technical article from Lowrance
During this time, a
few people were using large, cumbersome sonar units on fishing
boats. Working at low frequencies, these units used vacuum tubes
which required car batteries to keep them running. Although they
would show a satisfactory bottom signal and large schools of
fish, they couldn’t show individual fish. Carl and his sons
began to conceptualize a compact, battery operated sonar that
could detect individual fish. After years of research,
development, struggle and simple hard work, a sonar was produced
that changed the fishing world forever.
Out of this simple beginning, a new industry was
formed in 1957 with the sale of the first transistorized
sportfishing sonar. In 1959, Lowrance introduced “The
Little Green Box,” which became the most popular sonar
instrument in the world. All transistorized, it was the first
successful sportfishing sonar unit. More than a million were made
until 1984, when it was discontinued due to high production
costs. We’ve come a long way since 1957. From
“little green boxes” to the latest in sonar and GPS
technology, Lowrance continues to lead in the world of
People have been fishing for thousands of years. Every person
fishing has had the same problem - finding fish and getting them
to bite. Although sonar can’t make the fish bite, it can
solve the problem of finding fish. You can’t catch them if
you’re not fishing where they are - and the Lowrance sonar
will prove it
In the late 1950s, Carl Lowrance and his sons Arlen and Darrell
began scuba diving to observe fish and their habits. This
research, substantiated by local and federal government studies,
found that about 90 percent of the fish congregated in 10 percent
of the water on inland lakes. As environmental conditions
changed, the fish would move to more favorable areas. Their dives
confirmed that most species of fish are affected by underwater
structure (such as trees, weeds, rocks, and drop-offs),
temperature, current, sunlight and wind. These and other factors
also influence the location of food (baitfish, algae and
plankton). Together, these factors create conditions that cause
frequent relocation of fish populations.
How it Works
The word "sonar" is an abbreviation for "SOund, NAvigation and
Ranging." It was developed as a means of tracking enemy
submarines during World War II. A sonar consists of a
transmitter, transducer, receiver and display.
In the simplest terms, an electrical impulse from a transmitter
is converted into a sound wave by the transducer and sent into
the water. When this wave strikes an object, it rebounds. This
echo strikes the transducer, which converts it back into an
electric signal, which is amplified by the receiver and sent to
the display. Since the speed of sound in water is constant
(approximately 4800 feet per second), the time lapse between the
transmitted signal and the received echo can be measured and the
distance to the object determined. This process repeats itself
many times per second.
The frequencies most often used by Lowrance in our sonar are 192
- 200 kHz (kilohertz); we also make some units that use 50 kHz.
Although these frequencies are in the sound spectrum,
they’re inaudible to both humans and fish. (You don’t
have to worry about the sonar unit spooking the fish - they
can’t hear it.)
As mentioned earlier, the sonar unit sends and receives signals,
then “prints” the echo on the display. Since this
happens many times per second, a continuous line is drawn across
the display, showing the bottom signal. In addition, echoes
returned from any object in the water between the surface and
bottom are also displayed. By knowing the speed of sound through
water (4800 feet per second) and the time it takes for the echo
to be received, the unit can show the depth of the water and any
fish in the water.
Total System Performance
There are four facets to a good sonar unit:
- High power transmitter.
- Efficient transducer.
- Sensitive receiver.
- High resolution/contrast display.
We call this our "Total System Performance" specification. All
of the parts of this system must be designed to work together,
under any weather condition and extreme temperatures.
High transmitter power increases the probability that you will
get a return echo in deep water or poor water conditions. It also
lets you see fine detail, such as bait fish and structure.
The transducer must not only be able to withstand the high power
from the transmitter, but it also has to convert the electrical
power into sound energy with little loss in signal strength. At
the other extreme, it has to be able to detect the smallest of
echoes returning from deep water or tiny bait fish.
The receiver also has an extremely wide range of signals it has
to deal with. It must dampen the extremely high transmit signal
and amplify the small signals returning from the transducer. It
also has to separate targets that are close together into
distinct, separate impulses for the display.
