Marine background noise (ocean ambient noise) includes marine environmental and technical noise. Marine dynamic noise and biological noise are also called marine environmental noise. Generally speaking, dynamic marine noise is produced by waves, ocean currents, and wind. Biological noise is produced by marine organisms, such as fish, shrimp, and mammals.
Technical noise is caused by the machinery of the ship and the technical equipment of the port. Silent submarine research on marine noise is essential for the design and use of underwater acoustic equipment to reduce submarine noise, as well as the improvement of underwater weapons such as mines and torpedoes.
On the contrary, The noise of the oceans also has a close relationship with the combat operations of ships. The background noise of seawater is 90 decibels, and the seawolf submarine level noise is only 95 decibels. These are the so-called silent submarines.
Different sound source fields produce noise of different frequencies and sound levels. The noise of the same frequency range may be generated by one or more sound sources. The following describes the influencing factors of marine environmental noise according to the frequency range.
Very low-frequency noise
(1) Low-frequency vibration
Crustal movement, sound sources such as volcanic eruptions, tides, ocean turbulence, the static pressure effect of waves, and underwater creatures are the contributing factors of the underwater sound field. A very strong and almost continuous form of vibration is microseism, which has a quasi-periodicity of 1/7 Hz. Intermittent ground motions such as a single large earthquake and distant volcanic eruption are also sources of deep-sea low-frequency noise.
The non-linear interaction of back-propagating sea waves will produce environmental noise with a frequency below 5-10 Hz. Turbulence is formed by large or small irregular currents in the ocean. It will cause hydrophones and cables to vibrate or make noise, and its internal pressure produces acoustic effects.
The change of turbulent pressure will also radiate to a certain distance, i.e. noise will be generated in the seawater outside the turbulence. The sound spectrum produced by ocean turbulence is in the ten-octave frequency range of 1-20 Hz. The sound source of underwater organisms is also a component of extremely low-frequency noise sources in the ocean.
Some marine mammals, such as baleen whales, can emit low-frequency groans. The sound signal frequency of blue whales and fin whales is 10-20 Hz. The estimated sound source level can be Up to 190dB.
(2) Coupling of atmospheric sound sources
Atmospheric sound sources emit sound waves that are coupled to the underwater sound field, where thunder can produce an extremely low frequency sound spectrum below 30 Hz, and its energy can be detected below 10 Hz.
(3) Underwater blasting construction
During underwater blasting, the sound wave frequency at the moment of the explosion is slightly affected by the nature of the explosive and the charge. The frequency is 4～10Hz. then, affected by the friction and viscous force of water, the shock wave is gradually deactivated into a sound wave, and the frequency is also changing, and the frequency after the change is 10～105Hz.
Ultra-low frequency, very low-frequency noise
(1) Low frequency super electromagnetic noise
The ultra-low frequency electromagnetic noise that appears in the marine environment is mostly related to submarines. It is mainly produced by the components of the shaft frequency electric field and the power frequency electric field in the ultra-low frequency band and has a direct relationship with the speed of the submarine.
It is known that the effective means for submarine communication are mainly VLF (3～30kHz) and ultra-low frequency (30～300Hz) communication. When a submarine performs underwater ultra-low frequency communication, it will be interfered with by power frequency noise, which can be inferred around the submarine. The electromagnetic noise is ultra-low frequency electromagnetic noise.
(2) Ship navigation and geological prospecting
In sea areas where ships are frequently sailing, in the frequency range of 5 to 500 Hz, the natural noise spectrum is quite consistent with the maximum value of the ship’s radiation noise spectrum. In this low frequency, ship navigation is the main source of global ocean noise.
Geological exploration is also a contributor to the low-frequency sound of the ocean. It is the main means to detect the mineral reserves of the seabed. The air gun used in geological exploration mainly produces ultra-low frequency (5～300 Hz) noise.
(3) Various low-frequency sonars
The towed array low-frequency active monitoring system (SURTASS-LFA) is used in a vertical array working in the frequency range of 100-500Hz. The array has up to 18 sound source transmitters, and the working sound source level of each sound source is about 215dB.
