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Sounders work on the principle of sonar. This technology was originally developed as a means of tracking enemy submarines during World War II. Back then they sure weren?t sending down chunks of pilchard on a hook when an object showed up ? more likely it was a depth charge!
A modern day boat sonar unit consists of a transmitter, transducer, receiver, and display. The transmitters? job is to generate and send an electrical impulse to the transducer. The power of that transmitter is expressed in either Watts peak to peak, or RMS.
Basically the electronic gurus have devised a complicated formula they call ?Root Mean Squared? (RMS) to express the power output of the transmitter in a more manageable formate for we poor laymen. If you take a look at a few ratings you will discover that the RMS is 1/8 of the peak to peak rating.
The amount of power you require is dependent on the depth and conditions of the water you intend working in.
Most of the less expensive units on the market today have a RMS rating of 200 Watts (1600W peak to peak). Lowrance units (up to the X75 model) actually only have 75W RMS. These units are purpose built for the freshwater market, which is very large in the US.
The 200 Watt RMS unit can provide an affordable unit capable of giving the user a pretty good indication of bottom type and fish presence to a depth of up to 100 metres in saltwater. Having said that, I would point out that water conditions (along with the quality of the receiver) have a very big impact on the quality of the signal and the quality of the display. Variations will occur from day to day, area to area, dependant on water conditions. There is always a difference between salt and freshwater.
Basically the more power in the transmitter, the superior the signal. Power delivers not only greater depth but also a clearer picture of what?s below. So power matched to a quality receiver, makes the unit.
From the perspective of buying a new sounder, the more power you can afford the better. However, price and power do seem to go hand in hand.

Transducer:
The transducer receives the electrical impulse from the transmitter, converts it to a sound frequency and shoots that sound frequency down into the depths beneath the boat.
Sound travels at a very reliable constant speed in water (4800 feet per second in freshwater). As each sound wave hits an object, its echo rebounds to the transducer, which collects the echo and sends it to the receiver.
The timing of that return signal is used to calculate the different depths of the objects below. The clarity or sharpness of the return gives you an indication of the hardness of the structure below.
Transducers come with varying cone angles and frequency. Sadly the job of selection has been made more complicated by some manufacturers following a different standard of measurement for cone angle than the status quo. Both cone angle and frequency is important to your operation.

Cone Angle:
The sound pulse fired by a transducer is not dissimilar in shape to that of a torch beam. As with a torch, the wider the beam, the less distance out the beam travels. You will also note that a torch projects a stronger light down the centre of the beam with brightness diminishing towards the beam edges. The same goes for the sound pulses sent from a transducer.
To measure the cone angle, we have to rely on manufacturers using the same standards. However, they?d don?t all do this.
Cone degree angle should be measured by taking the radius of the cone to the point where the sound wave has reduced in strength by -3Db. From that measurement the angle can be deduced. The -3Db should be used because for every reduction of 3Db, sound power is halved.
Humminbird bypass the -3Db mark and uses the -10Db mark from which to take this measurement. Probably they are trying to give the impression of a wider cone angle for the same depth of penetration.
In practice the standard cone angle of 20 degree is basically the same cone angle Humminbird advertise as a 45-degree cone. They are both the same degree being displayed, but presented to you differently!
To my knowledge only Humminbird use the  -10Db mark from which to measure cone angle. A measurement to the -10Db mark is around 10 times weaker and 10 times less sensitive than the best reading at the centre of the frequency signal. One that is taken at the -3Db is only half as weak as that at the best reading in the centre. In other words, Humminbird?s 45-degree cone will gain you no better reading than the others product advertised at 20 degree will.
Cone angle is important in that the wider the delivered cone the less depth, to a given power/frequency. Also the wider the cone angle, the greater the area beneath your boat displayed on your screen. As an example if you operated in shallow water, you would opt for a wide cone angle, deep water a narrow angle.

Frequency:
Each sound wave is made up of a series of high and low pressure points. As the frequency decreases, the distance between the high and low pressure points increases.
It?s important to grasp this concept if you are operating a dual frequency unit. The frequencies operating around the 200kHz do not penetrate the water as efficiently as say the 50kHz frequencies. Therefore 200kHz are better suited to shallower water operation.
As the pressure points are closer together on the higher frequencies, they are more likely to catch a fish with the full spectrum of their pressure range. This gives you a clearer (stronger) reading of the fish.
A higher frequency of say around 200kHz is ideal for helping the receiver separate individual targets from each other. On the other hand the lower frequencies pressure groups are wider apart. As a result you can get the situation where a fish swimming through the sound wave is only hit by part of the frequencies pressure wave, resulting in a poor or even missed reading of that fish.
The lower frequencies, because of the wider pressure spread, can often blend a group of fish together showing them as one target.
So why bother with lower frequency? This blending effect actually works to your advantage when evaluating bottom type or conditions. It can also be extremely helpful when you encounter very poor water conditions as it can allow you added control in eliminating noise. Lower frequencies also have far greater depth penetration and are essential for working depths of greater than 150 metres.
If I haven?t confused at this point, I probably will now. As you laterally spread the projected frequencies, you weaken their penetration.
For example, a 200kHz frequency sent out by an 8-degree cone might give you a great reading at 150 metres. However, the same frequency sent out under identical conditions through a 16-degree cone might only give you a great reading to 100 metres.
Dual frequency sounders offer the user versatility. A dual frequency of say 50/200kHz offers the user the choice of better fish identification and separation, to limited depths, when using 200kHz. It also offers him better bottom definition through the use of the 50kHz and greater depths if he should require it.

