Waterborne sound

Water sound is sound , which in the water is transmitted. The associated specialty of acoustics is hydroacoustics . The term water- borne noise is occasionally used as a synonym for the technical application of hydroacoustics.

Since electromagnetic waves such as radio waves and light can propagate much more poorly in water than water-borne noise and have ranges from a few meters to a maximum of 100 m due to their stronger absorption , water-borne noise has many technical applications. This includes communication , navigation and positioning as well as the measurement of physical , chemical and biological quantities. The frequency range used here is around 10  Hz to 1 MHz.

Water-borne sound waves of sufficient transmission power can be perceived in the low-frequency range over any waterway on earth that is not interrupted by land.

history

An early statement on water-borne noise can be found in Leonardo da Vinci , who wrote in 1490: "If you dip a pipe in the water and hold the other end to your ear, you can hear ships at very great distances."

In 1827, Colladon and Sturm described a measurement of the speed of sound in the water in Lake Geneva . They wanted to determine the compressibility of the water.

Around 1900, water-borne noise began to be used as a navigation aid .

The American oceanographer Maury was probably the first to point out the use of sound for echo sounding of the water depth in 1855 . However, his own attempts were unsuccessful. As the inventors of the echo sounder in the years 1912 to 1914, Behm , Fessenden and Richardson are therefore considered to be completely independent of one another, but apparently all three under the impression of the sinking of the Titanic . Back then, the echoes of icebergs were thought to be easier to get than from the ocean floor.

At the end of the First World War, the knowledge was available that enabled echolocation of submarines with sound, but it was no longer used for technical purposes. In contrast, the passive localization of submarines was used as early as the First World War. This technology, which was initially known as ASDIC in Great Britain , was later referred to as sonar based on radar .

Submarine tracking took off considerably during World War II. Nevertheless, the declining effectiveness of the submarines in the Second World War was less due to the use of water-borne sound than to the newly invented radar and the combat against submarines from the air.

After the Second World War, the submarines, which up until then could be called "submersibles", which only dived briefly before an attack or when threatened, developed into real underwater vehicles that only appear briefly or as conventional submarines to charge the batteries snorkeling . Nuclear powered submarines or conventional AIP submarines can almost only be located with waterborne sound or sonar . Because of this, the development and research of waterborne sound and military sonar devices has made a considerable boost. The fact that sea ​​mines are now mainly detected with the help of sonar has contributed to this .

Basic water-borne sound phenomena

Example of typical profiles of temperature, speed of sound and pressure, measured at 9 ° N, 30 ° W

Sound waves are pressure waves that propagate longitudinally , i.e. That is, the molecules vibrate in the direction of sound propagation. So you need a medium in which they can spread. As with other waves, the following applies:

${\displaystyle \lambda ={\frac {c_{\text{S}}}{f}}}$

Where:

${\displaystyle c_{\text{S}}}$the speed of sound in water, the frequency of the sound and the wavelength of the sound in the water. ${\displaystyle f}$ ${\displaystyle \lambda }$

In the ocean, sound is much faster at around 1480 m / s than in the air, where, under normal conditions, it travels at around 340 m / s. The speed of sound increases with temperature , pressure and salinity . Since the pressure is almost linear with the depth, this is often used to calculate the speed of sound. There are some empirically determined formulas with which the speed of sound can be calculated relatively well. These formulas are all quite similar, a simple one being:

${\displaystyle c_{\text{S}}\,=\,(1449+4{,}6\ T-0{,}05\ T^{2}+1{,}4(S-35)+0{,}017\ D)\;\mathrm {\frac {m}{s}} }$,

where the temperature is in ° C, the salinity is in psu and the depth is in meters. ${\displaystyle T}$ ${\displaystyle S}$ ${\displaystyle D}$

For the dependence of the speed of sound one can say:

• In the upper layer, the temperature is decisive, as this variable changes the most.
• The change in temperature is very small below the temperature jump layer, here the depth is the determining parameter.
• The salinity has hardly any influence on the speed of sound, as it has an almost constant value of around 3.5% in most places in the ocean and the associated term is therefore very small. It can often be neglected.

Because salinity and temperature also lead to changes in density and horizontal changes in density do not exist statically in liquids, the water is stratified with only weak horizontal gradients . Correspondingly, the gradient of the speed of sound and thus the refractive index is also weak horizontally. As a result, sound beams that run vertically are hardly refracted. The sound spreads very regularly in the vertical direction, as in the free, unlimited medium without changes in the speed of sound.

Waterborne sound level

As with airborne noise, the sound pressure level is used as the most important measured variable for water- borne noise . Although the same methods are used formally, water-borne and air-borne sound levels are difficult to compare and repeatedly lead to misunderstandings among laypeople. Levels are logarithmic values and can only be interpreted in a meaningful way if the respective reference values ​​used are observed. As a reference value for the sound pressure level, 20 µPa is selected for airborne sound , but 1 µPa for waterborne sound  . The reference value at the airborne sound was based on the human hearing threshold is selected and corresponds to propagation of a plane wave in the propagation medium air a sound pressure of about 2 × 10 ^-5 Pascal (20 uPA). This reference sound pressure corresponds to an intensity of around 10 ⁻¹²  W / m².