Also, it is reported that the AN/SQS-53C tactical sonar of the U.S. Navy ship emits pulses in the 1～5kHz frequency band, and the working sound source level is 235dB; the AN/SQS-56 sonar emits pulses in the 5～10kHz frequency band, the working sound source The level is 223dB.
According to different applications, commercial sonars generally work in a narrow frequency band from 1 to 200 kHz or higher in the center, and the sound source level of some sonar transducers is as high as 250dB.
(4) Industrial and construction activities
From power plants located by the sea to piling, dredging, shipbuilding, canal sluice structure operations, and daily activities in ports, the categories are quite wide. Many activities (such as impact piling, power plant work, industrial machinery operation, mechanical movement in dredging, wind power Etc.) all produce various sound sources.
The drilling technology used in the oil and gas industry requires much equipment, such as drilling ships, rigs, drilling platforms, and supply ships and airplanes during drilling. Among all the drilling equipment used, the noise generated by the drilling ships is the noisiest, and its frequency span is 10Hz～10kHz (the 10～30Hz frequency range is extremely low frequency), the sound source level is as high as 190dB.
(5) Sea surface roughness and sea surface rainfall
In the frequency range of 500Hz～25kHz, the natural noise level is directly related to the sea state and is related to the local wind speed during the hydrophone measurement. In this frequency range, the noise generated by the sea surface roughness is the noise of the natural noise in the very high-frequency range. Rain on the sea surface also contributes to the sound waves in the ocean.
The study found that in the frequency range of 1～10kHz, the noise spectrum of heavy rain is close to “white noise”, while at 10kHz, the noise level of heavy rain exceeds 18dB when there is no rain. in the wide frequency range from several hundred hertz to 20kHz, the noise level of rain can increase the noise level of the natural environment up to 35dB.
(6) Atmospheric sound source, geological sound source, and sea ice effect
Thunder is a natural atmospheric sound source of ocean noise, which includes not only the extremely low-frequency sound spectrum below 30 Hz but also the ultra-low frequency and very low-frequency sound spectrum above 30 Hz to 1 kHz. Earth’s crust movement can produce extremely low-frequency noise. Among them, the underwater sound waves of earthquakes can extend the frequency above 100 Hz in a short distance and can last from a few seconds to a few minutes.
In addition to crustal movement, the movement of ocean currents on the seafloor causes sediment movement, which can generate environmental noise spectrums with frequencies above 1 to 200 kHz. The ice sheet on the sea can fundamentally change the ocean noise field. The relative movement of nearby ice blocks can generate sound waves in large ice floes.
The bursting caused by the mechanical stress of rigid ice releases higher intensity sound waves. The level of mechanical noise generated by ice cracks and ice wrinkles in glaciers is also very high. At a distance of 100m from the active ice ridge and a water depth of 30m, the density level of the sound pressure spectrum measured between 10 and 100 Hz is 97dB.
(7) Biological sound
(I) Fish and Marine Invertebrates
Many species of fish can emit sound waves through various mechanisms. The size of the sound waves varies with different ecosystems and time scales such as day and season. The sound waves emitted are 5 Hz to 5 kHz, most of which are pulse signals below 1 kHz. They are mainly used for communication, Predation, swimming, and other behaviors. Compared with fish, marine invertebrates have fewer vocal species, and sea urchins and crayfish are more studied.
When sea urchins eat food, teeth rubbing against coral reefs can produce sounds of about several hundred hertz. At the same time, their hard shells can also resonate with the sound. This noise can cause the spines on the sea urchin to vibrate at a certain frequency and increase the noise level of the surrounding environment by 20～ 30dB. Crawfish can emit sounds in the frequency range of 2 to 200 kHz in their colonies, and the range of 30 to 200 kHz is high-frequency noise.
(II) Marine Mammals
Although the sound frequencies of marine mammals cover a wide range, their sound frequencies are mainly concentrated in 30Hz～30kHz. Most large baleen whales, such as southern right whales, arctic whales, gray whales, and humpback whales can record vocalizations below 1kHz and sound source levels above 180dB.