The display must have high resolution (vertical pixels) and good
contrast to be able to show all of the detail crisply and
clearly. This allows fish arches and fine detail to be
Most Lowrance sonar units today
operate at 192 or 200 kHz (kilohertz), with a few using 50
There are advantages to each frequency, but for almost all
freshwater applications and most saltwater applications, 192 or
200 kHz is the best choice. It gives the best detail, works best
in shallow water and at speed, and typically shows less "noise"
and undesired echoes. Target definition is also better with
these higher frequencies. This is the ability to display
two fish as two separate echoes instead of one "blob" on the
There are some applications where a 50 kHz frequency is
best. Typically, a 50 kHz sonar (under the same conditions
and power) can penetrate water to deeper depths than higher
frequencies. This is due to water's natural ability to
absorb sound waves. The rate of absorption is greater for
higher frequency sound than it is for lower frequencies.
Therefore, you'll generally find 50 kHz used in deeper saltwater
applications. Also, 50 kHz transducers typically have wider
coverage angles than 192 or 200 kHz transducers. This
characteristic makes them useful in tracking multiple
downriggers. Thus, even when these downriggers are in
relatively shallow depths, 50 kHz is preferred by many
fishermen. In summary, the differences between these
|192 or 200 kHz
- Shallower depths.
- Narrow cone angle.
- Better definition and target separation.
- Less noise susceptibility.
- Deeper depths.
- Wide cone angle.
- Less definition and target separation.
- More noise susceptibility.
The transducer is the sonar unit's "antenna." It converts
electric energy from the transmitter to high frequency sound. The
sound wave from the transducer travels through the water and
bounces back from any object in the water. When the returning
echo strikes the transducer, it converts the sound back into
electrical energy which is sent to the sonar unit's receiver. The
frequency of the transducer must match the sonar unit's
frequency. In other words, you can't use a 50 kHz transducer or
even a 200 kHz transducer on a sonar unit designed for 192 kHz!
The transducer must be able to withstand high transmitter power
impulses, converting as much of the impulse into sound energy as
possible. At the same time, it must be sensitive enough to
receive the smallest of echoes. All of this has to take place at
the proper frequency and reject echoes at other frequencies. In
other words, the transducer must be very efficient.
The active element in a transducer
is a man-made crystal (lead zirconate or barium titanate). To
make these crystals the chemicals are mixed, then poured into
molds. These molds are then placed in an oven which "fires" the
chemicals into the hardened crystals. Once they've cooled, a
conductive coating is applied to two sides of the crystal. Wires
are soldered to these coatings so the crystal can be attached to
the transducer cable. The shape of the crystal determines both
its frequency and cone angle. For round crystals (used by most
sonar units), the thickness determines its frequency and the
diameter determines the cone angle or angle of coverage (see Cone
Angles section). For example at 192 kHz, a 20 degree cone
angle crystal is approximately one inch in diameter, whereas an
eight degree cone requires a crystal that is about two inches in
diameter. That's right. The larger the crystal's diameter - the
smaller the cone angle. This is the reason why a twenty degree
cone transducer is much smaller than an eight degree one - at the
Transducers come in all shapes
and sizes. Most transducers are made from plastic, but some
thru-hull transducers are made from bronze. As shown in the
previous section, frequency and cone angle determine the
crystal's size. Therefore, the transducer's housing is determined
by the size of the crystal inside.
For more information on transducer types and their
applications see The Transducer Selection Guide.
Speed and the Transducer
Cavitation is a major obstacle to achieving high speed
operation. If the flow of water around the transducer is smooth,
then the transducer sends and receives signals normally. However,
if the flow of water is interrupted by a rough surface or sharp
edges, then the water flow becomes turbulent. So much so that air
becomes separated from the water in the form of bubbles. This is
called "cavitation." If these air bubbles pass over the face of
the transducer (the part of the housing that holds the crystal),
then "noise" is shown on the sonar unit's display. You see, a
transducer is meant to work in water - not air. If air bubbles
pass over the transducer's face, then the signal from the
transducer is reflected by the air bubbles right back into it.
