How does a transducer work?

What is a Transducer?A good fishfinder depends on an efficient transducer tosend and receive signals. The transducer is the heart ofan echosounder system. It is the device that changeselectrical pulses into sound waves or acoustic energyand back again. In other words, it is the device thatsends out the sound waves and then receives theechoes, so the echosounder can interpret or “read”what is below the surface of the water.

How Does a Transducer Work?The easiest way to understand how a transducerfunctions is to think of it as a speaker and amicrophone built into one unit. A transducer receivessequences of high voltage electrical pulses calledtransmit pulses from the echosounder. Just like thestereo speakers at home, the transducer then convertsthe transmit pulses into sound. The sound travelsthrough the water as pressure waves. When a wavestrikes an object like a weed, a rock, a fish, or thebottom, the wave bounces back. The wave is said toecho—just as your voice will echo off a canyon wall.When the wave of sound bounces back, the transduceracts as a microphone. It receives the sound waveduring the time between each transmit pulse andconverts it back into electrical energy. A transducer willspend about 1% of its time transmitting and 99% of itstime quietly listening for echoes. Remember, however,that these periods of time are measured in

microseconds, so the time between pulses is very short.The echosounder can calculate the time differencebetween a transmit pulse and the return echo andthen display this information on the screen in a waythat can be easily understood by the user.

What Goes into the Making of a Transducer?The main component of a depth transducer is thepiezoceramic element. It is the part that convertselectrical pulses into sound waves, and when theechoes return, the piezoceramic element converts thesound waves back into electrical energy. Piezoceramicelements are most often in a disk form, but they mayalso be in the shape of a bar or a ring. A transducermay contain one element or a series of elements linkedtogether called an array. A transducer is made up of sixseparate components:

• Piezoceramic element or an array of elements• Housing• Acoustic window• Encapsulating material• Sound absorbing material• Cable

Echosounder B744V transducer

Echo Echo

Transmit pulse Transmit pulse

Time (seconds)

How does a transducer work?

How Does a Transducer KnowHow Deep the Water is?The echosounder measures the time betweentransmitting the sound and receiving its echo. Soundtravels through the water at about 1,463 m/s(4,800 ft/s), just less than a mile per second. Tocalculate the distance to the object, the echosoundermultiplies the time elapsed between the soundtransmission and the received echo by the speed ofsound through water. The echosounder systeminterprets the result and displays the depth of the waterin feet for the user.

How Does a Transducer KnowWhat the Bottom Looks Like?As the boat moves through the water, the echoes ofsome sound waves return more quickly than others.We know that all sound waves travel at the samespeed. When a sound wave in one section of thesound field returns more quickly than another, it is

because the wave has bounced off something closer tothe transducer. These early returning sound wavesreveal all the humps and bumps in the underwatersurface. Transducers are able to detect whether abottom is soft or hard and even distinguish between aclump of weeds and a rock, because the sound waveswill echo off of these surfaces in a slightly differentmanner.

How Does a Transducer See a Fish?The transducer can see a fish, because it senses the airbladder. Almost every fish has an organ called an airbladder filled with gas that allows the fish to easilyadjust to the water pressure at different depths. Theamount of gas in the air bladder can be increased ordecreased to regulate the buoyancy of the fish.Because the air bladder contains gas, it is a drasticallydifferent density than the flesh and bone of the fish aswell as the water that surrounds it. This difference indensity causes the sound waves from the echosounderto bounce off the fish distinctively. The transducerreceives the echoes and the echosounder is able torecognize these differences. The echosounder thendisplays it as a fish.

Cable Housing Sound absorbingmaterial

Encapsulatingmaterial

Piezoceramic elementAcoustic window

Air bladder

Softbottom

Softbottom

Hard rocky bottom

Can Fish Hear the Sound WavesProduced by a Transducer?No, the sound waves are ultrasonic. They are above(ultra) the sound (sonic) that human ears are able tohear. Humans can hear sound waves from 10 Hz to20 kHz. Most fish are unable to hear frequencies higherthan about 500 Hz to 1 kHz. The ultrasonic soundwaves sent out by Airmar transducers have frequenciesranging from 10,000 kHz to 2 MHz (200,000,000 Hz),clearly beyond the hearing of fish. However, mostpeople can hear the transmit pulses of our 10 kHztransducers; they sound like a series of clicks.