It is well known that water is much harder than air, so that with the same intensity the acoustically relevant particle velocity is much lower, but the sound pressure is higher. For this reason, a comparison of the sound pressures is not very meaningful anyway. If you want to compare, you should rather use the energy flux density (intensity). Because the sound pressure is more accessible in terms of measurement technology, it is still preferred. The reference intensity of the water-borne noise is around 0.65 · 10 ⁻¹⁸  W / m², corresponding to the reference pressure of 1 µPa used. The relationships between sound pressure and sound intensity depend on the one hand on the static pressure and on the other hand on the temperature and salinity of the water. The same level data for air and water at their respective reference pressure differ in their intensity by approximately 62 dB.

For a - anyway questionable - comparability, about 62 dB have to be deducted from the water-borne noise. Furthermore, frequency-weighted sound pressure levels are often given for airborne sound in order to take into account the frequency response of the human ear. This makes no sense under water and is therefore not used there, which further increases the difference between the apparently identical level values. The difficulty in comparing sound level information for airborne and waterborne noise ensures, for example, For example, when discussing the harmfulness of sonars and echo sounders to marine animals, it is often a source of confusion.

Modern high-performance sonars and echo sounders generate a sound pressure level of around 220 to 240 dB related to 1 µPa at a distance of 1 meter.

Sound absorption

Absorption for sea water of 35 ppT, 2 ° C and 10 atm

The sound absorption converts sound energy into thermal energy. According to the classical acoustic theory, this occurs on the one hand through the adiabatic temperature change as a result of the alternating sound pressure. Energy is extracted from the sound field through thermal conduction or thermal radiation. This effect is negligible in water. The other classic cause of absorption is internal friction during compression and decompression. It plays a role at high frequencies (over 100 kHz) and in fresh water.

However, these two so-called classic causes are below 100 kHz in seawater compared to relaxation damping , i.e. H. the damping caused by the delayed setting of the equilibrium volume when the pressure changes, negligible. Some components of the lake water are in different chemical states, the relationship of which depends on the pressure and is delayed, whereby acoustic energy is withdrawn from the sound field, which is converted into heat. This relaxation damping is caused at low frequencies (below 10 kHz) by boric acid and higher frequencies by magnesium sulfate . The absorption loss can be calculated with Technical Guides - Calculation of absorption of sound in seawater .

Despite these effects, the absorption attenuation in sea water is much lower than in air and decreases at lower frequencies. Absorption also occurs when it is reflected on the sea floor and e.g. B. on ice and bubbles in the water.

The sound decreases with distance from the sound source not only through absorption but also through divergence . In an infinite space of constant speed of sound, this leads to a decrease in sound intensity with the square of the distance (1 / r²) or the sound pressure with the linear distance (1 / r). Although this divergence attenuation is largely independent of frequency, it represents the main problem in calculating the sound field due to the layering of the speed of sound in the sea and the edge reflections.

Multipath propagation

Multipath propagation in sound transmission in shallow water

Because the sound beams are curved as a result of the temperature stratification (but also due to the pressure increase with depth and salinity stratification), they are usually reflected at greater transmission distances on the sea surface or on the bottom or both. There are therefore several transmission paths, but often no direct path without edge reflection. These different paths between the sound source and receiver have almost the same transit time, especially in shallow water, and are therefore often not perceived as different transmission paths at the receiver. But they have different phases, which leads to interference . Small changes in the transmission paths due to reflection on the rough edges, due to refractive index variations due to internal waves , for example, change these interferences, whereby the sound transmission fluctuates. However, particularly at greater distances in deep water, transit time groups that are clearly separated from one another also occur.

The multipath propagation is characteristic of the sound transmission in shallow water, but also plays a role in deep water over longer distances.

The picture shows a typical situation as it occurs in the North Sea in summer, although, contrary to reality, the sound velocity stratification is assumed to be horizontally unchangeable and the edges, apart from the noticeable slight undulation of the bottom, are smooth. The reality is incomparably more complicated than shown here. The rays look very steep because of the compression of the horizontal axis by a factor of about 36, but are actually quite flat. In particular, there are no flatter beams with this sound velocity profile.

Shallow water / deep water sound propagation

In shallow water, multipath propagation occurs even at relatively small horizontal distances. At lower frequencies, the acoustically soft edges on the sea surface and the acoustically hard edges on the sea floor also form a waveguide which , however, is attenuated in particular by incomplete reflection on the sea floor. At very low frequencies, the frequency falls below the cutoff frequency of the waveguide, which leads to very high attenuation.

In deep water, apart from an additional reflection on the surface of the sea with the shallow transmitter, the multipath propagation only plays a role at very great distances. Otherwise, at least the runtime differences are very large. Due to the increase in the speed of sound due to the pressure increase with depth, sound can be transmitted over any distance without reflection from the ground. Apart from near-polar waters (and marginal seas), the water temperature at the sea surface is always higher than in deep water, where it is only slightly above 0 ° C in deep water at the equator. There is therefore a sound transmission without edge reflections (see SOFAR channel ). The depth of the water at which the speed of sound is as great due to the rise in pressure as it is on the surface due to the effect of temperature is called the “critical depth”. Sonar experts only speak of deep water when the water depth is greater than the critical depth.