Since the air is so close to the transducer, these reflections
are very strong. They will interfere with the weaker bottom,
structure, and fish signals, making them difficult or impossible
The solution to this problem is to make a transducer housing that
will allow the water to flow past it without causing turbulence.
However, this is difficult due to the many constraints placed
upon the modern transducer. It must be small, so that it doesn't
interfere with the outboard motor or its water flow. It must be
easy to install on the transom so that a minimum of holes need to
be drilled. It must also "kick-up" without damage if struck by
another object. Again, the patented design of the HS-WS
transducer is Lowrance's latest improvement in high-speed
transducer technology. It combines high speed operation with easy
installation and will "kick-up" if struck by an object at high
The cavitation problem is not limited to the shape of the
transducer housing. Many boat hulls create air bubbles that pass
over the face of a transom mounted transducer. Many aluminum
boats have this problem due to the hundreds of rivet heads that
protrude into the water. Each rivet streams a river of air
bubbles behind it when the boat is moving, especially at high
speed. To fix this problem, mount the face of the
transducer below the air bubbles streaming from the hull. This
typically means you have to mount the transducer's bracket as far
down as possible on the transom.
Transducer Cone Angles
concentrates the sound into a beam. When a pulse of sound is
transmitted from the transducer, it covers a wider area the
deeper it travels. If you were to plot this on a piece of graph
paper, you would find that it creates a cone shaped pattern,
hence the term "cone angle." The sound is strongest along
the center line or axis of the cone and gradually diminishes as
you move away from the center.
In order to measure the transducer's cone angle, the power is
first measured at the center or axis of the cone and then
compared to the power as you move away from the center.
When the power drops to half (or -3db[decibels] in electronic
terms), the angle from that center axis is measured. The total
angle from the -3db point on one side of the axis to the -3db
point on the other side of the axis is called the cone
This half power point (-3db) is a standard for the electronics
industry and most manufacturers measure cone angle in this way,
but a few use the -10db point where the power is 1/10 of the
center axis power. This gives a greater angle, as you are
measuring a point further away from the center axis. Nothing
is different in transducer performance; only the system of
measurement has changed. For example, a transducer that has
an 8 degree cone angle at -3db would have a 16 degree cone angle
Although the half power point is the standard for measuring cone
angles, fish detection angles are much larger. Lowrance sonar
units have very sensitive receivers and can detect return echoes
from fish, structure or the bottom out to 60° or more. This
means that the fish detection angle is 60° even though the
cone angle is only 20°.
20 degree cone angle | 8 degree cone
Lowrance offers transducers with a variety of
cone angles. Wide cone angles will show you more of the
underwater world, at the expense of depth capability, since it
spreads the transmitter's power out. Narrow cone angle
transducers won't show you as much of what's around you, but will
penetrate deeper than the wide cone. The narrow cone transducer
concentrates the transmitter's power into a smaller area. A
bottom signal on the sonar unit's display will be wider on a wide
cone angle transducer than on a narrow one because you are seeing
more of the bottom. The wide cone's area is much larger than the
High frequency (192 - 200 kHz) transducers come in either a
narrow or wide cone angle. The wide cone angle should be
used for most freshwater applications and the narrow cone angle
should be used for all saltwater applications. Low
frequency (50 kHz) sonar transducers are typically in the 30 to
45 degree range. Although a transducer is most sensitive inside
its specified cone angle, you can also see echoes outside this
cone; they just aren't as strong. The effective cone angle is the
area within the specified cone where you can see echoes on the
display. If a fish is suspended inside the transducer's cone, but
the sensitivity is not turned up high enough to see it, then you
have a narrow effective cone angle. You can vary the effective
cone angle of the transducer by varying the receiver's
sensitivity. With low sensitivity settings, the effective cone
angle is narrow, showing only targets immediately beneath the
transducer and a shallow bottom. Turning the sensitivity control
up increases the effective cone angle, letting you see targets
farther out to the sides.