What is Frequency?Frequency is the number of complete cycles orvibrations that occur within a certain period of time,typically one second. Sound waves can vibrate at anyone of a wide number of frequencies. The easiest wayto understand frequency is to think of it in terms ofsounds that are familiar. For example, a kettle drumproduces a low-pitched sound (low-frequency). That is,

it vibrates relatively few times per second. Whereas, aflute produces a high-pitched sound (high-frequency). Itvibrates many more times per second than a kettledrum. The frequency of sound waves is measured in aunit called a Hertz. A Hertz is one cycle per second.For example: a 150 kHz transducer operates at150,000 cycles per second.

Selecting Frequencies

10 MHz

1 MHz

100 kHz

10 kHz

1 kHz

10 Hz

0

Fish hearing frequency range

Human hearing frequency range

Airmar transducer frequency range

How Does a Customer DecideWhat Frequency is Needed?Airmar transducers are often designed for 50 kHz(50,000 cycles per second) or 200 kHz (200,000 cyclesper second). Transducers can be designed to operateefficiently at any number of specific frequenciesdepending upon the application and performancerequirements of the customer. A higher-frequencysound wave will give the user a higher-resolutionpicture of what is present under the water, but therange will be short. Fishermen in more shallow lakes,who want a crisp clear picture of the bottom need ahigher-frequency transducer. Low-frequency soundwaves will not give the user as clear a picture of thebottom, but they have greater range for very deepareas where high-frequency sound waves cannotreach. A low-frequency unit will work well in thedepths of Lake Michigan or the ocean.

A higher-frequency transducer will put out quicker,shorter, and more frequent sound waves. Like theripples made when a small pebble is thrown into stillwater, small waves of sound move evenly out andaway from the source. Because they are just smallwaves, they will not travel far, and small obstacles willcause them to bounce back. Higher frequencies aremore sensitive to small objects and will send backdetailed information which will show as crisphigh-resolution pictures on the echosounder screen.The range of high-frequency sound waves, however, isshort. In fact, sound waves emitted by a 200 kHztransducer have a limited range of about 200 m (600’).

Selecting Frequencies

90°

80°

70°

60°

50°

90°

80°

70°

60°

50°

40° 40°30° 30°

20° 20°10° 10°0°

Beamwidth

Sidelobe

-30dB-20dB

-10dB

-3dB

90°

80°

70°

60°

50°

90°

80°

70°

60°

50°

40° 40°30° 30°

20° 20°10° 10°0°

Beamwidth

Sidelobe

-30dB-20dB

-10dB

-3dB

200 kHz

50 kHzNow, think of the large waves created by a largeboulder thrown into still water. Low-frequency soundwaves are like these large waves; they travel muchfarther than high-frequency waves. But becauselow-frequency waves are so large, they wash right oversmall obstacles. Low-frequency sound waves are not assensitive in detecting small fish or other small obstaclesas are high-frequency waves, and although they cansee to greater depths, they will not send back detailedinformation or clear crisp pictures.

This illustration shows the differences in beamwidth ofa transducer operating at both 50 kHz and 200 kHz.Notice the different way the fish appear as “marks” ateach frequency.

Transducer Style and Screen Images

In-Hull Transducers

Transom-Mount Transducers

M260 R199

M89B744V

P58 P66 TM258

Thru-Hull/External Transducers

SS260

P79

Transducer Style and Screen Images

600 W

Screen Images

1 kW 2 kW

600 W 1 kW 2 kW

600 W 600 W 1 kW

Transducer Style and Screen Images

3 kW

3 kW

1 kW

Transducer Styles and Mounting Methods

Choosing the Right TransducerThere are three main considerations when selecting atransducer.

