RESEARCH PAPER
Underwater Acoustic
Navigation Using SONAR
Principles, Design, Applications and other details
Presented by – Sylvia
Institute – Indian Institute of Technology (IIT), Delhi
Contact – 9306142593
Email – sylviamalik2k@gmail.com and me2190935@iitd.ac.in
Abstract
The paper goes through brief history of SONAR, how its design improved over time, the
principles it is based on and underwater acoustics using SONAR. A brief description of
SONAR along with its working is also discussed. The paper also covers the factors affecting
transmission loss and also the ambient noise, which is crucial in wave detection. Then at the
end, applications of SONAR and Acoustic Navigation in the modern world problems are
listed. The paper is presented in a very layman language, without going deep into the
complex mathematics.
History
Like many other great innovations, the principle of SONAR has also been taken from animals
like bats and dolphins using sound for communication and detecting objects.
Leonardo da Vinci was first to observe this when he used to detect vessels underwater by
inserting a tube in water and placing one ear on the outer end.
During the 1930s American engineers developed their own underwater sound-detection
technology. Americans began to use the term SONAR for their systems, coined by Frederick
Hunt to be the equivalent of RADAR but mainly for underwater systems.
What is SONAR?
SONAR or Sound Navigation and Ranging is a technology that uses acoustic waves to detect
location of objects underwater.
Detection of Acoustic Energy
The electrical energy from the transmitter is converted into acoustic energy. The device
which accomplishes this task is transducer.
If the transducer can only receive it is called a hydrophone, and if can only transmit it is
called a projector.
In many situations, the transducer can work both as transmitting and receiving device, which
is the usual case for sonar, so it is just called a transducer.
Active SONAR
In an active sonar system, the source of the acoustic wave is part of the sonar system. The
output of the transducer is measured as an SPL and is termed the source level (SL).
Passive SONAR
In a passive sonar system, the source is the target itself. The same terminology applies to
passive sources, namely the source level, SL.
Principle
BASIC IDEA
Water can be seen as weights connected by springs. The weights are the molecules of water,
and the springs are the inter-molecular forces of water molecules. To put it in simple words,
if we try to pull the water molecules apart or push them together, there will be resistance. So
the model seems good. We will call the longitudinal wave in water an acoustic wave.
When water molecules are pushed or pulled apart, they exert a restoring force that resists the
motion. Considering an extended body of water, the force will be felt locally as pressure, or
force per unit area. So the fundamental parameter of an acoustic wave is pressure. The
amplitude of the wave will be the peak pressure reached in one cycle.
Working
The simplest sonar devices send out a sound pulse from a transducer, and then measure the
time it takes for the sound pulses to be reflected back to the transducer. The distance to an
object can be calculated using this time difference and the speed of sound in the water
(around 1,500 meters per second).
The waves produced from the source travel forward as compressions and rarefactions are
passed from one point to its surrounding points. Each new point acts as a new source. The
rate of propagation of waves is called velocity.
Therefore at different points, same event s(x, t) will be observed. If t1, t2… are linked with
x1, x2….:
x2 – x1 = c(t2 - t1)
x3 – x2 = c(t3 – t2)
where c is speed of sound in that medium.
Intensity and power
When we say that the wave propagates, it is actually the energy associated with the wave that
is being transferred in the medium. It is called Acoustic Energy. The acoustic intensity for
𝑝2
wave of amplitude p is 𝐼 = 2𝜌𝐶 , rho is density and C is wave speed.
Velocity Factors
1. Temperature – The speed changes with temperature of water and as temperature is not
constant, the speed changes. Although very close to seabed, temperature can be
considered constant and hence the speed. Speed increases with increase in
temperature.
2. Depth- Density increases and temperature decreases as we go deeper, so speed
changes accordingly.
3. Salinity- Salinity is presence of salts, which usually increases the density of water and
hence increasing the speed.
