Ambient Noise. The background noise of the sea.

Transcription

1 Ambient Noise The background noise o the sea. When trying to detect a target or contact out in the ocean, one o the most diicult parts is to hear the target through all the background noise. This is just like trying to hear a riend talk while standing in a crowd o people at a noisy rock concert. But out in the ocean, what are the sources o the background noise? Major sources o background noise in deep water Tides A small contribution to ambient noise is the movement o water due to tides. This movement can create large changes in ambient pressure in the ocean. These changes will be most signiicant at very low requencies (<00 Hz) but will decrease in strength with increasing depth. Overall though, tides contribute little to the ambient noise level. Seismic Another source o very low requency noise is that due to the earth s seismic activity. The noise due to seismic activity is not signiicant though, above 0 Hz. As with tides, we will treat seismic sources as being insigniicant in our calculations o ambient noise levels. Turbulence This can be a signiicant actor in ambient noise levels below 00 Hz but generally, we will not consider the aect o turbulence in our calculations. Ship Traic In the North Atlantic, there can be more than 000 ships underway at any one time. The noise rom this shipping traic can sometimes travel up to distances o 000 miles or more. The requency range where this man-made noise is most dominant is rom 0 Hz to 000 Hz. Noise levels depend on area operating in and shipping density. Close proximity to shipping lanes and harbors increases noise levels. Shipping traic is one o the two dominant actors we will use to determine ambient noise levels. The below chart shows how shipping density varies throughout the oceans o the world. -

2 Sea State Sea State (or more importantly wind speed) is the dominant actor in calculating ambient noise levels above 500 Hz. The noise levels depend on sea state and wind speed. Less the 0 Hz, wind-generated turbulence induces pressure variations similar to acoustic pressure variations. Greater than 00 Hz, wind generated microbubbles in the shallow water layers burst and cause pressure changes. Wenz Curves For ASW operators to predict the ambient noise levels or a given condition and requency band, we have the Wenz Curves. Wenz Curves are plots o the average ambient noise spectra or dierent levels o shipping traic, and sea state conditions (or wind speeds ). Below is a complex example given in the Naval Warare Publication. At the end o this handout are the simpliied Wenz curves you will use or all homework, quizzes and exams. -

3 0-00 Hz Noise levels depend heavily on shipping density and industrial activities. Levels are typically in range o 60-0 db with very little requency dependence Hz Noise in this band is dominated by shipping (decreasing intensity with requency increases). A signiicant contribution is also rom sea surace agitation. Urick (86) developed a model or predicting this shipping noise: NL SHIPPING = NL 00 0log 00 Where NL 00 is 60-0 db based on shipping density -00 khz Sea surace agitiation is now the dominant actor, unless marine mammals or rain is present. Knudsen (48) presented a model to predict this contribution: NL SURF NL = NL K K 7log 000 IF < 000 Hz IF > 000 Hz NL K is in the below table, and is based on sea state. A new model has been developed by APL (4), it is more accurate but is more complex. -

4 Beauort Sea Windspeed NL K Force State Knots db Description Sea Condition Calm Sea like a mirror Light Air Ripples but without oam crests Light Breeze Small wavelets. Crests do not break Gentle Breeze Large wavelets. Perhaps scattered white horses Moderate Breeze Small waves. Fairly requent white horses Fresh Breeze Moderate waves, many white horses Strong Breeze Large waves begin to orm; white oam crests, probably spray Near Gale Sea heaps up and white oam blown in streaks along the direction o the wind Gale Moderately high waves, crests begin to break into spindrit 4-47 Strong Gale High waves. Dense oam along the direction o the wind. Crests o waves begin to roll over. Spray may aect visibility Storm Very high waves with long overhanging crests. The surace o the sea takes a white appearance. The tumbling o the sea becomes heavy and shock like. Visibility aected 56-6 Violent Storm Exceptionally high waves. The sea is completely covered with long white patches o oam lying in the direction o the wind. Visibility aected 64+ Hurricane The air is illed with oam and spray. Sea completely white with driving spray. Visibility very seriously aected. >00 khz Noise is dominated by electronic thermal noise (we will discuss causes later) = log NL TH The total ambient noise level is derived by calculating the level sum o the contributing noise actors given by the ollowing equation: NL = NL NL amient The appropriate level o shipping is selected based on location. The heavy shipping curves should be used when in or near the shipping lanes in the North Atlantic. The light-shipping curves should be used or more southerly, remote areas o the ocean. The regions below 0 Hz and above 00 khz are dominated by other actors that are quantiied by the solid lines. General Rules. NL generally decreases with requency increasing. NL decreases at great depths since most noise sources are at the surace.. Ambient noise is greater in shallow water (noise is trapped between sea loor and the ocean surace). ship -4 SS

5 Example For a sonar receiver set with a band width o 00 Hz, centered around 00 Hz, what is the ambient noise level? (Shipping is heavy, sea state is.) To calculate the upper and lower requency o the band: c 00Hz = From the Wenz Curves (end o handout): = = 56Hz = 56Hz ( + 00Hz) ISL ave shipping = 6 db ISL ave sea state = 67 db Thus: NL = NL NL tot ship SS ( ) ( ) NL = ISL + 0 log ISL + 0 log tot aveship avess NL = 8dB 87dB NL tot tot = db Transient Noise Just or passing interest, there are numerous other sources o noise in the oceans. Many o these sources are transitory in nature though which makes them hard to quantiy. They may only aect detectability o contacts or short periods o time. These sources may include but are certainly not limited to: Human industrial sources ashore particularly in coastal areas Biological actors including o snapping shrimp mostly in warm, shallow coastal areas generate intense broadband noise, = -0 khz, SL =60-0 db o whales, dolphins, etc echolocation and communication = Hz khz or whale songs, SL up to 88 db Echolocation khz similar to active sonar, SL up to db -5