For the distinction between shallow water and deep water, there are several criteria from an acoustic point of view, some of which are frequency-dependent but hardly dependent on distance and some are distance-dependent but frequency-independent.

1. Shallow water is when the transmission has the character of a waveguide with few acoustic modes . This is the case when the water depth is only a few wavelengths. As a rule of thumb, one can assume that shallow water is spreading when the product of water depth in m and frequency in kHz is less than 100.
2. Deep water is when the water depth is greater than the critical depth.
3. At high frequencies, one speaks of deep water transmission even if the horizontal distance between transmitter and receiver does not exceed the order of magnitude of the water depth. Then the re-routes are no problem.

These definitions are of course unsatisfactory because according to them the same area is sometimes deep water, sometimes shallow water. They are therefore only used in connection with the identification of the transmission conditions (for example "deep water conditions"). Sonar experts also use the term separations “blue water” more frequently for indisputable deep water, greater than the critical depth, which is usually 1000 to 2000 m. This is the high seas, especially in the deep-sea basins. “Green water” is the transition area, also characterized by the exclusive economic zone with water depths less than the critical depth, but deeper than the typical shelf edge up to 200 m water depth and finally “brown water”, as coastal water on the shelf base, also with additional problems for the propagation of sound through estuaries, strong tidal currents , etc. This “brown” shallow water, together with other waters with particular difficulties in the sonar area, such as fjords , is referred to as “confined water” in NATO .

Lloyd mirror effect

The Lloyd mirror effect is also known for electromagnetic wave propagation. It is important for water-borne noise because the water surface is a very soft boundary, i.e. the sound pressure is only negligibly low in relation to the speed of sound at the sea surface for water-borne noise conditions. This means that a sound wave turns its phase by 180 ° when it is reflected on the sea surface . As a result, the sound pressure at the surface is canceled out by interference. With increasing distance from the surface, this phase angle increases further, so that cancellation no longer occurs. The formula for this is at a large distance ${\displaystyle r}$

${\displaystyle {P \over P_{0}}=2\left(1-\cos {4\pi z_{1}z_{2} \over r\lambda }\right)}$

with the current sound pressure in comparison to sound pressure in the free field without borders, the distances from the transmitter and receiver to the surface and the acoustic wavelength . ${\displaystyle P}$ ${\displaystyle P_{0}}$ ${\displaystyle z_{1}}$ ${\displaystyle z_{2}}$ ${\displaystyle \lambda }$

At low frequencies in shallow water this effect is modified because the reflections on the sea floor cannot be neglected. A corresponding effect is based on the assumption that all modes have a sound pressure node on the sea surface, whereby a similar sound pressure decrease occurs, ultimately as a result of the same physical cause. The Lloyd-Mirror effect is also influenced by the sound velocity profile and can then often hardly be separated from the sound velocity profile effect of the shadow zone (“ zone of silence ”).

The significance of these effects lies in the fact that sound transmitters and receivers are often attached to the ship's hull, i.e. very close to the water surface.

Shadow zone

Most of the time the temperature is highest near the surface, so that the speed of sound is also higher there. The sound is therefore refracted downwards, so that no more rays can reach above the uppermost possible ray (boundary ray). This applies in deep water. In shallow water, this area can be brightened by the rays reflected from the ground. Above the borderline the shadow zone or the " zone of silence " begins . At lower frequencies, this acoustic shadow sets in softly as a result of diffraction.

This shadow zone also occurs when the water on the surface is completely mixed by the swell (mixed layer). In this case, the shadow zone begins in the so-called thermocline under the mixed layer (at which the constant temperature above it drops downwards), while at higher frequencies the sound can certainly propagate in the mixed layer, because here due to the increase in water pressure Sound beams are curved slightly upwards. The propagation of sound in the mixed layer is strongly dependent on the sea state because of the reflections on the sea surface. The scattering on the rough surface can lighten the shadow area a little.

So there are many opposing effects here: The mixed layer is mostly created by swell, which dampens the propagation of sound in the mixed layer, but brightens the shadow zone. The sound propagation in the mixed layer is only possible at higher frequencies because of the Lloyd Mirror effect. The higher frequencies are in turn more strongly influenced by the sea.

Which of the effects wins in each case can only be predicted with a lot of experience or with the help of numerical models.

Afternoon effect

Even before the Second World War, the American Navy had determined that the sonar conditions were very good in the morning, but they were worse in the afternoon, so that a location was often no longer possible. The reason for this is that in the morning, due to the nightly cooling, a mixed layer that was effective at the active sonar frequencies of 20 to 30 kHz that was common at the time had appeared. During the course of the day, the surface temperature rises again due to solar radiation, so that the mixed layer loses its sound-guiding ability and the shadow zone begins on the surface. This effect is most noticeable in poor wind and sunny weather in deep water.