Water and Bottom Conditions
The type of
water you're using the sonar in affects its operation to a large
degree. Sound waves travel easily in a clear freshwater
environment, such as most inland lakes.
In salt water however, sound is absorbed and reflected by
suspended material in the water. Higher frequencies are most
susceptible to this scattering of sound waves and can't penetrate
salt water nearly as well as lower frequencies. Part of the
problem with salt water is that it's a very dynamic environment -
the oceans of the world. Wind and currents constantly mix the
water. Wave action creates and mixes air bubbles into the water
near the surface, which scatters the sonar signal.
Micro-organisms, such as algae and plankton, scatter and absorb
the sonar signal. Minerals and salts suspended in the water do
the same thing. Fresh water also has wind, currents and
micro-organisms living in it that affect the sonar's signal - but
not as severely as salt water.
Mud, sand and vegetation on the bottom absorb and scatter the
sonar signal, reducing the strength of the return echo. Rock,
shale, coral and other hard objects reflect the sonar signal
easily. You can see the difference on your sonar's screen. A soft
bottom, such as mud, shows as a thin line across the screen. A
hard bottom, such as rock, shows as a wide line on the sonar's
Soft Bottom | Hard
You can compare sonar to using a flashlight
in a dark room. Moving the light around the room, it's easily
reflected from white walls and bright, hard objects. Moving the
light onto a darkly carpeted floor returns less light because the
dark color of the carpet absorbs the light, and the rough texture
scatters it, returning less light to your eyes. Adding smoke to
the room (children, don't try this at home!), you'll see even
less. The smoke is equivalent to salt water's effect on the sonar
Water Temperature and Thermodlcines
Water temperature has an important influence
upon the activities of all fish. Fish are cold-blooded and their
bodies are always the temperature of the surrounding water.
During the winter, colder water slows down their metabolism. At
this time, they need about a fourth as much food as they consume
in the summer.
Most fish don't spawn unless the water temperature is within
rather narrow limits. The surface water temperature gauge built
into many of our sonar units helps identify the desired surface
water spawning temperatures for various species. For example,
trout can't survive in streams that get too warm. Bass and other
fish eventually die out when stocked in lakes that remain too
cold during the summer. While some fish have a wider temperature
tolerance than others, each has a certain range within which it
tries to stay. Schooling fish suspended over deep water lie at
the level that provides this temperature. We assume they are the
most comfortable here.
Lowrance liquid crystal graph marking a thermocline on
Skiatook Lake near Tulsa, in Oklahoma, between 40 and 50 feet of
water. Notice how the thermocline stays consistent across
the body of water regardless of bottom contour.
The temperature in a lake is seldom the same from the surface to
the bottom. Usually there is a warm layer of water and a cooler
layer. Where these layers meet is called a thermocline. The depth
and thickness of the thermocline can vary with the season or time
of day. In deep lakes there may be two or more thermoclines. This
is important because many species of game fish like to suspend
in, just above, or just below the thermocline. Many times bait
fish will be above the thermocline while larger game fish will
suspend in or just below it. Fortunately, this difference in
temperatures can be seen on the sonar screen. The greater the
temperature differential, the denser the thermocline shows on the
After starting your boat, go to a protected cove and stop.
Leave the engine on. You may want to take a partner along to
operate the boat while you learn how to use the sonar. Press the
sonar unit's ON key and idle slowly around the cove. You'll
probably see a screen similar to the one to the left. The dashed
line at the top of the screen represents the surface. The bottom
shows in the lower part of the screen. The current water depth
(33.9 feet) shows in the upper left corner of the screen. The
depth range in this example is 0 to 40 feet. Since the unit is in
the automatic mode, it continually adjusts the range, keeping the
bottom signal on the display.
Every Lowrance LCG offers the convenience of our Advanced
Fish-Symbol I.D.™. Activated by the press of a button,
Advanced Fish Symbol I.D.™ lets your unit do the work of
interpreting return sonar signals. Advanced Fish Symbol
I.D.™ works in automatic mode only. If you turn it on while
in manual mode, it will switch to automatic mode. Fish and other
suspended targets are clearly displayed as fish-shaped symbols in
four different sizes.
