Underwater Acoustics

Objectives

• What is underwater Acoustics

• Hydrophone

• Underwater Communication

• DART & Robotic Crawler

• Hearing Underwater

• Underwater Navigation & Tracing

• Sonar

• Acoustical Oceanography

# What is underwater Acoustics?

• Underwater acoustics is the study of the propagation of sound in water and the

interaction of the mechanical waves that constitute sound with the water and its

boundaries.

• A sound wave propagating underwater consists of alternating compressions and

rarefactions of the water. These compressions and rarefactions are detected by a

receiver.

• It’s same as any other wave studding based on analyzing of frequencies , kind of media

that allow propagation and loss parameters.

Absorption of waves

• The key of understanding absorption is to identify and recognize the media we are dealing with.

• AMBIENT NOISE : noise in water can form a real challenge for waves to travel over them, noise of

different power and frequencies compared within air or space.

• Bottom pressure : plankton and bacteria lives in that layers of water witch will cause additional

reflections and resistance.

• Motion layers : liquids generally divided themselves naturally into sub-layers that differ from each

other on motion exposition meaning that waves should adopt the change each time it crosses the

layer.

Absorption of waves (cont.)

• Viscosity : head of loss reasons defined as “measure of its resistance to gradual deformation

by shear stress or tensile stress”.

• Viscosity is a property arising from collisions between neighboring particles in a fluid that are

moving at different velocities.

• Viscosity express the rate of how can a water resist a moving's within .. That also related to our

wave.

• Temperature is a basic factor on the viscosity prosperity, when temperature increases viscosity

decreases and versa is true.

• You can imagine what viscosity property means by comparing honey and water for example.

# Hydrophone

• Hydrophone are designed to be used underwater for recording or listening to

under water sound

• hydrophone are based on a piezoelectric transducer that generate electricity when

subjected to a pressure change.

• transducers can convert a sound signal to electrical signal since sound is a

pressure wave

• From late in world ware I until the introduction of the active sonar, hydrophone

were the sole method for submarines to detect targets while submerged.

PIEZOELECTRIC TRANSDUCER

• Piezoelectric means electricity resulting from pressure.

• It is a device that transform a one type of energy to another by taking

advantage of piezoelectric properties of certain crystal or other materials.

• Piezoelectric material is subjected to stress or force , it generate an electricity

potential or voltage proportional to the magnitude of the force .

• This type is ideal to convert an mechanical energy or force to electric

potential.

Directional hydrophones

• Focused transducers

_ uses a signal transducers element with a dish or conical-shaped sound reflector to focused the

signals

_ Can be produced low-cost omnidirectional type

_ Must be used while a stationary, as a reflector impedes its movement through water

• Array of hydrophones

_ Multiple hydrophones can be arranged in an array.

_ It will add the signals from the desired direction while subtracting signals from other directions.

_ Hydrophones are arranged in a “line array”, but may be in two-or three -dimensional arrangements.

Array of hydrophones• Sound is transmitted by the ship and reflect off the

submerged submarine. The reflected sound reaches

hydrophone A first, then hydrophone B , and finally

hydrophone c .The time –of- arrival- difference

between hydrophones in the array is used to

determine the direction to the submarine.

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• Hydrophones located around the pacific ocean monitor the ocean for sound of seismic event. The sound made by seismic event are also used to accurately located the

event.• using the sound made by the seismic event ,

scientist can tell if the event is an earthquake or a

volcanic eruption .

# Underwater Communication

• Technique of sending and receiving message below water. most commonly

employed using hydrophones.

• Wired underwater communication is not feasible :-

1.Breaking of wires

2.Significant cost of deployment

3. very bad over long distance

• Wireless underwater communication :-

• Why Wireless underwater communication is more Difficult ????

1- multi-path propagation 2- time variations of the channel

Underwater Communication (cont.)

3- small available bandwidth , Radio waves propagate under water at extremely low frequencies (30Hz-

300Hz) & require large antennae and high transmission power.

4-strong signal attenuation , Attenuation underwater is the loss of beam intensity due to intrinsic

absorption by water, dissolved impurities, organic matter

• NOW ... We can’t use an Electromagnetic Waves

• Acoustic waves longitudinal waves that propagate by compression and dilation are passed on from

one point to the surrounding points because of the elastic properties of the propagation medium .

• Frequency : number of vibrations per second

• Typical frequencies associated with Underwater Acoustics are 10 Hz to 1 MHz

Frequencies above 1 MHz are rarely used because they are absorbed very quickly. Why ???

• Attenuation is approximately linearly proportional to frequency .

• (K) Is attenuation coefficient , in sea water it depends on :-

1- Viscosity of pure water 2- MgSO4 3 - B(OH)3

High frequency high attenuation maximum range for wave

propagation is reduced

Basic acoustic communication model

• Acoustic modem Converts digital data into special underwater sound signals

and vice versa .

• Employ high performance error detection and correction coding scheme which

reduces bit error rate to less than 10-7 .