Decibel
The acoustic values like pressure, energy, energy are quantified on a logarithmic scale and
reported in decibels (dB). In 10log(P1/P2), a 10 dB difference would mean P1 is 10 times
P2. In underwater acoustics, pressure reference is usually in microPascal. Pref = 1uPa .
Sound Pressure Level (SPL)
For working purposes, we can use a term called the sound pressure level, or SPL(where
reference value is 1uPa) which is defined as:
Combining Sound levels
If we need to add two source levels (SPL's), we cannot simply add them algebraically. For
example, if SPL1 = SPL2 = 60 dB, you cannot add the levels to get SPL 120 dB. The correct
answer is 60 dB combined with 60 dB gives 63 dB. The correct way would be:
SPL = 10 Log{10SPL1/10 + 10SPL2/10}
Propagation Loss
The water is an imperfect acoustic medium, acoustic energy can lost due to absorption and
scattering thus resulting in transmission loss. Transmission loss is generally influenced by
non-environmental factors such as transmitter and receiver depth, transmission range and
frequency.
Absorption loss and geometric spreading loss on the acoustic wave propagation will give the
transmission loss value which can be expressed in equation as:
TL = 20 1og R + α R
Where the R factor (Distance) is entered because the α is usual expressed in dB / km and α is
absorption coefficient value.
The absorption coefficient is one of the parameters that can affect the transmission loss. The
value of the absorption coefficient increases with increasing frequency, and the value of
transmission loss increases with distance.
Ambient Noise
The introduction of noise from other sources is a significant factor in sonar performance.
There are a wide variety of noise sources present in the underwater environment. Ambient
noise does not come from a particular direction or source. The noise conditions are uniform
in a small local area and SPL will be same everywhere. So, the ambient noise is specified as a
particular SPL.
The most obvious contribution to the ambient noise is the action occurring on the surface of
the ocean which include the waves, current, local sea storms, sailing ships etc.
The greater the size of the waves, the greater the ambient noise contribution. The waves are
driven by the winds, so there is a direct correspondence between the steady wind speed and
the sea state.
The condition of the ocean surface is quantified by a number ranging from 0-9. The Beaufort
Wind Scale which ranges from 0-12 is the standard measure of the consistent wind speed and
indicated the surface disturbance.
The greater the wind speed or sea state, obviously the greater the ambient noise contribution.
The frequency of the noise from sea state tends to be greater than 300 Hz.
Another major contributor to ambient noise is shipping. This noise, in contrast to the noise
from sea state, will be at low frequency (< 300 Hz).
One possible ambient noise source is the marine life like shrimp, whales, dolphins which also
use sound waves for communication.
Uses of Acoustic Waves Underwater
•
Primary function of SONAR systems is to detect and locate obstacles, used in
fisheries but mainly in military.
•
For transmitting signals underwater such as messages between submarines,
commands to remotely operated systems or data acquired by scientific
instrumentation
•
Sediment profilers – used to study the seabed structure
•
For Seismic Exploration - Seismic exploration involves the use of low frequency
sound (< 100 Hz) to probe deep into the seabed.
•
Due to its excellent propagation properties, underwater sound is used as a tool to aid
the study of marine life. Echo sounders, also referred to as hydroacoustics is also used
for fish location, quantity, size, and biomass.
•
Oceanography- Large scale ocean features can be detected by acoustic tomography.
Bottom characteristics can be measured by side-scan sonar and sub-bottom profiling.
References
•
https://fas.org/man/dod-101/navy/docs/es310/uw_acous/uw_acous.htm
•
https://books.google.co.in/books?hl=en&lr&id=VTNRh3pyCyMC&oi=fnd&pg=PR1
1&dq=Underwater+acoustic+navigation+using+SONAR+principle,+design,+applicat
ions+and+other+details&ots=mtvsALJpM&sig=fuYEwAPBfl3Am5n44CywYhfkb9Q#v=onepage&q&f=false
•
https://en.wikipedia.org/wiki/Sonar
•
The paper is result of reading and studying from various sources on the internet and
learning from classroom, therefore some similarities are bound to be present. I have
tried by best to present everything in a simple way.