• Transducer mounting style• Type of boat and its hull material• Application or expectation for the transducer

Transducer Mounting StyleTransducers are typically mounted in one of three ways:through the hull, inside the hull, or on the transom.

Through the HullThe transducers in this mounting style fall into twocategories. There are “flush” thru-hull sensors that sitflush or nearly flush with the boat hull. They arerecommended for smaller boats with a minimumdeadrise angle. And they are often installed on sailingvessels because they produce minimal drag.

External thru-hull transducers extend beyond the hull’ssurface and usually require a fairing to aim the soundbeam vertically. They are right for larger un-traileredvessels. When external mounts are installed with aHigh-Performance Fairing, the transducer face is flushwith the surface of the fairing and parallel to thewaterline, resulting in a truly vertical beam, puttingmaximum energy on the target. This installation, whenmounted in “clean water,” forward of propellers andrunning gear, produces the most effective signal return,since nothing on the vessel interferes with thetransducer’s active face.

Inside the HullAn in-hull transducer is installed inside a boat hullagainst the bottom and sends its signal through thehull. Some people prefer this mounting style, because itis not necessary to drill through the vessel. A unitcannot be damaged when a boat is trailered, thetransducer is not exposed to marine growth, and thereis no drag. Additionally, a transducer can be installedand serviced while the vessel is in the water. Mostin-hull transducers are mounted inside a liquid filledtank that is first epoxied in place. As long as the waterflow below the transducer is “clean”, it will give greathigh-speed performance.

On the TransomTransom-mounts are attached to the back (transom) ofa boat hull. Trailered boats typically use this mountingstyle, since it is out of the way of the rollers. Somepeople prefer a transom-mount, because it is easy toinstall and remove a unit—especially if a kick-up bracketis used. Kick-up brackets move a transducer out of theway to prevent damage from floating debris when aboat is underway. Also, they protect the transducerwhen a boat is trailered, or when it is kept in the waterfor a long period of time. A transom-mount installationgives better performance than an in-hull at boat speedsbelow 30 knots.

Transducer Styles and Mounting Methods

P79

Plastic Bronze Stainless steel

Type of Boat and its Hull MaterialThe type of boat and the material used to make thehull must be taken into consideration when selectingthe transducer mounting style. Each has its limitations.

Thru-Hull/External MountThru-hull transducers will work with any engine type:inboard, outboard, or I/O. And these transducers areright for power and sailboats alike. There are thru-hullunits for every hull material.

Thru-hull units are not recommended in two situations.• Plastic thru-hull housings cannot be used in a

wooden boat. Wood swells as it absorbs water, so itmay crack the housing.

• Bronze thru-hull housings cannot be used inaluminum boats. The interaction between thealuminum and the bronze, especially in thepresence of salt water, will eat away thealuminum hull and/or the bronze housing.

In-Hull MountIn-hull transducers will work with any engine type:inboard, outboard, and I/O. These transducers performwell on both power and sailboats.

Thick aluminum hulls are not recommended for in-hullinstallations, because there is too much signal losstransmitting through the metal. In addition, this

Transom MountTransom-mount transducers can be used with any hullmaterial. However, they will not work on a vessel withan inboard engine due to the turbulence forward ofthe sensor. And because of excessive heeling,transom-mounts are not recommended for sailboats.

installation will not work on wooden boats or coredfiberglass hulls (foam, balsa wood, or plywood layerssandwiched between an inner and an outer skin).These materials contain air bubbles that reflect andscatter sound pulses before they reach the water.

Some in-hull transducers, such as the P79, can be usedon small aluminum-hull boats up to 6.7 m (22’) with ahull thickness of 0.38 mm (0.150”) or less. In theseinstallations, the transducer face is epoxied directly tothe hull.