SOFAR channel

→ Main article : SOFAR channel

The already mentioned interaction of the increase in the speed of sound on the sea surface due to the temperature and at great depths due to the increase in pressure means that sound propagation, namely multipath propagation, is possible without reflection at the edges. The axis of this channel, i.e. H. the lowest speed of sound, occurs at a depth of approximately 1000 m. Sound propagates in a fiber optic cable similar to light . Because of the considerable dimensions in deep water, this also applies to very low frequencies, which are hardly absorbed in free water. This enables very large transmission ranges, which are ultimately only limited by the edges of the world's oceans. This sound channel in deep water is called the SOFAR channel (Sound Fixing And Ranging). It plays a major role in the extensive surveillance of the sea with stationary passive sonar systems SOSUS and in acoustic ocean tomography.

The SOFAR Canal only exists in deep water. In the Baltic Sea there is also a similar sound channel in the basins in summer, whereby the increase in the speed of sound below is not caused by the pressure, but by the interaction of higher salinity and higher temperature (Baltic Duct). Because this channel has much smaller dimensions, it only works at significantly higher frequencies and is also often disturbed there by the great variability of the stratification.

Convergence zone expansion

At greater water depths than the critical depth, i.e. when the temperature-related higher speed of sound is exceeded due to the pressure increase in the depth, a phenomenon occurs that the sound from the source on the surface initially dips into the depths (with shadow formation already at very short intervals of a few km) and at a greater distance, typically somewhere between 50 and 70 km, appears strongly bundled again. This so-called convergence zone acts like a new sound source, so that the convergence zones repeat themselves in multiples of this distance as rings around the original sound source. Initially this was an important effect for passive sonar location. The low-frequency towed arrays are particularly effective for the convergence zones.

The convergence zones have only recently become usable for active sonar with the introduction of LFAS (Low Frequency Active Sonar), because at higher frequencies the absorption attenuation in the water is too great for echo location.

The convergence zone location is only possible at water depths greater than the critical depth, which is why the critical depth is of particular importance for sonar location.

Acoustic effect of bubbles

With the mass of the surrounding water and the compressibility of the enclosed gas, bubbles form a high quality oscillation system . At the resonance frequency, the backscatter cross section is very large compared to the geometric cross section of the bubble. Bubbles are therefore extremely effective acoustically as a cause of the scattering of sound, but also for the attenuation and for the generation of noise.

The resonance frequency of a bubble a mm in diameter at a depth of z m (water pressure) is

${\displaystyle f_{r}={3260 \over a}{\sqrt {1+0{,}0982\cdot z}}\,\mathrm {Hz} }$,

so a bubble 1 mm in diameter near the surface of the water has a resonance frequency of 3.2 kHz.

Below the resonance frequency, the scattering cross-section decreases with the fourth power of the frequency. Above it finally goes back to the geometric cross-section.

In practice, the following bubbles are of particular importance

• the bubbles washed in by the breaking sea, whereby even the smallest foam heads are effective
• the swim bladders of fish
• the bubbles in the wake of ships
• rising bubbles from methane gas reservoirs

The breaking sea is the main cause of the natural ambient noise in the water in the medium frequency range and can cause very strong sound attenuation, especially in shallow water.

The fishfinder sonar uses the backscatter effect of the swim bladders. Special research plumb bobs , e.g. B. the Simrad EK60, use multiple frequencies, which makes it possible to determine the number of fish by size class, because the different size classes resonate at different frequencies.

Wake- seeking torpedoes use upward echo sounders at two frequencies, one with a strong resonance scattering effect and a second at a very high frequency, corresponding to a bubble diameter of a few µm, at which the number and effectiveness of the bubbles is relatively low. This allows the torpedo to know when it is crossing the wake.

Ambient noise in the sea

Noise is the term used to describe all undesired, observed sound events in contrast to the expected "signals". A distinction must be made between self noise, the noise that is generated by the sound receiver or its platform itself, and the ambient noise that is also present without the sound receiver.

The intrinsic noise can be real water-borne noise that occurs on the receiver (hydrophone) or the carrier platform (ship), or structure-borne noise that is transmitted to the sound receiver via its attachment, or it can be caused by electrical interference directly at the receiver.

The ambient noise can be divided into natural sound sources or those that can be traced back to the activities or facilities of humans (man made noise), and on the other hand, a distinction must be made between constant, slowly fluctuating sound and short sound events ( transients ), which are also natural or man-made.

According to Urick, five areas are distinguished in the frequency range:

1. Below 1 Hz. Here one receives essentially "pseudo-sound", i. H. Pressure fluctuations, which are not due to wave-like propagation, but to the hydrostatic pressure due to the changing water level or flow-induced pressure fluctuations due to the Bernoulli effect . The effects of earthquakes, which can be real sound (with propagating waves), also fall into this frequency range as transients. A systematic frequency response cannot be specified.
2. Between 1 Hz and around 20 Hz (10 to 30 Hz), the ambient noise spectrum drops by around 8 to 10 dB / octave. At 1 Hz it is about 120 dB rel. 1 µPa and 1 Hz bandwidth. Here, too, Bernoulli's pressure fluctuations due to turbulence are assumed to be the main cause. In shallower water, the swell also directly causes such pressure fluctuations.
3. Depending on the density of sea ​​traffic , a relative maximum between 10 Hz and 200 Hz (with heavy ship traffic and shallow seas also up to 1 kHz) is obtained, primarily due to the noise of distant ships. But even in the few places where the far-reaching noise of ships is imperceptible (some areas of the South Pacific), the noise in this area falls less with increasing frequency, because distant noises of waves crashing on the coast and from the pack ice border can then be heard. The relative maximum at around 80 Hz is around 70 dB rel. 1 µPa and 1 Hz and with heavy ship traffic (without a single ship in the vicinity of the receiver) about 90 dB rel. 1 µPa and 1 Hz.
4. Above about 200 Hz to about 20 kHz, in heavy seas up to about 100 kHz, the ambient noise is caused by the breaking sea ("ringing" of bubbles in the breaking sea) or, in the case of heavier rain, by the bubbles generated by the rain fall. But even in this frequency range, the noise from nearby ships predominates (mainly cavitation noise from the ship's propeller or, for example, the noise from the towing gear of fishermen when they are nearby).
5. Above 20 kHz, thermal noise finally wins through Brownian molecular motion . At 100 kHz it is about 25 dB rel. 1 µPa and 1 Hz, increases with frequency and is not very variable due to the small range of variation of the water temperature in the sea.
   • The sound just below the surface of the water, individual bubbles can be heard in the breaking sea. ^{? / i}
   • At a great distance from the sea surface, the sound of the bubbles becomes blurred, and because of the greater attenuation of higher frequencies, the sound sounds a little deeper. ^{? / i}
   • Sound of a freighter, mostly cavitation sound of the four-bladed propeller. ^{? / i}
   • Sonar signals recorded in stereo with two hydrophones ^{? / i}
   • Twitter of a herd of pilot whales. The crackling background noise comes from the clicks to the echo location ^{? / i}

reverberation

When reverberation is called continuous reflections of sound waves (acoustic reflections) in a closed room or within a limited range, of course. Echoes, on the other hand, are reflections from individual objects.

In the case of airborne sound, the reverberation is characterized by the reverberation time . In the case of water-borne noise - in which the reverberation is only important for active location (and multistatic) location - one is interested in the distance course of the reverberation compared to the target echo.

One often hears the rule of thumb among sonar operators that the achievable location range is as large as the reverberation range. This sounds plausible because the propagation attenuation of the sound is decisive for both the echo and the reverberation. On the other hand, particularly strong reverberation - which therefore goes far - can mask the target echo. This creates an inverse relationship between the reverberation distance - i.e. H. the distance (or time) at which the reverberation disappears in the sound - and the maximum localization distance.

Depending on whether the reverberation distance or the maximum localization distance is greater, one speaks of reverberation-limited or noise-limited range. If the range is limited, increasing the transmission level does not increase the range. In shallow water, the range is mostly limited to reverberation and in deep water, rather to noise. For this reason, sonar systems that are optimized for shallow water usually have a lower transmission power than typical deep water sonar systems.

There are essentially two options for reducing the reverberation effect. One is that the echoes from location targets experience a Doppler shift due to the travel speed of the target , which is greater than the Doppler frequency range of the reverberation, at least in the case of objects approaching quickly. This differentiates the target from the reverberation by means of “Doppler selection”. The other possibility is to increase the time and angle resolution of the system, because this reduces the simultaneously illuminated area and thus the reverberation relative to the target. In addition to the increased demands on the technology of the sonar system, this measure comes up against two limits: On the one hand, signals with a high time resolution generally have a low frequency resolution and can therefore make less use of the Doppler effect. On the other hand, the reverberation may lose its diffuse character at high resolution. In the aftermath, individual local peaks emerge as “false targets”, which are difficult to distinguish from the target sought.

A distinction is made between three types of reverberation according to the place of their origin, surface reverberation, volume reverberation and floor reverberation.

In shallow water, the bottom reverberation usually plays the greatest role. The bottom is the boundary that is hit more and at a steeper angle by the sound velocity profile (see picture of the multipath propagation in shallow water). The reverberation is very different depending on the type of floor. The rougher the floor, the more reverberation it creates. The spatial wave number spectrum determines the frequency dependence of the reverberation via the Bragg equation . In the case of sand, the reverberation therefore takes z. B. between a few 100 Hertz and a few Kilohertz. Craggy rocky ground leads to a strong, but not very diffuse reverberation with a strong false target character. Schlickboden usually has a very smooth surface, but can produce strong reverberations. This is based on the fact that sound penetrates the silt well and the reverberation is then determined by gas bubbles in the silt or the roughness of the structures under the silt. Because the attenuation of the sound in the mud is highly frequency-dependent, this means that low-frequency reverberation is preferred.

The reverberation from the sea surface plays a role especially in winter (no temperature increase of the water towards the sea surface). It is caused more by reflections on bubbles washed in by the sea than by the rough sea. Because it only depends on the wind (and the swell associated with the wind), but hardly on the location, Chapman and Harris presented a very useful empirical description of surface reverberation as early as 1962.