Whales and dolphins were recorded in early March 2000 (the breeding season) The frequency of occurrence is the highest sound level of 100-150Hz, 250-350Hz, 600-650Hz. Toothed cetaceans (dolphins and dental whales) can emit a whistling sound of 1-25kHz, and some species of dolphins can also emit a whistling sound of 129Hz-30kHz.
High frequency noise
(1) High frequency sonar
High-frequency sonar (above 10kHz) is used in weapons and anti-weapon applications at distances of tens to thousands of meters. The high-frequency sonar used by the mine-hunting system ranges from detection of tens of kilohertz to positioning of hundreds of kilohertz. The system uses pulse signals and points Very strong.
Compared with military sonars, civilian sonars generally work at higher frequencies, among which commercial sonars are designed for special purposes, such as obstacle detection, depth sounding, and fish detection. According to different applications, commercial sonars generally work in a narrow frequency band with a center frequency of 1 to 200 kHz or higher.
(2) Thermal noise
The thermal noise generated by molecular disturbance is the main noise source of high-frequency noise in the deep-sea natural environment. Through the analysis of the Wenz spectrum level diagram, it can be concluded that the frequency of thermal noise is above 10kHz and mainly concentrated above 30kHz.
(3) Biological sound
Marine organisms that can make high-frequency sounds are mostly limited to marine mammals, and crawfish can emit high-frequency noise with a frequency band of 30 to 200 kHz in their colonies. The high-frequency sound of marine mammals is mainly used for echo localization.
Toothed whales (dolphins and dental whales) can make a population-specific clicking sound, with peak energy much higher than 100kHz, and can emit whistle sounds with harmonics up to 100kHz. Harbor seals can emit 40kHz high-frequency sounds in the air, and dolphins use ultrasonic waves with frequencies above 200-350kHz for “echo location” underwater (read more about dolphin sound detection).
The State Key Laboratory of Sound Field Acoustic Information of the Institute of Acoustics obtained marine environmental noise including typhoon processes in the South China Sea and the Western Pacific Ocean in the autumn of 2017. The noise spectrum of the typhoon passed through and the wind speed comparison chart provided by the National Ocean Forecast Center is shown in the following figure.
(4) Noise field
(I) Directivity of the noise field
The marine environmental noise field composed of different types of noise sources in different seasons in different regions of the ocean has different characteristics. It is usually expressed by statistical parameters such as the power spectrum of the noise, the spatial distribution of the amplitude, and the spatial correlation.
The power spectrum of marine environmental noise is continuous. The spatial distribution of various underwater noise sources is irregular, but they all have their directionality. Changes in the sea and hydrological conditions and different seabed characteristics caused by sea and meteorological conditions will affect the directivity of the noise field.
Also, changes in wind speed and shipping volume, intermittent biological noise, temporal and spatial changes in temperature and salinity, and the effects of tides, day and night, and seasons all make the noise field show obvious periodic or random changes. Since the underwater noise field has directivity, the research on the temporal and spatial correlation characteristics of the field is very important.
(II) Noise field in high seas and shallow seas
A large amount of measured data shows that the environmental noise spectrum of the deep ocean is quite stable. Shallow sea noise is affected by hydrological conditions, seabed characteristics, near-shore industrial facilities, and fish migration, and varies greatly with regions and seasons.
Noise sources such as industrial facilities and tides near bays and ports have strong day and night and seasonal changes, so the environmental noise spectrum levels of different bays, ports, and nearby areas can only be roughly drawn. Nevertheless, the average spectrum of environmental noise obtained from a large number of long-term samples is still of great practical value.
(III) The influence of noise field on sonar
For the design and use of sonar equipment and underwater acoustic instruments, marine environmental noise is a source of interference. It is necessary to measure the power spectrum characteristics, directivity, and time and space correlation characteristics of the marine environmental noise field in different sea areas and different seasons, and take corresponding measures in the hydrophone array and circuit design to increase the signal-to-noise ratio gain.
The marine biological noise spectrum is an important criterion for marine biological research and marine fishing. The development of detection and data processing technology, the relationship between environmental noise and reception depth. There has been great progress in research on the exploration of sources such as the directionality of the noise field, the noise spectrum under ice, the statistical characteristics of the instantaneous value of the noise level, and the noise below 10 Hz or even below 1 Hz.