# Acoustics/Sonar

**SONAR** (**so**und **n**avigation **a**nd **r**anging) is a technique
that uses sound propagation under water to navigate or to detect other
vessels. There are two kinds of sonar: active and passive.

## History

The French physicist Paul Langevin, working with a Russian émigré
electrical engineer, Constantin Chilowski, invented the first active
sonar-type device for detecting submarines in 1915. Although
piezoelectric transducers later superseded the electrostatic transducers
they used, their work influenced the future of sonar designs. In 1916,
under the British Board of Inventions and Research, Canadian physicist
Robert Boyle took on the project, which subsequently passed to the
**Anti-** (or **Allied**) **Submarine Detection Investigation
Committee**, producing a prototype for testing in mid-1917, hence the
British acronym **ASDIC**.

By 1918, both the U.S. and Britain had built active systems. The UK
tested what they still called ASDIC on *HMS Antrim* in 1920, and started
production of units in 1922. The 6th Destroyer Flotilla had
ASDIC-equipped vessels in 1923. An anti-submarine school, *HMS Osprey*,
and a training flotilla of four vessels were established on Portland in
1924.

The U.S. Sonar QB set arrived in 1931. By the outbreak of World War II,
the Royal Navy had five sets for different surface ship classes, and
others for submarines. The greatest advantage came when it was linked to
the Squid anti-submarine weapon.

## Active sonar

Active sonar creates a pulse of sound, often called a \"ping\", and then
listens for reflections of the pulse. To measure the distance to an
object, one measures the time from emission of a pulse to reception. To
measure the bearing, one uses several hydrophones, and measures the
relative arrival time to each in a process called beamforming.

The pulse may be at constant frequency or a chirp of changing frequency.
For a chirp, the receiver correlates the frequency of the reflections to
the known chirp. The resultant processing gain allows the receiver to
derive the same information as if a much shorter pulse of the same total
energy were emitted. In practice, the chirp signal is sent over a longer
time interval; therefore the instantaneous emitted power will be
reduced, which simplifies the design of the transmitter. In general,
long-distance active sonars use lower frequencies. The lowest have a
bass \"BAH-WONG\" sound.

The most useful small sonar looks roughly like a waterproof flashlight.
One points the head into the water, presses a button, and reads a
distance. Another variant is a \"fishfinder\" that shows a small display
with shoals of fish. Some civilian sonars approach active military
sonars in capability, with quite exotic three-dimensional displays of
the area near the boat. However, these sonars are not designed for
stealth.

When active sonar is used to measure the distance to the bottom, it is
known as echo sounding.

Active sonar is also used to measure distance through water between two
sonar transponders. A transponder is a device that can transmit and
receive signals but when it receives a specific interrogation signal it
responds by transmitting a specific reply signal. To measure distance,
one transponder transmits an interrogation signal and measures the time
between this transmission and the receipt of the other transponder\'s
reply. The time difference, scaled by the speed of sound through water
and divided by two, is the distance between the two transponders. This
technique, when used with multiple transponders, can calculate the
relative positions of static and moving objects in water.

### Analysis of active sonar data

Active sonar data is obtained by measuring detected sound for a short
period of time after the issuing of a ping; this time period is selected
so as to ensure that the ping\'s reflection will be detected. The
distance to the seabed (or other acoustically reflective object) can be
calculated from the elapsed time between the ping and the detection of
its reflection. Other properties can also be detected from the shape of
the ping\'s reflection:

-   When collecting data on the seabed, some of the reflected sound will
    typically reflect off the air-water interface, and then reflect off
    the seabed a second time. The size of this second echo provides
    information about the acoustic hardness of the seabed.
-   The roughness of a seabed affects the variance in reflection time.
    For a smooth seabed, all of the reflected sound will take much the
    same path, resulting in a sharp spike in the data. For a rougher
    seabed, sound will be reflected back over a larger area of seabed,
    and some sound may bounce between seabed features before reflecting
    to the surface. A less sharp spike in the data therefore indicates a
    rougher seabed.