Volume reverberation is caused by scattering bodies in the water column. It is usually much weaker than the reverberation from the edges and is primarily observed in deep water and in echo sounders. Its main cause is plankton at high frequencies and fish at medium to low frequencies, or more precisely, the air-filled swim bladders of fish. The distribution of the reverberation sources in the water is not uniform, according to the preferred location of plankton (which the fish also follow). Under certain conditions, for example in the Baltic Sea Sound Canal or when large schools of fish are present, the volume reverberation is also effective in shallow water.

Technical applications

A distinction is essentially made between application goals:

• Depth measurement: echo sounder
• Location: of submarines (underground hunting), mines (mine hunting), with active and passive sonar
• Fish finder for finding fish and schools of fish
• Underwater imaging: side view sonar
• Underwater communication: underwater telephone
• Underwater navigation: e.g. B. Posidonia
• Determination of the flow velocity
• Ocean Acoustic Tomography

Echo sounder

The traditional echo sounder is called a navigation sounder to distinguish it from the special forms because it serves as a standard equipment of a ship for safe navigation. It usually has a limited depth range because at great depths the water depth is irrelevant for navigation, and it is usually not stabilized against the ship's movements and is not very focused.

Research plumb bobs or survey plumb bobs are built much more complex. For precise location and depth determination, they are tightly bundled (Narrow Beam Sounder, shelf edge plumb bob) so that the depth is not measured as an inclined distance on sloping seabeds. They must therefore also be stabilized with regard to the ship's movements.

Nowadays, fan-shaped plumb bobs are mainly used for research and measurement. With them, the directional resolution is shared between the transmitter (high resolution in the forward direction, wide in the transverse direction) and the receiver (several receiving lobes by electronically pivoting the directional lobes simultaneously for many directions - hence fan). They allow extensive coverage of the sea floor instead of profile lines under the ship. For measurement purposes, the current sound velocity profile on site and the current water level must be taken into account.

Sediment-penetrating solders allow sound to pass through the upper sediment layers, depending on the power and frequency. They are relatively low-frequency and represent a smooth transition in technical construction and in the application to seismic systems ( seismic ). Sediment-penetrating plumb bobs are low-frequency because the attenuation in the ground is very large and approximately linearly proportional to the frequency.

There are also small hand plummets in the shape of a flashlight for sport boaters and divers.

Military sonars

• Ujagdsonar
• Submarine sonar
• Mine hunting sonar, mine avoidance sonar
• Gun sonar

The hunting sonar distinguishes between active sonar (echo location) and passive sonar. Recently, mixed forms have also been used: bistatic sonar, i. That is, the transmitter and receiver are arranged on spatially separate platforms.

After the Second World War, active sonars at the bow of the ship initially played a major role (bow sonar, hull mounted sonar). They were then supplemented by towed array sonar (TA) in the early 1970s . The further noise reduction also in the case of nuclear-powered submarines made a return to active sonar (echolocation) necessary, today mainly through low-frequency towing sonars, in which the towed array has a low-frequency, towed transmission unit ("Active Adjunct"). Today, however, the old towed arrays with their equipment for very large bandwidths (partly 10 Hz to 1 kHz) are rarely used for this purpose, which could not distinguish between right and left in the direction. Instead, cardioid (triplet) or twin arrays are used, which allow this distinction, but are limited in the frequency range to the transmission frequency. These systems work differently than conventional active sonar systems, which were operated in the medium frequency range (3 kHz to 20 kHz), and in the low frequency range (100 Hz to 3 kHz).

During the Cold War , the American Navy installed stationary passive sonar systems on the ocean floor at various locations around the world for the large-scale surveillance of enemy submarines. These systems, which were kept strictly secret at the time, are called SOSUS (Sound Surveillance System). This system was only announced after the end of the Cold War and is used today for civilian purposes, unless it has since been switched off.

The requirements of mine hunting sonar systems are significantly lower in terms of range (<1 km), but the higher the resolution, so that the comparatively small targets can be reliably identified and, if possible, also classified. Accordingly, they work at very high frequencies between 100 kHz and 1 MHz. The requirements for mine avoidance sonar systems, which are only supposed to warn the carrier (e.g. frigate or submarine) of mines, especially anchor mines, in good time, in order to enable an evasive maneuver, are somewhat lower. Mine avoidance sonars use frequencies similar to mine hunting sonars, but are generally more modest, particularly with regard to classification capabilities.

The active or passive sonar system that many torpedoes have at their disposal is called a weapon sonar. Strictly speaking, the acoustic sensors of mines should also be referred to as passive weapon sonars. However, there they are recorded as acoustic sensors.

Due to the lack of space and the lower range requirement, the frequencies of the weapon sonar are significantly higher than the underground hunting sonar, but usually lower than those of the mine hunting sonar. There are active and passive sonars as torpedo sonars, whereby active sonars are usually used compared to submarines and passive sonars are used compared to surface ships.

Well-known research institutions for water- borne noise in Germany are the Bundeswehr Technical Center for Ships and Naval Weapons, Maritime Technology and Research , in Europe the Center for Maritime Research and Experimentation of NATO and in the USA the Naval Research Laboratory (NRL) and the Naval Undersea Warfare Center (NUWC). In other countries, mostly centralized military research institutions are involved in waterborne sound research.