• modem receives data from its link in sleep mode and then switches to transmit

mode and transmit the data to save power

# Deep-ocean Assessment and Reporting of Tsunamis (DART)

• NOAA (National Oceanic and Atmospheric Administration) has created an expansive network of 39 DART buoy stations, with 32 located in the Pacific and 7 in the Atlantic Basin.

• Can be used to provide early warnings of tsunamis generated by undersea earthquakes.

• Pressure data are transmitted to a near-by surface buoy via an acoustic data link using underwater modems.

- The data are then relayed to researchers on land in real-time via satellite.

• Researchers can also request real-time data independent of the automatic detection system. The data are used to provide early warnings of a tsunami before it comes ashore.

Deep Ocean Assessment & Reporting of Tsunami

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VIDEO

DETECTING UNDER WATER OBJECTS(Robotic Crawler)

• A robot crawler carries a modem, a camera, and a digital signal-processing unit.

• Traversing the seafloor, searches for an object.

• When object found, sends an acoustic signal to a ship or shore based station

• Can then be commanded to take a still frame photo, compress it and transfer the image to an acoustic signal that is sent back to the investigator

• Can carry sensors into very shallow. water, or even into the surf zone.

• Equipped with a camera and modem, can be controlled from substantial distances via acoustic communications.

LIMITATIONS

• Battery power is limited and usually batteries can not be recharged easily.

• The available bandwidth is severely limited.

• Underwater sensors are prone to failures because of fouling, corrosion, etc.

• Highly affected by :

_environmental and natural factors such as heterogeneities of the water column

_variations of sound velocity versus depth, temperature and salinity

_multiple and random sea reflections and significant scattering by fish & bubble clouds

# Hearing Underwater

• Normal Hearing

• why a person’s own voice sounds different

to them when it is recorded؟؟

• Hearing by bone-conduction , where sound is conducted to the inner

ear through the bones of the skull

• sound localization : In air, sound localization occurs by detecting the delay between

sounds arriving at each ear. If bone conduction is the only mechanism for hearing then

localization would not be possible because the sound at each ear would be similar.

• At low frequencies tympanic conduction appears to predominate and this may explain

why sound localization is more acute at these frequencies. At high frequencies, bone

conduction is considered to be the dominant

• both bone conduction and tympanic sound conduction produce hearing underwater,

the so-called dual path theory.

Sources of noise underwater1-Ambient underwater noise levels :: from turbulence and pressure fluctuations – waves - offshore oil exploration

2-Ambient dive site noise levels :: Compressor - a hydraulic drill -

3-Self-generated breathing noise and helmet noise :: breathing noise (Bubble noise ) - helmet there is also the

noise from the air flow -Communications

4-Tool noise :: underwater tools -

5-Compression chamber noise :: Noise during compression and decompression cycles of a compression

chamber

Diver audiometric surveys

• Molvaer and Lehmann [36] investigated 160 professional divers and compared them with a standard population of

non-divers, with age grouped according to decades between 20 and 60 years. As expected, hearing sensitivity

decreased with age/diving experience, and also with smoking and subjectively assessed noise exposure. The study

indicated that while hearing acuity in younger divers was better than an age-matched general population, hearing in

the older age groups was the same as the non-diving population. .

• thus the requirement to control noise exposure and reduce the risk of long term damage.

Actions to Reduce Noise

• The legislation normalized to an eight-hour working day, five days a week

• using a knowledge of hearing sensitivity underwater since hearing is less

sensitive underwater, a higher level of noise is tolerable.

• Hooded divers :divers with a wet earwearing diving hoods hearing is

further protected by the attenuation factor of the hood.

• Hearing protection devices

‘Wet’ ear/‘Dry’ ear effect

• As hearing is more sensitive in air than in water , it is assumed that a given noise level is more damaging

to the ‘dry’ ear than the ‘wet’ ear.

# Underwater Navigation and Tracing * We use underwater acoustic positioning system:

- is a system for the tracking and navigation of underwater vehicles or divers by means of acoustic distance and/or direction

measurements, and position triangulation.

- it used in underwater work, including oil and gas exploration, ocean sciences, salvage operations and military activities.

* Method of operation:

Acoustic positioning systems measure positions relative to a framework of baseline stations(transponders), which must be

deployed on the sea floor.

* Classes of underwater positioning system :

They are categorized into three types:

1-Long-baseline (LBL) systems.

2-Short-baseline (SBL) systems.

3-Ultra-short-baseline (USBL) systems.

1-Long-baseline (LBL) systems:Long baseline systems determine the position of a vehicle or diver by acoustically measuring the distance from a vehicle

or diver interrogator to three or more seafloor deployed baseline transponders.

In figure 1, a diver mounted interrogator (A) sends a signal, which is received by the baseline transponders (B, C, D). The

transponders reply, and the replies are received again by the diver station (A). Signal run time measurements record the

distances A-B, A-C and A-D, which are used to compute the diver position by position search algorithms.