### Sonar and marine animals

Some marine animals, such as whales and dolphins, use echolocation
systems similar to active sonar to locate predators and prey. It is
feared that sonar transmitters could confuse these animals and cause
them to lose their way, perhaps preventing them from feeding and mating.
A recent article on the BBC Web site (see below) reports findings
published in the journal *Nature* to the effect that military sonar may
be inducing some whales to experience decompression sickness (and
resultant beachings).

High-powered sonar transmitters may indirectly harm marine animals,
although scientific evidence suggests that a confluence of factors must
first be present. In the Bahamas in 2000, a trial by the United States
Navy of a 230 decibel transmitter in the frequency range 3 -- 7 kHz
resulted in the beaching of sixteen whales, seven of which were found
dead. The Navy accepted blame in a report published in the Boston Globe
on 1 January 2002. However, at low powers, sonar can protect marine
mammals against collisions with ships.

A kind of sonar called mid-frequency sonar has been correlated with mass
cetacean strandings throughout the world's oceans, and has therefore
been singled out by environmentalists as causing the death of marine
mammals. International press coverage of these events can be found at
this active sonar news clipping Web
site. A lawsuit was filed in Santa Monica, California on 19 October 2005
contending that the U.S. Navy has conducted sonar exercises in violation
of several environmental laws, including the National Environmental
Policy Act, the Marine Mammal Protection Act, and the Endangered Species
Act.

## Passive sonar

Passive sonar listens without transmitting. It is usually employed in
military settings, although a few are used in science applications.

### Speed of sound

Sonar operation is affected by sound speed. Sound speed is slower in
fresh water than in sea water. In all water sound velocity is affected
by density (or the mass per unit of volume). Density is affected by
temperature, dissolved molecules (usually salinity), and pressure. The
speed of sound (in feet per second) is approximately equal to 4388 +
(11.25 × temperature (in °F)) + (0.0182 × depth (in feet) + salinity (in
parts-per-thousand)). This is an empirically derived approximation
equation that is reasonably accurate for normal temperatures,
concentrations of salinity and the range of most ocean depths. Ocean
temperature varies with depth, but at between 30 and 100 metres there is
often a marked change, called the thermocline, dividing the warmer
surface water from the cold, still waters that make up the rest of the
ocean. This can frustrate sonar, for a sound originating on one side of
the thermocline tends to be bent, or refracted, off the thermocline. The
thermocline may be present in shallower coastal waters, however, wave
action will often mix the water column and eliminate the thermocline.
Water pressure also affects sound propagation. Increased pressure
increases the density of the water and raises the sound velocity.
Increases in sound velocity cause the sound waves to refract away from
the area of higher velocity. The mathematical model of refraction is
called Snell\'s law.

Sound waves that are radiated down into the ocean bend back up to the
surface in great arcs due to the effect of pressure on sound. The ocean
must be at least 6000 feet (1850 meters) deep, or the sound waves will
echo off the bottom instead of refracting back upwards. Under the right
conditions these waves will then be focused near the surface and
refracted back down and repeat another arc. Each arc is called a
convergence zone. Where an arc intersects the surface a CZ annulus is
formed. The diameter of the CZ depends on the temperature and salinity
of the water. In the North Atlantic, for example, CZs are found
approximately every 33 nautical miles (61 km), depending on the season,
forming a pattern of concentric circles around the sound source. Sounds
that can be detected for only a few miles in a direct line can therefore
also be detected hundreds of miles away. Typically the first, second and
third CZ are fairly useful; further out than that the signal is too
weak, and thermal conditions are too unstable, reducing the reliability
of the signals. The signal is naturally attenuated by distance, but
modern sonar systems are very sensitive.

### Identifying sound sources

Military sonar has a wide variety of techniques for identifying a
detected sound. For example, U.S. vessels usually operate 60 Hz
alternating current power systems. If transformers are mounted without
proper vibration insulation from the hull, or flooded, the 60 Hz sound
from the windings and generators can be emitted from the submarine or
ship, helping to identify its nationality. In contrast, most European
submarines have 50 Hz power systems. Intermittent noises (such as a
wrench being dropped) may also be detectable to sonar.