6 Weather rain o Rain drops impacting sea surace and implosion o air bubbles caused by rain, = -00 khz, max 0 khz, SL can be up to 0 db above sea surace noise -6

7 Problems. What is the principal cause o ambient noise at requencies a. to 0 Hz b. 0 to 500 Hz c. 500 to 50,000 Hz d. above 50,000 Hz. Using the Wenz curves, determine the isotropic ambient noise level or an area with heavy shipping. Assume that wind speeds are 4 knots and we are interested in the noise level at exactly 00 Hz (use a Hz bandwidth).. The SONAR receiver onboard ship operates in a requency range rom 50 Hz < < 000 Hz. Using the Wenz curves, determine the isotropic ambient noise level in the operating band o the receiver. Assume that winds are light as 4-6 knots and shipping traic is moderate. (Note: You will have to determine an average ISL rom the Wenz curves and calculate the appropriate band levels.) 4. List three intermittent sources o ambient noise. 5. Using the wenz curves or average deep water ambient noise, estimate the band level noise or heavy shipping and sea state 6 or the ollowing conditions: a. Noise received in a band between 0 and 50 Hz. b. Noise received in a band between 000 and 5000 Hz. -7

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9 Lesson Major Sources o Noise Wentz Curves Sea State Dominant actor above 500 Hz Ship Traic Dominant actor 0 to 000 Hz Minor Sources Tides Turbulance Seismic Transients NWP Wenz Curves Hz Wentz Curves NL 00 is 60-0 db depending on shipping. Below 00 Hz NL is the same. Above 00 Hz, the noise decreases with requency NL SHIPPING = NL 00 0log 00

10 Lesson -00 khz Beauort Beauort Sea Sea Windspeed WindspeedNL K Force Force State State Knots Knots db Description Description Sea Condition Sea Condition Calm Calm Sea like Sea a mirror like a mirror Light Air Light Air Ripples Ripples but without but without oam crests oam crests Light Breeze Light Breeze Small wavelets. Small wavelets. Crests Crests do not do break not break Gentle Gentle Breeze BreezeLarge wavelets. Large wavelets. Perhaps Perhaps scattered scattered white horses white horses 4 4 Moderate Moderate Breeze Breeze Small waves. Small waves. Fairly requent Fairly requent white horses. white horses. NL SURF NL = NL K K 7log 000 IF < 000 Hz IF > 000 Hz Fresh Fresh Breeze BreezeModerate Moderate waves, waves, many white many horses white horses Large waves Large waves begin to begin orm; to white orm; oam white crests, oam crests, Strong Strong Breeze Breeze probably probably spray spray Sea heaps Sea heaps up and up white and oam white blown oam blown in streaks in streaks along Near Gale Near Gale along the direction o the wind Moderately Moderately high waves, high waves, crests crests begin to begin break to into break into Gale Gale spindrit spindrit High waves. High waves. Dense Dense oam along oam the along direction the direction o the o the wind. Crests wind. Crests o waves o waves begin to begin roll to over. roll Spray over. Spray Strong Strong Gale Gale may aect may visibility aect visibility NL alls at 7 db per decade above 000 Hz Very high Very waves high waves with long with overhanging long overhanging crests. crests. The The surace surace o the sea o the takes sea a takes white a appearance. white appearance. The tumbling The tumbling o the sea o the becomes sea becomes heavy heavy and and Storm Storm shock shock like. Visibility like. Visibility aected aected Exceptionally Exceptionally high waves. high waves. The sea The is sea completely is completely covered covered with long with white long patches white patches o oam o lying oam lying Violent Violent Storm Storm in the direction in the direction o the wind. o the Visibility wind. Visibility aected aected Hurricane Hurricane The air The is illed air is with illed oam with and oam spray. and spray. Sea completely Sea completely white with white driving with driving spray. spray. Visibility Visibility very very seriously seriously aected. aected. Wentz Curves Above 50 khz Thermal Agitation o water molecules Thermal noise in electronics 6 db per octave increase in noise Total Noise NLambient = NLship NLSS Example For a sonar receiver set with a width o 00 Hz, centered around 00 Hz, Shipping is heavy, Sea state is, What is the ambient noise level?

11 Lesson Wentz Curves Transients Turbulence & Currents Human industrial activity Biological Activity Snapping Shrimp -0 khz, SL = 60-0 db Whales, dolphins Whale songs 5 khz, SL = 88 db Echolocation khz, SL = db Weather Rain 00 khz, SL(0 khz) is 0 db above sea state noise General Noise Rules NL generally decreases with requency increasing NL decreases at great depths since most noise sources are at the surace. Ambient noise is greater in shallow water (noise is trapped between sea loor and the ocean surace). c = From the Wenz Curves: Hz = ( + 00Hz) ISL ave shipping = 6 db ISL ave sea state = 67 db 00 = 56Hz = 56Hz NLtot = NLship NLSS NL = ISL + 0log ISL + 0log NL 8dB 87dB NL ( ) ( ) tot aveship avess tot = tot = db