Fishfinder sonar

These are high-frequency active sonars that locate echoes from swim bladders. Most fish finders do not use the acoustic resonance effect of the swim bladders, but work at higher frequencies. However, especially for research purposes (e.g. Simrad EK 60) there are multi-frequency sonars that use the resonance frequency to differentiate between the size of the fish.

Imaging sonars

Initially, this was only understood to mean side-scan sonar (SSS). Some modern high-resolution plumb bobs represent real competition to the SSS in terms of the image quality that can be achieved. In addition, the trend is towards mixed forms between these two originally quite different processes.

Schematic sketch of the side viewing sonar

The fan plumb line works like a traditional echo sounder with plumb lines, but several of them are recorded simultaneously in parallel, creating a two-dimensional depth profile, i.e. basically (with the depth) a three-dimensional image.

The SSS detects a line on the sea floor for each ping sent , which is modulated by the local backscatter strength of the sea floor along this lateral strip. With the sequence of pings, a “picture of the sea floor” is created. B. sand and silt clearly differentiated. Individual objects on the ground are also characterized by strong backscattering with subsequent interruption of the backscattering by the formation of shadows. This results in the sequence of pings with the “lines” per pings, images with the intuitive impression of black and white photos. However, they do not contain any direct depth information. The height of objects can be estimated from the length of the shadow.

Modern developments at SSS, for example through the use of the interferometric effect, also allow the evaluation of depth information. Since, on the other hand, fan-shaped plumb bobs are further developed through the skillful use of the echo signals via pure distance determination, the two methods are converging more and more.

Range and resolution are interchangeable, complementary requirements for an SSS. Usual high-resolution SSS with frequencies from 200 kHz to 1 MHz are very limited in range, at high frequencies to a few 10 m. In marine research, however, SSS in the lower kHz range are also used, which can reach ranges of almost 100 km in deep water. An essential parameter in the SSS is also the height of the system above ground. Basically, the closer the device is driven above the ground, the more photo-like the image. But because horizontal rays are curved by the sound velocity profile, the closer the device is operated above the ground, the shorter the range.

Underwater communication

Although water-borne noise does not sufficiently meet today's communication requirements, electromagnetic waves are practically unsuitable for transmission distances over (depending on the cloudiness of the water) from 10 to 300 m, so water-borne noise has no alternative. The underwater telephone, UT or Gertrude , was used as early as the Second World War . It was an analog voice transmission using SSB technology in the upper sideband with 9 kHz as the carrier and the frequency range of 300 Hz to 3 kHz used in telephony at the time. Reception was poor, especially in shallow water, due to the multipath transmission. Only this one frequency channel was available. In the meantime, transmission methods from modern mobile radio technology are used which, because of the poor propagation conditions and the low available bandwidth (predominantly frequencies between 5 kHz and 40 kHz) only allow small amounts of information and ranges.

Acoustic underwater navigation, releaser

Although the use of water-borne sound began with navigation aids, there is practically no absolute (coordinate-determining) navigation method any more. This is where the electromagnetic surface navigation methods have established themselves. Only local relative systems are now used, in particular to determine the location of a submerged system relative to an above-water platform.

For this purpose, one or more sound sources are used, with whose reception (more common with several receivers than with several transmitters) the position can be determined with centimeter accuracy. This allows the exact location of devices that are set off from a ship (e.g. ROVs ) or are being towed. But AUVs and underwater landers are also positioned with systems that work in a similar way relative to the mother platform or to submarine reference stations.

Such systems have also long been used for dynamic positioning; That is, to keep floating platforms stationary by means of a controlled drive (more precisely than would be possible by anchoring). They were of great importance for floating drilling platforms and drilling ships. In the meantime, however, they have largely been replaced by GPS systems, for which no floor sensors have to be precisely positioned.

So-called releasers, in which location determination is only a secondary function, are of great importance in marine research . They are used with anchored submerged measurement systems, especially when they are not marked by a surface buoy. Depending on the signal sent, a response signal is sent to you, which is used on the one hand for function control and on the other hand to determine the location of the system, or the connection to a basic weight is released so that the system can float.

Acoustic flow measurement

The most important measuring device for flow measurement today is the Acoustic Doppler Current Profiler (ADCP), an active sonar that uses the Doppler frequency shift of the reverberation of scattering bodies in the water (mainly plankton ) to determine the local flow velocity. Basically the resolution is due to the reverberation blur relation

${\displaystyle \Delta v_{n}\Delta z=c^{2}/2f_{0}\,}$

limited, where the resolution of the flow velocity, the distance resolution , the speed of sound and the center frequency of the transmitted signal are. The higher the frequency, the better the achievable resolution and the shorter the length of the flow profile because of the frequency-dependent damping. The resolution can be increased with the help of a large transmission signal bandwidth compared to the reverberation uncertainty relation (broadband ADCP, BB-ADCP). ${\displaystyle \Delta v_{n}}$ ${\displaystyle \Delta z}$ ${\displaystyle c\approx 1480\mathrm {m/s} }$ ${\displaystyle f_{0}}$

Ocean Acoustic Tomography (OAT)