Passive sonar systems may have large sonic databases, however most
classification is performed manually by the sonar operator. A computer
system frequently uses these databases to identify classes of ships,
actions (i.e., the speed of a ship, or the type of weapon released), and
even particular ships. Publications for classification of sounds are
provided by and continually updated by the U.S. Office of Naval
Intelligence.

### Sonar in warfare

Modern naval warfare makes extensive use of sonar. The two types
described before are both used, but from different platforms, i.e.,
types of water-borne vessels.

Active sonar is extremely useful, since it gives the exact position of
an object. Active sonar works the same way as radar: a signal is
emitted. The sound wave then travels in many directions from the
emitting object. When it hits an object, the sound wave is then
reflected in many other directions. Some of the energy will travel back
to the emitting source. The echo will enable the sonar system or
technician to calculate, with many factors such as the frequency, the
energy of the received signal, the depth, the water temperature, etc.,
the position of the reflecting object. Using active sonar is somewhat
hazardous however, since it does not allow the sonar to identify the
target, and any vessel around the emitting sonar will detect the
emission. Having heard the signal, it is easy to identify the type of
sonar (usually with its frequency) and its position (with the sound
wave\'s energy). Moreover, active sonar, similar to radar, allows the
user to detect objects at a certain range but also enables other
platforms to detect the active sonar at a far greater range.

Since active sonar does not allow an exact identification and is very
noisy, this type of detection is used by fast platforms (planes,
helicopters) and by noisy platforms (most surface ships) but rarely by
submarines. When active sonar is used by surface ships or submarines, it
is typically activated very briefly at intermittent periods, to reduce
the risk of detection by an enemy\'s passive sonar. As such, active
sonar is normally considered a backup to passive sonar. In aircraft,
active sonar is used in the form of disposable sonobuoys that are
dropped in the aircraft\'s patrol area or in the vicinity of possible
enemy sonar contacts.

Passive sonar has fewer drawbacks. Most importantly, it is silent.
Generally, it has a much greater range than active sonar, and allows an
identification of the target. Since any motorized object makes some
noise, it may be detected eventually. It simply depends on the amount of
noise emitted and the amount of noise in the area, as well as the
technology used. To simplify, passive sonar \"sees\" around the ship
using it. On a submarine, the nose mounted passive sonar detects in
directions of about 270°, centered on the ship\'s alignment, the
hull-mounted array of about 160° on each side, and the towed array of a
full 360°. The no-see areas are due to the ship\'s own interference.
Once a signal is detected in a certain direction (which means that
something makes sound in that direction, this is called broadband
detection) it is possible to zoom in and analyze the signal received
(narrowband analysis). This is generally done using a Fourier transform
to show the different frequencies making up the sound. Since every
engine makes a specific noise, it is easy to identify the object.

Another use of the passive sonar is to determine the target\'s
trajectory. This process is called Target Motion Analysis (TMA), and the
resultant \"solution\" is the target\'s range, course, and speed. TMA is
done by marking from which direction the sound comes at different times,
and comparing the motion with that of the operator\'s own ship. Changes
in relative motion are analyzed using standard geometrical techniques
along with some assumptions about limiting cases.

Passive sonar is stealthy and very useful. However, it requires
high-tech components (band pass filters, receivers) and is costly. It is
generally deployed on expensive ships in the form of arrays to enhance
the detection. Surface ships use it to good effect; it is even better
used by submarines, and it is also used by airplanes and helicopters,
mostly to a \"surprise effect\", since submarines can hide under thermal
layers. If a submarine captain believes he is alone, he may bring his
boat closer to the surface and be easier to detect, or go deeper and
faster, and thus make more sound.

In the United States Navy, a special badge known as the Integrated
Undersea Surveillance System Badge is awarded to those who have been
trained and qualified in sonar operation and warfare.

In World War II, the Americans used the term **SONAR** for their system.
The British still called their system **ASDIC**. In 1948, with the
formation of NATO, standardization of signals led to the dropping of
ASDIC in favor of sonar.

