hundred thousand tons of shipping a month. Tactically and technologically, Germany's opponents today know much more about fighting submarines than they did in 1917.

In the last war anti-submarine warfare started from scratch. At one time Britain tried to train seals to hunt submarines. Various more practical expedients were tested--mine barriers, nets,-"mystery ships" (disguised trawlers and other craft which pretended to flee from submarines, then suddenly unmasked guns when the pursuing U-boats came close). Most effective defense against submarines was found to be the convoy. But the British wanted to hunt down the subs and destroy them. The problem was that of a blind man groping for a frog in a fishpond. So the British decided to use ears instead of eyes --mechanical and electrical ears.

"Fix." Water is an excellent conductor of sound, much better than air. As in air, abound wave in water registers against a diaphragm as a series of mechanical impulses. One early type of hydrophone was like a crude telephone. A rubber diaphragm immersed in the water received the impulses, transmitted them to a carbon-granule chamber, thence through wires to the earphones.

Finding the direction, or bearing, of a U-boat was not enough to locate it. But if each of three patrol vessels, say a mile apart, picked up submarine sounds, determined the bearing, then communicated their bearing readings to the other two by wireless telephone, three direction lines could be drawn on a chart and the point where the lines intersected was the quarry's approximate location. Another such "fix," obtained a few minutes later, would show the submarine's course and speed.

The "binaural principle" of acoustical direction-finding is basically the same as that which enables a human being (with good hearing in both ears) to tell approximately whence a sound comes. The compression peaks of a sound wave coming in at an angle reach the near ear a tiny fraction of a second sooner than the far ear--and the hearing mechanisms are so sensitive that they translate this minute time difference into a sense of direction. The simplest directional hydrophone is a rotatable bar with a receiver at each end, each receiver connected with one of the listener's ears. If a sound comes in at an angle, the slight time lag in the receivers causes the listener to hear it louder in one ear than in the other. He rotates the bar until the sound volume is equal in both ears; then the bar is perpendicular to the direction of the sound source. In antisubmarine practice, it was soon found impracticable to rotate the detector, whether attached to the hull of the patrol ship or towed behind. So the detector was kept stationary and the effect of rotation was obtained by lengthening the path from one receiver to one ear, shortening the other, until the sound volume was equal in both ears. This was called "binaural compensation."

These early devices have been vastly improved in accuracy and sensitivity. In the early 19305 the standard equipment on U. S. destroyers was the MV Tube, which carried a number of button microphones and handled the binaural compensation electrically, so that the bearing of the sound-source was read off on a dial. Such instruments pick up the characteristic beat of a submarine propeller 20 or 30 miles away (under favorable conditions as much as 50 miles) and determine its bearing within one degree or less. Nowadays the navies of major powers, including the U. S., try to keep the technical details and performance figures of their latest sound installations as secret as possible, but it is generally known that they have taken full advantage of recent improvements in electronics, such as powerful vacuum-tube amplifiers, microphones with low noise levels, unidirectional microphones, electrical filters which can cut out sounds in undesired frequency ranges.

Ashcans. After destroyers or subchasers have located a submarine, the problem is to overtake and annihilate it. No. 1 destructive weapon is the depth bomb--big cans of TNT (in naval lingo, "ashcans") which can either be rolled into the water from rails on the stern or propelled in sidewise arcs from depth charge throwers. Depth bombs have detonators which work by water pressure--that is, when the water pressure outside the bomb (proportional to depth) increases to a certain point, the bomb goes off. The bomb does not have to come into actual contact with the submarine; it wreaks its havoc by concussion transmitted through the water.

Since the location of a submerged submarine, even under the most favorable circumstances, is known only approximately, it is good tactics to drop several bombs in a "pattern." One simple but effective pattern is to drop the first bomb from the stern, then a little farther along to shoot two bombs to each side from depth charge throwers, then another bomb from the stern. If the sub is inside the diamond thus formed, it is likely to fare badly. More elaborate patterns can be laid down by several destroyers working together, to catch a sub which has turned right or left after the last "fix."

Airplane pilots on submarine patrol, who can sometimes see the silhouettes of submerged subs, also carry depth bombs of various weights. Last week the British Ministry of Information announced that a U-boat had been spotted and sunk by a bombing plane (see p. 35).

One uncertainty of submarine patrollers in the last war, as it may be in this, was that they did not always know whether their depth bomb barrage had finished off their prey or not. An oil slick might appear if a bomb exploded so close to the sub as to smash it completely, but such bull's-eyes were exceptional. Vice Admiral Sims, in The Victory at Sea, told how a British destroyer, equipped with U. S.-made listening devices, located and pursued a U-boat, bombed it with depth charges. At first there was absolutely no sign of success. Then a huge German sailor broke water beside the destroyer. Taken aboard and questioned, this dripping giant said he did not know what had happened, but he supposed he had been blown out through the conning tower. His captors assumed, with some reason, that the submarine had been sunk.

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