*Advantage:

very high positioning accuracy and position stability.

(transponders are installed in the reference frame of the work site itself).

*Disadvantage:

requires multiple transponders.

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*Example:

In 1960s, the Soviets were developing underwater navigation systems including

seafloor transponders to allow nuclear submarines to operate precisely while staying submerged.

2) Short-baseline (SBL) systems:- SBL systems do not require any seafloor mounted transponders and are suitable for tracking underwater targets from boats or ships.

- Short baseline systems determine the position of a tracked target by measuring the target's distance from three or more transducers(sonar) that are, for

example, lowered over the side of the surface vessel from which tracking operations take place.

- In figure (2), baseline transducer (A) sends a signal, which is received by a transponder (B) on the tracked target. The transponder replies, and the reply is

received by the three baseline transducers (A, C, D). Signal run time measurements record the distances B-A, B-C and B-D. The

resulting target positions are always relative to the location of the baseline transducers.

- SBL positioning system is combined with a GPS receiver and an electronic compass which determine the location and orientation of the boat, which are

combined with the data from the

SBL system to establish the position of the tracked target in earth coordinates.

*Advantage:

- good accuracy.

- require only one transponder.

*Disadvantages:

- accuracy dependent on shipboard.

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*Example:

In 2007 underway in Antarctica, the Moss Landing Marine Laboratory is using a PILOT SBL system to guide the SCINI remotely operated vehicle. figure(3)

3) Ultra-short-baseline (USBL) systems:- A complete USBL system consists of a transceiver, which is mounted on a pole under a ship, and a transponder on the seafloor.

- A computer is used to calculate a position from the ranges and bearings measured by the transceiver.

- An acoustic pulse is transmitted by the transceiver and detected by the transponder, which replies with acoustic pulse. This return pulse

is detected by the shipboard transceiver. The time from the transmission of the initial acoustic pulse until the reply is measured by the

USBL system and is converted into a range.

- To calculate the position, the USBL calculates both a range and an angle from the transceiver to the transponder. Angles are measured

by the transceiver. A method called “phase-differencing” within this transducer array is used to calculate the angle to the

subsea transponder to determine position.

*Advantage:

&requires only one transponder.

*Disadvantage:

&low accuracy.

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*Example:

*** Use in autonomous underwater vehicles.

# Sonar

• Sonar is advice that is used to detect underwater objects using sound waves.

• In this system a sound pulse is generated and sent underwater through transmitter.

• Sound waves are reflected by the underwater object which are received at receiver.

• The time taken by sound wave to come back is recorded.

• And by knowing the speed of sound wave in water the distance can be easily calculated by formula.

• Distance =speed x time

Type of Sonars

• Sonar is of two types :

_ Active sonar _

- uses sound transmitter and receiver .

- Creates a pulse of sound often called “aping” and then listens for reflections (echo) of the pulse and there

are three modes of operation :

- Monostatic mode : when the transmitter and receiver are the same place.

- Bistatic mode : when the transmitter and receiver are separated by the some distance.

- Multistatic mode : when more transmitters (or more receivers) are used , again spatially separated.

Type of Sonars

• Passive sonar

_ Listens without transmitting.

_ Has a wide variety of techniques for identifying

the source of a detected sound.

_ Have large sonic database but sonar operator

classify signals by use of computer and use these

data base to identify classes of ships and action.

Applications & Limitations

Applications

It used to fined the actual depth of the sea .

Sonar systems are used to find the lost ships and submarines .

These are used in ocean surveillance systems .

Thy are used for under water security .

Limitation

It has adverse effects on marine animals like dolphins and whales .

The sonar systems generate lot of noise .

High intensity sonar sounds can create a small temporary shift in the hearing threshold of some fish .

# Acoustical Oceanography

• is the use of sound in studying sea, its boundaries and

its contents.

• Equipment used in oceanography:

1) Rainbow echo sounder

_Measure the depth of water and It was used until 1995

2) fathometer

_It’s used to measure the depth of water

_It use sound wave

3) Electronic sonar

• Not just detect the depth but also define the objects

underwater and classifies it.

Echo Sounding & Hydrograph

• Echo sounding :

It also can used in studding fish

beside fishing.

• Hydrograph :

_ It’s an application of echo sounding

_ It’s used to study underwater graph

_ It use dual frequency “ two different

_ frequencies” at the same time

Echo Sounding & Hydrograph (Cont.)

• The lower frequency is around 24 KHz

• the high frequency in the depth of 100m is around 200 KHz, but in deeper water it use high

frequency as 33 KHz, because lower frequencies are less susceptible to attenuation in water.

• this “dual frequency” are discrete .

• The two return signal don’t typically interfere with each other

• The advantage of dual frequency echo sounding is to identify the vegetation layer and rock layer

THANK YOU

Members

Mahmoud Abdallah Mohamed Ahmed

Mostafa ElZaref Mohamed Hasan

Islam Elsayed Haithum Hussein

Hasan Youssef Ahmed Abd ElRauf