P58

Transducer Styles and Mounting Methods

B744V P66

R199

DST800

SS260

R99

Application or Expectation for the Transducer

Cruising and SailingIf time on the water is spent sailing or cruising, ahigh-power transducer is not needed. Single-frequency200 kHz transducers with 200 W to 600 W RMS powerwill be more than enough in this application. Accuratedepth readings will range from 61 m to 152 m(200’ to 500’) depending on the depth instrument.

Recreational FishingIf the application is recreational fishing, a 600 Wtransducer will do the job. These transducers haveenough power to read bottom in over 305 m (1,000’)of water and have 50 kHz and 200 kHz dual-frequencycapability. Typically matched with small to mid-sizefishfinders, a 600 W transducer is perfect for bottomfishing, marking bait, and marking game fish.

Tournament Sport FishingA 1 kW to 2 kW transducer is a must for tournamentsport fishing. These powerhouses will give the user acrystal clear screen on medium to large fishfinders. Themultiple elements that make up the transducer candistinguish schools of fish as closely-spaced individualtargets and can distinguish fish close to the bottom.These transducers are so precise; fish are no longerconcealed by their surroundings.

Many of the 1 kW and 2 kW transducers have Airmar’sexclusive Broadband Ceramic Technology. The 200 kHzelement produces the highest resolution available todaywithout sacrificing sensitivity.

Commercial FishingThese transducers are available in frequencies from24 kHz to 200 kHz and power from 1 kW to 4 kW.Units feature high-efficiency designs producing superiorfishfinding and clear and distinct images of both thebottom and closely-spaced fish.

Transducer Styles and Mounting Methods

Navigation andOcean Survey

CommercialFishing

TournamentSport Fishing

CS229

RecreationalFishing

Cruising andSailing

M42

Navigation and Ocean SurveyThis broad product group features transducers rangingin power from 10 kHz to 2 MHz and sized from smallportable units for harbor survey to multi-frequencyarrays used in deep-sea sounding. Arrays of more than100 piezoceramic elements have been designed andmanufactured. Airmar can produce dual-beam andsplit-beam transducers, linear phased-array transducers,and multi-beam transducer arrays.

An example of a hydrographic product, the SwathBathymetry Transducer, has a transmitting array thatemits a fan-shaped beam that is very narrow in onedirection and very broad in the other. Echoes arereceived by 30 separate transducer elements. This gives30 sets of data, yielding a great amount of detail aboutthe bottom. One Swath is used to monitor theever-shifting channels in the Mississippi River

600 W vs 1,000W High-Performance Units

B744V600 W

SS2601 kW

Transducer Comparison: 600 W versus 1 kWThe photos below clearly show the screen resolution differences between a single-element 600 W transducer and amultiple-element 1 kW transducer.

Frequencies Number of Beam RatedElements and Width RMS PowerConfiguration (@-3dB) (W)

45° 600 W

12° 600 W

Frequencies Number of Beam RatedElements and Width RMS PowerConfiguration (@-3dB) (W)

50 kHz-AE 19° 1 kW

200 kHz-BH 6° 1 kW

50 kHz

200 kHz

Single-element600 W transducer

High-performance1 kW transducer

50 kHz

200 kHz

50/200 kHz-A

600W vs 1,000W High-Performance Units

Xducer I.D.™ FeatureAirmar’s exclusive Transducer ID feature allowsechosounders to query the connected transducergathering important operating characteristics. With thisdata, the echosounder and transducer function as aprecisely-tuned system. A Transducer ID enabled sensorcontains an embedded microcontroller thatcommunicates with the connected echosounder via asingle conductor in the transducer cable. The principaldata transmitted is intended to identify the type andconfiguration of the transducer. Then the echosoundercan alter its parameters of operation to optimizeperformance and to protect the transducer fromoverdrive. The Transducer ID feature also providesimportant information to installers and technicians such asserial number and housing style. Listed below is asummary of the information that the Transducer IDfeature can provide to future fishfinders.

