At the lower frequencies the dominant source of ambient noise is the cumulative effect of ships that are too far away to be heard individually. The spectrum of the noise radiated from ships as observed at great distances differs from the spectrum at close range due to the effect of frequency-dependent attenuation.

Sea state is a critical factor in both active and passive detection. Inactive sonobuoy detection, waves 6 feet or greater will start to produce a sea-state-limited situation. For shipboard sonar systems, location of the sonar dome, ships speed, course, and relation to the sea all have an effect. The limiting situation is generally sea state 4 or 5. For passive detection, the noise level created by wind waves of 10 feet or greater will result in a minimum of antisubmarine warfare (ASW) operational effectiveness, depending on the type of sensor.

WIND-GENERATED NOISE. Sea-state noise generated by surface wave activity is usually the primary component over a range of frequencies from 300 Hz to 5 kHz. It maybe considered to be one of the most critical variables in active and passive detection.

SEA-STATE NOISE LEVELS. The wind-generated noise level decreases with increasing acoustic frequency and increases with increasing sea state (approximately 6 dB for each increase in sea state). It is very important to understand that all sound-sensor ranges are reduced by additional noise, and that there can be a 20-dB spread in background noise between various sea states.

Ambient noise is also produced by intermittent and local effects such as earthquakes, biologics, precipitation, ice, and breakage of waves.

PRECIPITATION. Rain and hail will increase ambient-noise levels at some frequencies (usually between 500 Hz and 15 kHz). Large storms can generate noise at frequencies as low as 100 Hz and can substantial y affect sonar conditions at a considerable distance from the storm center.

ICE. Sea ice affects ambient-noise levels in polar regions. Provided that no mechanical or thermal pressure is being exerted upon the ice, the noise level generally is relatively low during the growth of ice. According to investigations carried out in the Bering Sea, the noise level should not exceed that for a sea state 2, even for winds over 35 knots. The exception to this rule is extremely noisy conditions due to entrapped air.

BIOLOGICS. Biological noise may contribute significantly to ambient noise in many areas of the ocean. The effect of biological activity on overall noise levels is more pronounced in shallow coastal waters than in the open sea. It is more pronounced in the tropics and temperate zones than in colder waters. By far the most intense and widespread noises from animal sources in shallow water observed to this time are those produced by croakers and snapping shrimp. Fish, more than crustaceans (crabs, lobsters, shrimp), are the source of biological noise in most of the open ocean.

Mammal sounds include a much greater range of frequencies than do the sounds of either crustaceans or fish. They have been recorded as low as 19 Hz (whale sounds) and as high as 196 kHz (porpoise sounds).

EVALUATING THE IMPACT OF BIOLUMINESCENCE

LEARNING OBJECTIVES: Identify the primary sources of bioluminescence in the oceans. Recognize distinguishing features of sheet, spark-type, glowing ball, and exotic light display luminescence.

Plankton organisms are chiefly responsible for bioluminescence in the sea. The smallest forms are luminescent bacteria that usually feed on decaying matter or live in various marine animals. However, with a supply of the proper nutrients, luminescent bacteria can develop in great masses in the sea, causing a general bluish-green glow in the water. The glow is usually diffused and barely detectable, although exceptionally bright displays caused by luminous bacteria occasionally are observed in coastal regions near the outflow of large rivers. The light given off frequently outlines the current front where the river and ocean meet.

Bioluminescent displays may be classified according to their appearance. They are sheet, spark-type, glowing ball, and exotic light.

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