In tomographic methods is consistently to physical-mathematical methods used for obtaining three-dimensional fields or for the mapping of slices using inverse method. Acoustic marine tomography has this basic approach in common with other tomographic methods. A large number of sound transmitters and receivers are used to evaluate the transit time of the received signals in order to determine the three-dimensional speed of sound and the temperature distribution within the enclosed field. However, the term tomography raises expectations that acoustic tomography cannot meet. The sound waves do not propagate in a straight line, which means that the relationship between the travel time and the temperature stratification is not linear. This makes inversion more difficult. One is therefore dependent on a functional model of the water stratification, which has a limited number of free parameters that can be determined with the help of the measurement. Since the results can only be exact if the underlying model is exact (“you can only determine something exactly what you already know”), the OAT is only suitable for large-scale phenomena and for investigating changes over time. This demanding, complex method has proven itself particularly for the investigation of variability. Walter Munk and Carl Wunsch are considered pioneers of the OAT .

The big experiment ATOC (Acoustic Thermography of the Ocean Climate) in the Pacific, with which the necessary large-scale averaging to determine global warming should be achieved, became relatively well known . It makes extensive use of the OAT methods.

Waterborne sound and whales

Given the superiority of water-borne noise as a communication medium under water, it is not surprising that animals also use water-borne noise. Well-known examples are whale songs for communication or the echolocation of whales or dolphins. Due to the technical use, there are conflicts of use. As far as we know today, this particularly affects marine mammals. You are definitely dependent on hearing as the most important sense. In the northern hemisphere, the underwater noise is now permanently characterized by a diffuse noise from the ships in the frequency range between 100 and 300 Hz, provided that it is not drowned out by very strong natural noise from the sea surface during storms.

The loudest noises in the sea include seaquakes , submarine volcanoes and calving icebergs, high-energy seismic sonars (so-called air guns, also known as sonic bombs), which are used to search for oil and natural gas in the sea floor. They are extremely loud (up to over 220 dB) and are generated around the clock every few seconds.

However, technical facilities are also increasingly coming under criticism. Although the damage caused by technical noise is relatively minor, it is unknown how large the number of unreported cases is.

Meanwhile, can be regarded as certain that medium-frequency military sonar equipment in the frequency range of 2 to 5 kHz to strandings of beaked whales may result.

In several individual events, around 50 beaked whales have stranded and died as a result since 1996 . Around 10 whales were affected in each of the events. The cause is a panic emergence reaction of the deep-diving beaked whales, which lead to the diving disease , an embolism caused by the peeling of nitrogen. The risk to whales from sound is significant, but still low compared to the risk from whaling (including bycatch ) and from collisions with ships.

literature

• X. Lurton: An Introduction to Underwater Acoustics, Principles and Applications . 2nd Edition. Springer / Springer Praxis Books, 2010, ISBN 3-540-78480-2 .
• RJ Urick: Principles of Underwater Sound . 2nd Edition. McGraw-Hill, New York 1975, ISBN 0-07-066086-7 .
• HG Urban: Handbook of water- borne sound technology, STN Atlas Electronics, Bremen 2000.
• PC Wille: Sound Images of the Ocean , Springer Verlag, 2005. ISBN 3-540-24122-1 .
• JR Apel: Principles of Ocean Physics. Academic Press, London 1987. ISBN 0-12-058866-8
• Robert J. Urick: Principles of Underwater Sound. McGraw-Hill, New York 1967, 1975, 1983. ISBN 0-07-066087-5
• PC Wille: Sound Images of the Ocean . Springer Verlag, 2005, ISBN 3-540-24122-1 (to the imaging sonars)

Individual evidence

1. ↑ ^{a b c d} R. J. Urick: Principles of Underwater Sound. Mc Graw-Hill New York ea 1975
2. ↑ JD Colladon, JK Sturm: About the compressibility of liquids. In: Annals of Physics and Chemistry , Volume 12, 1928, pp. 161-197
3. ↑ ^{a b c} G. H. Ziehm: Kiel - An early center of water-borne noise . In: Deutsche Hydrogr. Z. , Supplementary Booklet Series B No. 20, 1988
4. ^ RE Francois, GR Garrison: Sound absorption based on ocean measurements . Part II: Boric acid contribution and equation for total absorption . In: J. Acoust. Soc. At the. , 72, 1982, pp. 1879-1890
5. ^ RP Chapman, JH Harris: Surface Backscattering Strengths Measured with Explosive Sound Sources . In: J. Acoust. Soc. At the. , 34, 1962, p. 547
6. ↑ Accoustic Release Transponders . ( Memento from March 19, 2014 in the Internet Archive ) (PDF) Boating Brochures
7. ↑ NPAL Acoustic thermometry
8. ↑ Underwater noise: whales under constant stress , on greenpeace.de
9. ^ D'Amico A. (Ed.) 1998. Summary Record, SACLANTCEN Bioacoustics Panel. La Spezia, Italy, June 15-17, 1998. Saclant Undersea Research Center, M-133
10. ↑ Sonic bombs kill marine mammals
11. ^ PL Tyack et al .: Extreme Diving of Beaked Whales . In: Journ. Experim. Biology , 206, 1960, pp. 4238-4253
12. ^ R. Thiele: Sonar: Danger for whales? . In: Marineforum , 3, 2007, pp. 33-36