Unit Selection:
Most brands or models of fishfinders offer a number of choices in cone angle and frequency. If I were to be placed on the spot I would make the following recommendations:
Generally I?ve found a 16 to 25-degree transducer (Humminbird 35 to 55-degree) matched with 200kHz, 200W RMS a good all round performer for fresh or saltwater. In other words fishing large lakes, rivers, estuaries and inshore up to 80 metres.
For those fishos looking for greater depths (between 80 and 150m) will need to reduce the cone angle for the 200kHz to 8-degree (Humminbird 18-degree). Either that or increase power output to 500W RMS.
Any depth beyond 150 metres the 200kHz becomes less reliable and you will need 50kHz. Frankly, in all situations, I would opt for the dual frequency, having the 50kHz aboard is fantastic for picking bottom type in all depth ranges.

Receivers:
It costs considerably more to produce a better quality, more sensitive and efficient receiver, than it does to produce a higher output transmitter. So you can have all the power in the world, but the unit?s performance is critically related to the quality of its receiver.
Interestingly I haven?t come across any of the mainstream recreational sonar manufacturers who give any information on their receivers! That?s why TBF runs sounder tests ? so we can find out how really good they are.
We regularly come across sounders boasting 300W RMS, however when put to the test the receivers are not up to the job and the unit performs like a 150W RMS unit with quite often heaps of noise interference on the screen.
Buyer beware, we have all been conditioned into looking at the RMS, power output, but you can be sure there are units out there that do not have the quality receiver to back up that output.
Some manufacturers dress up the appearance of quality through their display, others trade off pixels and even power to improve receiver quality.
The difference between a good sonar and not so good comes back to the quality of the receiver. The cheaper units will invariably have a lower quality receiver.
Remember that the difference will not be a variation in power output, nor the amount of pixels in the display. It won?t even be the cone angle, but it will be the quality, sensitivity and general capability of the receiver that will make or break the performance of the unit.
The functions and chores of the receiver are extremely wide ranged. Not only does it have to have the ability to detect and amplify the faintest of signals, but it must have the capacity to dampen the extremely high transmit signal. It also has to separate signals that are close together into distinct, separate impulses for the display. The receiver quality is the most overlooked item on the modern fishfinder, yet is critical to the performance standard of the unit.

Display:
Basically you have two choices in displays - Liquid Crystal Graph (LCG) or Cathode Ray Tube (CRT) colour. Although colour offers a far wider diversification of options in the set up of presentation and therefore better interpretation, I am going to stick to the more popular and compact display unit, the LCG.  For size and weight, the LCG screen delivers a remarkable performance.
Night performance has been greatly improved with most LCG units. However make sure you test this function, using a dark room before you buy. The LCG offers remarkable readability in direct sunlight, a problem that can be experienced with CRT (colour).
LCG units generally are exceptionally convenient for fitting into small boat dash panels because they are so compact.
Dimensional measurement of the actual screen will always relate to dollars spent and available space on the boat. Generally the larger the screen the easer to view. However the larger screen will not display any more detail than is available on the smaller screen. Larger screens come into their own when operating split screen functions, or when you want to display large volumes of written material such as speed, temp, position etc., Watch the larger screens for their backlight function, many we have viewed are lacking in this department.
Pixel resolution is a factor when considering a unit. At the cheaper end of the market you get 128v x 64h pixel (block type image) to a higher resolution of 240v x 360h (very fine image). Bear in mind that the cost of achieving one feature is often at the expense of another. So look very carefully at the superfine units, and if the results aren?t reflected in the price, you can bet the quality of another factor in the unit has been sacrificed.
Raytheon, as an example, have compromised and come up with the very successful combination of 240v x 64 pixels in their lower priced (smaller screened) units and provide 240v x 128h pixels. The display produced is quite adequate, providing quite fine detail.
Meanwhile, Lowrance have opted to provide a resolution of 128v x 64h in their cheaper sets with an excellent 160v x 160h, right up to 200v x 320h in their dearer sets. However, be aware that Lowrance quote Peak to Peak power ratings and you will need to divide the figure given by 8 to get a RMS rating comparison with other units.
You will note that the higher resolution screens are at the expense of a considerable amount of power output for dollars spent. 
You will often see advertising suggesting units are completely sealed, or totally waterproof. Fact is that?s impossible to achieve in good old Australian conditions. With temperature extremes, expansion and contraction will provide a high possibility of such units displaying foggy screens in no time at all.
Units designed to breathe, quite often provide you with the best chance of long service life and are certainly easier to service should moisture become a problem.

Summary:
Clearly feature trade-offs are common place in this extremely competitive market. Sadly the trend to use different information standards is not making it any easier to choose a unit.
The two difficult areas are measuring of the cone angle and the method of quoting power. Also the total lack of receiver standard on all recreational sounder packaging is also another challenge for the buyer.
So when next considering a sounder, take the time to convert all power ratings and cone angles to a common denominator. Then weigh up the cost factor of the resolution being offered.
Talking to another sounder user is handy, but often very limiting in usefulness. Most users really have very little knowledge of what the oppositions units are like in comparison. Some might be shocked if they ever saw another unit in action alongside their favourite unit.
So keeping in mind the factors outlined above when looking for a new unit. Doing your homework can mean getting the right unit first go, and thereby improving your fish capture dramatically.
Good Luck.