• Airmar part number• Housing style• Serial number• Ceramic element

configuration• Date of manufacture• Acoustic window• Impedance matching

configuration• Nominal frequency(s)• Best transmit frequency(s)• Power rating• Beam pattern

The image above shows the depth and beamwidthdifferences between the single-element, 600 W, B744V andthe multiple-element, 1 kW, SS260.

High-Performance FairingAchieve maximum fishfinder performance by installing an Airmar transducer with a High-Performance Fairing. EachHigh-Performance Fairing is custom designed to match its transducer model. The fairing assures a vertical beam whichresults in strong return echoes. Additionally, the streamlined shape reduces drag and minimizes turbulence over the faceof the transducer. At speeds above 30 knots, screens continue to display clear images and solid bottom tracking.

Without High-Performance Fairing With High-Performance Fairing

The photos above show a boat-test comparison of a transducerinstalled with a High-Performance Fairing versus a standard fairing.The same transducer model was used. One transducer wasinstalled on the port side of the boat with a High-Performance Fairing,and the other was installed on the starboard side with a standardfairing. Using a switchbox, we were able to swap from onetransducer to the other. At speed, the significant resolution andclarity on the fishfinder screen when using the transducer with aHigh-Performance Fairing is clearly depicted.

200 kHz, 25 knots 50 kHz, 36 knots50 kHz, 23 knots

Without afairing, thebeam isangledimproperly.

With a fairing,the beam isangledproperly.

Advantages of a High-Performance Fairing Block

High-Performance Fairing OnlyHigh-Performance Fairing

Standard FairingHigh-Performance Fairing

Standard Fairing

Benefits of BroadbandTransducer Technology

Airmar is the first to introduce affordable BroadbandTransducers. This is an enabling technology that providesbetter fish detection today and will lead to dramaticadvances in echosounder performance in the future.While these transducers are more costly to manufacture,the present and future benefits are huge.

Broadband Transducers enhance fish detection onvirtually all of today’s fishfinders. They give betterdefinition; it is far easier to distinguish among individualfish and between fish and the bottom.

Airmar achieves superior results by using a new ceramicmaterial. It lets transducers operate over a range offrequencies while maintaining sensitivity. TheseBroadband Transducers are, by definition, low-Q devices.In other words, they exhibit very low ringing. There is littlevariation from transducer to transducer. Additionally,Broadband Transducers are relatively immune to theeffects of aging, so their frequency range remains stableover time.

Benefits TodayManufacturers now market echosounders that can adjustoperating frequency and power output. While these arepremium products, the designs are a precursor of thingsto come. With the ability to adjust frequency, anechosounder can operate Airmar’s broadband ceramicsanywhere in the 160 kHz to 260 kHz band. By selectingdifferent operating frequencies, two or morehigh-frequency sounders can work simultaneouslywithout interference. The frequency also can be adjustedto the mission. Lowering the operations frequencyincreases the beamwidth and depth capability; raising thefrequency narrows the beamwidth, increases echodefinition, and improves high-speed performance.

Future BenefitsHere is where it gets really exciting. In today’s fishfinders,good fish detection is obtained by transmitting a longpulse. This puts more energy on the target. With a longpulse, closely-spaced fish cannot be separated—you get abig blob. Fish close to the bottom appear attached to thebottom and are difficult or impossible to detect.

Airmar’s broadband transducers enable frequencymodulated (FM) transmissions; a.k.a. CHIRP or codedtransmissions. Using FM transmissions, you can achieveboth the benefits of long pulse, more energy on target,and short pulse, segregation of closely-spaced fish andidentification of fish on or close to the bottom. This isbecause the coding of the transmission is known and thereturn echoes are similarly coded. The technique is alsoknown as pulse compression. In summary, fishfinders ofthe future with FM transmissions will have dramaticallyimproved target resolution and signal-to-noise ratio.Airmar’s broadband transducer technology will enable thisto happen.

Figure of Merit

10

0-10

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-60

-70140 160 180 200 220240 260

Frequency (kHz)

dB(3)