Advanced Fish Symbol I.D.™ is designed to give a
simplified, easy to interpret display of suspended targets that
are assumed to be fish. After gaining experience with your sonar,
you will probably turn it off much of the time so you can see all
of the detailed information on fish movement, thermoclines,
schools of baitfish, weed beds, bottom structure, etc.
ASP™ (Advanced Signal
Processing (ASP™) is another exclusive Lowrance
innovation that uses sophisticated programming and advanced
digital electronics to continually monitor the effects of boat
speed, water conditions and other interference sources - and
automatically adjusts the sonar settings to provide the clearest
ASP™ sets the sensitivity as high as possible while keeping
the screen free of "noise." It
automatically balances sensitivity and noise rejection. The
feature can be turned off and on and will work whether the
sonar is in automatic or manual mode. With ASP™
operating behind the scenes you'll spend less time making routine
sonar adjustments and more time spotting fish.
The sensitivity controls the
ability of the unit to pick up echoes. A low sensitivity level
excludes much of the bottom information, fish signals, and other
target information. High sensitivity levels enable you to see
this detail, but it can also clutter the screen with many
undesirable signals. Typically, the best sensitivity level shows
a good solid bottom signal with GRAYLINE® and some surface
clutter. When in the automatic mode, the sensitivity is
automatically adjusted to keep a solid bottom signal displayed,
plus a little more. This gives the unit the capability to
show fish and other detail. In automatic mode, the unit also
adjusts sensitivity automatically for water conditions, depth,
etc. When you adjust the sensitivity up or down, you are
biasing up or down the normal setting the unit's automatic
control would choose. With ASP™ enabled, the automatic
mode picks the proper sensitivity level for 95% of all
situations, so it is recommend to always use this normal mode
first. But, for those unusual situations where it is
warranted you can bias it up or down. You can also turn off
the automatic sensitivity control for special uses.
To properly adjust the sensitivity while the unit is in the
manual mode, first change the range to double its current
setting. For example, if the range is 0 - 40 feet, change it to 0
- 80 or 0 - 100 feet. Now increase the sensitivity until a second
bottom echo appears at twice the depth of the actual bottom
signal. This "second echo" is caused by the echo returning from
the bottom reflecting off the surface of the water, making a
second trip to the bottom and returning. Since it takes twice as
long for this echo to make two trips to the bottom and back, it
shows at twice the depth of the actual bottom. Now change the
range back to the original scale. You should see more echoes on
the screen. If there is too much noise on the screen, back the
sensitivity level down a step or two.
GRAYLINE® lets you distinguish
between strong and weak echoes. It "paints" gray on targets
that are stronger than a preset value. This allows you to
tell the difference between a hard and soft bottom. For
example, a soft, muddy or weedy bottom returns a weaker symbol
which is shown with a narrow or no gray line. A hard bottom
returns a strong signal which causes a wide gray line.
If you have two signals of equal size, one with gray and the
other without, then the target with gray is the stronger
signal. This helps distinguish weeds from trees on the
bottom or fish from structure.
GRAYLINE® is adjustable. Since GRAYLINE® shows the
difference between strong and weak signals, adjusting the
sensitivity may also require a different GRAYLINE® level.
You may see fish arches while trolling with the unit in a 0 -
60 foot scale, however it it much easier to see the arches when
using the zoom feature. This enlarges all echoes on the screen.
Turning the zoom feature on gives you a screen similar to the one
at left. The range is 8 - 38 feet, a 30-foot zoom. As you can
see, all targets have been enlarged, including the bottom signal.
Fish arches (A & B) are much easier to detect, and important
structure (C) near the bottom is magnified. This also shows small
fish hanging just beneath the surface clutter (D). The above
steps are all that's required to manually adjust your sonar unit
for optimum fish finding capability. After you've become more
familiar with your unit, you'll be able to adjust the sensitivity
properly without having to look for a second echo.
Read more - Sonar Tutorial Part II